THE CHEMICAL BONDING AND APPARENT OVERCO...

THE CHEMICAL BONDING AND APPARENT OVERCONSOLIDATION IN THE NAMAK MARINE CLAY DEPOSIT IN KOREA Jeong-Yun Won, Ph.D., Dep. of Civil Eng. – The University of Texas at Austin, Austin, Texas, USA Pyoung-Wuck Chang, Professor, Dep. of Rural System Eng. – Seoul National University, Seoul, Korea Young-Hwan Son, Ph.D. Dep. of Rural System Eng. – Seoul National University, Seoul, Korea ABSTRACT Indicators of cementation such as geochemical composition, pore water chemistry, pore size distribution, and magnetic susceptibility, were examined for a marine clay deposit in Namak, Korea to investigate the causes of apparent overconsolidation in the clay. Undisturbed samples were continuously retrieved from two separate borings. Although there has been no leaching or weathering process in the clay deposit, overconsolidation ratios in the upper part were found to be greater than 1. Agents of chemical bonding were compared to consolidation properties of the marine clay. The qualitative degrees of cementation were evaluated by stress state charts and compression index ratios. Although quantitative relationships between chemical bonding agents and overconsolidation ratio (or degree of cementation) could not be established, there seems to be strong correspondences between the chemical components (sulphate and potassium) in the pore water and the cementation. RÉSUMÉ La composition géochimique, l’organisation des pores de l’eau, la distribution de la taille de ces pores et les sensibilités magnétiques ont été étudiées dans le dépôt d’argile marin de Namak, Corée pour rechercher les causes de l’évidente dureté de l’argile. Des échantillons ont été prélevés dans 2 points d’extraction séparés. Bien qu’il n’y ait pas eu d’action de l’eau ni d’action du climat sur le dépôt d’argile, on a découvert que la dureté de la partie supérieure était supérieure à 1. La structure atomique a été comparée à la dureté du dépôt. Le ciment a été évalué par des graphiques montrant l’étirement et la compression excercés sur le dépôt. Les relations quantitatives entre la structure des atomes et le niveau de dureté n’ont pu être établies, il semble qu’il y ait des relations étroites entre les composants chimiques (sulfate et potassium) des pores de l’eau et la formation du ciment. 1. INTRODUCTION Since 70% of the land in the Korean Peninsula consists of mountainous area, there is a shortage of land available for agriculture. Accordingly, the widely distributed tidal flats along the southern and western coasts have been reclaimed since the mid-thirteenth century. Recently, reclamation programs have been carried out for the construction of harbour facilities and industrial / housing complexes. Currently, a major construction project to develop Namak New City is underway on a reclaimed area. Because a large portion of the townsite is composed of soft marine silty clay, estimation of the settlement due to the surcharge loading was a critical design calculation. Besides economical benefits, ecological and social issues made it essential to optimize borrowing of fill material from the mountain adjacent to the townsite. Thus, consolidation properties of the soft marine clay and its thickness were the key items during the site characterization program. An extensive site characterization program was undertaken to describe the stratigraphy and mechanical

properties of the soft marine clay over the 9 km2 townsite. Results from the comprehensive field and laboratory tests raised issues about the interpretation of the upper part of the deposit, where the overconsolidation ratios (OCR) were consistently greater than 1. From a geotechnical point of view, there was no classical geotechnical explanation for the overconsolidation phenomena in the townsite (e.g., geological stress history, groundwater fluctuation, and cementation), which was investigated by Won (2004). The apparent overconsolidation of a soil can be defined as overconsolidation phenomenon if it is not caused by the removal of a previous loading. Cementation has been studied by numerous researchers as a cause of apparent overconsolidation (Bjerrum, 1972; Sangrey 1972; Kelly et al. 1974; Moore et al. 1977; Quigley 1980; Jamiolkowski et al. 1985; Ohtsubo et al. 1995; Fukue and Nakamura 1999). There seems to be a variety of processes associated with chemical bonding in clays and these processes seem to depend on clay type, locality, and geological features. As Asaoka et al. (2000) pointed out, three different concepts, namely, structure, secondary compression, and overconsolidation are closely related to each other. Although structural effect

