Ch 7 Foundation Engineeringbartlett/CVEEN5305/Ch. 7 - Site Chracterization... · Ch 7 Foundation...

© Steven F. Bartlett, 2011

Ch. 7 Lecture Notes○

Sections 7.1-7.7 (Salgado)○

Reading Assignment

Other Materials

Problems 7-7, 7-6, 7-16 b○

Homework Assignment 7

Ch 7 Foundation EngineeringWednesday, August 17, 2011 12:45 PM

Ch. 7 - Site Chracterization


Site Investigation Strategy

1. Obtain an understanding of the potential geologic and geotechnical conditions of the proposed site by reviewing regional studies. Also, consult with other professionals who have worked in the area to obtain their opinion regarding potential conditions and issues at the site.2. Obtain aerial photography (historical and recent), geologic and soil maps and other sources of subsurface information.3. Obtain drawing of the facilities planned for construction including the foot print of the building, foundation locations, areas of cut and fill, utilities, property lines, etc.4. From this information, develop a first phase of investigation to improve your understanding about key issues and obtain the data necessary to complete preliminary engineering analyses.5. Perform the assessment and calculations of the phase 1 data to understand the sensitivity of the analyses to key soil and groundwater input. The potential range of outcomes should be estimated by varying the key inputs within the possible ranges suggested by the characterization data.

a. Safe or adequate performance expectedb. Unsafe or with inadequate performance likelyc. Unknown condition or performance

6. From the phase 1 assessment, decide if the conditions at the site and the performance of the geosystem falls with the following categories:

7. For case 6a, decided if additional data or analyses are needed, if any, to complete the design. For case 6b, decide what design modification and/or ground improvement is necessary to obtain acceptable performance. In some cases other foundation types and construction methods will need to be used. In a few instances the planned site will have to be abandoned and another location selected. For case 6c, decide what key conditions must be better defined to complete the design and plan a phase II investigation.

Site InvestigationsWednesday, August 17, 2011 12:45 PM



Required Information

a. X, Y, Z coordinates for each borehole locationb. Legendc. North arrowd. Scalee. Date

1. Site map or plan view with facility locations and borehole locations

a. Soil layers with soil type and thicknessb. Depth to groundwaterc. Representative cross-section of cuts, fills and embankment

2. Detailed and representative cross sections including the following:

a. CPT tip, sleeve, friction ratio and pore pressure versus depthb. SPT N values versus depthc. Unit weight versus depth. Liquid limit, plastic limit and moisture content versus depth

3. Plots of basic soil properties versus depth

4. Plots of consolidation properties

a. Preconsolidation stress versus depthb. Compression indices versus depthc. Coefficient of consolidation versus depth

a. Effective stress phi angle versus depth (sands and non-plastic silts)b. Undrained shear strength versus depth (clays and plastic silts)c. Effective stress parameters versus depth (sands and clays for long-term stability)

5. Plots of shear strength properties versus depth

Site Investigations (cont.)Wednesday, August 17, 2011 12:45 PM



Borehole Map and LogWednesday, August 17, 2011 12:45 PM



>

Depth PlotsWednesday, August 17, 2011 12:45 PM



Hollow Stem Auger

Mud Rotary Drilling with bit

Coring

Drilling Techniques (Comparison)Wednesday, August 17, 2011 12:45 PM



Mud rotary drilling is most commonly used to create a stable hole that can then be used

for other purposes, such as to create water wells, or for seismic testing.Slim hole rotary with the rods in compression

As mentioned previously, drilling works by applying downward pressure to a bit to cut

into ground. This method uses the pressure of compressed rods to push down on the bit.

The rods are slightly smaller than the bit and are supported by the hole. When the weight

of the rods becomes greater than the pressure needed on the bit, the driller reverses the

drill pull down to hold the drill string back.

Advantages

This method can be used by all sizes of drill rigs at any angle.

Disadvantages

The holes can deflect easily, and need special drilling techniques and equipment to keep

the hole straight.Single tube reverse circulation

Reverse circulation is used for drilling large diameter holes. Mud is pumped down the

outside of the hole to the bit. A centrifugal pump sucks the fluid and drill cuttings up

inside the drill string, where it is discharged into a large settling pit

Pasted from <http://toolboxes.flexiblelearning.net.au/demosites/series7/706/units/mud/index.htm>

Drilling Techniques (Mud Rotary)Wednesday, August 17, 2011 12:45 PM


http://toolboxes.flexiblelearning.net.au/demosites/series7/706/units/mud/index.htm


Pasted from <http://www.geology.sdsu.edu/classes/geol552/hollowstem.htm>

a helical or spiral tool form is used to move material from the subsurface to the surface

•

a bit at the bottom cuts into the subsurface material •spiral augers on outside convey the material to the surface while spinning •

