Gmax of Fine-Grained Soils at Wide Void Ratio Range,Focusing on Time-Dependent Behavior

Effects of creep as well as initial water content at deposition on the pseudo-elastic shear modulus, G_max, of fine-grained soil are studied in a consolidometer equipped with bender elements. The state boundary (SB) concept is discussed in void ratio (e)-G_max-Effective Vertical Stress (σ′_v) space and the condition of metastability associated with short-term creep and long-term natural creep are exemplified from laboratory creep tests and in-situ soil test results, respectively. In addition to the conventional laboratory reconstitution method adopted for almost all clay samples that use an initial water content of twice the liquid limit, the soil samples from Minato-Mirai (MM) site were also prepared by directly consolidating the slurry with a water content ranging from 1.5 to 5.0 times the liquid limit. The larger primary metastable region or higher degree of on-depositional structuration associated with the samples at higher initial void ratio is confirmed. However, no appreciable difference was found in the increase in G_max with time, represented by N_g, between the samples prepared by the two different methods. In addition to the highly plastic MM clay, the study of N_g also covered other clayey soils of different origin and plasticity index range from 29 to 78. Metastability index, MI(G_max)^e that manifested the aspect of structuration and déstructuration reaches a maximum value at the end of sustained loading and vanishes slowly with any increase in stress level. By using liquidity index instead of void ratio in the e-log G_max plot, the metastability index, MI(G_max)^LI, is found to represent a wider variety of soils with minimum scatter. A slight stress level dependency of metastability increase was observed yielding smaller values at higher stresses. For the present test conditions and duration, subsequent stressing of 1.5-2.0 times the creep stress brought about the complete déstructuration of the creep-added soil-structure formed in the previous creep step.     

Comparison of Mechanical Behavior of Two Overconsolidated Clays: Yamashita and Louiseville Clays

A site investigation program was carried out at the Yamashita site, Yokohama, Japan, by means of laboratory as well as field tests. It was found from the investigation that the clayey layer is overconsolidated with an OCR of about 2.0. It is evident from geological consideration that this overconsolidation may not have been caused by stress change, but was probably due to structure effects such as cementation or delayed consolidation. The physical and mechanical properties of the Yamashita clay is compared with those of Louiseville clay, a Champlain Sea Clay widely distributed in Quebec, eastern Canada, and a well known cemented clay. Louiseville clay was retrieved by Japanese sampler and transported to the Port and Harbour Research Institute (PHRI) for laboratory investigation. Comparison of the two clays shows that strength properties such as the stress-strain curve or stress path for both clays are very similar to each other. However, the pattern of the e-log p curve measured by Constant Rate of Strain oedometer, particularly beyond the yield consolidation pressure, is remarkably different between the two clays.     

Ageing Effects on Consolidation Properties—Based on the Site Investigation of Osaka Pleistocene Clays—

Soil samples were recovered from the Osaka basin, where Holocene as well as Pleistocene marine clays are thickly deposited. It is found from X-ray diffraction analysis that the component of clay minerals for these day layers is almost identical through all depths. 'This fact suggests that the origin of these sediments has not been drastically changed through at least more than 1,000,000 years. Using data obtained by oedometer tests, this paper examines ageing effects on consolidation properties. It is found that void ratio for the Pleistocene clays keeps large even under high in situ effective overburden pressure (p_vo′). Salinity of pore fluid decreases with depth and becomes nearly zero at depths deeper than 70 m. However, the change in salinity does not give any influences on the index properties, suggesting that the relatively large void ratio for the Pleistocene clays is caused by other reasons than leaching. It is also found that the Pleistocene clays reveal no particular differences in consolidation properties, but fundamentally similar to those of Holocene clays'. There is no distinguished difference in the overconsolidation ratio (OCR), non-linearity of the e-log p' relation after yield consolidation pressure including the maximum compression index (C_cmax) and the hydraulic conductivity (k). The only prominent difference in the Osaka Pleistocene clays, compared to the Holocene clay, is relatively small void ratio due to the large p_vo′.     