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(Leroueil and Vaughan 1990) can be found in most natural soft clays, the fundamental mechanism that causes soil structure to affect the engineering properties of a soil is still not be understood. In general, the clay structure refers to a combination of clay “fabric” arrangement and physico-chemical “bonding” (i.e. cementation) between clay particles. However, in this paper, these two terms will be used separately to indicate discriminable mechanism. In this paper, as a part of the through investigation on the Namak site, chemical bonding and its effects soil structure will be discussed. The results of consolidation tests on undisturbed samples obtained during the investigation on the Namak site, which will be discussed in more detail later, will be compared with other chemical properties measured with the pore water and soil bulk. To reinforce the conclusions, evidences of leaching or weathering process at the site will be discussed. Then, qualitative relationships between the prominent chemical components and the indices for the degree of cementation will be discussed. 2. SITE INVESTIGATED The Namak site is located in the estuary of the Youngsan River in the south-western part of the Korean Peninsula (see inset of Figure 1). Its geographical features are partly isolated by numerous islands, so the effects of waves are relatively low and those of tides are great (its tidal range is about 2.7 m). The Youngsan sea dyke, completed in 1981, transformed the marshes or submerged lands to onshore paddy lands. The Namak site had been used as paddy land for more than 20 years.

$GREH�6\VWHPV

Figure 1. Location of the boring points in the Namak site

From the extensive site characterization program, two types of stratigraphy were identified. Two locations, noted on Figure 1, were identified as having representative features for each type of stratigraphy, labelled as MP-1 and MP-2. Boring logs for the MP-1 and MP-2 points are shown in Figure 2. The thicknesses of the marine clay deposit were 13.9 m for the MP-1 and 20.6 m for the MP-2. Ground water tables in the paddy area which had been mainly measured in winter (draught season) were about 1.0 m at maximum. This marine deposit has been deposited since approximately 8,000 years before present during monotonic sea level rise (e.g. transgression). More detailed site descriptions and information about the depositional environments of the study area were given in Won (2004). Only the marine deposit in this study area will be discussed hereafter.

Figure 2. Boring logs for the MP-1 and the MP-2 points 3. TESTING PROGRAM 3.1 Sampling In order to reduce sample disturbance, boring holes were drilled using 100-mm diameter casing. Then high pressured water was injected into the borehole prior to sampling to wash out the remaining slime. A fixed piston


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type sampler (76-mm inner diameter) was used to retrieve the undisturbed samples. In the laboratory, specimens of approximately 120 mm in length were cut for mechanical tests and were coated by low melting point paraffin in a specially prepared bronze mould to prevent changes in water content and aging effects during storage, as recommended by Lessard and Mitchell (1985). The thickness of the paraffin coating was about 3 mm. The other parts of the sample were sectioned in half longitudinally, and were used for core observations and sub-samplings for the various other analyses. Core colour was checked based on the Standard Colour Chart. 3.2 Geotechnical Tests Particle size analyses were carried out using the Sedigraph, which uses X-ray scattering and density changes with time to calculate equivalent spherical diameters of soil. Results of particle size analysis were presented in terms of f=(-)log2D (mm) scale. Using this scale, its range of f=(-)1~3 is for sand (D>0.074 mm), f=4~8 is for silt, and f=9~ is for clay (D<0.002 mm). Samples for particle size analysis were well preserved to maintain the natural water contents. Organic materials were dissolved by H2O2 solutions and soil particles were dispersed using sodium metaphosphate, several times. Atterberg limits were determined by the Swedish type (60o, 60g) fall cone apparatus with a bronze ring of 60mm in diameter, as proposed by Feng (2000). Distilled water was added from the natural water content state or after short air drying. Liquid and plastic limits were determined at the penetration depth of 10.0 and 1.0 mm (Koumoto and Houlsby 2001), respectively. Oedometer tests were performed on 60-mm in diameter and 20-mm in height undisturbed specimen, in which the load incremental ratio was 1, and the duration for each increment of load was 24 hours. Yield stresses (sy’) of the samples were determined using the Casagrande method, and coefficients of consolidation were evaluated by Taylor method. 3.3 Physico-Chemical Tests The pore water naturally existing within the clay sample was collected by squeezing the soil in a soil press. Then, the suspended solid particles were separated from the removed water using a centrifugal separator. Collected pore water was stored in the refrigerator at a temperature of 10oC. The salinity and pH of the pore water were measured with electronic conductivity meter and pH meter, respectively. Exchangeable cations (Ca2+, Mg2+, and K+) and anions (Cl- and SO4