AUGER DRILLING

center of auger is hollow like a straw when the inner drive rods and plug are removed

•

during drilling or formation cutting, the center is filled with rods connected to a plug at the bottom bit

•

once the desired drilling depth is reached, the center plug and rods can be pulled out - leaving the hollow augers in place

•

the hollow augers hold the borehole open for sediment sampling and well installation

•

HOLLOW-STEM AUGER

Pasted from <http://www.geology.sdsu.edu/classes/geol552/hollowstem.htm>

Drilling Techniques (Auger and Hollow Stem Auger)Wednesday, August 17, 2011 12:45 PM


http://www.geology.sdsu.edu/classes/geol552/hollowstem.htm

http://www.geology.sdsu.edu/classes/geol552/hollowstem.htm


Drilling Techniques (Rock Coring)Wednesday, August 17, 2011 12:45 PM



Standard Penetration Test (SPT)Wednesday, August 17, 2011 12:45 PM



Standard Penetration Test (SPT) (cont.)Wednesday, August 17, 2011 12:45 PM



(Old - Not Used) (Donut Hammer)(Ch = 45/60)

(Safety Hammer)(Ch = 60/60)

Ch = ER hammer/ER safety hammer = ER hammer/60 percent where ER = energy ratio




Cr = 0.75 for rod length < 4m

Cr = 0.85 for 4m <= rod length < 6m

Cr = 0.95 for 6m <= rod length < 10 m

Cr = 1 if rod length => 10 m

Correction for rod length

Correction for borehole diameter and liner

Cd = 1 For 66 to115 mm

Cd = 1.05 For 116 < diameter <= 150 mm

Cd = 1.15 For 151 < diameter < 200 mm

CS = 1.0 ( with liner)CS = 1.2 (without liner)




PA = reference stress (100 kPa) or 1 tsf




Relative Density from SPT

DR% / 100% = N60 / (A+BC*v'/PA)

Applicable for DR between 35 and 85 percent

And v' between 50 and 250 kPa

27 <= A < 46 or average A = (27+46/2) = 36.5B = 27C = Ko/KoNC




Peak Drained Friction Angle from SPT (sand)Wednesday, August 17, 2011 12:45 PM



Thin Walled Sampler (Shelby Tube)

Shelby Tube attached to a piston sampler

Undisturbed Soil and Rock SamplingWednesday, August 17, 2011 12:45 PM



Double Barrel CoreSampler for Rock

Undisturbed Soil and Rock Sampling (cont.)Wednesday, August 17, 2011 12:45 PM



Srm = CwSir

Weakness in Rock MassWednesday, August 17, 2011 12:45 PM



Cone Penetrometer TestWednesday, August 17, 2011 12:45 PM



Small truck mounted rig

Pasted from <http://www.conetec.com/site_investigation_equipment/cpt_trucks.aspx>

Large truck mounted rig

Cone Penetrometer Test (cont.)Wednesday, August 17, 2011 12:45 PM


http://www.conetec.com/site_investigation_equipment/cpt_trucks.aspx


Trailer rig

Track mounted rig (all-terrain)




Typical CPT data

Clay likematerials

Sand likematerials

NoteHighPorePressuresIn Clay




Soil behavior type plotted as a function of depth to show the soil layers present in the subsurface profile




Seismic cone penetrometer testing (SCPT) done in conjunction with standard CPT

Interpretation of SCPT

Seismic Cone Penetrometer TestWednesday, August 17, 2011 12:45 PM



Critical state friction angle for sandy materials as a function of lateral (i.e., horizontal) effective stress and normalized tip resistance.

Engineering Properties from CPTWednesday, August 17, 2011 12:45 PM



Estimating effective lateral stress from normalized CPT resistance.(Needed from previous critical state friction angle charts.




Estimating horizontal coefficient of consolidation from CPTU (cone penetrometer dissipation tests.

t90 = 2875s

t90 = 48 min

d

0.15 d

t90 / t50 = 0.848/0.196t50 = 48 min * (0.196/0.848)t50 = 11 min

u2 (shoulder mounted piezometer is most common)

T = ch t/H2




Undrained shear strength, su




Nk = 14 recommended for Lake Bonneville silty clay and clayey silt

NC and OC Clay (Ladd 1977)○

(su/v')OC = (su/v')NC*OCR0.8

Adjusting su for overconsolidation




BlankWednesday, August 17, 2011 12:45 PM


Ch 7 Foundation Engineeringbartlett/CVEEN5305/Ch. 7 - Site Chracterization... · Ch 7 Foundation...

Documents

Transcript of Ch 7 Foundation Engineeringbartlett/CVEEN5305/Ch. 7 - Site Chracterization... · Ch 7 Foundation...