Variability of Physical and Consolidation Test Results for Relatively Uniform Clay Samples Retrieved from Osaka Bay

Samples collected from both Holocene and Pleistocene layers in Osaka Bay were examined in this study. The objective of this study is to evaluate variations of soil parameters in a sample length of about one meter. The retrieved samples from EL. (elevation) -37.5 to -38.5 m and from EL.-125.5 to -126.5 m with a length of about one meter were divided into every 25 mm long to trim consolidation specimens. Variations of soil parameters obtained from laboratory tests were evaluated. Clay microfabric was also evaluated by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The clay layers are relatively homogeneous with variation coefficient CV in a range of 0.025 to 0.12 for the consolidation parameters (e, p′_c, C_c and c_v). CV of void ratio e is only 0.025 to 0.056. In contrast, CV values of the other consolidation parameters are significantly greater than this, but the average is less than 0.1. It can be concluded that CV of soil consolidation parameters for the homogeneous natural clay deposits dealt in this study is less than about 0.1. These variations are mainly influenced by sedimentary environment. Even the soil is said to be homogeneous, because the specimen size for the laboratory tests is sufficiently large in consideration of microscopic heterogeneity obtained from SEM and MIP.     

Mechanical Behavior of Bentonite-Sand Mixtures as Buffer Materials

For the purpose of establishing the method for estimating in-situ mechanical behavior of artificial buffer materials, stress-deformation behavior of bentonite-sand mixtures were investigated through oedometer test, consolidated undrained triaxial compression test and expansive stress-strain measuring test by changing the clay content as 30, 50, 70 and 100%, and by changing the range of initial dry density of mixture from 1.4 to 1.8 g/cm³. Oedometer test results suggest that the magnitude of consolidation yield stress almost coincides with the maximum expansive stress (p′_s)_max irrespective of bentonite-sand mix proportion, initial density of mixture and the magnitude of molding stress at the specimen making. Strong correlation between consolidation stress and initial tangent modulus during undrained triaxial compression test is observed, and it is found that the reduction rate of rigidity is hardly dependent on the specimen making method, molding stress and the consolidation stress. From the two series of expansive stress-strain measuring tests, it is recommended to perform the measurement of expansive stress by feed back system with the load cell installed at the base of the specimen. A unique relationship is found between the maximum expansive stress (p′_s)_max versus bentonite specific volume v_b, which is defined as the specific volume calculated by excluding the volume of sand particles. The line showing the unique log v_b versus log (p′_s)_max relationship can be recognized as the state boundary line prescribing one-dimensional expansive stress-strain behavior of the bentonite-sand mixtures.     

A unified approach to link small-strain shear modulus and liquefaction resistance of pumiceous sand

Natural pumiceous (NP) sands containing pumice particles, a type of volcanic soil, are commonly found in the central part of the North Island in New Zealand. The pumice particles are highly crushable, compressible, lightweight and angular, making engineering assessment of their properties problematic. In this paper, several series of bender element and undrained cyclic triaxial tests were performed on reconstituted and undisturbed NP sands to determine their small-strain shear modulus (G_max) and cyclic resistance ratio (CRR). Furthermore, similar tests were also conducted on normal hard-grained sands (e.g., Toyoura sand) for the purpose of comparison. The results showed that the NP sands have considerably lower G_max compared to normal sands, resulting in their higher deformability during the initial stages of the cyclic loading test. The high angularity of NP sands play an important role toward the end of the cyclic loading and contributed to their higher CRR. Next, the ratio of CRR/G_max for each sample was correlated to a level of strain denoted as cyclic yield strain (ε_ay), which was found to be significantly dependent on the percentages of pumice particles present in the natural soils. On the other hand, the ε_ay was found to be less sensitive to the consolidation stress (σ′_c) and the relative density (Dr) of the materials. For example, over different values of σ′_c and Dr, NP sands have substantially higher values of cyclic yield strain due to their lower G_max and higher CRR when compared with those of ordinary sands.     