2-) in the pore water were measured using inductively coupled plasma

– mass spectrometry (ICP-MS) at Seoul National University. Magnetic susceptibility (MS) is defined as the ratio of the induced magnetization to the applied magnetic field. MS values are influenced by the types and contents of magnetic mineral, and by the size and shape of the particles. A profile of the MS values can be used as an index of weathering, leaching, and/or mineral compositions. Cubes for MS measurement were used to sample the core at 10~20 cm intervals. Low field magnetic susceptibility was measured with a Bartington magnetic susceptibility meter (MS2). Powdered sediment samples were digested with chemical solutions to investigate the geochemical elements of bulk sediments. The samples were analyzed using inductively coupled plasma - emission spectrometry (ICP-ES). Total organic carbon (TOC) and total nitrogen (TN) were measured in CHN analyzer. The calcium carbonate contents (CaCO3) were indirectly evaluated using TOC and TN values. Specimens for mercury intrusion porosimetry (MIP) were cut from undisturbed samples and frozen in a deep freezer (-66oC) for 24 hours, then dried in the freeze dryer (-54 0C) for 24 hours. Successive intrusion technique (Delage and Lefebvre 1984) was used to measure the pore distribution characteristics using AutoPore IV 9500. 4. TEST RESULTS 4.1 Geotechnical Tests Profiles for the particle size of the deposit were shown in Figure 3 in terms of mean f, sorting and skewness. Values of sorting and skewness of the MP-1 indicate that the depositional environments like fluid velocity in the upper part and the lower part, have been apparently different. It was shown that the lower part of the MP-1 has more clay fraction than the upper part, whereas comparatively constant particle distribution can be found in the MP-2. Consistencies of the clay deposit are shown in Figure 4 for the MP-1 and MP-2 respectively. The ranges of consistencies of the marine deposit were 43.7~67.4 % for liquid limit, 20.9~41.3 % for plasticity index, and the natural water contents were between 44.1~72.9 %. Natural water contents were about the same to the liquid limits which is typical in the marine deposit in coastal area in Korea. Liquidity indices (0.58~1.58) shown in Figure 4 (c) correspond to the normally consolidated range. It can be hardly seen the differences in consistency between the upper and the lower marine deposit.


229

Mean f , M z

6 . 57 . 07 . 58 . 08 . 5

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0

S o r t i n g

2 . 02 . 12 . 22 . 3

Skewness

- 1 . 2- 0 . 8- 0 . 40.0 0.4

( a )( b )( c )

M P - 1M P - 2

Figure 3. Particle size distributions; (a) mean particle size, (b) sorting, and (c) skewness Values of salinity and pH of the pore water from the natural soils are shown in Figure 5 (a) and (b), respectively. In consideration of the representative salinity of sea water around the Korean Peninsular of 32 g/L, profiles of salinity in the marine deposit indicate that there has been no significant leaching action after or during deposition. Constant liquid limits and plasticity indices with depth (Figure 4) also can be the evidence for non-leaching deposition. The basic factor governing the chemical stability of minerals in the clay is the pH value of the pore water (Bjerrum 1967). The constant values of pH in Figure 5 (b) indicate that the chemical actions between the clay particle and pore water have been stable.

