CoHocDat - Le Xuan Mai

8/2/2019 CoHocDat - Le Xuan Mai

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Li ni u

p ng vi yu cu i mi trong chng trnh o to ca Trngi Hc Bch Khoa i vi cc nghnh Xy dng Cu ng, Xy dng Dn

dng - Cng nghip v Xy dng thu li - Thu in thuc cc h o to dihn v va hc va lm, ng thi nhn k nim 30 nm thnh lp Trng iHc Bch Khoa Nng. B mn C s k thut Xy dng khoa Xy dng Cung Trng i Hc Bch Khoa thuc i Hc Nng cho xut bn Giotrnh C hc t.

Cun sch ny dng lm ti liu phc v cho cng tc ging dy v hctp ca sinh vin cc nghnh Xy dng, ng thi c th lm ti liu tham khocho cc K s thit k Nn- Mng.

Ngy nay, cc bi ton C hc t c nghin cu pht trin rtnhiu, khi lng kin thc v C hc t rt ln. Khi bin son cun sch ny,chng ti c gng nu ln nhng vn c bn ch yu nht, ng thi bm st

cc tiu chun hin hnh nc ta v gii thiu cc phng php ang c pdng rng ri cc nc Ty - u, ngi c c th d dng nm bt v thchnh c. Chng ti mong rng mc ngn gn v d dng, cun sch nycng s c ch cho nhng ngi hc C hc t trc y hoc ang nghincu v C hc t, n s cng c li cc kin thc C hc t so vi nhng tiliu c gii thiu.

Khi bin son ni dung chng VI, cc tc gi c kt hp s dng chnlc nhiu ni dung trong cun ( Th nghim t hin trng v ng dng phntch Nn Mng) ca GS.TS. V Cng Ng - ThS. Nguyn Thi do Nh Xut bnKhoa hc v k thut xut bn nm 2003. Ni dung cc chng khc c tham

kho theo nhiu gio trnh c lit k ti mc cc ti liu tham kho.Cc tc gi xin c gi gm vo cun gio trnh ny lng bit n su sci vi cc thy gip cho cc tc gi c c thnh qu ngy hm nay:

GS.TS. V Cng Ng, GS.TS. Dng Hc Hi, TS. Hong Truyn, TS.Nguyn Hng Sn. ng thi cc tc gi cng xin chn thnh cm n tp th cnb khoa Xy dng Cu ng, Trng i Hc Bch Khoa - i Hc Nng ginh cho cc tc gi s quan tm, gip c hiu qu cun gio trnhhon thnh v sm ra mt bn c.

Do C hc t pht trin qu nhanh trong thi gian qua, do kinhnghim v kin thc c hn, nn chc chn bn thn nhng ngi vit khng

th nm

c y nhng din gii hay, nhng kt qu tt ca n, nn khngth trnh khi cc thiu st. Mong cc bn ng nghip trong chuyn nghnh chdn cho. Mi s gp v ni dung cun sch s c tip nhn vi lng bit n.a ch gp gi v: B mn C s k thut Xy dng khoa Xy dng Cung.

Trng i Hc Bch Khoa - i Hc Nng54 Nguyn Lng Bng - Qun Lin Chiu - TP NngEmail: xdcauduong @yahoo.com

Cc tc gi.


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Mc lcTrang

Li ni u 3

Mc lc 4Cc th nguyn thng dng 7M u 8Chng i: bn cht vt l ca t v phn loi t 31. S hnh thnh ca t 31.1Qu trnh phong ha 31.2Cc dng trm tch 31.3Cc nh hng ca mi trng a - Vt l n tnh cht ca t. 42. Cc thnh phn cu to ca t v tc dng ln nhau gia chng 52.1. Thnh phn rn cng 52.2. Thnh phn nc trong t 102.3. Thnh phn kh trong t 122.4. Cc tc dng qua li gia cc thnh phn trong t 13

3. Kt cu v C cu ca t 173.1. Kt cu ca t 173.2. C cu ca t 18

4. Cc ch tiu vt l ca t 20

4.1. Cc ch tiu vt l xc nh bng th nghim 204.2. Cc ch tiu vt l xc nh bng tnh ton 224.3. Cc ch tiu xc nh trng thi ca t 24

5. Phn loi t 286. Mt s tnh cht c l thng xy ra trong t 326.1. Tnh dnh ca t 326.2. Tnh co v n ca t 326.3. Tnh tan r ca t 336.4. Hin tng tikxotrofia ca t 33

6.5. Hin tng bin long ca t ct 346.6. Tnh m cht ca t 356.7. Tnh thm ca t 38

Cc v d mu 40

chng ii: xc nh ng sut trong nn t 44

.1Khi nim. 44

.2Phn b ng sut do ti trng ngoi gy ra. 44

2.1 Bi ton c bn - tc dng ca lc tp trung. 44

2.2 Phn b ng sut trong trng hp bi ton khng gian. 48

2.3 Phn b ng sut trong trng hp bi ton phng. 57


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.3Phn b ng sut trong nn t c xt n tnh khng ng nht v tnh khngng hng ca t 65

3.1 Trng hp di nn t l lp t cng. 67

3.2 Trng hp nn t gm hai lp, lp di l lp mm yu. 70

.4 Phn b ng sut tip xc di y mng. 71

4.1 Trng hp bi ton khng gian. 72

4.2 Trng hp bi ton phng. 74

4.2.1 Trng hp mng cng hnh bng chu ti trng trung tm. 75

4.2.2 Trng hp mng cng hnh bng chu ti trng lch tm. 76

.5 Phn b ng sut do trng lng bn thn ca t gy nn. 76

5.1 Trng hp t nn ng nht. 76

5.2 Trng hp t nn gm nhiu lp c tnh cht khc nhau. 775.3 Trng hp t nn c mc nc ngm. 77

5.4 Trng hp nc c p. 79

CHNG III: BIN DNG V LN CA NN T 91

1. Khi nim chung. 912. Tnh bin dng ca t. 92

2.1. Cc nghin cu v tnh cht bin dng ca t. 922.2. Cc c im bin dng ca t. 99

2.3. Cc nhn t ch yu nh hng n bin dng ln ca t. 102

3. Tnh ton ln cui cng ca nn t. 103

3.1. Trng hp c bn: ln ca t trong cc trng hp th nghim nn. 103

3.2. Tnh ton ln cui cng ca nn t di mng cng trnh. 107

4. L thuyt c kt thm v tnh ton ln theo thi gian. 125

4.1. L thuyt c kt thm ca K.Terzaghi v phng trnh vi phn c kt thm.

1264.2. Tnh ton ln ca nn t theo thi gian trong iu kin bi ton mt chiu.

129

4.3. Tnh ton ln ca nn t theo thi gian trong iu kin bi ton phng vbi ton khng gian. 139

chng iV: cng v n nh ca nn t 1451. Khi nim chung. 145

2. Sc chng ct ca t. 146

2.1. Sc chng ct cc hn ca t, nh lut ct ca t. 1462.2. Cc yu t nh hng n sc chng ct ca t. 152

2.3. T bin ca t st v s nh hng ca n n cng chng ct. 155


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3. Trng thi cn bng gii hn ti mt im trong nn t v iu kin cn bnggii hn mohr - coulomb 1593.1 Trng thi cn bng bn v trng thi cn bng gii hn ti mt im bt ktrong nn t. 159

3.2 iu kin cn bng gii hn Mohr - Coulomb. 1594. Xc nh sc chu ti ca nn t 1624.1. Phng php tnh ton da vo l lun nn bin dng tuyn tnh kt hp viiu kin cn bng gii hn ( da vo s pht trin cu vng bin dng do). 164

4.2 Phng php tnh ton da vo l thuyt cn bng gii hn. 1714.3. Phng php tnh ton da vo gi thit mt trt trc: 192

5 n nh ca mi dc 1945.1. iu kin n nh ca t trn mi dc. 196

5.2. Phn tch n nh mi dc theo phng php mt trt cung trn hnh tr. 199

CHNG 5: tnh ton p lc t ln lng tng chn.

1. Khi nim chung. 2111.1. Phn loi tng chn t 211

1.2. p lc t v iu kin sn sinh ra p lc t. 2121.3. Cc l thuyt tnh ton p lc t ln tng chn. 2142. Phng php xc nh p lc tnh ca t ln tng. 215

3. L thuyt p lc t ca C.A.Coulomb. 2153.1. Tnh ton p lc ch ng ln nht ca t theo l thuyt C.A.Coulomb. 216

3.2. Tnh ton p lc b ng nh nht ca t tc dng ln lng tng chn. 223

4. Cc phng php da vo l thuyt cn bng gii hn. 2244.1. Tnh ton p lc t theo l lun W.J.R.Rankine. 224

4.2. Tnh ton p lc t theo l thuyt V.V.Xclovski. 2305. Tnh ton p lc t ln tng chn trong cc trng hp thng gp. 232

5.1. Tr

ng hp ti trng ngoi tc dng ln mt t. 2325.2. Trng hp lng tng gy khc v mt t phng. 235

5.3. Trng hp t p sau tng gm nhiu lp. 235

5.4. Trng hp t p sau tng c nc ngm. 236

6. Nhn xt phm vi p dng l thuyt p lc t ln tng chn. 240

7. Mt s vn cn ch khi tnh ton p lc t ln tng chn. 2417.1. Vic chn cc ch tiu c l ca t p. 241

7.2. nh hng ca s n t v p lc thy ng. 2437.3. Bin php lm gim p lc t ln tng. 243

Chng VI. Cc th nghim t hin trng.6.1. Th nghim xuyn tiu chun (SPT). 244


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6.2. Th nghim xuyn tnh. 248

6.3 Th nghim nn ngang trong l khoan (PMT). 256

6.4 Th nghim nn ngang DMT (DILATOMETER). 261

6.5. Th nghim ct cnh (VST). 273

6.6. Th nghim bn nn hin trng. 275

Ti liu tham kho

Cc th nguyn thng dng:

100kPa = 100kN/m2 = 1 bar = 1 pa 1kG/cm2 = 10 t/m21tsf = 2 ksf


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CHNG I Trang 8

M u1. nh ngha v i tng nghin cu:

c hc t l mt ngnh ca c hc ng dng chuyn nghin cu v t.Hu ht cc cng trnh xy dng u t trn t, ngha l dng t lm nn cho cccng trnh, s khc cc cng trnh nh nn ng, , p t th li dng t lm

vt liu xy dng. V vy, mun cho cc cng trnh c tt, ngha l cng trnh nnh, bn lu v tit kim th nht thit phi nm r cc tnh cht ca t khi dngn lm vt liu xy dng hay lm nn cho cc cng trnh xy dng.

Nh vy i tng nghin cu ca c hc t l cc loi t thin nhin, lsn phm ca qu trnh phong ha cc gc lp trn cng ca v qu t. Miloi phong ha c tc dng ph hy gc khc nhau v n to ra cc loi t khcnhau. c im c bn ca t l mt vt th gm nhiu ht rn ring r khng gnvi nhau hoc gn kt vi nhau bng cc lin kt c sc bn nh hn nhiu ln sovi sc bn ca bn thn ht t. Do qu trnh hnh thnh t m chng tn ti

rng trong t v rng ny li c kh nng thay i di nh hng ca tc ngbn ngoi. Ngoi ra trn b mt ht t c nng lng, chng gy ra cc hin tngvt l v ha l phc tp, dn n lm thay i cc tnh cht vt l v c hc cat. V vy khi nghin cu t phi nghin cu n ngun gc hnh thnh v cciu kin t nhin m t tn ti.2. c im v ni dung ca mn hc:

C hc t l mn hc cn vn dng cc hiu bit v t t cc mn khoa hckhc c lin quan nh a cht cng trnh, th cht hc... V ng thi vn dng cckt qu ca cc ngnh c hc khc nh c hc cc vt th bin dng (l thuyt nhi, l thuyt do, l thuyt t bin). Trn c s ca cc l thuyt ny, C hc t xy dng c cc l thuyt ring ph hp vi cc qu trnh c hc xy ra i vit. Tuy vy ngoi cc nghin cu l thuyt, cc nghin cu thc nghim, thcnghim v cc quan trc thc t cng ng vai tr quyt nh trong nghin cu sdng t trong xy dng.

T cc nghin cu l thuyt v cc nghin cu thc nghim, C hc t tptrung gii quyt cc nhim v v ni dung c bn sau:

- Xc lp cc quy lut c bn v cc qu trnh c hc xy ra i vi t, ngthi xc nh c cc c trng tnh ton ng vi cc qu trnh xy ra .

- Nghin cu s phn b ng sut trong t, quan h gia ng sut v bin

dng di tc dng ca ngoi lc.- Gii quyt cc bi ton v bin dng, v cng , v n nh cc nn t,

v mi dc cng nhbi ton p lc t tc dng ln tng chn.3. S lc lch s pht trin ca mn hc

C hc t l mn hc c hnh thnh chm hn nhiu so vi cc mn hcng dng khc, nhng t lu loi ngi c nhng nghin cu v t, tuy nhin dox hi lc hu nn cc kin thc v t xy dng ch nm mc nhn thc cmtnh, cha c nng cao thnh nhn thc l ln. Nhiu nh khoa hc c nhngcng hin to ln v c cng xy dng nn mn c hc t ngy nay. y ch

gii thiu hai nh bc hc c cng lao ln n s pht trin ca c hc t.Cng trnh khoa hc u tin ca C hc t l ca C.A Coulomb (1736 -

1806) thiu t k s cng binh, vin s vin khoa hc Php, nm 1773 a ra l


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CHNG I Trang 9

lun ni ting v cng chng ct ca t v cng l ngi u tin xy dngc phng php xc nh p lc t ln vt chn. Tri qua hai th k v cho nngy nay, cc phng php ca ng vn c s dng rng ri.

S hnh thnh ca c hc t nh mt mn khoa hc c lp vi h thnghon chnh v cc phng php ring bit ca n c xem nh bt u t nm

1925, khi K.Terzaghi (1883-1963) cho xut bn cun C hc t trn c s vt lca t.Nm 1963 Hi ngh khoa hc quc t v C hc t - Nn mng hp ln th

nht v sau c 4 nm hp mt ln. Hi ngh C hc t - Nn mng v cc hitho khoa hc lin quan cng c t chc nhiu nc v khu vc.

n nay, C hc t tr thnh mt mn khoa hc vi nhiu ni dungphong ph, gm nhiu lnh vc khc nhau nhm p ng s pht trin mnh m cacng nghip, xy dng.