基于原位十字板强度的沿海地区软黏土超固结比分析

超固结比OCR是评价软黏土变形和强度特性的重要参数。基于天津、上海、连云港、深圳、中山和珠海这6个沿海地区软黏土的原位十字板强度,系统地分析这些地区软黏土的固结状态,并探讨其形成机制。研究结果表明:(1)基于原位十字板强度得到的OCR要明显大于室内一维压缩试验的结果,室内压缩试验会显著低估沿海软黏土的OCR;(2)6个地区的OCR随深度的变化规律较为一致,OCR随深度增加,逐渐减小至某一稳定值,浅层均表现出一定的超固结现象,而深层的固结状态则表现出一定的复杂性;(3)现有模型能够解释我国沿海软黏土超固结的成因以及变化规律,浅部的超固结现象源于以风化胶结为主的"表层作用",而深部的超固结则源于次压缩。考虑到软黏土突出的取土扰动问题,这种基于原位十字板强度的软黏土OCR评价及成因分析方法值得在我国沿海地区推广。     

Unloading behavior of clays measured by CRS test

The unloading behavior of clays was studied by the Constant Rate of Strain (CRS) test, for three clays: two of them are reconstituted and the other was intact. In the conventional CRS test where the stress monotonically increases, the distribution of the pore water pressure in a specimen is assumed to be parabolic, the effective stress is calculated and then the compression behavior is evaluated. However, this assumption cannot be directly applied the unloading condition. In this study, the pore pressure distribution under unloading was simulated by a cubic polynomial under the assumption that hydraulic conductivity does not change in the unloading process. A unique relation in the e–log σ′_v relation was found, irrespective of both the magnitude of stress or strain and the compression index, C_c, at the unloading test, when the consolidation pressure is normalized by σ′_vmax, which is the consolidation pressure before the unloading test. In addition, the creep strain, which is gained by constant loading before the unloading test, was shown to have a great effect on the unloading behavior: that is, the soil behaves stiffly when subjected to a constant load for a prolonged period of time. A strain rate dependency in the unloading process was also observed particularly for heavily unloaded specimens. The unloading behavior was also investigated by the conventional constant load test. The test results show reasonable agreement with those obtained from the CRS test.     

Mechanisms of quasi-preconsolidation stress development in clays: A rheological model

This paper deals with the development of a mechanistic model for the ageing consolidation behavior of clays with the focus on aspects related to the development of quasi-preconsolidation pressure. The initial use of such pressure in design met with criticism, but field and laboratory evidence, which highlights its significance, continues to accumulate. A nonlinear rheological model is used to numerically simulate the consolidation process of clay in laboratory tests and to identify the basic mechanical parameters that contribute to the development of the quasi-preconsolidation phenomenon. Methods to identify the parameters of the model from oedometer tests are described. It is shown that while the variation in soil modulus can be characterized by a linear form in the virgin compression region, it is nonlinear in the recompression region and is best characterized by a hyperbolic function. Changes to the modulus in the recompression region, due to ageing, is shown to be the dominant cause of the development of the quasi-p_c phenomenon. Observed results as well as numerical simulations demonstrate that specimens that had aged longer show increased quasi-p_c values. While the variation in soil modulus controls the EOP curve of clays, the observed time effects, such as the “vanishing p_c” phenomenon, are controlled primarily by changes in soil viscosity. However, this has no bearing on the development of the quasi-p_c phenomenon.     

Stiffness decay characteristics and disturbance effect evaluation of structured clay based on in-situ tests

The structure of the sedimentary clay influences its mechanical behavior in non-negligible ways. This paper proposes an effective approach for investigating the in-situ stiffness characteristics from shear modulus and strain decay curves (G–γ curves) based on self-boring pressuremeter and seismic dilatometer tests. To evaluate the excavation disturbance effects on the structured clay, the stiffness parameters from pre-bored pressuremeter tests are compared with the results of self-boring pressuremeter tests. The result indicates that the complete in-situ G–γ curves can be acquired by integrating the strain-dependent tangent shear modulus G_t from self-boring pressuremeter tests and the small-strain modulus G₀ from seismic dilatometer tests. Simultaneous observations of the G–γ curves with hyperbolic shapes in semi-logarithmic coordinates at the same strain scale show the similarity of the stiffness decay mode of the soil at different depths. The increase in the measured values of G_t and G₀ with depth can be attributed to the improved consolidation pressures and cemented strength in the structured clay. Additionally, the G/G₀–γ curves measured by the in-situ tests generally agree well with the results predicted by the Stokoe model. The excavation disturbance weakens the stiffness of the structured clay, as evidenced by G_t from the pre-bored pressuremeter data being significantly smaller than that from self-boring pressuremeter tests at the same depth. Based on quantitative analysis, the disturbance degree computed from the measured results has a low sensitivity to the soil depth and a strong negative correlation with the strain level of the soil. This study provides an effective method for predicting the stiffness parameters of structured soil based on in-situ tests.     