Consistency (%)

0 2 04 06 08 0

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0

Consistency (%)

2 04 06 08 0

Liquidity index

0 . 51 . 01 . 52 . 0

( a )( b )

LL

PL

M P - 1M P - 2

(c)

N a t u r a lWater Content

Figure 4. Atterberg limits of (a) MP-1 and (b) MP-2 points, and (c) liquidity indices of the MP-1 and MP-2 points

Salinity (g/L)

5 1 01 52 02 53 03 5

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0( a )

pH

7 . 07 . 58 . 08 . 5

MS (m3/g)

0 . 20 . 40 . 60 . 81 . 01 . 2

( b )( c )

M P - 1M P - 2

Figure 5. (a) Salinity and (b) pH of the pore water, and (c) magnetic susceptibility (MS) of the soil bulk Profiles of the magnetic susceptibility (MS) measured from bulk soil samples are shown in Figure 5 (c). Although the values of MS can be affected by various factors, the profile of MS in a boring point can be used to predict the weathering history. High MS values of the surface crust in the MP-2 exemplify the weathering evidences. The crust in the MP-2 is basically the same soil as the underlying layer, but the soil has different colour and lower water content. In comparison with the profile of mean particle size shown in Figure 3 (a), MS values of the lower part in the MP-1 seems to be affected by the particle size. Unlike the quick clays in North America Glacial clay deposits, it seems that, in the Namak site, there has been no leaching action which can cause changes in yield stresses and liquid limits (Cox 1968) after deposition. Profiles of sample quality designations (SQD), overconsolidation ratios and compression indices (Cc) for the marine deposit are shown in Figure 6. To compare and evaluate the overconsolidation ratio, sample depths were normalized by thickness of the marine deposit (13.9m for the MP-1 and 20.6m for the MP-2). The values of SQD were calculated from volumetric strain in oedometer tests. SQD values which are fallen within A~C grade indicate that OCR values may have been underestimated. In spite of sample disturbances, values of OCR in the upper part of the marine deposit are consistently greater than 1. The results in Figure 6 indicate that the boundary between the upper overconsolidated layer (OCR>1) and the lower normally consolidated layer is about half depth of the marine deposit. Among the compression indices changing with the consolidation pressure, those shown in Figure 6 (c) were the maximum values. Unlike the profiles of the OCR and SQD, the profile for the compressibility (Cc) of the deposit shown in Figure 6 (c)


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seems to have no basic trend with depth and minimal differences between the upper and the lower parts.

S Q D0 1 2 3 4

No

rma

lize

dD

ep

th(z

/H)

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

1 . 1

( a )

( b ) ( c )

A B C D

O C R1 2 3

Cc

0 . 40 . 60 . 81 . 0

( b )( c )

M P - 1M P - 2

Figure 6. Results from oedometer test; (a) sample quality designation, (b) overconsolidation ratio, and (c) compression index

0 . 0 10.1 1 10 100

Normalized Consolidation Pressures, s' / sy'0 . 0 10.1 1 10

No

rma

lize

dV

oid

Ra

tio

,e

/e

o

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

MP-1 1.8m MP-2 2.7m

MP-1 10.7m

MP-2 17.7m

Figure 7. Consolidation curves for the Namak clay deposit The representative consolidation curves for the marine clay specimens are shown in Figure 7, in which void ratios and consolidation pressures were normalized by the initial void ratios and the yield stresses, respectively. Brittle yields around the yield stress and inverse S-shape consolidation curves exhibit typical structured clay behaviours. The degree of structure, however, does not seem to be high.