Vit Nam , C hc t c bt u nghin cu t nm 1956. n nay ing nhng ngi lm cng tc nghin cu C hc t trng thnh c v cht

lng v s lung, sc gii quyt cc bi ton a dng v phc tp do thc t xydng cc cng trnh ra. Tuy vy do iu kin kinh t v x hi cn hn ch nntrang thit b chuyn nghnh u t cha y v ng b, v vy vic pht trinkin thc v cng ngh v C hc t cn i hi nhng n lc ln hn.


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CHNG I Trang 10

chng i: bn cht vt l ca t v phn loi t1. s hnh thnh ca t1.1. Qu trnh phong ha:

S ph hoi v lm thay i thnh phn ca gc di tc dng vt l, hahc ca cc yu t khc nhau gi l qu trnh phong ha. Do tc dng ca phong

ha nn cc khi ca nham thch quyn khng th gi nguyn c trng thiban u ca n, m lun thay i, b v vn, b ri ra, b cc dng nc v gi cuni, hnh thnh cc lp t ph quanh phn ln mt ngoi ca v qu t. Do vy, khis dng t lm nn cng trnh, lm mi trng, hoc vt liu xy dng, cn phixt n s bin i khng ngng xy ra cc lp t bn trn ca v qu t.

Da vo c trng bin i ca gc v s nh hng ca cc tc nhnphong ha, c th chia ra phong ha vt l, phong ha ha hc v phong ha sinhhc. Trong , theo quan im v xy dng, ch c phong ha vt l v phong haha hc l ng c quan tm nghin cu.

Phong ho vt l:Sinh ra ch yu c lin quan vi s thay i ca nhit ,gy nn n nhit khng u v th tch, lm cho cc gc b ph hoi v phn vnra thnh nhng ht to nh khng u nhau, nhng khng lm thay i v thnh phnha hc ca khong vt. Do sn phm ca phong ha vt l to ra cc loi t ri( dm, cui si, cc ht c t, v.v ) c thnh phn khong vt tng t vi gc.

Phong ho ho hc:Sinh ra l do cc tc nhn nh nc, xy, axit cacbonicv cc axit khc ha tan trong nc, lm cho cc gc b ph hoi km theo sthay i thnh phn khong vt mi n nh hn, to ra cc loi t st khc nhauc kch thc ht nh v cc k nh, phn ln khng phn bit bng mt thngc. Cc nhm ht st nh ny phn ln cha nhiu ht n khong thuc ba nhmkhong vt - Mnmrilonit, Ilit v Kaolinit. Tt c nhng khong cht ny u ccu to tinh th bn mng, nhng c nng lng b mt khc nhau, Mnmrilonithot ng mnh hn c v Kaolinit l yu nht.

Thng thng qu trnh phong ha vt l v ha hc xy ra cng mt lc v

h tr cho nhau. vng kh hu kh lnh th phong ha vt l l ch yu, cn vngkh hu nng m, nh nc ta chng hn, th phong ha ha hc ng vai tr quantrng hn.

Cc sn phm cui cng ca s phong ha c th nm ngay ti ch hnhthnh ban u ca n hoc c th b di chuyn i ch khc bi dng nc hoc gi

v to thnh cc dng trm tch ca t.1.2. Cc dng trm tch ca t:

- Trm tch tn tch (Eluvian) : L trm tch ca nhng sn phm phong hacc lp v nm ngay ti ch hnh thnh ban u ca n. c im ni bt l baogm cc ht c dng gc cnh nhn sc khng th phn loi theo kch thc ht, vthnh phn thch hc ni chung rt ging gc. nc ta, do kh hu nhit inn qu trnh phong ha ha hc xy ra mnh lit hn v bin cc loi gc thnhcc loi t st c mu , nu, vng, thng gi l t Laterit. Qu trnh Laterit hany l qu trnh hnh thnh t ch yu nc ta.

- Trm tch sn tch (Deliuvian) : Ch yu c tch ly li sn dc vchn sn dc, cng nh cc khonh thp st ng chia nc. Trm tch ny cto thnh do nc ma cun tri cc sn phm ri xp ca phong ha t nhng vng


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CHNG I Trang 11

cao hn a xung. c im gm cc loi t ri rc, cc ht t nh ln vinhng ht rt ln, khng n nh, thng hay b trt l theo mt lp gc bndi, c b dy ca lp t rt khng ng u.

- Trm tch bi tch (Aluvian): l tt c cc sn phm c to thnh bngmi cch sng, hp thnh cc trm tch cc thung lng c, hin i v lng sng.

c im ca loi trm tch ny l c tnh phn lp theo quy lut v thnh phn htca chng, t cc lp bn trn thng l t loi st v ct mn, n cc lp bn dithng c cu to bi t ct ln t si v cui.

- Trm tch tam gic tru v h sng tru: c hnh thnh do sng mang vtliu n v lng ng vng ca sng v cc khc sng cht. Trm tch ny cc trng bi s tn ti cc lp bn st, bn hu c cha c nn cht my, ctmn, ct pha st ... Cc t thuc loi ny thng c dy v din tch phn b ln,to thnh mt khi do c tnh nn ln.

- Trm tch bin: L s tch ly di y bin cc vt liu do dng ncmang n. Thnh phn v tnh cht ca loi trm tch bin ny ph thuc rt nhiu

vo s tn ti cc cht hu c thc vt v ng vt sng di y bin. Trm tchny ch yu l cc t st v t bn ph bin trn mt din tch rt rng ln vc c trng bi nhng tnh cht rt khc nhau ty theo tui v lch s hnh thnhca chng.

Vi s m t tm tt cc loi trm tch trn, th thy r rng cc t trongthin nhin rt khc nhau, v bn cht vt l ca chng cc k phc tp. T qutrnh hnh thnh ca t n hon cnh hin ti ca chng, tt c nhng yu t to nn nhng tnh cht c o ca cc t thin nhin.1.3 nh hng ca mi trng a - vt l n tnh cht ca t.

Vi cc vn trnh by trn, c th thy r rng mi trng a - vt lc nh hng rt ln n s hnh thnh ca t, nn khi nghin cu t khng thtch ri nhng iu kin lch s t nhin hnh thnh v tn ti ca t c.

Chng hn, ty theo tui v ton b lch s trc y ca s hnh thnhchng, cc loi t st thin nhin c nhng tnh cht rt khc nhau. V d: cc tst Cambri tui khong 500 triu nm th chc chn rng, trong thi gian di chu tc dng ca nhng p lc ln thay i, b p mt nc trong tng b phn vb kh i, v.v... Cc t st ny tri qua mi qu trnh ha hc v ha - l xyra, ngay c nhng qu trnh xy ra vi nhng tc rt nh hon ton khng th

nhn bit

c trong mt khong thi gian t

ng i ngn. Mt khc, cc qu trnhdnh kt cc k chm xy ra trong mt thi gian di cng c nh hng n kt cuv c cu ca t loi st . Tt c cc qu trnh to nn tnh cht hon tonc bit ca cc t st Cambri so vi cc t st khc. Theo kt qu nghin cu thcc t st ny c th coi nh vt liu cng nht n hi, c kh nng chu ti ln.Tri vi cc t st Cambri, cc loi t st (trm tch bin, h, m) hin ithng cn t c nn cht, chng thng c trng thi n nho v c sc chu tikhng ng k.

i vi cc t ct cng vy, chng ph thuc rt ln vo iu kin hnhthnh ca chng, c loi ct trng thi rt cht, c loi th li ri xp, thm ch c

loi ct trng thi huyn ph d sinh hin tng ct chy.


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CHNG I Trang 12

Do , khi nghin cu cc t thin nhin c xt n tc dng tng h cachng vi mi trng xung quanh v s bin i lin tc cc tnh cht ca chng, thcn phi ch nhiu n lch s ca chng, ngha l ch n cc iu kin v dinbin ca qu trnh hnh thnh cng nh hon cnh a - vt l ca s hnh thnh t.Qu trnh "ha " c mt ngha quan trng trong s hnh thnh cc tnh cht mi

ca t. Cc hin t

ng ti kt hp (s nn cht, s dnh kt) v kin to (ch yu ls trt xung ca mt phn v qu t) c th to nn nhng iu kin c kh nnglm thay i thnh phn v kt cu ca t, hn na, cng vi nhng p lc v nhit thch hp, chng c th dn ti hin tng bin cht, ngha l thay i hon toncc ri bng cch kt dnh li, kt tinh li cc ht khong vt ca chng n khithnh cc khi lin.

Do cc tnh cht ca t ph thuc rt nhiu vo nhng tc dng ca mitrng xung quanh, nn trong C hc t, khi chn cc s tnh ton cn phi xtn hon cnh t nhin m t tn ti. Cn vic xc nh cc c trng tnh ton cat th cn m bo sao cho cc mu t th nghim phn nh c trng thi tn ti

t nhin ca n. p ng c yu cu trn, cc mu t dng th ngim phic gng lm sao m bo cho kt cu ca n t b ph hoi nht.

2. Cc thnh phn cu to ca t v tc dng lnnhau gia chng

Nh trn trnh by, t thin nin l mt vt th phn tn bao ph phn lnb mt ca v qu t. Do khi nghin cu cc t thin nhin cn phi ch rngchng l mt h thng phc tp, c tc dng tng h ln nhau gia cc thnh phnrn (cng), lng v kh.

Trong trng thi t nhin, quan h gia cc nhm ht ring r c ngha cbn v c bit l s c mt ca s lng cc ht rn nh v cc k nh trong t,chng c din tch b mt ring ln nht v do c hot tnh cao nht.

Trng hp tng qut, t gm ba thnh phn: Cc ht khong cht rnthng chim phn ln th tch ca t, th lng chim mt phn hay ton bkhong trng gia cc ht rn ca t v thnh phn kh chim phn cn li trongcc l rng ca t, gm ch yu l khng kh. Cc tnh cht ca nhng thnh phnny, t l s lng gia chng trong t, cc tc dng in phn t, ha - l, c hcv cc tc dng tng h khc gia cc thnh phn ca t quyt nh bn cht

ca t.2.1. Thnh phn rn (cng) ca t:Thnh phn rn ca t ch yu gm cc ht khong vt nguyn sinh hoc

th sinh, thng gi l ht t, c kch thc t vi xentimet n vi phn trm,phn nghn milimet. Cc tnh cht ca t ph thuc vo thnh phn khong chtca chng.

2.1.1. Thnh phn khong ca t:Thnh phn khong cht ca t ph thuc ch yu vo thnh phn khong

ca gc v vo mc tc dng ca phong ho i vi cc gc y. Ty theo

mc tc dng ca phong ha khc nhau, thnh phn khong s khc nhau, ngayc khi do cng mt loi gc sinh ra, do n c nh hng khc nhau n tnh


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CHNG I Trang 13

cht vt l v c hc ca t. Cc khong vt to thnh t trong thin nhin c thphn thnh hai nhm nh sau: Khong vt nguyn sinh v khong vt th sinh.

Cc khong vt nguyn sinh: Thng gp trong t thin nhin l Fenpat,thch anh v mica. Cc ht t c cha thnh phn khong ny thng c kch thcln. i vi cc nhm ht ln thng t khi khc nhau v tnh cht c - l ca

chng, ngay c nhng loi t c lch s khc nhau, ng thi thnh phn khongcng khng c nh hng nhiu ti tnh cht c - l ca chng.Cc khong vt th sinh: Chia thnh hai loi khc nhau ty theo tnh cht

ha tan trong nc. Trong s cc khong vt th sinh khng ha tan trong nc,thng gp nhiu nht l Mnmrilnit, Ilit v Kaolinit, cc khong vt ny cn gil khong vt st, v chng l thnh phn ch yu ca cc ht st (nh hn 0,005mmv loi c bit nh hn 0,002mm). Cc khong vt ny c cu to kt cu phn tdng tm r rt, nhng tnh hot ng keo b mt rt khc nhau. i vi thch anh,tnh hot ng keo b mt gn bng khng, i vi Kaolinit khong 0,4, i vi Ilitl 0,9 v Mnmrilonit t 1,5 n 7,2 ty theo n cha ion canxi (Ca++) hay ionNatri (Na+). T c th thy rng, thnh phn khong cht ca t ch nh hngch yu n cc ht t nh v cc nh, v rng cc ht t cng nh th t din tch(m2/g) ca chng cng ln, do hot tnh keo ca khong vt c pht huy y nht, m nh trn trnh by, hot tnh keo ca cc loi khong vt khc nhauth rt khc nhau, dn n tnh cht c - l ca t cng khc nhau.

Cc khong vt th sinh ha tan trong nc thng gp l: Canxit, micatrng, thch cao v mui m,v.v...

2.1.2. Thnh phn ht ca t:Trong t nhin t do cc ht to nh c thnh phn khong vt khc nhau hp

thnh. Kch thc ca cc ht thay i trong mt phm vi rt rng ln, t hng chc,hng trm xentimet nh cc hn tng, cui, n vi phn trm, vi phn nghnmilimet nh ht st. Ht t cng nh th t din tch cng ln, do nng lngmt ngoi cng ln v tnh cht ca t cng phc tp. Cn i vi t ht to th lrng gia cc ht ln, nn tnh thm nc ln hn t ht nh. iu ni ln rtnhiu tnh cht c - l ca t c lin quan n thnh phn ht ca t. Tuy vycng cn lu rng chng ta khng th nh gi mt cch nh lng nh hngca thnh phn ht n tnh cht ca t c, bi v tnh cht ca t cn do nhiuyu t phc tp khc quyt nh, hn na ty iu kin c th nh hng ca chng

cng rt khc nhau.Khi nghin cu thnh phn ht ca t, trc ht phi tin hnh phn tch htt phn chia tt c cc loi ht c kch thc cc ht khc nhau thnh tngnhm. Trong mi nhm kch thc c th thay i trong mt phm vi nht nhnhng c bn chng c nhng tnh cht c - l gn ging nhau. Mi nhm nh vygi l nhm ht. Lng cha tng i ca cc nhm ht trong t (Tnh theo phntrm trong tng khi lng t kh) gi l thnh phn ht ca t hay cn gi lthnh phn cp phi ca t.

Hin nay, ty theo tng nc v ty theo mc ch s dng m gii hnng knh khi phn chia cc nhm ht v t l gia cc nhm ht khi phn loi t

cng c t nhiu khng hon ton thng nht. nc ta, vic phn chia cc nhmht theo mc ch xy dng hin nay thng dng bng phn loi (I-1) sau y:


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CHNG I Trang 14

Bng (I - 1): Phn nhm theo ng knh ht

Nhm ht Phnnhm

Kch thc ht(mm)

Tnh cht chung

Ln >800Va 800 - 400

ln hc

Nh 400 - 200Rt ln 200 - 100Ln 100 - 60Va 60 - 40

dmcui

Nh 40 - 20Th 20 - 10Va 10 - 5Sn, siNh 5 - 2

Tnh thm ln, khng dnh, dng cao canc mao dn rt nh, khng gi c nc

Th 2 - 0,5

Va 0,5 - 0,25Ht ct Nh 0,25 - 0,05

D thm nc, khng dnh, dng cao ca

n

c mao dn khng ln, gp n

c khng nra, khi kh khng co li, ri rc, khng thhin tnh do, tnh nn ln nh.