Main Factors Governing Residual Effective Stress for Cohesive Soils Sampled by Tube Sampling

The residual effective stress (p'_r) was measured for various clayey soils collected from various parts of the world, including Japan. All samples studied in this paper were retrieved by the same sampling method, i.e., using the Japanese standard sampler. However, measured p'_r/σ'_vo, where σ'_vo is the in situ effective overburden pressure, considerably varied for different sites as well as with depth. This paper examines main factors governing the p'_r value, focusing on location of the sample in the sampling tube; transportation of the soil samples; time duration between retrieval of the sample and extrusion of the sample from the sampling tube; overconsolidation ratio (OCR); clay content and plasticity index (I_p). In addition, the p'_r values are correlated to the volume change generated when the in situ σ'_vo is applied in the oedometer test, which is extensively used for assessment of the sample quality. The largest measured value of p'_r was found at one third of the sample length from the cutting edge of the sampling tube. The effects of the transportation and the time duration from the sampling to the extrusion of the sample are not prominent for the p'_r value. Any clear relations between p'_r and Δe/e_o are not found, where Δe and e_o are the void ratio change caused by applying σ'_vo and the initial void ratio, respectively. Among factors examined in this paper, OCR is the most effective factor: i.e., as OCR increases, p'_r/σ'_vo ratio increases for every studied site. However, when compared at different sites, the p'_r/σ'_vo ratio at the same OCR is considerably different. In spite of some exceptions, there exists a tendency that p'_r/σ'_vo ratio increases with the increase in the clay content as well as I_p.     

Field and Laboratory Measurements of Small Strain Stiffness of Decomposed Granites

Non-linear stress-strain characteristics and stiffness-strain relationships of sedimentary soils and sands at small strains have been reported by many researchers. Research work on the behaviour of weathered or decomposed granites at small strains, however, has rarely been reported. This paper compares some stiffness measurements of decomposed granites from field investigations involving crosshole seismic, self-boring pressuremeter (SBPM), and high pressure dilatometer as well as results from laboratory tests using bender element and internal transducers. The in-situ crosshole measurements show that the elastic stiffness of Moderately Decomposed Granite (MDG, approximately 7000 MPa) is about 25% greater than that (about 5500 MPa) of Highly Decomposed Granite (HDG), which is in turn approximately 18 times higher than that (about 300 MPa) of Completely Decomposed Granite (CDG). This is likely attributable to the materials' different bond strengths and structures. A new method has been adopted to interpret the SBPM data. Measured data from crosshole seismic and self-boring pressure meter tests for CDG are found to be consistent. Bender element laboratory tests on CDG indicate that the measured A-coefficient in the expression of G₀/p_r = A(p/p_r)ⁿ lies between the results from clay and sand as reported in the literature. However, the measured n-value for CDG is generally larger for clays and sands. The measured bender element results are consistent with data from internal transducers. Highly non-linear characteristics of CDG were observed in both the laboratory and field tests. Generally the elastic stiffness of CDG as determined by laboratory tests is about 50-80% of that from field tests. Some possible reasons are discussed.     

Effect of initial water content on undrained shear strength of K0 consolidated clay

This study investigates the effect of initial water content on the pore pressure response and undrained shear behavior of K₀ consolidated reconstituted clay. A series of K₀ consolidated undrained triaxial compression tests were conducted on reconstituted Lianyungang clay. Results were compared to those obtained by isotropic consolidated undrained triaxial tests. The testing results showed that the K₀ consolidated undrained strength envelope of reconstituted soil content is a straight line passing through the origin regardless of the initial water content. The initial water content would affect the undrained strength of K₀ consolidated clay as decreased normalized undrained shear strength was observed with clay at higher initial water content. The slope C of normalized pore pressure and stress ratio is affected by the consolidation method, where C is found to be a soil constant for K₀ consolidated clay and the value would be higher with clay under K₀ consolidation. The pore pressure increases with increasing initial water content at a certain axial strain under given consolidation pressure, and the difference in excess pore pressure increases with the increasing consolidation pressure. Pore pressure coefficient at failure (A_f) increases as the initial water content increases, where a trendline can be well fitted between the pore coefficient at failure and the ratio of initial water content to the liquid limit of clay. The undrained strength indexes, i.e., effective cohesion and effective internal friction angle have decreasing tendency with increasing initial water content; however, changes in the total strength indexes of soil in this study are insignificant with varying initial water content.     