0 . 11 10

Normalized Consolidation Pressures, s' / sy'0 . 11 10C

oe

ffic

ien

tso

fC

on

so

lida

tio

n,

v(c

m2/s

ec)

10 -4

10 -3

10 -2

( a ) ( b )

c

Figure 8. The variations of coefficient of consolidation with consolidation pressures; (a) MP-1 and (b) MP-2 The abrupt changes of the coefficient of consolidation (cv) around the yield stresses (Figure 8) implying that the marine deposit has a structured characteristics. The difference between cv values in the MP-1 and the MP-2 is attributed to the effect of the laminated macrofabric found in the MP-2. In Figure 8, consolidation pressures also were normalized by the yield stresses. 4.2 Physico-Chemical Tests Pore diameter distribution curves obtained from mercury intrusion porosimetry (MIP) are shown in Figure 9. It must be noted that MIP does not give a measurement of pore sizes, but of pore entrance sizes (Delage and Lefebvre 1984). Although this limitation, from Figure 9, it can be seen that the samples from the upper part have larger portions of inter-aggregate pore (> about 1 mm) than those from the lower part. Delage and Lefebvre (1984) reported that, during consolidation, only the largest existing pores collapse under a given effective pressure, and that small intra-aggregate pores are not affected under a large pressure, i.e. the compressibility of the clay at a given consolidation pressure appeared to be related to the largest pore size existing at such pressure. In this regard, the upper part of the marine deposit could be expected to have greater compression indices (i.e. compressibility) and low OCRs. However, as shown in the previous Figure 6, the opposite trends were observed. In comparison with the MP-1, samples from the MP-2 seem to have more portion of big pore and this can be explained by the fact that the mean particle sizes of the MP-2 were greater (i.e. coarser) than those of the MP-1 (see Figure 3).


231

Entrance Pore Diameter (mm)

1 0- 21 0- 11 00 1 01 1 02 1 03

Intr

ud

ed

Vo

lum

e(m

L/g

)

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8MP-1 3.4mMP-1 12.8mMP-2 4.5mMP-2 6.0mMP-2 17.2m

Figure 9. Pore size distribution curves for the clay deposit

Cl- (g/L)

1 21 41 61 81 02 0

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0

SO 4

2 - (g/L)

0 . 11 10

M P - 1M P - 2

(a) (b)

Figure 10. Profiles of anion of the pore water; (a) chloride and (b) sulphate

Ca2 +

(mg/L)

0 1 0 02 0 03 0 04 0 0

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0

Mg2 +

(mg/L)

4 0 06 0 08 0 0

K+ (mg/L)

2 0 03 0 04 0 05 0 0

M P - 1M P - 2

(a) (b) (c)

Figure 11. Profiles for ion of the pore water; (a) calcium, (b) magnesium, and (c) potassium

The concentration of different ions in the pore water, specifically chloride (Cl-), sulphate (SO4

2-), calcium (Ca2+), magnesium (Mg2+), and potassium (K+) were measured and plotted in Figure 10 and Figure 11. In conjunction with the geochemical components of the soil particles, profiles of pore water chemistry can be related with the chemical bonding between clay particles or aggregates. From Figure 10 and Figure 11, profiles of the sulphate anion and potassium ion seem to show distinct differences between the upper and the lower part of the marine deposit. Profiles of chloride anion seem to be affected by high salinity of the pore water. Profiles of the calcium and magnesium ions seem to have little relation with consolidation properties in this site.

Al (%)

3 4 5 6 7 8

De

pth

(EL

,m)

- 2 2

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

-8

-6

-4

-2

0

Fe (%)

2 . 02 . 53 . 03 . 54 . 0

Mg (%)

0 . 80 . 91 . 01 . 11 . 2

( a )( b )( c )

MP-1 MP-2

Figure 12. Profiles of geochemical components; (a) aluminium, (b) iron, and (c) magnesium Among the numerous geochemical components, profiles of most prominent components such as aluminium (Al), iron (Fe), and magnesium (Mg) are shown in Figure 12. Aluminium is known as a conservative major component of clays and is supposed to have a strong relationship with particle size. As shown in Figure 12 (a), profiles of the aluminium component show similar trend compared with profiles of mean particle size shown in Figure 3 (a). The distinctions between the upper and the lower part of the deposit can be found in profiles of the iron and magnesium components and its boundary is consistently about the half of the thickness of the marine deposit. 5. DISUCUSSION Information about the origins of sediments and changes in depositional environments can be obtained by analyzing geochemical components of the soils particles. In Figure 13, profiles of the total organic content, K+/Na+ ratio, and C/N ratio are shown with