Th 0,05 - 0,01Ht bi Nh 0,01 - 0,002

Tnh thm nh, hi dnh khi t, nc maodn dng ln tng i cao v nhanh, gpnc n ra, kh khng co nhiu.

Ht st < 0,002

Hu nh khng thm nc, tc dng ca ncmng mng r rt, lc m c tnh do, tnhdnh ln, gp nc n ra nhiu, kh co linhiu, tnh nn ln ln.

Th nghim phn chia cc nhm ht t gi l th nghim phn tch ht,ty theo kch thc ht to nh m k thut phn tch c khc nhau. Ni chung trongnhng phng php phn tch thnh phn ht, ch yu chng ta mi ch dng hailoi chnh nh sau:

- Phng php dng ry: Phng php ny dng cho cc loi t ht ct vln hn. Ngi ta dng mt h thng ry c ng knh l to nh khc nhau, tincho vic s dng thng ngi ta dng loi ry c ng knh l trng vi gii hn

ng knh ca cc nhm ht phn chia nh trn. nc ta dng ry nh nht l

0,1mm, cn Bc M v mt s nc Ty u ngi ta nh s ry theo s lng ltrn mt ins vung, ry nh nht l No200 tng ng vi kch thc mt l l0,074mm. Do nguyn nhn ny 0,074 c cc nc trn xem l bin tiu chungia vt liu ht th v ht mn.

- Phng php thu lc: Phng php ny da trn c s nh lut Stokes,trong tc ca cc ht hnh cu lng chm trong mi trng lng l hm s cang knh v trng lng ring ca ht t. Trong s cc phng php th nghimda trn nguyn l ny, nc ta thng dng nht l phng php t trng k,dng xc nh thnh phn ht ca t ht bi v ht st. Ni chung phn tch ht

ca t st l mt vn ht sc phc tp, hin nay cn nhiu vn ch a cnghin cu k cng, chng ta cn c bit lu ti. Cch tin hnh c th ca tngphng php c th xem trong cc ti liu hng dn th nghim t v cc ti liu


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CHNG I Trang 15

c lin quan. Nu trong t ng thi c c hai nhm ht nu trn th phi kthp c hai phng php th nghim trn xc nh.

Kt qu th nghim phn tch ht ca t c biu th bng ng cong cpphi ca t, v trn h trc ta bn logarit, trong trc honh biu th logaritca ng knh ht cn trc tung th biu th lng cha phn trm ca nhng ht c

ng knh nh hn mt

ng knh cho no . Chng hn theo kt qu phntch, biu din bi ng cong cp phi I ca t hnh (I -1) th lng cha ht bil 72%, lng cha ht ct 17% v lng cha ht st l 11%.

(17%)

(72%)

(11%)0,001(mm)

100

1,0100 10 0,1 0,01

50 I) D10=0,0046mmD60=0,041mmCu=9

II) D10=0,17mm

D60=0,40mmCu=2,35

II I

A' 11%A

B' B

83%

100% 100%Ht cui Ht si Ht ct Ht bi Ht st

tng

Ht ct

Ht bi

Ht st

Hnh I - 1:ng cong tch ly ht

ng cong cp phi ca t c dng xc nh tn gi, ng knh chiu v h s khng ng u ca t. xc nh tn t, sau khi v c ngcong cp phi (ng cong tch ly ht), cn tm ra lng cha tng i ca ccnhm ht ct, ht bi v ht st trong t. Da vo kt qu v dng cc bngphn loi t (bng I-5) xc nh tn ca loi t ang xt ng thi lm c s

cho vic nh gi cc tnh cht c - l ca n.H s khng ng u c k hiu l Cu v c xc nh theo cng thc:

10

60

D

DC u = (I - 1)

D10 l ng knh m nhng ht bng n v nh hn n chim 10%, cn D60l ng knh m nhng ht c kch thc bng v nh hn n chim 60% tng khilng t kh. i vi loi t trn hnh (I -1) ng knh ny tng ng vi im Bv D60 = 0,041mm. H s khng ng u ca mt loi t cng ln, th t ccu to bi cc ht c kch thc cng khng u nhau, ngc li khi Cu cng nh

th t cng u ht. Thng thng trong thc t h s khng ng u ch p dngcho cc loi t ri. Cc loi ct si, ct th v ct va, nu c h s khng ngu ln hn 3 th c gi l ct khng u, v c xem l c cp phi tt, v lcny cc l rng gia cc ht ln c cc ht nh xen k v lp kn, lm cho chtca t tng ln v tnh thm gim i, ng thi t c tnh ln nh v kh nngchng ct ln khi chu tc dng ca ti trng ngoi.

Nh phn trn trnh by, gia kch thc cc ht t v thnh phn khongca chng c mi lin quan mt thit vi nhau. V d : vi nhng ht c kch th cln hn ht ct thng c thnh phn khong ging vi gc, cc ht c kchthc ca ht ct thnh phn khong vt ca chng thng thuc loi nguyn sinh,trong cc ht ln thng cha cc khong vt km n nh, d b ph hy dophong ha gy nn nh Fenft, Mica en, v.v... Cn cc ht nh th phn ln c


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CHNG I Trang 16

cha khong vt n nh, kh b phong ha nh thch anh. T c th thy rng,mc d cng thuc loi ht ct, nhng t gm cc ht c kch thc ln nh khcnhau th dn n nhng tnh cht c - l cng khc nhau. i vi cc ht st th chyu do khong vt th sinh to nn, trong c cc ht kch thc tng i lnthng l nhng ht khong vt loi Kaolinit, cn nhng ht c kch thc nh ht

keo l nhng ht khong vt loi Mnmrilorit, nhng ht c kch th

c trung bnhgia hai loi trn th thng l nhng ht khong vt loi Ilit.2.1.3. Hnh dng ht t:

Hnh dng ht t rt khc nhau t dng hnh cu n dng tm mng v hnhkim, do m tnh cht ca t s khc nhau khi hnh dng ca cc ht khc nhau.

Thng thng cc nhm ht c kch thc ln nh ht ct tr ln th c hnhdng trn nhn hoc sc cnh. Trong trng hp ny hnh dng ca ht t s c nhhng nhiu n tnh cht ca t, chng hn trong nhm cc ht ct gm nhng htthch anh c gc cnh sc nhn, nh chng c th xen k vo nhau xp xpc cht hn so vi nhm cc ht c cng kch thc nhng c dng trn nhn.

i vi nhng nhm ht c kch thc nh (nh ht st hay ht keo), bngknh hin vi in t ngi ta xc minh rng hu nh tt c chng u c hnhdng bn tm r rt hoc l trong nhng trng hp ring c dng hnh kim phngdi. Trong trng hp ny hnh dng ca ht t t lm nh hng n tnh cht cat m tnh cht ca t ch ph thuc ch yu vo thnh phn khong ca chng.Nh trn trnh by, thnh phn khong ca cc ht t li c quan h mt thitvi mc phn tn ca chng, mc phn tn ca t khc nhau dn n chngc t l din tch khc nhau.

Theo kt qu phn tch cc mu t (Bng I -2) cc ht thuc nhm ht st c

t din tch rt ln, do nng lng mt ngoi ca chng cng rt ln v to nncho chng nhiu tnh cht ring bit khc.Bng I -2 : T din tch ca cc khong vt st

Kch thc thc t tnh bng anstron(0,001)

Tn khong vtT s kch thc

cc chiu Chiu di v chiurng

Chiu dyT din tch tnh

bng m2/g

Mnmrilonit 100 100 1 1000 - 5000 10 - 5 800Ilit 20 20 1 1000 - 5000 50 - 500 80

Kaolinit10 10 1

1000 - 20000 100 - 1000 10

Theo bng (I -2) c th thy rng b mt (t din tch) ca nhng nhm htst rt nh (nh khong st Mnmrilonit) t ti vi trm mt vung trong mtgam t. iu quan trng cn phi ch na l cc khong vt nhm Mnmrilonitkhng nhng ch c t din tch ln m cn c kh nng hp th ln nht v tnh nmnh nht khi gp nc. iu ny c th c gii thch bi cu trc tinh th cachng.

Trn hnh (I -2) trnh by cu to mng li tinh th (kt cu phn t) caKaolinit v Mnmrilonit. Mng tinh th n v ca Kaolinit c nm lp in t vi

mng li tinh th bt ng.V gia hai tinh th n v tip gip nhau c lin kt cht [gia - 6(O) v

+6(OH)] nn chng kh tch ri nhau, lm cho Kaolinit t n khi gp nc. Cn


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CHNG I Trang 17

mng tinh th ca Mnmrilonit c by lp in t v mng li tinh th di ngc, v lp in t ca hai tinh th n v gn nhau quay vo nhau l cc in tca xy c in cng du [gia-6(O)v -6(O)] v gia chng c lc y, nn chngd b tch ri nhau, cng chnh v th nn n d cho cc phn t ca nc chuivo gia lm dn rng cc mng li tinh th ca Mnmrilonit ra, lm cho ht

khong vt ny c tnh n ln khi gp n

c.

+6(OH)

-6(O)

4(Al)

4O+2(OH)

+4(Si)

-6(O)

+6(OH)

-6(O)

4O+2(HO)

+4(Si)-6(O)

4(Al)

4O+2(HO)

-6(O)

Truc b

Truc

c

Truc b

Truc

c+4(Si)

7.2Ao

9.3Ao

a) Kaolinit a) Monmorilonit Hnh I - 2: Cu to mng tinh th ca Kaolinit v Mnmorilonit

2.2. Thnh phn nc trong t:

Trong cc t thin nhin lun lun cha mt lng nc nht nh no .Nc l mt thnh phn c tc dng rt cht ch vi cc ht t, nht l i vi ccloi t ht nh v c cha cc cht hu c. Do mi lin quan tc dng t ng h lm nh hng rt ln n tnh cht c - l ca t. Ty theo dng tn ti canc trong t, nc c tc dng khc nhau v dn n hnh thnh cc tnh chtkhc nhau ca t, do cn phi phn loi nc trong t trc khi nghin cu nh

hng ca n n cc tnh cht ca t.Ty theo nhim v, mc ch c th ca tng ngnh m vic nghin cu nctrong t theo tng kha cnh khc nhau. Theo quan im v xy dng th ch yunghin cu s nh hng ca nc i vi tnh cht xy dng ca t nn. Da voquan im ny c th phn loi nc trong t theo s sau:

- N

c trng lc

- Nc trong khong vt ca ht tc kt hp mt ngoi ht t - Nc ht bm

- Nc lin kt - Nc lin kt mnh- Nc lin kt yu

- Nc t do : - Nc mao dn- N

Nctrongt

2.2.1. Nc trong khong vt ca ht t:

L loi nc tn ti nhng v tr nht nh trong mng tinh th ca khongvt di dng ion (H+ v OH-) hoc di dng phn t (H2O). N ch c th tch rakhi khong vt nhit cao (4000 - 5000), thc ra n l mt b phn ca khongvt loi nc ny t nh hng n tnh cht c - l ca t, nn theo quan im xydng th khng c ngha quan trng.

2.2.2. Nc kt hp mt ngoi ht t:

Nc kt hp mt ngoi ht t l loi nc c gi li trn b mt ht tdo tc dng ha hc, ha - l v in phn t. Tnh cht ca loi nc ny khc hnvi nc thng thng, n khng chu chi phi bi trng lc, v cng khng truyn


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CHNG I Trang 18

p lc thy tnh. Ty theo mc bm cht ca nc vo ht t, nc kt hp mtngoi c th phn ra nh sau:

- Nc ht bm : L loi nc bm rt cht vo ngay mt ngoi ca ht t.N khng c kh nng ha tan cc loi mui, khng th trc tip di chuyn t htt ny sang ht t khc, t trng ca loi nc ny ln hn 1 v c gi tr khong

1,5. L

ng cha n

c ht bm ph thuc vo tng loi t, vi t ct l 0,5%, vit st pha l 5 7% v i vi t st l 10 20%. Khi t st ch c nc ht bmth t trng thi rn.

- Nc lin kt: L loi nc bao ngoi nc ht bm v c th phn rathnh hai loi: nc lin kt mnh v nc lin kt yu.

Nc lin kt mnh: L nc c gi li trong t bi cc lc ht phn t,nc ny p cht vo t, c khi dng p lc hng chc kG/cm2 cng khng th tchn ra khi ht t c. Khi thnh to nc lin kt mnh thng thot ra mt lngnhit kh ln, bi v khi kt hp cc phn t nc b ht cht vo trn mt ht v mtnng lc hot ng t do, do ng nng bin thnh nhit nng phng ra ngoi.N khng chu tc dng ca trng lc, ch khi n hp th y nhit nng th nmi thot khi ra khi mt ht trng thi hi nc. V vy nc lin kt mnh cnhng tnh cht khc hn nc thng thng, t trng ca n t 1,2 n 1,5. mtng ng vi b dy ln nht ca nc ht bm v nc lin kt mnh gi l mphn t ti a. Nc lin kt mnh c th chuyn t ht t c b dy mng ncln n ht t c b dy mng nc nh di tc dng ca lc ht phn t. Khitrong t tn ti (c mt) nc lin kt mnh th t trng thi na rn v cha thhin tnh do.

Nc lin kt yu: L phn bc ngoi cng ca nc lin kt, chim phn ch

yu trong mng nc lin kt. Khi thnh to nc lin kt yu ny khng pht nhit,iu chng t rng cc phn t nc trong lp ny khng b mt qu nhiu ngnng bin thnh nhit nng nh lc thnh to nc lin kt mnh. Nhng lc htgia ht t v cc phn t nc trong lp nc lin kt yu cng kh ln, do ncng c nhng tnh cht khc hn nc thng thng. T trng ln hn 1, cng ra xalp nc lin kt mnh th cc phn t nc trong lp nc ny sp xp cng t stcht hn. V vy, cc phn t nc trong lp nc ny c th di chuyn chm chpt ht ny sang ht kia m khng cn phi qua trng thi hi. Nc lin kt yukhng chu tc dng ca trng lc, nhit kt tinh nh hn 0 0C. Khi t c cha

loi nc ny v nu kt cu ca t b ph hoi th t s th hin tnh do, nhngnu trng thi thin nhin th d t c cha nc lin kt yu, t st cng khngxut hin tnh do, V.A.Priklonxki gi l trng thi do ngm.