Characterization of Yangsan Clay,Pusan, Korea

It has been reported by previous researchers that the preconsolidation pressure (p_c) of Kimhae (Yangsan) clay, which is thickly deposited along the Naktdong River, Pusan, Korea, is considerably less than its in-situ effective overburden pressure (p'_vo)-The question has arisen whether such a low p_c value is due to underconsolidation or to the unreliability of laboratory tests, including the use of low quality sample. As a cooperative research program between PHRI (Port and Harbour Research Institute) and Pusan National University, extensive in-situ and laboratory investigations were carried out at Yangsan, using a Japanese sampler and Japanese sampling technique. It is found that the p_c values at the site are slightly greater than p'_vo, which is different from the test results from the previous researchers. However, the overconsolidation ratio (OCR) at this site is quite small compared to similar aged normally consolidated clays deposited in Japan.     

STRENGTH AND CONSOLIDATION PROPERTIES OF BUSAN NEW PORT CLAYS

The sedimentary environment and the effects of sample disturbance on strength and consolidation properties of Busan New Port clays are examined through microfossils and radiocarbon age analyses and unconfined compression, K₀-consolidated undrained triaxial compression and extension tests and consolidation tests. In this study, only one or two samples, 74 mm in diameter and 100 mm in height, obtained from different depths, are used for the whole series of tests to provide small-sized specimens. The sedimentation rates of Busan New Port clays were (3.7-7.8) mm/year and higher than those for the coastal areas of the USA, Thailand and Japan. The in-situ undrained shear strength and consolidation parameters were estimated using Shogaki's method and compared with those of other test results and evaluated. Busan New Port clays are lightly overconsolidated clays. It can be seen that the consolidation settlements, which were greater than those estimated, were observed in Holocene Busan clay, are caused by the underestimation of the compression index and coefficient of consolidation values caused by sample disturbance.     

Undrained Shear Strength of Cement-Treated Soils

In order to evaluate the effects of cementation on the mechanical properties of cement-treated soil, a series of isotropic consolidation and undrained triaxial compression shear tests were performed for cement-treated specimens of Ariake clay, Akita sand, Rokko Masado and Toyoura sand. This paper evaluates factors affecting the shear strength of these cement-treated soils. The following conclusions are obtained: 1) Cement-treated soil has a normally consolidated line in e-ln p' space which depends on the mixing cement content. The consolidation yield stress, p'_y, of cement-treated soil increases with increasing cement content and initial specimen density. 2) Changes in cohesive strength due to cement-treatment can be represented by a tensile effective stress, p'_r. Strength properties can then be normalized by the augmented consolidation stress, (p'_c+p'_r). 3) The shear strength properties of quasi-overconsolidated clay can be represented by the yield stress ratio, R=(p'_y+p'_r)/(p'_c+p'_r). 4) The undrained shear strength of cement-treated soils can be represented as a power law relation of the yield stress ratio, R, and the augmented consolidation stress.     

Development of a New Cone Penetrometer and its Application to Great Depths of Pleistocene Clays

A new piezocone was developed and applied to Osaka Pleistocene clay layers as deep as 250 m. Its geometry is the same as that of the reference procedure defined by the ISSMFE and standard of JGS: i.e., the projected cross sectional area is 10 cm²; the angle of the cone is 60°; the pore water pressure is measured at the shoulder. This piezocone does not measure the skin friction. Due to great depth at the investigated site, the capacity of the point resistance (q_t) and the pore water pressure is as large as 30 MPa and 20 MPa, respectively. The test was carried out, using a borehole drilled prior to the penetration, because of stiff sand or gravel layers and large skin friction between the rod and the ground. The q_t measured by the cone penetration test (CPT) was correlated to the yield consolidation pressure (p_y) measured by the Constant Rate of Strain (CRS) oedometer test for the soil sample recovered near the point of the CPT investigation. The cone factor for the p_y value (N_pc) was defined by (q_t-p_vo)/p_y, where p_vo is the in situ total overburden pressure. The range of observed N_pc value is relatively narrow and between 2.5 and 2.8, which is in the middle of the range of N_pc factors measured in Holocene clays in the various areas in the world as well as Japan. The overconsolidation ratio (OCR) was also derived by CPT. Variation of the OCR estimated by the CPT is nearly equivalent to that measured by the CRS oedometer. It may be concluded from this investigation that the consolidation properties, especially the p_y value, derived from samples recovered from great depths, are quite reliable as design parameters.     

