Assessment of the Liquefaction Susceptibility of Fine-Grained Soils

Observations from recent earthquakes and the results of cyclic tests indicate that the Chinese criteria are not reliable for determining the liquefaction susceptibility of fine-grained soils. Fine-grained soils that liquefied during the 1994 Northridge, 1999 Kocaeli, and 1999 Chi-Chi earthquakes often did not meet the clay-size criterion of the Chinese criteria. Cyclic testing of a wide range of soils found to liquefy in Adapazari during the Kocaeli earthquake confirmed that these fine-grained soils were susceptible to liquefaction. It is not the amount of “clay-size” particles in the soil; rather, it is the amount and type of clay minerals in the soil that best indicate liquefaction susceptibility. Thus plasticity index (PI) is a better indicator of liquefaction susceptibility. Loose soils with PI<12 and wc/LL>0.85 were susceptible to liquefaction, and loose soils with 120.8 were systematically more resistant to liquefaction. Soils with PI>18 tested at low effective confining stresses were not susceptible to liquefaction. Additionally, the results of the cyclic testing program provide insights regarding the effects of confining pressure, initial static shear stress, and stress-path on the liquefaction of fine-grained soils.     

Evaluation of Cyclic Softening in Silts and Clays

Procedures are presented for evaluating the potential for cyclic softening (i.e., onset of significant strains or strength loss) in saturated silts and clays during earthquakes. The recommended procedures are applicable for fine-grained soils with sufficient plasticity that they would be characterized as behaving more fundamentally like clays in undrained monotonic or cyclic loading. The procedures are presented in a form that is similar to that used in semiempirical liquefaction procedures. Expressions are developed for a static shear stress correction factor and a magnitude scaling factor. Guidelines and empirical relations are presented for determining cyclic resistance ratios based on different approaches to characterizing fine-grained soil deposits. The potential consequences of cyclic softening, and the major variables affecting such consequences, are discussed. Application of these procedures is demonstrated through the analysis of the Carrefour Shopping Center case history from the 1999 Kocaeli earthquake. The proposed procedures, in conjunction with associated liquefaction susceptibility criteria, provide an improved means for distinguishing between the conditions that do and those that do not lead to ground deformations in fine-grained soil deposits during earthquakes.     

Determination of Strength and Index Properties of Fine-Grained Soils Using a Soil Minipenetrometer

This paper describes the use of a soil minipenetrometer (SMP) to determine the strength and index properties of fine-grained soils. The SMP has been developed to allow both fall cone and quasi-static penetration tests to be performed. Displacement controlled quasi-static penetration tests can be used for the direct measurement of undrained shear strength, both for remolded and undisturbed samples. In addition the quasi-static penetration test can be used to define an additional lower plastic limit parameter, the PL100, which represents the moisture content of a fine-grained soil with an undrained strength 100 times that at the liquid limit. This approach offers the advantage that removal of the coarse fraction is not required to estimate the PL100.     

Determining Intrinsic Compressibility of Fine-Grained Soils

Results of laboratory oedometer tests on reconstituted specimens of four clays prepared at different initial water contents, ranging from the liquid limit to 1.75 times the liquid limit, show that the intrinsic compression line may not be “unique” for a given soil. This suggests that the “intrinsic” parameter Iv, which is based on the constants of intrinsic compressibility, e100?, (void ratio corresponding to σv′ = 100?kPa), and Cc?, (e100??e1000?), may in fact not be a truly intrinsic parameter of the soil, but is dependent on sample preparation. The positioning of the normalized compression curve in e–log–σv′ space is significantly influenced by the initial remolding water content, therefore resulting in differing values of e100? for a given soil depending on the initial water content. The influence of initial water content was greater for kaolinitic and illitic clay than for montmorillonitic clay. It is hypothesized that the difference in behavior may be attributed to differences in mineralogy. The results illustrate that caution should be used when comparing tests results from widespread sources and suggest that a standard level of initial water content be used to evaluate the intrinsic compressibility.     

Liquefaction Susceptibility Criteria for Silts and Clays

New liquefaction susceptibility criteria for saturated silts and clays are presented that are based on the mechanics of their stress-strain behavior and which provide improved guidance for selecting engineering procedures for estimating potential strains and strength loss during seismic loading. Monotonic and cyclic undrained loading test data for silts and clays show that they transition, over a fairly narrow range of plasticity indices (PI), from soils that behave more fundamentally like sands (sand-like behavior) to soils that behave more fundamentally like clays (clay-like behavior), with the distinction having a direct correspondence to the type of engineering procedures that are best suited to evaluating their seismic behavior. It is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine-grained soils exhibiting sand-like behavior, whereas the term cyclic softening failure be used to describe similar phenomena in fine-grained soils exhibiting clay-like behavior. For practical purposes, clay-like behavior can be expected for fine-grained soils that have PI ≥ 7, although a slightly lower transition point for soils with a CL-ML classification (perhaps PI ≥ 5 or 6) would be equally consistent with the available data. Issues related to the practical application of these criteria are discussed.     