2.2.3. Nc t do:Nc t do l loi nc ngoi phm vi tc dng ca lc in phn t ca

ht t do n c th chuyn dch trng thi lng di tc dng ca ngoi lcnh p lc thy tnh v p lc mao dn v c th phn thnh nc mao dn v nctrng lc.

- Nc mao dn: Nc mao dn l nc chim mt phn hay ton b l rngca t v c b mt gii hn bi cc mt khum. Khi t cha bo ha nc, ccmt khum ca nc to thnh trong lng t, cn khi t bo ha nc th ccmt khum to thnh trn b mt mc nc mao dn (Hnh I-3).


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CHNG I Trang 19

Trong t t st chiu cao mao dn c tht ti vi mt (v kch thc l rng nh). Ctnc mao dn c th to ra mt p lc ph cho t.Cn trong t ct, do kch thc l rng ln nnchiu cao mao dn nh xem nh khng ng k.

Tuy nhin, khi cc ht nh, vic hnh thnhnhng mt khum v do lc cng b mt lm chocc t ct ht nh dng nh c mt dnh no khi t.

hk

MNN

mt t

hhk

q=o.hk mt khum lom

i bao hoamao dn

mt chun

Pk

a) b)- Nc trng lc: Nc trng lc l loinc tn ti trong cc l rng ca t v c nhng Hnh I-3

tnh cht thng thng ca nc trng thi lng ni chung. Nc ny vn ngdi tc dng ca trng lc. Theo quan im xy dng, i vi loi nc ny chngta cn lu xt n cc vn sau: kh nng ha tan v phn gii ca nc, nhhng ca p lc thy tnh i vi t v nh hng ca lc thm do s chuyn

ng ca nc trong t i vi tnh n nh ca t.2.3. Thnh phn kh trong t:

Nu cc l rng trong t thin nhin m khng cha y nc, th kh(thng l khng kh) s chim ch trong cc l rng y. Cn c vo s nh hngca kh i vi tnh cht c - l ca t, th c hai dng kh c bn trong t cnphi xt n l kh t do v kh ha tan trong nc. Cc kh t do li chia ra lm khthng thng vi kh tri v kh khng thng thng vi kh tri - gi l kh kn.

Thnh phn ca cc kh trong t c th khc bit rt nhiu so vi kh tri,chng hn nh trong cc kh sinh ra trong qu trnh sinh ha (mtan v nhng kh

khc) c th ng vai tr quan trng, cng nh trong c cha nhiu xitcacbonc ln Sunfua v cc kh khc m cc kh ny khng ng k trong khng kh.

Cc kh thng thng vi kh tri c nhit v p sut gn ging nhit v p sut ca khng kh im ang xt. Khi c tc dng ca ti trng ngoi lnt th cc kh ny d dng thot ra khi t.

Cc kh kn (Kh khng thng vi kh tri) thng gp trong cc t dnh, chyu l trong t st. S xut hin ca cc kh kn trong t st c lin quan ti kchthc to nh khc nhau ca cc ng l rng chng cht phc tp trong t. Loikh kn ny tn ti trong cc ng l rng ca t, trong tnh trng b cch ly

khng c kh nng vn ng, th

ng thy khi c cc mng n

c, cng nh

n

cmao dn v nc t do bt ng chon mt phn l rng.S tn ti kh kn trong t dnh c nh hng ln ti tnh cht c hc ca

t, cc bc kh ny s lm gim tnh thm ca t, lm tng tnh n hi v c nhhng ti qu trnh nn ln ca t di tc dng ca ti trng ngoi.

Kh ha tan trong nc, ty theo thnh phn ha hc c th nh hng khcnhau ti thnh phn khong vt ca t. Mt s kh to ra qu trnh xy ha, mt skhc li to ra qu trnh cacbonat ha t, v.v... Khi nhit tng ln v p lc gimi, kh ha tan trong nc cc l rng tch ra v tr thnh kh t do, lc xy ras ph hoi kt cu ca t. iu ny cn c xt ti khi o h mng, v khi ly

mu cc l khoan ln phn tch cc tnh cht c hc ca t trong phng thnghim.2.4. Cc tc dng qua li gia cc thnh phn trong t:


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CHNG I Trang 20

2.4.1. Lc in phn t v v hydrat ca t:Nh trnh by phn trc, ht t c kch thc cng nh th t din tch

ca n cng ln v ti mt mc phn tn nht nh no ca cc ht t, th cclc in phn t (hot nng) trn b mt chng s th hin mt cch mnh m, cclc ny s quyt nh bn cht tc dng gia cc ht khong vt vi nhau, cng nh

gia chng vi mi tr

ng n

c. Nguyn nhn ca s hnh thnh cc lc in phnt ny c th gii thch nh sau:Mng tinh th ca cc khong vt st c cu to bi cc nguyn t cc

nguyn t ha hc, b tr theo mt quy lut nht nh nh hnh (I -2). Phn lnnguyn t bn trong mng li tinh th cc ht khong cht trung ha, ngha lin dng ca ht nhn trong chng cn bng vi in m ca cc in t. Khi mtnguyn t trung ha mt hay nhn thm mt hoc mt s in t, th n tr thnhmt ion mang in dng (cation) hay in m (anion). Nu bn trong cc htkhong vt cc ion c in tch khc du cn bng, th ngc li cc in tch t dokhng c cn bng bi in tch ca cc ion khc. V vy m ht khong vt tr

nn nh nhng vt mang in. Theo ti liu th nghim cho thy rng i vi cc htst, in tch trn mt ngoi ca chng thng l c du m (rt t khi mang dudng). Chnh do trn b mt ca ht khong vt mang in tch t do nh vy, nnkhi cc ht st trong mi trng nc lun lun c xy ra cc tc dng tng hvt l v ha hc nht nh v to thnh hot tnh hot ng b mt ca chng.Cng in tch b mt ca cc ht ch yu ph thuc vo thnh phn khong vmc phn tn ca ht. Nu c cc ht khong ca mt cht bt k nh, n mc tnh hot ng ca n c nh hng cn bn n cc tnh cht c - l ca chng,th ngi ta ni rng cht trng thi keo, v hot tnh b mt gi l hot tnh

keo. Kch thc cc ht keo t thay i trong khong t 1 0,1micron, kh nngca cc ht chuyn ng Brao (do cc sc y phn t to ra) v kh nng ng ttrong nc khi c cht in gii l nhng tnh cht keo in hnh.

Hot tnh b mt ca cc ht khong cht th hin rt r qua cc tc dngtng h ca chng i vi cc phn t nc. Nh bit, nc gm nhng phn tlng cc, mt bn l ion hyr (H+) mang in dng v mt bn l ion (OH-)mang in m. V th khi nm trong in trng do cc ht khong vt st to ra, thcc phn t nc lng cc u b ht v pha ht st v c sp xp c nh hngtrong in trng, hnh thnh mng nc kt hp mt ngoi ht t nh trn trnh

by. Lc ht in phn t ca cc ht khong ch c tc dng mnh trong mtphm vi nht nh gn mt ht t. Tr s ca cc lc ny xc nh theo cu to phnt ca ht, gn b mt ca ht khong vt th n rt ln, nhng xa dn mt ht tn gim rt nhanh cho n bng khng. Phm vi tc dng ca cc lc in phn tcng ph thuc vo thnh phn nc trong t v theo ti liu ca Gtbe cc lcin phn t c tc dng trong khong t vi lp n vi chc lp phn t nc,chiu dy chung ca lp ny theo riaghin l nh hn 0,1.


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Cc phn t nc c hp th vo b mt cc ht khong vt, n lt n liht cc lp khc to thnh lp v hyrat bao quanh cc ht khong vt v c nhhng ln n cc tnh cht c - l ca t.

Trn hnh (I-4) c trnhby s tc dng tng h

ca cc lc in phn t trnbin phn cch ht rn vinc, cng nh s ca slin kt (nh hng) ca ccphn t nc trong intrng ca ht t.

c)

Lcinphnt

a) b)

Nc hut bam

Nc lin kt manhNc lin kt yu

Khoang cach

Hat tNhng lp phn t tip

xc trc tip vi b mt ccht khong vt chu nhng lcko mnh nht, cho nn nng

cc phn t nc v ion lln nht v chng rt kh ding. Ra xa dn, cc lc inphn t gim nhanh nn nng v ion gim xung vchng cng c tnh di ngln. Cho n mt khong cch Hnh I-4:

a)S b tr cc phn t nc trong in trng caht t

b) S b tr cc phn t n

c xung quanh ht tc) Quan h gia lc ht in phn t v khong cchk t mt khong vt

no th cc phn t nccng s khng nh hng v

trng thi t do i vi cclc b mt. Trn hnh (I -4) ctrnh by s lc biu bini ca cc lc in phn t ty theo khong cch n b mt ht t. t s nyc th nhn xt rng, tr s ca lc in phn t gim nhanh theo tng cakhong cch, chng mi micron cc lc ny c tr s khng ng k, khng vtqu trng lc ca cc ht phn b. T c th thy rng trng thi ca nc trongt c quan h cht ch vi lc ht in phn t ca ht t v cng chnh l cs phn loi nc trong t c trnh by phn trn.

dy ca nc kt hp mt ngoi (nc mng mng) c nh hng rt lnn tnh cht ca t st. V vy, nm c cc nhn t nh hng n dy calp nc ny tc l nm c quy lut bin i tnh cht c - l ca t, y l mtvn quan trng c ngha thc t ln. Chng ta bit rng, bt c mt ht stno khi tc dng vi nc u sinh ra tng in kp (hay gi l lp lng in), tuycch thnh to tng in kp c khc nhau ty tng loi khong vt v mi trngnc xung quanh. in th nhit ng v in th in ng cng thay i ty theothnh phn khong vt, thnh phn v nng ion cng nh pH ca mi trng.V vy cng l nhng nguyn nhn ch yu nh hng n dy ca lp ncmng mng. Chng hn khi cc cation nm trong phm vi lp nc mng mng,

cng vi b mt ht t mang in tch m to thnh tng in kp. in th lnnht (Nhit ng lc ) l cc Cation c nh ca ht khong, s h thp (gimdn) in th theo chiu ny lp nc mng mng n mc in th dung dch


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CHNG I Trang 22

nc t do th tng ng vi th in ng (gi l th nng in ng ). C ththy rng th nng in ng cng ln th b dy lp nc mng mng cng cngln. mt khc, v chu sc ht ca lc in phn t, nn lp nc mng mng c tnhnht ln hn nc thng thng.

Khi khong cch ac ca hai ht t (hnh I -5) nh hn tng s bn knh nh

hng ca lc ht in phn t ad&cd th nc trong phm vi edfb ng thi phichu nh hng lc ht in phn t ca c hai ht, do tnh nht ca n tng iln.

Khong cch gia hai ht cng nh th r rngtnh nht ca nc mng mng gia hai ht cng lnv cc ht cng kh b x dch. Ngc li, b dync mng mng cng ln, th khong cch gia ccht cng ln, th tnh nht ca nc mng mng giahai ht cng nh, v cc ht cng d dng b x dch

ln nhau. L lun nc mng mng ny hin nay

hat a d cbt t

hat

Hnh I-5c dng kh rng ri gii thch mt s tnh chtc bit ca t dnh nh c, tnh dnh, tnh co, tnh m cht .v.v...

B dy nc mng mng cn ph thuc vo nng ion trong dung dch(mi trng xung quanh) v vo tnh a nc ca ht khong. Nng ion trongdung dch ln th in th in ng gim xung, do b dy ca nc mngmng cng gim v ngc li. i vi tnh a nc ca ht khong, th nh thc t cho thy nh hng ca yu t ny th hin r rt cc t st thuc loiMnmrilonit c kh nng ht nc ln cn cc t st thuc loi Kaolinit t htnc th n th hin khng r rt.

Do nc mng mng chu nh hng ca lc ht in phn t, nn khng thdng phng php thot nc thng thng (trng lc) rt nc ra c.Nhng nu c tc dng ca in trng, chng hn nh khi cm cc cc in dngv m vo trong t st v cho dng in chy qua, th lc s xut hin hintng "vn in" v "thm in" lm cho cc ht t cng vi lp nc ht bmxung quanh mang in tch m chuyn dch v pha cc dng ca in trng "vnin cn nc mng gm nhng cation th chuyn v cc m "thm in". Nu ticc m b tr thit b ht nc th c th rt nc i c. Nguyn l "thmin", thot nc ny trong thc t thng c dng rt nc l rng, v do

lm tng cng chu lc ca cc tng t kh thm nhcc tng t st yu.2.4.2. S trao i ion trong t:

thy r c tc dng ca s trao i ion trong t, cn phi xt n khnng hp th ca t i vi cc cht cng, lng v kh khc nhau, cc ion, phn tv cc ht keo t mi trng xung quanh. S hp th trong t, c bit l trong tst, c bn cht phc tp v thng gm mt s qu trnh din bin ng thi.K.K.Geroytx (1933) ngh phn bit nm dng hp th: c hc, vt l, ha - l,ha hc v sinh hc. Nhng dng hp th quan trng hn c l hp th ha - l hoccn gi l kh nng trao i ion. Chnh kh nng ny lm nh hng nhiu n

tnh cht ca t.Nc cha trong cc l rng ca t t nhiu u c ha tan cc loi muikhc nhau, nn khi tip xc vi cc ht khong vt s xy ra hin tng: Cc cation


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CHNG I Trang 23

b hp th ra khi dung dch tham gia ha hp vi thnh phn khong ca t, thayvo ch chng c mt s lng tng ng cc cation khc t lp khuych tn cathnh phn khong vt chuyn sang dung dch. Gia cc cation ca lp khuych tnthuc thnh phn khong vt ca t v cc cation ca dung dch bao gi cng c s

Hp th trao i cht. Khi trao i ion, th trn b mt ht khong vt ca t c

nhiu bin i mnh m v dn n nh h

ng cc tnh cht ca t nh

l tnhthm, tnh do, lc mao dn, v.v...Trong iu kin t nhin, trng thi trao i ion trong t st gp ch yu l

cc Cation: H+, K+, Na+, Ca++, Mg++, t hn l Fe3+, Al3+ t hn na l mt s anionca cc axt ftphoric, Silicic, cacbonic v cc axit khc. Trong ring hai cationCa++ v Na+ thng c cha trong cc t dnh nhiu hn c.

Cc nhn t nh hng n kh nng trao i ion ca t bao gm thnhphn khong, mc phn tn, c im kin trc ca ht t, nng ca chtin gii trong dung dch, tr s pH ca dung dch v tnh cht ca ion trao i.