232

normalized depth. Depositional environment such as water temperature can be estimated by the organic content. For example, the organic content in the Northern Hemisphere like Quebec clay is less than 1 %, that in Japan clays is 2~8% (Locat et al. 1996), whereas approximately 0.3% in Korean clays (Chough 1983). Most of the clays in the Northern Hemisphere were deposited in glacial regions at a time when the terrain was incapable of supporting very organic material and the sea water was too cold to support life (Cox 1968). Although relatively warm sea water can be assumed around the study area, organic contents (TOC) shown in Figure 13 (a) was about 0.3% without variation. Bjerrum (1967), explaining the base exchanges due to the weathering in Norwegian clays, suggested that the ratio of potassium ions to sodium ions (K+/Na+) increase from approximately 0.2 at the instance of first deposition to approximately 2 after weathering. In the Namak site, K+/Na+ ratios shown in Figure 13 (b) are rather constant (0.03~0.07) with normalized depth indicating there has been no weathering process.

TOC (%)

0 . 20 . 40 . 60 . 81 . 01 . 2

No

rma

lize

dD

ep

th(z

/H)

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

1 . 1

( a )

M P - 1M P - 2

K+/ N a

+

0 . 0 40 . 0 60 . 0 8

C/N ratio

1 02 03 04 05 0

( b )( c )

M P - 1M P - 2

Figure 13. Indices for the depositional environment; (a) total organic content, (b) ratio of potassium to sodium, and (c) ratio of carbon to nitrogen In addition to the organic content, the ratio of carbon to nitrogen (C/N ratio) of the organic materials can be used as an indicator whether the sediment was from marine or from continental supply. Generally, C/N ratio of the organic materials produced by marine biogenic product show the value of about 10, whereas that originated by nearby continental supply shows C/N ratio of more than 10. In consideration of the estuary environment of the study area, C/N ratios shown in Figure 13 (c) are implying that the source sediment of the Namak marine clay deposit could be both marine and inland (upstream).

Different variables reflecting the pore water chemistry and overconsolidation ratio are compared in Figure 14. Sulphate ions in the pore water can play a role as a cementation agent in connection with ferric sulphate. Similarly, the potassium ion as a cementation agent can be explained by ion exchange with sodium. Among other chemical components of the pore water, sulphate (SO4

2-) and potassium (K+) ions seem to have meaningful relationship with overconsolidation ratios. On the other hand, it was hard to find the relationship between overconsolidation ratio and geochemical component of the soil bulk.

O C R

1 2 3

No

rma

lize

dD

ep

th(z

/H)

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

1 . 1

SO 4

2 - (g/L)

0 1 2 3

K+ (g/L)

0 . 20 . 30 . 40 . 5

M P - 1M P - 2

(a) (b) (c)

Figure 14. Comparison between (a) OCR and (b) sulphate and (c) potassium ions in the pore water It seems possible to measure the absolute values of chemical components of pore water in marine clay by means of precise measuring instruments. Comparing chemical components from different marine deposits, however, should be done with great restriction, because chemical components in the sea water will depend on locality, weather, and oceanic current and so on. Further, it is reasonable to assume that cementation depends not only on pore water chemistry but also numerous factors. In this regard, quantitative correlation between overconsolidation ratio (or degree of cementation) and values of chemical component in the pore water was not made in this study. Tsuchida et al. (1991) have introduced the compression index ratio defined as rc = Ccmax / Cc*, where Ccmax is the peak value of compression index (Cc) and Cc* is the Cc at which consolidation pressure is five times of Pccmax (Pccmax is the consolidation pressure when Cc=Ccmax). They introduced the compression index ratio to represent the aging effect of natural clays. The ranges of the compression index ratio they have reported were 1.1~3.0 for Japanese alluvial marine clays, 3.2 for Mexico City clay, 4.2 for Canadian marine clay, 6.0 for Leda clay.