Measured and predicted performance of prefabricated vertical drains (PVDs) with and without vacuum preloading

This paper presents the effectiveness of vacuum preloading in accelerating the consolidation of PVD improved soft Bangkok clay by comparing with the corresponding results without vacuum preloading. Laboratory tests were conducted using a large scale consolidometer having diameter of 300 mm and height of 500 mm with reconstituted specimens installed with prefabricated vertical drains (PVD) with and without vacuum preloading. In addition, field data were collected from Second Bangkok International Airport (SBIA) site improved by PVD with and without vacuum pressures. Analyses were carried out to compare the compressibility parameters (C_h and k_h/k_s) by back-calculation of laboratory and field settlements using Hansbo (1979) method. From the laboratory tests, the horizontal coefficient of consolidation (C_h) values from reconstituted specimens were 1.08 and 1.87 m²/yr for PVD without and with vacuum pressure, respectively and the k_h/k_s values were 2.7 for PVD only and 2.5 for vacuum-PVD. After the improvement, the water contents of the soft clay were reduced, thereby, increasing its undrained shear strengths. Similarly, the field data analysis based on the back-calculated results showed that the k_h/k_s were 7.2 and 6.6 for PVD without and with vacuum, respectively. The C_h values increased slightly from 2.17 m²/yr for PVD only to 3.51 m²/yr for vacuum-PVD. The time to reach 90% degree of consolidation for soils with vacuum-PVD was one-third shorter than that for soils with PVD only because of higher C_h values. Thus, the addition of vacuum pressure leads to increase horizontal coefficient of consolidation which shortened the time of preloading. The PVDCON software was found to be useful to predict the settlements of the PVD improved ground with and without vacuum preloading.     

Performance of Band Shaped Vertical Drain for Soft Hai Phong Clay

Performance of band shaped prefabricated vertical drain (PVD) installed into soft Hai Phong clay with a 110 cm triangle arrangement is reported together with the engineering properties of the clay investigated by field and laboratory tests. Stationary piston sampling was carried out to obtain high quality undisturbed soil samples for laboratory tests and reliable engineering characteristics of the clay. It was assumed for the design of PVD spacing and preloading that the ratio of apparent value of horizontal coefficient of consolidation c_h(ap) to vertical coefficient of consolidation c_v is equal to 1.0. The settlement monitored in the field, which clearly showed that the actual settlement was faster than expected, resulted in the c_h(ap) value 1.5 times as much as c_v determined by the laboratory test.     

Reconsideration of the settlement behavior of peat from view point of hydraulic conductivity

This paper deals with the settlement of peat from the view point of hydraulic conductivity (k). The validity of using the oedometer tests, including the calculation method, for measuring k is carefully examined by numerical analysis as well as a test combined with an oedometer and a hydraulic conductivity test. It is found from these studies that the conventional oedometer test (JIS A 1217, 2009) can be evaluated to measure k for peat with the same accuracy as that for usual clays, provided that the incremental load ratio is unity. The significant difference in the characteristics of k for peat and usual clayey soils is their relation between the compression index (C_c) and the hydraulic conductivity change index (C_k). As a result, rather than remaining constant during consolidation, the coefficient of consolidation (c_v) of peat decreases considerably with increasing consolidation pressure (p), while the c_v coefficient for usual clayey soil is almost constant at the normally consolidated stage. The influence of c_v dependent on p is studied by a numerical analysis for the one dimensional consolidation problem as well as for the ground improved by vertical drain. It is found that if the incremental consolidation pressure (Δp/p) is large, careful judgment is required when adopting conventional consolidation analyses, especially in case of the vertical drain.