Kh nng trao i ca cc khong vt th sinh mnh hn cc khong vt

nguyn sinh, ng thi trong cc khong vt th sinh, ring loi cc mng tinh thkm vng chc nh Mnmrilonit li c kh nng trao i mnh so vi loi c mngtinh th bt ng nh Kaolinit. Trong cng mt loi khong vt th mc phn tncng ln th kh nng trao i cng mnh, v lc nng lng mt ngoi ln. Nut b ph hoi kt cu thin nhin th kh nng trao i cng mnh, lc ny cclin kt gia cc ht b ph hng.

V pha dung dch trong t m xt, th khi tr s pH gim nh, nng cation H+ s tng ln lm cho cc cation khc kh xm nhp vo trong mng nc,do kh nng trao i ca t i vi cc cation ny s b gim i.

Thc nghim cn cho bit rng, nng cht in gii trong dung dch tngln th kh nng trao i ion cng tng. Ngoi ra, tr cation H+, cn i vi cccation khc, khi ha tr cng ln th kh nng trao i ion cng s mnh ln, ngthi trong cc cation cng ha tr th kh nng trao i tng ln cng vi s tng cabn knh ion. V vy c th sp xp cc cation theo trnh t t trao i mnh n traoi yu nh sau: Al+++> H+>Ca++>Mg++>K+>Na+.

S trao i ion trong t c nh hng rt ln n cc tnh cht c - l caloi t st. Bit c sc cha hp th ca t v thnh phn ion b hp th, c thphn on gn ng v tnh cht ca t v nhng bin i c th ca chng khithay i iu kin ha - l. Tuy vy hin nay ch mi c trng c nh hng nymt cch nh tnh, vn cn cha thit lp c quan h nh lng gia tnh chtca t vi thnh phn cation b hp th. Chng hn, st Mnmrilni cha Natridi dng cation trao i (b hp th) c th hp th nc hai ba ln ln hn so vist nh vy cha Canxi. St Mnmrilnit Natri c tnh trng n, tnh nn ln, do ln hn nhiu, bn b hn nhiu, thm nc b hn so vi stMnmrilnit Canxi. Ngoi ra da vo s trao i ion trong t, ngi ta c th cito c mt s tnh cht ca t nn nhm phc v cho vic xy dng cc cngtrnh c an ton v r tin. V d, nu t st hp th nhiu ion Na+ v Ca++, thtnh ht nc, tnh do, v tnh nn ln ca n kh ln. gim cc tnh cht y

ngi ta cho mt dng in mt chiu chy qua t m cc dng ca n cu tobng Al. Bi v cation Al+++ s y cc cation Ca++ v Na++ ra ngoi v hp th vot, lm cho t c tnh ht nc v tnh nn ln gim i rt nhiu.


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CHNG I Trang 24

3. C cu v kt cu ca t:

Thnh phn rn (cng) ca t l mt tp hp gm cc ht to nh khc nhau,cc tnh cht ca t khng nhng ph thuc vo thnh phn ca t m cn phthuc vo hnh thc tn ti ca t nh cch sp xp cc ht, mi lin kt gia cc

ht, v.v.... Cng nh gia cc tp hp to thnh t, tc l ph thuc vo c cu vkt cu ca t.3.1. Kt cu ca t:

Kt cu ca t l s sp xp c tnh quy lut ca cc ht hoc cc m htt c ln v hnh dng khc nhau trong qu trnh trm tch. Kt cu ca cc tthin nhin l yu t quan trng nht, xc nh cc tnh cht c - l ca t nh lnn v mi trng xy dng cng trnh, v n xc nh bn v tnh bin dngca t di tc dng ca cc lc ngoi.

Cc lc in phn t tc dng gia cc gia cc ht khong vt, cc lc tcdng gia ht v nc cng nh t s ca chng vi trng lng ht c ngha quantrng khi to thnh kt cu ca t thin nhin. Cc tnh cht ca mi trng lngng c nh hng c bn n kt cu to thnh ca cc khong cht chm lng vr rng lc ny, kch thc cng nh hnh dng v thnh phn khong vt ca ccht t chm lng u c ngha quan trng.

Da trn c s thnh to v s nh hng ca n n tnh cht ca t thinnhin, ngi ta thng phn tch kt cu ca t ra lm ba loi c bn, bao gm ktcu ht n, kt cu t ong v kt cu bng.

3.1.1. Kt cu ht n:Hnh thnh do s chm lng t do ca cc ht tng i th (ln hn

0,05mm) trong mi trng nc. Trong trng hp ny, trng lng ca ht ln hnlc ht gia chng nn chng c sp xp theo cch ht n da vo ht kia (hnh I-6). Vi loi kt cu ht n ny thng thy trong cc loi t ct tng i th,t ct, si cui.

i vi cc t ct, th trong loi kt cu n ny c th phn bit thnh hailoi ht khc nhau l kt cu ri xp v kt cu cht. cc t ct c kt cu ri xpth cc ht trng thi khng n nh, ng thi gia cc ht c khi c cc l rng

tng i ln, vt qu kch thc ca chng. cc t ct c kt cu cht th tnhhnh xy ra ngc li. Nh trn ta bit kt cu ca t c quan h mt thit vi

iu kin trm tch v ln ca c ht, do m trong thin nhin t gp loi ktcu cht cc ht nh, m gp nhiu cc loi ht to.3.1.2. Kt cu t ong:

Hnh thnh do s lng chm t do ca cc ht tng i nh trong nc dotrng lng cc ht khng thng ni lc ht phn t gia chng nn chng khngth tip tc chm xung m lng ng li ngay ch u tin chng chm n lptrm tch, do to thnh kt cu t ong. Vi dng kt cu ny, cc ht t trngthi khng n nh. ( Hnh I-6b)

3.1.2. Kt cu bng:

Nu cc ht khong vt lng chm trong n

c c kch th

c ca ht keo thchng s pht sinh chuyn ng Brao v chng s trng thi huyn ph mt thigian lu. Nu thm vo hn hp mt cht in gii (Ca2Cl, hoc l cht khc) th lc


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CHNG I Trang 25

y gia cc ht gim i v cc ht c kh nng xch gn li, ri lin kt vi nhauthnh cc m ht hnh thnh kt cu bng nh hnh (I - 6c). Loi kt cu ny rtkhng n nh v thng gp trong trm tch bin.

t trong thin nhin do cc ht ln nh khc nhau to thnh, v vy kt cuca t khng n gin nh cc kt cu c bn ni trn. Thng thng trong cng

mt loi t c th gp c hai hoc ba loi kt cu .Trong qu trnh to thnh v tn ti kt cu, trong t v nht l trong cc tst hnh thnh ra cc lin kt ni ti, gn lin cc ht hoc cc m ht khc nhaugi l lin kt kt cu

S c mt ca cc lin kt kt cu trong t, cng ca chng, sc bn vtnh n hi ca chng l nhng yu t quan trng xc nh phm cht ca t vnn t di cc cng trnh.

Cc lin kt kt cu hoc l xut hin khi cc ht chm lng trong nc hocl hnh thnh trong qu trnh tn ti. Da theo thi gian xut hin m cc lin kt ktcu phn bit ra l lin kt nguyn sinh (s cp) v lin kt th sinh (th cp).

Lin kt nguyn sinh to ra bi cc lc phn t tc dng tng h gia ccht khong vt cng nh gia cc ht khong vt vi nc. c im ca lin ktny thng l c tnh n hi v tnh do nht.

Lin kt th sinh xut hin do kt qu ca s gi i ca cc ht keo, s ktdnh li ca chng v do nhng qu trnh kt dnh ca cc mui ha tan trong nc.i khi cc lin kt th sinh gi l cc lin kt gia cng (Theo N.Ia.nhixp).Cc lin kt cng ca t u thuc lin kt ny. c im ca chng l c tnhcng ch b ph hoi khi cc ht c mt chuyn v tng i vi ln nht nh.Khc vi cc lin kt nguyn sinh, cc lin kt th sinh b ph hoi theo dng gy

dn v mt khi b ph hoi th khng th phc hi li c.Chng ta cn lu rng, d thuc loi lin kt nguyn sinh hay th sinh, cclin kt trong t u c cng km xa cng ca bn thn ht t. Chnhiu ny cng vi c im ca t l mt mi trng ri, lm cho t khc hn ccloi vt liu lin tc v mt tnh cht c - l ca chng.

Nh trn trnh by, kt cu ca t nh hng rt ln n tnh cht c - lca t, cho nn m bo c nhng s liu phn nh ng tnh hnh t nhin cat, cc th nghim t phi c tin hnh vi cc mu t khng b ph hoi ktcu t nhin. Tuy nhin, v hn ch v mt k thut ly mu cng nh do tnh chtca cng vic th nghim, nn cc mu th nghim trong phng hu nhu b phhoi kt cu t hay nhiu, do kt qu nghin cu cc tnh cht ca t trongphng th nghim c hn ch v mc chnh xc. Chnh v vy, nn hin nay ngita ang t ra phng hng cn p dng nhiu cc phng php th nghim trctip ngoi hin trng th mi mang li kt qu chnh xc, phn nh ng thc tiu kin lm vic ca t thin nhin.3.2. C cu ca t:

C cu ca t l tp hp tt c nhng c im c trng cho tnh khngng b nht trong s phn b cc yu t kt cu v c hc ca n trong cc lp t.Trong qu trnh hnh thnh ca n, c cu ca t chu tc ng ca nhng iu

kin hnh thnh cc trm tch t, chng hn nh tnh tun hon khi chm lng ccht trong nc chy v nc nm yn cng nh chu tc ng ca nhng bin isau v phng v tr s ca p lc bn ngoi, lm cho cc lp t b un cong


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CHNG I Trang 26

hoc hnh thnh dng phin. Ngi ta thng chia C cu ca t thnh nhng dngc bn sau y:

3.2.1. C cu lp: L loi c cu ph bin nht v th hin r rt trong cc trm tchlng sng, m, h v bin cn nc, bao gm cc lp ct, st xen k nhau, ty stn ti v cch sp xp ngi ta c th phn bit thnh c cu lp ngang v c cu

lp nghing. C cu lp ca cc t c biu hin r rt v ton b tnh a dng can lm cho cc t khng ng hng, ngha l cc tnh cht c - l ca nhng ty (chng hn nh tnh thm, sc chng ct, tnh n hi, v.v...) s rt khc nhautheo cc hng khc nhau.

3.2.2. C cu Pocphia: Th hin cc loi trm tch mnh ln, bao gm cc ht th(si, sn ,ct) v cc ht st phn tn u tham gia vo sc bn ton phn chng cctc dng ca cc lc ngoi, nhng cc tnh cht nh l tnh nn ln, tnh thm, scchng ct, tnh n hi ca t s ch yu ph thuc vo cc tnh cht ca vt liuphn tn nh c nhng mnh th nm trong .

3.2.3. C cu lin: Thng gp cc loi t st v t bn c trong qu trnh acht chu nhng p lc ln, cng nh mt s t hong th v st pha hong thcha nn cht nhng dnh kt bng cc mui.

V c cu ca t c nh hng rt ln ti tnh cht c - l ca n nn khithm d v thit k cn phi ch n yu t ny mt cch thch ng. Hnh (I -7)s gii thiu cc loi c cu bn ca t.

a) Kt cu n a) Kt cu t ong a) Kt cu bng Hnh I-6:Cc dng kt cu ca t

a) Dang c cu lp b) Dang c cu pocphia c) Dang c cu lin

Hnh I-7: Cc dng c cu ca t


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CHNG I Trang 27

.4. Cc ch tiu vt l ca tThng thng t c ba thnh phn to nn l rn - lng - kh. Trong t

nhin t l gia ba thnh phn ny thay i rt nhiu, c bit l thnh phn nc,c khi cha y trong cc l rng ca t. T l gia ba thnh phn ny thay i thtrng thi vt l cng thay i theo. T l phn phi v trng lng v th tch ca

ba thnh phn trong t gi chung l cc ch tiu tnh cht vt l ca t.Trong cng tc xy dng, nh gi c tnh cht v kh nng chu lcca t, ngi ta phi da vo cc ch tiu vt l v c hc ca n. Nhng ch tiu chc (tnh nn ln, tnh chng ct, v.v...) ca t s c trnh by trong cc chngc lin quan vi ch tiu ny, y ch trnh by cc ch tiu vt l ca t.

Vic xc nh tr s cc ch tiu vt l ca t l mt vn rt quan trng, vn c dng rng ri trong tnh ton thit k cng trnh. Ty theo tng loi t, tytheo nguyn nhn to thnh v iu kin tn ti ca t m cc ch tiu vt l cacc loi t rt khc nhau. Trong cc ch tiu vt l ca t, c loi th phi trc tiplm th nghim mi xc nh c - gi l cc ch tiu xc nh bng th nghim, cloi th c th tnh ton t cc ch tiu th nghim m ra - gi l cc ch tiu tnhton, ngoi ra trong cc ch tiu ny c nhng ch tiu dng nh gi (hay xcnh) trng thi ca t, ta c th a v mt nhm gi l cc ch tiu xc nh trngthi ca t.

tin cho vic nghin cu cc ch tiu vt l ca t, ta cng thng nhtdng cc s quy c trn hnh (I - 8) v cc k hiu sau y:

VK, Vn, Vh, Vr,V: L th tch kh, nc, ht rn, l rng, v th tch ca tonb mu t ; Qn , Qh, v Q: l trng lng phnnc, ht rn v ton b mu t cn m, n l th

tch ht, l rng trong mt n v th tch.

Ht t

Nc

Kh Vk

Vn

Vh

Vr

V

Q

k

Qn

Qh

Q

Trng lng Th tch

4.1. Cc ch tiu vt l xc nh bng th

nghim

4.1.1 Dung trng ca t:Hay cn gi l trng lng th tch t

nhin ca t, l trng lng ca mt n v thtch t trng thi t nhin, n v thng dngl g/cm3 hay T/m3 N/cm3, KN/ m3, xc nh theocng thc :

Hnh I - 8: S quy c bapha ca t v t l gia chng

=VQ (I-2)

T nh ngha c th thy rng dung trng ca t ph thuc vo thnh phnkhong, rng cng nh lng cha nc ca t. Khi xc nh cn dng munguyn dng v ty theo tng loi t m chn dng cc phng php th nghimcho thch hp. t loi st, ht nh dnh kt, d ct, ta c th dng phng php daovng, t vn to, t cha si, cui khng ct c bng dao vng th ta nn dngphng php bc parafin. Ngoi thc a trong iu kin nht nh ta c th o h,xc nh trng lng v th tch t trong h o xc nh dung trng ca t.

Thng thng tr s dung trng ca cc loi t trong thin nhin nh sau:t ct t (1,45 1,85)T/m3; t ct pha, st pha t (1,40 1,65)T/m3; t st phakhong 1,75T/m3; t st b nn cht t (1,8 2,1)T/m3.