233

From Figure 15 (b), the values of rc of the Namak marine clay range from 1.1 to 2.1 and they are smaller than those of aged clays like Leda clay. Although range of OCR values is 2~3 in the upper part of the marine deposit, from the values of rc, the degree of cementation in the Namak clay does not seem to be so high. Contrary to the trends of rc in Japanese clays, those in the Namak marine clay show a decrease with depth. Relatively high values of rc in the upper layer in the Namak marine deposit indicate that the degree of cementation in the upper layer is greater than that in the lower layer. One of the most common cementation agents could be calcium carbonate content, CaCO3 (Kelly et al. 1974; Jamiolkowski et al. 1985; Fukue and Nakamura 1999). As shown in Figure 15 (c), however, calcium carbonate content does not seem to have meaningful relation to overconsolidation ratio. As Fukue and Nakamura (1999) pointed out, calcium carbonate content seem to have more close relation with shear strength than consolidation properties.

O C R

1 2 3

No

rma

lize

dD

ep

th(z

/H)

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

1 . 1

rc

1 . 21 . 41 . 61 . 82 . 0

C a C O3 (%)

0 . 30 . 60 . 91 . 21 . 5

( a )( b )( c )

Figure 15. Comparison between (a) overconsolidation ratio and (b) compression index ratio, rc, and (c) calcium carbonate content

Vertical Effective Stress, sv o' , kPa1 0- 11 00 1 01 1 02 1 03 1 04

Vo

idIn

de

x,I v

- 3

- 2

- 1

0

1

2

3

4

5

6

S C L

I C L

quick and

c l a y s

range of

carbonate

c o m p r e s s i o n

range ofs e d i m e n t a t i o n

O v e r c o n s o l i d a t e dclays

M P - 2M P - 1

Figure16. Stress state for the Namak marine clay To evaluate the degree of cementation, the data obtained from oedometer tests are plotted in stress state chart (Figure 16). Stress state chart was originally introduced by Burland (1990) and was modified by Chandler (2000). Values of void index Iv were evaluated using the experimental equation proposed by Chandler (2000), where liquid limits were determined from fall cone tests. In stress state chart, most data of marine clay are plotted within the sedimentation compression region. It can be seen that the Namak marine clay is normally consolidated clay in geological meaning. In other word, overconsolidation ratios greater than 1 were not caused by removal of overburden stress. The data points plotted in the range of quick and carbonate clays are from the upper part of the MP-2 point. 6. CONCLUSIONS Chemical bonding in a soil was investigated as a possible explanation to the apparent overconsolidation in the clay deposit in Namak, Korea. Chemical bonding in the clay was evaluated by analysing the pore water chemistry and geochemical components in the soil bulk. From the study the following conclusions were made. It has been found that there has been no leaching and weathering actions after deposition in the site. This was supported by profiles of salinity, pH, liquidity index and magnetic susceptibility. Without geological event, in the upper layer in the clay, overconsolidation ratios (OCR) were constantly greater than 1. Compressibility in the upper layer, which was evaluated by mercury intrusion porosimetry, was greater than the lower layer. Some indices such as organic content, K+/Na+ ratio, and C/N ratio were found to be good


234

indicators for the depositional environment. The degrees of cementation were qualitatively evaluated by the compression index ratio and the stress state chart. Among the numerous chemical components of the pore water, sulphate (SO4

2-) and potassium (K+) ions showed relationship with OCRs. Geochemical components of the soil bulk and calcium carbonate content seemed to have little correlation with OCR. References Asaoka, A., Nakano, M., Noda, T., and Kaneda, K. (2000)

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