Improvements in Frost Heave Laboratory Testing of Fine-Grained Soils

We present a laboratory system designed for studying frost heave in fine-grained soil. The system consists of: a modified refrigerator, a frost heave test cell, a laser for measuring heave, a differential pressure transducer for measuring water intake, and platinum resistance temperature detectors for measuring pedestal temperatures. The frost heave cell allows for visual observation of the sample, and accommodates pretest sample consolidation, freezing tests using a variety of freezing methods, triaxial tests on frozen soil, and thaw consolidation tests. The modified refrigerator maintains the specified temperature ±0.5°C during the full length of the test. Test results indicate repeatability of frost heave ratios ξ to within ±7%, and average heave rates to within ±0.05?mm/h. Results from frost heave tests conducted on five fine-grained soils indicate that: (1) a soil removed of its colloidal organic content becomes less frost susceptible; (2) the geomorphologic history of a “regional” soil is a critical factor influencing its frost susceptibility; and (3) ξ is dependent on overall clay content and is most sensitive to chlorite content.     

Relationship between the Undrained Shear Strength, Water Content, and Mineralogical Properties of Fine-Grained Soils

The relationship between the undrained shear strength of fine-grained soils and the water content can be described with a nonlinear function in which the type of soil is determined by two parameters. It is well known that these parameters depend mainly on the mineral compositions of soils; these relationships, however, have not yet been investigated. The findings described in this paper define those mineralogical properties of soils which determine the values of both parameters. Experimentally obtained results suggest that the parameters primarily depend on the size of the clay minerals, their quantity in soil composition, and the interlayer water quantity in the expanding clay minerals. As this dependence is well defined, the parameters, and thus the undrained shear strength at different water content, can be defined from knowledge of these mineralogical soil properties.     

Liquefaction Resistance of Sandy Soils under Partially Drained Condition

In order to simulate the effect of drainage on soils adjacent to gravel drains that are installed as countermeasure against liquefaction, several series of cyclic triaxial tests were performed on saturated sands under partially drained conditions. The condition of partial drainage under cyclic loading was simulated in the laboratory using triaxial testing equipment installed with a drainage control valve to precisely regulate the volume of water being drained from test specimens. Effects of both drainage conditions and loading frequencies on cyclic response were incorporated through the coefficient of drainage effect, α*. Experimental results showed that for sand exhibiting strain softening, the partially drained response was controlled by the critical effective stress ratio while for sand showing strain hardening behavior, the controlling factor was the phase transformation stress ratio. Moreover, test results indicated that the minimum liquefaction resistance under partially drained conditions can be used as a parameter to describe the liquefaction resistance of sands improved by the gravel drain method. From these results, a simplified procedure for designing gravel drains based on the factor of safety (FL) concept was proposed.     

Establishing Soil-Water Characteristic Curve of a Fine-Grained Soil from Electrical Measurements

Application of a pressure membrane extractor (PME) to establish soil-water characteristic curve (SWCC) of fine-grained soils, in 0–1,500 kPa range, is well established. However, this technique requires testing of several identical specimens, corresponding to same or different pressure(s), and their subsequent removal from the PME chamber for moisture content determination. This turns out to be a cumbersome process and even the results are considered less accurate, by the research fraternity. This is mainly due to the fact that removal of the specimen before equilibration time may not incorporate the influence of the applied pressure, precisely. This calls for the development of an alternate technique that can be employed for measuring the instantaneous moisture content of the specimen when it is pressurized, sequentially, without removing it from the PME chamber. In this context, the utility of electrical measurements (i.e., the voltage) across two points in the specimen for determining moisture content was investigated and its details are presented in this paper. This technique has been found to be quite promising and hence can be employed for acquisition of the data which would yield the moisture content of the specimen, without removing it from the PME chamber, easily and quickly. Validity of the methodology has been demonstrated by comparing the obtained SWCC vis-à-vis those obtained by conducting studies using a dewpoint potentiameter, WP4, and by employing the fitting function and a pedo-transfer function available in the SoilVision database.     