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CHNG I Trang 28

4.1.2. m ca t: m ca t l t s gia trng lng nc cha trong t v trng lng

ht t trong mt khi t t nhin, n v thng tnh l phn trm, c khi dng sthp phn v c xc nh theo biu thc sau:

W = 100.h

n

Q

Q(I -3)

Tr s m ca t thay i ph thuc vo lng nc cha trong t, phthuc vo mt phn b ca ht, tc l ph thuc vo kt cu ca t.

m t nhin ca t c tr s thay i trong khong rt rng t vi phntrm n vi chc phn trm. xc nh tr s m ca t ngi ta tm trnglng ca mu t trc v sau khi sy nhit 1000C1050C n trng lngkhng i, t bit c trng lng nc (Qn) v trng lng ht (Qh). Khi thnghim m ca cc t bnh thng, nhit sy kh thng l 1000C1050C,nc t do v nc kt hp mt ngoi b bc hi ht v xem nh t kh tuyt i.

Nhng i vi t cha nhiu cht hu c,nhit sy kh 1000

C1050

C l qu cao.Ngoi thc a tit kim thi gian, c th dng phng php t cn, b

mu t vo hp, nh cn cho ti bo ha ri t, theo kinh nghim t chng baln th kt qu gn ging nh phng php sy kh. Phng php ny khng pdng cho t c cha cht hu c.

4.1.3.Trng lng ring ht t (h):Hay cn gi l trng lng th tch ht t, l trng lng ca mt n v th

tch phn ht (ct) t ca chng, n v tnh bng: g/cm3 T/m3, N/m3, KN/m3.Trng lng ring ht t c xc nh ch yu bng phng php t trng, theo

nh ngha trong vt l hc th t trng ca t bng: =

ho

h

V

Q

., h =

h

h

V

Q(I - 4)

T trng ca t khc vi trng lng ring ht t l, n l i lng khngth nguyn cn tr s th bng nhau.

T trng ca t khng ph thuc vo l rng v m ca t m chph thuc v t trng ca cc ht khong vt c trong t. T trng ca cc t thinnhin thay i t 2,5 - 2,8. t cha mn hu c th t trng nh hn t cha cckhong vt nng (pirit, st oxt) th t trng ln hn, t trng ca t st thng ln

hn t trng ca t ct. T trng

c xc nh bng ph

ng php bnh o t trng.Khi xc nh t trng ca t cha cc mui d ha tan phi dng cc cht lng huc trung tnh thay cho nc, nhdu ha, bezen, v.v...

Tr t trng ra, khi xc nh dung trng v m ca t bng th nghim,phi dng cc mu t nguyn dng. Tuy nhin, v trong qu trnh ly mu vchuyn ch cc mu kh trnh cho chng khi b thay i dung trng v m,nn kt qu th nghim cng khng trnh khi sai s, l cha k n sai s do kthut thao tc trong khi lm th nghim. Nn hin nay ngi ta ang c xu hngtm cch xc nh trc tip cc ch tiu dung trng v m ca t ngay hintrng. Phng php dng cht ng v phng x xc nh dung trng v m

ca t hin trng, hin nay ang c p dng v mang li mt s kt qu vkinh nghim tt.


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CHNG I Trang 29

4.2. Cc ch tiu vt l xc nh bng tnh ton:

4.2.1.Dung trng kh (k):Dung trng kh l trng lng ca ht t trong mt n v th tch t t

nhin v l ch tiu biu th cht ca t. n v thng dng g/cm3 hoc T/m3

N/cm3, KN/m3 v c xc nh theo biu thc sau:

k =V

Qh (I -5)

Tr s ca dung trng kh thng thay i trong khong (1,2 1,9)T/m3, trs ca dung trng kh c th xc nh qua tnh ton t dung trng v m ca t.

Theo nh ngha ca m c th vit:

W =( )

WV

V

Q

Qk

k

k

h

n

+=

=

1.

(I- 6)

Tr s dung trng kh ph thuc vo rng ca t v l mt ch tiu ktcu ca t. V vy ngi ta thng dng biu th trng thi kt cu ca t, khi

k cng ln tc l t cng cht, k cng nh th t cng xp.4.2.2. rng ca t:

rng ca t hay cng c th gi th tch l rng trong mt n v th tcht t nhin. Theo nh cch gi trn c th nh ngha nh sau: rng ca t lt s gia th tch l rng trong t v th tch t trng thi t nhin, n vthng dng l phn trm, nhng khi tnh ton thng biu th bng s thp phn.

n = 100.V

Vr (I-7)

Ch tiu ny c th xc nh c da trn c s bin i t cc biu

thc trn:n = 1- m = 1 -

0.

k (I-8)

4.2.3. H s rng ca t:H s rng ca t l t s gia th tch l rng trong t v th tch ht t

trong mu t , ch tiu ny thng c biu th bng s thp phn, theo nhngha c th vit:

e =h

r

V

V(I- 9)

T biu thc (I -9) c th khai trin thm nh sau:

e =( ) n

n

Vn

Vn

VV

V

r

r

=

=

11

.(I -10)

Tng t c th vit:

n =e

e

VeV

Ve

V

V

hh

hr

+=

+=

1.

.(I-11)

H s rng ng vai tr quan trng c bit trong C hc t (khi tnh lnv.v...), v vy xc nh h s rng cn phi lm chnh xc n mc c th lm c.Ni chung trong thc t thng dng e biu th mc rng ca t thin nhinthun li hn nhiu khi dng rng n. V d trong trng hp no i chng nath tch ht cng l mt tr s khng i. T biu thc (I -9) c th nhn thy rng,


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CHNG I Trang 30

tr s e cng ln th th tch l rng trong t cng ln v do c ng chng ctcng gim nh, tnh nn ln cng ln, v ngc li.

4.2.4. m ton phn ca t: m ton phn ca t, thc ra l mt ch tiu ph tr ca C hc t.

m ton phn ca t ngha l m v l thuyt ng vi lc nc cha y cc l

rng trong t, do c biu thc xc nh l:Wtf=

( )( ) hh

n eVn

Vn

Q

ohabQ

0

0

0 ....1

..=

= (I - 12)

4.2.5. bo ha ca t:Theo nh ngha, bo ha l t s gia th tch nc trong mt khi t v

th tch l rng ca t trong khi t , hay cng c th nh ngha nh sau. bo ha l t s gia m t nhin v m ton phn, ch tiu ny thng cbiu th bng s thp phn, theo nh ngha c th vit:

G =r

n

V

V

Hoc G =0.

.

e

W

W

W h

tf

= (I- 13)

4.2.6. Dung trng bo ha ca tDung trng bo ha l dung trng ca t khi cc l rng ca t cha y

nc. Trong trng hp ny t ch gm hai thnh phn to nn, l ht rn vnc, d ta c biu thc xc nh nh sau:

bh =( )V

ohabQQ nh + (I- 14)

4.2.7. Dung trng y ni ca tDung trng y ca t l dung trng ca t khi b ngp di mt nc t

do, tc l bng t s gia trng lng ni ca phn th rn trong khi t v th tchca khi t :

n =( )

eV

VQ hh

+

=

1

. 000 (I -15)

Ni chung tt c cc ch tiu trn u c th da vo cc nh ngha v ccch tiu v s lin h v th tch v trng lng gia cc thnh phn rt ra cc

biu thc cn thit. n gin ha trong vn tnh ton ngi ta lp sn chomt s cc biu thc hay ng dng nh trong bng (I -3).


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CHNG I Trang 31

Bng I- 3: Biu thc tnh i cc ch tiu thng dng

STT Ch tiu cn xc nh Cng thc

1

2

3

H s rng

( )

n

ne

e

we

k

h

o

=

=

+

=

1

1

101,01

4 rnge1

en

+=

5

6

bo ha

( )

+

= W

W

G 01,01

01,0.

0

e

WG

=

.01,0

7

8Trng lng ring ht n1

kh

=

oh .=

9

10Trng lng ring kh

( )n1hk =

Wk

01,01+=

11

12Trng lng ring y ni

n( )

e

o

+

=

1

1

n onn = Ghi ch: H s 0,01 t trc i lng W l chuyn t s phn trm sang s

thp phn.

4.3. Cc ch tiu nh gi trng thi ca t:

Trc khi xy dng cc cng trnh trn mt nn t no cng vic u tini vi chng ta l cn phi tm hiu trng thi vt l ca nn t , nh gi sb tnh cht ca n v cc mt nh cng v bin dng, v.v....

Hin nay ngi ta thng dng hai khi nim c bn ni ln trng thi vtl ca t nn l: cht i vi t ri v st i vi t dnh.

4.3.1. i vi t ri:

4.3.1.1. Ch tiu nh gi cht ca t ri:


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CHNG I Trang 32

cht t nhin ca cc t ri l ch tiu quan trng nh gi trng thica t ri khi lm nn cho cc cng trnh. V nhng loi t ri ny hon tonkhng c tnh do, cho nn trng thi vt l ca n c biu th bng cht l hpl nht, n c xc nh t cc s liu th nghim trong phng v hin trng.

Theo cc ti liu tnh ton v thng k cc h s rng trng thi cht nht

v xp nht

c xc nh trong phng th nghim i vi cc loi ct thch anhthuc cc ngun gc khc nhau, sau i chiu v iu chnh li vi cht tnhin ca n ngi ta lp bng (I-4) phn loi cht ca t ct theo h s rngnh sau:

Bng I - 4: Tiu chun v cht ca t ct

chtLoi ct

Cht Cht va Xp

Ct si, ct to, ct va e < 0,55 0,55 e 0,70 e > 0,70

Ct nh e < 0,60 0,60 e 0,75 e > 0,75

Ct bi e < 0,60 0,60 e 0,80 e > 0,80

Ngoi ra ngi ta cn a ra ch tiu cht tng i k hiu l D nhgi trng thi ca t ct v c nh ngha nh sau:

minmax

max

ee

eeD

= (I - 16)

Trong : emax - L h s rng ca t ct trng thi xp nht, c xc

nh trong phng th nghim bng cch nh ct kh vo bnh c vch o dungtch khng c chn ng, t xc nh c v tnh ekmin max.

emin - L h s rng ca t ct trng thi cht nht c xc nhtrong phng th nghim, bng cch ct vo bnh c vch o dung tch, rung cht

t xc nh c v tnh ekmax min.

e - L h s rng ca t ct trng thi t nhin.

Cn c vo cht tng i D ngi ta nh gi cht ca t ct nh sau:

D 1/3 t ct xp

1/3< D 2/3 t ct cht va

2/3< D 1 t ct cht

Vic xc nh cht ca t ct bng th nghim trong phng vn cn nhiunhc im do bin php thc hin trng thi xp nht, cht nht cha m bochnh xc, cn mang tnh ch quan. Hn na h s rng t nhin eo ca ct cngkh xc nh c v khng ly c mu t nguyn dng. Khi c th dng ccphng php th nghim ti hin trng xc nh cht ca t ct trng thi

t nhin ca n, trong thc t thng dng cc phng php th nghim xuyn ngv xuyn tnh s c gii thiu chng VI.


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CHNG I Trang 33

Bng (I-5) v (I-6) gii thiu trng thi ( cht) ca t thng qua kt quth nghim hin trng bng xuyn ng v xuyn tnh.

Bng I - 5 : cht theo th nghim xuyn ng

S ln ba ri (N) cht tng i (D) Trng thi ca t

1 4 < 0,2 Rt xp5 9 0,2 0,33 Xp

10 29 0,33 0,66 Cht va

30 50 0,66 1,0 Cht

> 50 > 1 Rt cht

Bng I - 6 : Sc khng mi xuyn theo cht ca t ct (100 Kpa)

Ct th Ct va Ct nh su

(m) Cht Cht va Cht Cht va Cht Cht va

5 150 150 - 100 100 100 - 60 60 60 - 30

10 220 220 - 150 150 150 - 90 90 90 - 40

4.3.1.2. Ch tiu nh gi m ca t rii vi t ri, mc kh hay m, t nh hng n cng ca t, nn

n gi nguyn c kt cu t nhin ca n khi m thay i. Tuy vy, chn su t mng cc cng trnh trn nn t ct th c trng ny li rt cn. V vytheo qui phm v nn dng bo ha phn loi trng thi ca t ct nh sau:

G 0,5 : Thuc t hi m

0,5 < G 0,8 : Thuc t m

G > 0,8 : Thuc t bo ha nc

4.3.2. i vi t dnhi vi t dnh th khng th tch ri trng thi cht ring r vi trng

thi m, i vi t dnh khi lng nc cha trong t thay i th trng thi vtl ca t s thay i. Cc loi t dnh c th tn ti trng thi cng khi m rtnh, v khi m tng dn ln th t bt cng chuyn dn sang trng thi do,

m tip tc tng na th t s dn n trng thi chy nho. Cc trng thi vt l ca t phn nh mt phn b ca ht t trong mi trng nc. nh gitrng thi ca t dnh ngi ta dng c trng st (hay c), k hiu l B.Cng thc nh ngha ca st l:

dnh

d

WW

WWB

= (I - 17)

Trong : W, Wnh, Wd - L m t nhin, gii hn nho v gii hn do ca t

- Gii hn nho (Wnh): l m ranh gii gia trng thi do v trng thi

nho. N

c xc nh bng cch nho t t nhin vi n

c cho n trng thi mkhi th qu chu tiu chun nng 76gam, u nhn 300. Mi chy ln su vo t10mm trong 5 giy. Hoc Ty u v M th dng cch lm ca Casagrande: Cho


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CHNG I Trang 34

t nho trn vo mt bt chm cu (r = 54mm, t dy 8mm), ct t thnh haiphn mt rnh hnh ch V, lm rung bng cch nng ln th ri xung. Nu sau 25ln nh vy hai m t va chp vo nhau tht ti gii hn nho, em xc nh mca t y th l gii hn nho ca t (Hnh

I - 9).

8mm

2mm

Hnh I-9

r=54mm

- Gii hn do (Wd) l m ranh giigia trng thi cng v trng thi do. Nc xc nh bng m ca t m ta cth ln t trn mt tm knh m cho kh dnv hnh thnh cc dy t c ng knh 3mm,di 10cm khi cc dy t ny bt u rn nt.

Theo quy phm hin nay ngi ta phn trng thi ca t dnh theo cc trngthi nhbng (I - 7).

Bng I - 7 : nh gi trng thi ca t dnht v trng thi st B

t ct pha ( ct) :- Rn B < 0- Do 0 B 1- Chy (nho) B > 1

t st pha v st ( st, st):- Rn B < 0- Na rn 0 B 0,25

- Do 0,25 < B 0,5- Do mm 0,5 < B 0,75- Do chy 0,75 < B 1- Chy ( nho) B > 1

Trng thi ca dnh t cn c th xc nh bng phng php xuyn tnhhin trng. Tuy vy, khi s dng cc kt qu cng cn phi kim tra li theo chtiu st xc nh qua m.