Sand Replacement Method for Determination of Shrinkage Limit of Fine-Grained Soils

Mercury is one of the many hazardous substances that has been recognized and banned by many natural codes of practice and governments. Because many laboratory works in research and practice require the use of mercury, safe alternative materials and procedures are being researched. One of the Atterberg limits dealing with volume stability of soils in the field is the shrinkage limit. The conventional method followed by many national codes of practice involves the use of mercury to measure the volume of dry soil pat. This paper proposes sand replacement method to determine the shrinkage limit of soils in the laboratory. This method uses sand of uniform gradation to determine the volume of dry soil pat. The proposed method is simple, safe, free from the limitations of the conventional mercury and wax methods, and eco-friendly. The shrinkage limit values obtained from the proposed sand replacement method compare very well with those from the mercury displacement method.     

Monotonic and Cyclic Behavior of Two Calcareous Soils of Different Origins

The behavior of two calcareous soils—Goodwyn (GW) and Ledge Point (LP)—is studied through a series of monotonic and cyclic triaxial tests. These two soils are selected because they represent two extreme formation conditions in terms of their depositional environments, physical characteristics, and grain strength. The experimental investigation included isotropic compression tests to high stress levels, undrained monotonic shearing tests, and undrained cyclic shearing tests under one-way and two-way loading conditions. Tests were performed on samples with different initial conditions. The experimental results show that, although the overall qualitative stress-strain behavior of both GW and LP soils is similar to that of other silicious soils, significant quantitative differences are observed between the two soils and also between calcareous and silicious soils, especially in terms of volumetric reduction during compression, monotonic and cyclic shear strength, and the strain required to mobilize the strength. This paper explores the mechanical behavior of the two calcareous soils and highlights the similarities and differences between their behavior and also between calcareous and silicious soils.     

Influence of Underlying Weak Soil on Passive Earth Pressure in Cohesionless Deposits

Finite-element simulations demonstrate the influence of underlying weak soil on mobilization of passive pressures in cohesionless deposits. Traditional passive earth pressure theories with typical angles of interface friction may overestimate passive forces in such cases. Simple analytical models that incorporate the underlying weak soil using traditional passive earth pressure concepts are shown to agree reasonably with the finite-element simulations. The studies presented herein are relevant for cases in which cohesionless soil deposits overlie soft clay, liquefiable sand, or other weak layers.     

Simplified Cone Penetration Test-based Method for Evaluating Liquefaction Resistance of Soils

This paper presents a new simplified method for assessing the liquefaction resistance of soils based on the cone penetration test (CPT). A relatively large database consisting of CPT measurements and field liquefaction performance observations of historical earthquakes is analyzed. This database is first used to train an artificial neural network for predicting the occurrence and nonoccurrence of liquefaction based on soil and seismic load parameters. The successfully trained and tested neural network is then used to generate a set of artificial data points that collectively define the liquefaction boundary surface, the limit state function. An empirical equation is further obtained by regression analysis to approximate the unknown limit state function. The empirical equation developed represents a deterministic method for assessing liquefaction resistance using the CPT. Based on this newly developed deterministic method, probabilistic analyses of the cases in the database are conducted using the Bayesian mapping function approach. The results of the probabilistic analyses, expressed as a mapping function, provide a simple means for probability-based evaluation of the liquefaction potential. The newly developed simplified method compares favorably to a widely used existing method.     

Simplified Relationships for Particle-Size Distribution and Permeation Groutability Limits for Soils

The particle-size distribution of soil with mean particle size and fines content are used not only in soil classifications but also in a number of other soil property relationships. In this study, two simple relationships (hyperbolic [tan?h(x)] and S-curve) were investigated to represent the particle size distribution of soils. The parameters of the hyperbolic model were correlated to various soil parameters such as the mean particle size, particle size range, and fines content. There was no direct correlation between Fredlund (four-parameter model) and S-curve model parameters and the soil parameters. The predictions of the two (hyperbolic) and three (S-curve) parameter models were compared to the four-parameter model (unimodal) using limited soil data from the literature and the agreements were good. The hyperbolic model was used to map the Unified Soil Classification System. A recent study had quantified the relationship between the grouting pressure and the fines content in nonplastic soils. Also in the current practice, upper and lower particle-size distribution limits are used in determining the groutability of soils. In this study, the relationship between grouting pressure and fines contents of the soil was generalized using the hyperbolic particle-size distribution model and verified with a groutability study using an acrylamide grout. Based on limited data in the literature, the groutability of soils was defined using a new set of parameters, grouting pressure, fines content, and mean particle size diameter of the soil.     

Two-Surface Plasticity Model for Cyclic Undrained Behavior of Clays

Based on a new type of kinematic hardening and the theory of critical state soil mechanics, a two-surface model is herein developed for predicting the undrained behavior of saturated cohesive soils under cyclic loads. The anisotropic hardening rule works in two steps: (1) introducing a new concept, memory center, to take into account the memory of particular loading history; and (2) regulating the movement of the bounding and loading surfaces according to the direction of loading paths in stress space. Conventional triaxial tests have been performed on reconstituted clay samples in the laboratory. The proposed model is verified with respect to the observed behavior of soil samples. It is shown that like a multisurface model, this model can realistically describe some important responses of clays subjected to both monotonic and cyclic loading, while incorporating the memory of particular loading events.     

Undrained Shear Strength of K0 Consolidated Soft Soils

On the basis of critical state soil mechanics, this study derives theoretical formulas for predicting the undrained shear strength of K0 consolidated soft soils in triaxial compression and extension. Although the modified Cam-clay model is often utilized to predict the undrained shear strength of soft clays, it is applicable mainly to isotropically consolidated soils. Because of the anisotropy under K0 consolidation, an inclined elliptical yield surface is chosen, which is different from those methods based on the original Cam-clay model. The inclined elliptical yield surface is testified to be appropriate to the K0 consolidated soft soil and results in a better prediction of undrained strength, especially for the triaxial extension test. It is concluded that the analytical solutions obtained in this paper are in good agreement with the available test results and back-analysis of slope failures. On the basis of the investigation of soil properties, a simple formula is proposed for calculating the mean undrained shear strength along the failure surface.     

Evaluation of Cyclic Shear Strength of Two Cemented Calcareous Soils

The mechanism controlling the cyclic shear strength of cemented calcareous soils was investigated based on the results obtained from monotonic and cyclic triaxial tests on two different types of calcareous soil. Undrained cyclic triaxial tests performed on artificially cemented calcareous soils with different loading combinations showed that the effective stress path moved towards or away from the origin, due to the generation or dissipation of pore pressure with progressive cycles. Previous investigations have shown that the Peak Strength Envelope or the State Boundary Surface or the Critical State Line forms a boundary beyond which effective stress paths during cyclic loading cannot exist. However, in this study it was observed that the maximum stress ratio (ηmax) obtained from monotonic tests defined the boundary for the cyclic tests. Based on the information obtained from this study, an approach for evaluating the cyclic shear strength is proposed. It was observed that the modified normalized cyclic shear strength had a strong linear relationship with the logarithm of the number of cyclic to failure irrespective of confining pressure, type of consolidation and stress reversal.     

Cyclic Softening of Low-Plasticity Clay and Its Effect on Seismic Foundation Performance

During the 1999 Chi-Chi Earthquake (Mw = 7.6), significant incidents of ground failure occurred in Wufeng, Taiwan, which experienced peak accelerations ～ 0.7?g. This paper describes the results of field investigations and analyses of a small region within Wufeng along an E–W trending line 350?m long. The east end of the line has single-story structures for which there was no evidence of ground failure. The west end of the line had three to six-story reinforced concrete structures that underwent differential settlement and foundation bearing failures. No ground failure was observed in the free field. Surficial soils consist of low-plasticity silty clays that extend to 8–12?m depth in the damaged area (west side), and 3–10?m depth in the undamaged area (east side). A significant fraction of the foundation soils at the site are liquefaction susceptible based on several recently proposed criteria, but the site performance cannot be explained by analysis in existing liquefaction frameworks. Accordingly, an alternative approach is used that accounts for the clayey nature of the foundation soils. Field and laboratory tests are used to evaluate the monotonic and cyclic shear resistance of the soil, which is compared to the cyclic demand placed on the soil by ground response and soil–structure interaction. Results of the analysis indicate a potential for cyclic softening and associated strength loss in foundation soils below the six-story buildings, which contributes to bearing capacity failures at the edges of the foundation. Similar analyses indicate high factors of safety in foundation soils below one-story buildings as well in the free field, which is consistent with the observed field performance.     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Seismic Behavior of Soil-Pile-Structure Interaction in Liquefiable Soils: Parametric Study

Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide range of frequencies of harmonic excitations. The inertial interaction attributable to a structure is analyzed for a system consisting of a four-storied portal frame on the pile group-soil subsystem. The responses of the structure are investigated for harmonic excitation and transient excitations. The importance of consideration of nonlinearity and liquefaction of the soil medium for analysis and design of a pile-supported structure is highlighted. Results from an analysis considering a practical soil-pile problem are presented to demonstrate the applicability of the developed algorithm for a practical problem.