Bng I - 8: Xc nh trng thi ca t theo kt qu xuyn tnh

Sc khng mi xuyn (Kpa) Trng thi ca t10.000 Cng

10.000 - 5.000 Na cng

5.000 - 2.000 Do

2.000 - 1.000 Do mm

< 1.000 Do chy


34/289

CHNG I Trang 35

5. phn loi t

t c thnh to trong t nhin rt khc nhau v ngun gc, thnh phn,cu trc v tnh cht ca n. Do khng th nghin cu v xc nh cc tnh chtca t nu khng h thng ha chng li theo mt quy nh nht nh, tc khngphn loi chng. V nu c phn loi chng, th mi c c s la chn cc phng

php nghin cu ph hp vi thc t t nhin hn, t c phng hng ng ns dng t vo mc ch xy dng cc cng trnh. phn loi t, khi chn ccc trng cn phi ch lm sao cho cc c trng ny tha mn cc yu cu sau:

- Phn nh c mt cch khch quan v y cc c im ca t l mtsn vt t nhin, thng xuyn chu nh hng ca mi trng xung quanh.

- Phi thch hp vi mc ch s dng t vo xy dng cng trnh.

phn loi t ri v t dnh, hin nay ngi ta u xut pht t quanim cho rng, cc loi t d c lch s hnh thnh khc nhau, nhng chng c

thnh phn ht hoc ch s do ging nhau th chng c nhiu tnh cht c l gingnhau, cc tnh cht c l ny cho php cung cp c c trng xc nh v chtlng xy dng i vi tng loi.

i vi t ri, do tnh dnh khng c nn ln v thnh phn cp phi caht t c kh nng phn nh c y cc tnh cht c l ca t nh tnh thm,tnh nn ln v cng chng ct, v.v... Do theo qui phm hin nay, vic phnloi t ri theo bng (I - 9).

i vi t dnh: Nh phn tch trn, trng thi do l mt trng thi cbit quan trng ca cc loi t dnh, tnh do ca t dnh l kt qu tc dng tngh gia ht t v nc. Do khi phn loi t dnh lm nn cng trnh ngi tada vo ch tiu ch s do l thch hp nht, ch s do k hiu l hoc l IP(Index plastic) theo bng phn loi (I - 9)

=IP = Wnh - Wd (I - 18)

Trong :Wnh , Wd - l m gii hn nho v m gii hn do ca t.

Bng I - 9 : Phn loi t theo quy phm Vit Nam QPXD 45-78 (CHNII 15-74)

S th t Tn t Cn c phn loi

t ri Hm lng c ht1 Tng ln Ct ht c d > 200mm chim trn 50%

2 Dm cui Ct ht c d > 10mm chim trn 50%

3 Si sn Ct ht c d > 2mm chim trn 50%

4 Ct sn Ct ht c d > 2mm chim trn 25%

5 Ct th Ct ht c d > 0,5mm chim trn 50%

6 Ct va Ct ht c d > 0,25mm chim trn 50%

7 Ct nh Ct ht c d > 0,1mm chim trn 75%8 Ct bi Ct ht c d > 0,1mm chim t hn 75%


35/289

CHNG I Trang 36

t dnh Ch s do IP

9 ct IP < 7

10 st 7 IP < 17

11 St IP 17

Trong t thin nhin cn gp mt loi t c kt cu km n nh gi l tbn. c im ca loi t ny l c kt cu d b ph hoi khi chu tc dng ca titrng ngoi cn rt nh.

V vy trong tiu chun xy dng, cc t ny c xp thnh mt loi ringbit, bao gm cc t dnh giai on u hnh thnh, c to bi trm tch ktcu trong nc c cc qu trnh vi sinh vt v trng thi t nhin c m vtqu m gii hn nho v h s rng vt qu cc tr s trong bng (I - 10). Tnbn c qui nh theo ch s do nu phn trn.

Bng I - 10 : Phn loi t bnLoi bn H s rng

- Bn ct e 0,9

- Bn st e 1,0

- Bn st e 1,5

Vic phn loi t c th khc nhau vi cc nc khc nhau, v ngay cngmt nc, cc nghnh khc nhau. Vi mc ch s dng t khc nhau, c th c

nhiu cch phn loi khc nhau.Theo AASHTO (American Association of State Highway and Transportion

officials): Cc t c chia thnh 8 nhm, t v c c chia thnh 7 nhm tngt t A-1 n A-7. Cc nhm ny c chia thnh 12 nhm ph. t c thnh phnhu c cao c xp loi A-8. Cc t c cha vt liu ht mn u c nh rthm bng cc ch s nhm ca n, ch s cng cao, t cng km thch hp tc lkm n nh.

Ch s nhm c xc nh:

G(n) = (F-35)[0,2+0,005(Wnh-40)]+0,01(F-15)(IP-10) (I-19)Trong : F - Phn trm qua ry N0200 ( 0,074mm), biu th bng mt s nguyn.

Wnh - Gii hn nho (chy)

IP - Ch s do ( Index Plastic).

Cc c trng ca nhm v ph nhm khc nhau v trnh t phn loi ctrnh by theo bng (I-11).

Theo h thng phn loi thng nht cu M USCS ( Unified Soil

Classification System - USCS-ASTM. D2487), cc t ht th c chia thnh:

+ Si v t si c k hiu l G.

+ Ct v t ct c k hiu l S.


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CHNG I Trang 37

Mi loi si v ct c chia nh thnh 4 nhm:

+ Vt liu tng i sch, cp phi tt, k hiu W (Well graded).

+ Vt liu cp phi tt vi cht gn kt rt tt, k hiu l C ( Clay).

+ Vt liu tng i sch, cp phi xu, k hiu P ( Poor graded).

+ Vt liu th cha ht mn, khng bao gm cc nhm trc k hiu M ( Silt)t ht mn chia lm ba nhm:

+ t ct rt mn v bi khng hu c, k hiu N.

+ t st khng hu c, k hiu C.

+ t st v bi hu c, k hiu O.

Mi nhm trong ba nhm ca t ht mn ny, li c chia nh theo gii hn chy:

+ t ht mn c Wnh< 50 c tnh do thp, k hiu L ( Low plasticity)

+ t ht mn c Wnh> 50 c tnh do cao, k hiu H ( High plasticity)

t nhiu hu c nht m ly, than bn c tnh nn ln ln, khng chianh m xp thnh mt nhm, k hiu Pt, cc c trng lin quan ca cc nhm khcnhau cho theo bng (I-12).

Bng I-11: Phn loi t v cc hn hp nhm ht t ca Hip hi ng b qucgia M M-145 ( AASHTO).

Phn loai chung Vt liu hat ( 35% hoc thp hn qua ry N 200) Vt liu set-bui( ln hn 35%qua ry N 200)

Phn loai nhomA-1 A-2

A-3 A-4 A-5 A-6A-7A-7-5A-7-6A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7

Phn tch bng ry% qua ryN 10N 40N 200

o

o

omax 15

max 50max 30 max 50

max 25 max 10min 51

max 35 max 35 max 35 max 35 min 36 min 36 min 36 min 36

c trng cua phnqua ry N 40

o

Ch s deo IP

Gii han chay(Wnh)

max 6

khngdeo

max 10

max 40

max 10

min 41

min 11

max 40

min 11

min 41

max 10

max 40

max 10

min 41

min 11

max 40

min 11

min 41

ch s nhom (G) 0 0 0


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CHNG I Trang 38

Bng I 12:H thng phn loi t thng nht

( Unified Soil Classification System USCS-ASTM D.2487)

Phn chia rng Ky hiu Tn goi in hnh Tiu chun phn loai theo phong th nghim

t soicui hn50%trn ry

N 200(0,074mm)

t soicui >50% lnghat congknh lnhn mtry N 4(4,76mm)

o

o

Soi cuisach khnghoc t cohat nho

Soi cui cp phitt, soi cui lncat, khng hoc that nho

GW

GP

Soi cui cp phikem, soi cui lncat, t hoc khngco hat nho

Ham lngsoi cui, catxac nh tng congcp phi.Tuy theoham lnghat nho hnmt ry N200 t soicui phnbit nh sau:

Cu=D60D10 >4 [(D60)x(D10)] =1-3

(D30)Cc=

2

Khng at nhng ch syu cu cho GW

Soi cuiln hat nho(hat nhoang k)

GMu

GC

d

Soi cui ln buicat ( trong xydng ng phnbit GMd vaGMu; LL 28 lau)

Soi cui ln setcat

Cac gii hanAtterberg thphn ng Ahoc IP7.

Cac gii hanAtterberg trnng A, IP trongkhoang 4 - 7 cn cotn kep.

o

Cac gii hanAtterberg thp

hn ng Ahoc IP6D60Cu= =1-3(D30)

* t hn 5%:GW, GP,SW, SP

** Ln hn12%: GM,GC, SM, SC

*** T 5%n 12%:

cn dungmt tn goikep

ML

Ky hiu

Bui va setco gii

han chay< 50t hat

nho>50%qua mtry N 200(0,074mm)

o

Phn chia rng

Bui v c va cat rt mn, catnho ln bui set, deo nho

Tiu chun phn loai theo phong th nghimTn goi in hnh

Set v c, deo t thpn trung bnh, set ln soi

cui, set ln cat, set ln bui

CL

Bui hu c, set ln bui huc deo thpOL

Bui v c, cat nho nhiu mica hocdiatomic, t bui, bui an hiMH

Set v c, deo cao, set beoCH

Set v c, deo t trungbnh n cao, bui hu cOH

Bui va setco giihan chay> 50

t co lnghu c cao

PtThan bun hoc cac cht cohu c cao

0 10 20 30 40 50 60 70 80 90

10

2030

40

50

60

IP

CH

OH va MH

ML va OLCM-M

CL

Biu tnh deo

Chs

de

o

Gii han chay

ng

A


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.6. mt s tnh cht c - l thng xy ra trong t

6.1. Tnh dnh ca t

Nh chng ta bit, ch c cc loi t dnh mi c tnh dnh cn i vicc loi t ri hon ton khng c tnh dnh. Tnh dnh l kh nng chu lc ko(d rng lc rt nh) ca cc loi t dnh. S d cc loi t ny c th chu c

lc ko, v gia cc ht t dnh c nhng lin kt kt cu gi cht n li vi nhau,cng chnh nh c lc dnh m cc khi t st trong thin nhin c th gi cmi dc thng ng n mt chiu cao nht nh no , chiu cao ny s c tnhton chng IV. Da vo bn cht v nguyn nhn to thnh n, ngi ta c thphn ra thnh hai loi nh sau:

Lc dnh do lc ht phn t gy ra: Da theo l thuyt nc mng mng trnh by, th lc ht phn t c th pht sinh trc tip gia hai ht t vi nhau,hoc pht sinh qua cc ion (cation) trung gian gn lin hai ht vi nhau, hoc c thdo sc cng mt ngoi ca cc mng nc mao dn c trnh by trn. Lcdnh ny c c tnh c bit l c kh nng phc hi li sau khi b ph hoi nu c

c nhng iu kin nh

lc hnh thnh ban u. Loi lc dnh th hai l do cclin kt xi mng v cc lin kt kt tinh c sn gia cc ht, lc dnh ny c tothnh do kt qu ca s ha gi cc cht keo, s kt tinh hoc ti kt tinh cc loimui ha tan trong nc, v.v... Thuc loi lc dnh ny th khng c kh nng phchi sau khi b ph hoi - loi lc dnh ny c tnh cht cng, dn, ngc li viloi lc dnh do lc ht phn t li c tnh n hi v tnh do nht.6.2. Tnh co v n ca t

Tnh co v n ca t st l hai mt ngc nhau ca mt qu trnh. Khi lngnc cha trong t thay i th s sinh ra hin tng co v n ca t, tc l, tnhco l kh nng gim th tch trong qu trnh bc hi nc, cn tnh n l kh nngtng th tch ca t dnh khi ngm nc.

Trong t nhin hin tng trng n thng rt hay gp do nc ma, ncmt t, nc di t chy vo cc vng t dnh trng thi kh, c bit ccvng kh hu kh ro. Hin tng trng n thng lm cho cng ca tgim xung, ton b khi t b bin dng v ph hng, do gy ra nhiu tc hicho cc cng trnh xy dng trn nn t , ph hoi tnh n nh ca b dc, bng, gy bng nn, v.v...

Khi t st b kh th nc trong mng nc bao quanh dn dn b bc hi,trong qu trnh , trc ht nc mao dn b bc hi trc v khi t bt u coli. Mng nc bao quanh mng dn lm tng nng ion trong tng ion tri du,do lc ht ca cc ion i vi ht st xung quanh tng ln, vt qu lc y

gia cc ht st do tch in cng du gy ra. V vy cc ht t b ht li gn nhauhn v khi t cng co thm na, v n mt lc no vic gim th tch ngngli (tng ng vi khi lc y gia chng t ti mt th cn bng mi vi lc ht)th trn mt khi t xut hin cc vt nt c hnh chn chim. chnh l bn chtca tnh cht co rt th tch khi kh ca t loi st. m ca t ng vi thiim gi l gii hn co. Khi t co th chuyn v ca cc im trong khi t xyra khng ging, do gy ra tnh hnh ng sut khng u v lm cho khi t ntn, cng gim i, tnh thm tng ln, v vy khi dng t p , p p, hayp ng th cn phi ch n hin tng ny. Ngc li, khi t st kh, mngnc ti ch tip xc gia cc ht c chiu dy nh. Sau nu gp nc c nng

ion thp hn nng ion trong tng tri du gia cc ht, th mt s ion trongtng tri du s chy ra ngoi nc t do. Do tc dng ca lc ht in phn t cacc ht t nn cc phn t nc s thm nhp vo nhng ni mng nc c chiu


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dy nh v tch cc ht t ra, gy nn hin tng trng n ca t. Tc dng nca t c nh hng ln n cc cng trnh xy dng, do cn phi c ch thch ng trong khi thit k v xy dng cng trnh. i vi cc tng chn chnghn, khi t p sau tng n ra th s xut hin mt p lc ph thm tc dng lntng, nh hng n tnh n nh ca n. Tuy nhin, khng phi lc no hintng n ca t cng c hi, tri li cng c trng hp c th li dng tnh n ca

t phc v cng trnh. iu ny c th tht r trong thc t, ngi ta c th dngp lc phun dung dch st Mnmrlnit c tnh n ln vo cc l rng hoc khe nt b knh v t nn lm gim tnh thm cho b knh v t nn.

Tnh cht trng n v co rt c lin quan mt thit vi s bin i dyca mng nc bao quanh ht t, v vy cc nguyn nhn nh hng n tnh chttrng n v co rt ca t s bao gm cc nhn t ch yu nh: Thnh phnkhong vt, thnh phn ht, thnh phn ha hc v nng ion trong dung dch, ktcu, c im mi lin kt gia cc ht v rng ca t.6.3. Tnh tan r ca t:

Tnh cht tan r ca t l tnh cht ca t khi b vo nc th pht sinh hin

tng mt ht tnh dnh v tr thnh mt khi ri rc. Hin tng tan r c th l dohin tng trng n pht trin m thnh. Trong nhng iu kin nht nh khi hintng trng n pht trin n mt lc no , th do mng nc khuch tn dy lnn tr s ln nht, cc ht st khng cn ht chung cc ion hay cc phn t ncna, lm mt mi lin kt keo nc gia cc ht v cc ht st b r ri trong nc.

Cc loi t st c tnh thm nh, ni chung tc tan r chm, hin tngtan r thng pht trin dn dn t ngoi vo trong, t khi tan r thnh tng cc. Nut c l rng ln th trc ht nc ngm vo cc l rng ny gy ra ng lc khngng u trong khi t v u tin t s b tan r dc theo cc mt mm yu thnhtng cc to nh hnh dng bt k. Cc t loi st do hm lng ht st cao nn tcng ca tng khuch tn n tnh cht tan r kh r rt, thi gian mng n ckhch tn t n dy ln nht di hn, lc ht gia cc ht cn tng i ln. Vvy, hin tng tan r trong t loi st l mt qu trnh pht trin tip theo qu trnhtrng n.

Hin tng tan r c lin quan mt thit vi hin tng trng n cho nn ccnhn t nh hng ca n cng l thnh phn khong vt, thnh phn ht, thnhphn ha hc v nng ion trong trong dung dch cha trong t v nc, kt cuv c cu ca t.

Khi nh gi tnh cht tan r ca t ngi ta thng dng cc ch tiu sau:- Thi gian tan r: l thi gian tan r hon ton ca mt mu t nht nh.- c im v cc hin tng xy ra trong qu trnh tan r.

- Tc tan r, tnh bng hm lng phn trm phn t b tan r so vi mut ban u trong mt n v thi gian.

Tnh tan r ca t c mt ngha thc t rt ln khi nh gi t lm vtliu p p, p , p ng, nh gi n nh ca b dc, b knh o, nh gitnh cht chng xi l ca t v.v... Cn c vo c tnh tan r ca t ta c th phnon mc nh hng n vic thi cng, s dng cc cng trnh m t rabin php x l, phng cho thch hp.6.4. Hin tng Tikxotrofia trong t:

Trong thc t c th gp trng hp: Khi di nh hng tc dng ca mtti trng ng, mt s t st v t bn c th chuyn sang trng thi chy nho ri

bin thnh dung dch, lc ny t hon ton mt ht lc dnh kt cu, v nu saumt thi gian khng tc dng ti trng ng na, cc t y li hon ton phc hicc c tnh c nh l kt cu, st, rng, v.v... Qu trnh c th lp i lp


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li nhiu ln. Hin tng xy ra nh trn gi l hin tng Tikxotrofia. t mangtnh cht ny gi l t Tikxotrofia.

Qua thc nghim ngi ta nhn thy rng, hin tng Tikxotrofia ch xyra khi c y cc iu kin sau:

- t cha nhiu ht phn tn nh nh ht keo, c bit l cha nhiuBentnt hay ni r hn l cha nhiu loi khong vt Mnmrilnit.

- t phi bo ha nc.- t chu tc dng ca ti trng ng.Da vo cc iu kin trn c th nhn thy rng hin tng Tikxotrofia

thng hay xy ra nhng loi t trm tch tr, cha nhiu ht keo.V hin tng c nh hng rt ln ti iu kin n nh ca cc cng

trnh xy dng, do trong xy dng cn phi quan tm ch n, c bit l khithi cng ng cc c th gy ra nh hng hoc lm hhng cc cng trnh ln cn.6.5. Hin tng bin long ca t ct:

Hin tng bin long (chy lng) ca t ct c th gp cc t ct nh nonc, khi chu ti trng rung nhng iu kin nht nh. Nu cc loi ny c

dng lm nn cho cc mng my, hoc lm vt liu p cc , p th trong nhngiu kin y, t nn s b n ra ngoi, hoc khi t p s b sp, gy nhhng tai hi cho cng trnh.

Hin tng bin long ny c gio s N.M.Gerxevanov v gio sN.N.Maxlov ch r rng: t ct nh bo ha nc pht sinh hin tng chy lng(bin long) l v, khi c tc dng ca ti trng ng th p lc nc l rng xuthin t ngt v c tr s ln ti v tr tc dng ca ti trng ng , ri truyn irt nhanh ln ton b khi t bo ha nc. Trong nhng trng hp nu tr s plc vt qu trng lng bn thn ca t trong nc (ng vi dung trng yni), th cc im tip xc gia cc ht b ph hoi v t chuyn sang hon tonnhmt dung dch (hnh I - 10).

Cng chng ct ca t ct lc ny hunh bng khng v c khi t hon ton mt scchu ti, dn n s ph hoi cng trnh. V nu nhngng tc dng ti trng ng th p lc nc lrng ca n gim xung, cc ht ct li dch li gnnhau v sp xp theo mt dng kt cu cht hntrc, ngha l rng ca n nh hn trng thiban u. Cn c vo cc ti liu nghin cu ngi tathy rng hin tng bin long d xy ra hn c cc t ct c hnh dng trn nhn, ng knh D10 ca ht b hn 0,1mm, h s

khng ng u Cu < 5 v rng (n) vo khong 0,44 ng thi trong t c chamt t ht st.

Hnh I-10

Hin tng bin long c nh hng ln n cc cng trnh xy dng, do vic phng, trnh hin tng ny xy ra l s cn thit i vi chng ta v hinnay thng dng cc bin php sau y:

- Gim bt cng ca ti trng ng.- Lm tng cht ca t ct.- Tng cng kh nng thot nc ca t ct.- Ci thin tnh hnh phn b ng sut trong t.Ni chung cc bin php thng khng p dng ring r nhau, m c p

dng kt hp cht ch vi nhau th mi mang li hiu qu ln nht.6.6. Tnh m cht ca t:


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t c tnh cht l: di tc ng c hc nh rung, nn, nn cc ht t sdch chuyn to thnh mt kt cu mi cht hn. Tnh cht ny ca t rt thun licho vic dng t lm vt liu xy dng nhng cng trnh bng t nh p nnng,,p. Trong thc t khi lm nn ng, p p, p v gia c nnthng cn phi m t ti mt cht cn thit cho cc cng trnh nu trn

bn vng, n nh v cc tnh thm, tnh nn ln, v.v... gim i. Cc yu t chyu nh hng n vic m cht t l: cp phi ca t, m ca t, cng mnn. Trong phn ln cc trng hp ngi ta khng th to ra c mt cp phinh mun (bng cch pha trn cc c ht) m ch c th chn bi khai thc t p cng trnh. Ngha l chp nhn mt cp phi t nhin tng i thch hp chovic m cht. Nh vy, vic nghin cu tnh m cht ca t ch cn tm mi quanh gia dung trng kh ( k), m (W) v cng m (A), t c th chn cdung trng kh, m tha mn vi yu cu ca cng trnh, ng thi ng vi cngm nn nh nht. Vic la chn ny da trn kt qu th nghim m cht t.

Nguyn l th nghim:Nu t kh, m nh th cng m nn stiu hao vo vic khc phc ma st gia cc ht mkhng lm cho t cht li c bao nhiu. Ngc linu t qu t, m cao th mng nc lin ktcng dy y cc ht xa nhau, lm cho t khng thcht c. Nh vy vi mt cng m nn xc nh,cn phi c mt m tt nht khi m nn t st c cht cao nht.

Dng c th nghim trong phng (Hnh I - 11)

l mt hnh tr trn bng kim loi, bn d

i c tmlt, bn trn c vnh ko di c th tho ri c.Qu m c hnh tr trn x vo mt cn thng v ding, cn c tc dng dn hng v khng ch caori, ngoi ra l cn v cc dng c th nghim m.

Cch th nghim:

Chun b mt lng t dng, khong 6-8kg, hong kh trong khng kh, nghin nh (ch khng lm v ht), loi b nhng ht to qu 20mm. Dng mt ng o th tch olng nc a vo mu t. Ti nc vo mu t bng cch phun ma trn rtcn thn trong thi gian lu m bo mu t m u. Nu t l loi st th phmu bng vi m trong nhiu gi trc khi em m nn.

Hnh I-11

Vng ph

Mu t

Tm lt

Nm vnkho, m

Ci m

Cn dn hng

Qu m

Cn ci khi cha c mu t. Cho mu t vo ci tng lp, thng lm nmlp. Vi mi lp u m (bng cch th ri qu m mt chiu cao xc nh) 55nht m, ch sao cho cc nht m phn b u trn mt mu. Sau khi m xong,tho vnh ko di mt trn, gt mu t bng mt ci. Cn li ci m c cha yt. Cui cng tho mu t ra khi ci, ly 2-3 mu nh (trn - 10g), em thnghim xc nh m.

Lp li th nghim vi mu t th hai, gia tng lng nc phun ti vo mut (khng qun trn cn thn v tht lu): tip tc th nghim cho 5-6 mu. Vi mi


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mu th nghim ta xc nh c trng lng th tch t (t) v tng ng vi nl m W, hoc trng lng th tch t (kh) k v m W.

Kt qu th nghim c th hinbng ng cong, trn th c trctung biu th k = f(W), trc honh biuth W (Hnh I -12). Theo ta xc nhc m m nn tt nht ng vimt cng m nn xc nh. Nu thayi cng m nn ta thy: Khi tng cngm nn ln th m tt nht nh imt cht (im cc tr dch v bn tri),tt c cc ng cong k = f (W) utim cn mt ng thng gi l ngbo ha.

W(%)

k(Kg/cm )

Wopt

ng

congnoncG

=1

n=10

0

n=60

n=30

n=20

G=0,8

3

Hnh I-12: Cc

ng cong m chtTh nghim m cht nu trn

ngi ta thng gi l th nghim Proctor (tn ngi xut th nghim u tin).Da vo nguyn l th nghim m cht nu trn, sau ny ngi ta ci bin i,nn cc nc khc nhau c quy nh khc nhau v th nghim m cht.

* Th nghim proctor thng thng:

Ci m cht c ng knh bng 10cm, chiu cao ci m l 12,70cm, thtch ci m l 1000cm3. m c ng knh y l 10cm, trng lng qu m2,5kg. Tm lt y c ng knh 10cm.

Cch th nghim :

- Cho qu m ri t do vi chiu cao 30,5cm, s lp t m l 3, chiu dylp t l 4cm, s nht chy m cho mi lp l 25 chy, nng lng n v5,4kG.cm/cm3.(Nng lng n v c tnh l: trng lng qu m nhn vi chiucao ri nhn vi s nht m cho mi lp nhn vi s lp t m nn, kt qu chia cho th tch ci m).

* Th nghim proctor ci tin:

Ci m cht c ng knh bng 12,5 cm, chiu cao ci m 12,70cm , thtch ci m 2224cm3. m c ng knh y l 5,08cm, trng lng qu m4,54kg.

Cch th nghim:

- Cho qu m ri t do vi chiu cao 45,7cm, s lp t m nn l 5, chiudy mi lp 2,5cm, s nht m cho mi lp l 55, nng lng n v25KG.cm/cm3.

* Th nghim proctor do cng binh M ci tin:

Ci m cht c

ng knh l 15,24cm, chiu cao ci m 12,70cm, th tchci m l 2317cm3. m c ng knh y l 5,08cm, trng lng qu m l4,54KG, tm lt y dy 5,08cm v ng knh l 15,24cm.


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Cch th nghim:

- Cho qu m ri t do vi chiu cao 45,7cm, s lp t m l 5, chiudy mi lp 2,5cm, s nht m 55, nng lng n v 24,5KG.cm/cm3.

* Th nghim CBR ( California Bearing Ratio)

M v mt s nc, trong xy dng ng t thng dng ch s CBR (vit tt ca tn California Bearing Ratio - Ch s chu ti CBR), l t s biu thbng phn trm gia p lc to s xuyn ngp mt tr xuyn trong t ta xt vi plc to s xuyn ngp nh th trong vt liu tiu chun. Kch thc tr xuyn, tc v su xuyn c chun ho.

Dng c th nghim ( hnh I-13): Tr xuyn tit din 3 inch2 ( 19,35cm2), dikhong 20cm, gn vo mt gi c gn lc k v ng h o chuyn v; mt ccu vitme vi tay quay nng hp mu to ra s xuyn ngp ca tr xuyn.

Hp mu l mt ci m chtkiu Proctor Cng binh M ci tin,nhng chiu cao ci ln hn, bng 7inch (17,78cm); ngoi ra c nhng tmcng vnh khuyn t trn mu ttrong ci m dng gia ti ln mut, tr xuyn i qua l tm cc tm.

Cch th nghim: u tin mut c m cht nh th nghimProctor Cng binh M ci tin vi m sai lch 60,5% m m nn ttnht. Dng cc tm vnh khuyn gia tiln mt mu bng ti trng t chutrong t nhin, nhng trng lng tithiu cc tm vnh khuyn l 10 lbs(4,54kG). a hp mu vo di trxuyn, tc dng lc 10 lbs tr xuyntip xc vi mu. G ng h ochuyn v t ln thnh ci m v a

v 0. Bt u qu trnh p tr xuyn,thao tc sao cho tc xuyn l khngi v ng bng tc tiu chun hol 1/20 inch trong mt pht(1,27mm/pht). Trong qu trnh xuynghi chp p lc xuyn ng vi cc su xuyn 0,64; 1,27; 1,91; 2,54; 5,08v 7,62mm.

Hnh I-13: Dng c th nghim CBR

02 4 6 8 10 12 14

10

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Titrngt

cdng(kG/cm)2

Hnh I - 14: ng cong ng sut bin dng

Kt qu o c trnh by trn th th nghim CBR ( Hnh I - 14)

Tnh c cc ch s chu ti:


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CHNG I Trang 45

%10000,70

p )54,2( v %10000,105

P )08,5

Trong : P(2,54) v P(5,08) - p lc lm tr xuyn su 2,54 v 5,08 mm.

70,00 v 105,00 - p lc lm tr c cng su xuyn nh th i vi

vt liu tiu chun ly lm gc so snh.Gi tr u c ly lm tr s chu ti CBR tr khi gi tr sau ln hn. Trong

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