Evaluation of Remolded Shear Strength and Sensitivity of Soft Clay Using Full-Flow Penetrometers

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites.     

某堆场试验区软基处理工程效果分析

试验区采用真空联合堆载法进行软基处理,室内土工试验表明,地基的力学性质有一定的提高,处理后淤泥层含水率、孔隙比及压缩系数分别下降了9.32%、6.59%和2.68%;而湿密度、φ、c及压缩模量分别提高了1.09%、40.48%、10.02%和13.53%;现场十字板抗剪强度试验结果表明,埋深15m以上淤泥不排水十字板抗剪强度平均增长了25.9%。通过现场沉降监测和孔隙水压力监测的试验数据,试验区经110d的真空联合堆载法处理,理论估算地基平均固结度≤90%。处理措施需要进一步优化。     

Strength Measurement for Near-Seabed Surface Soft Soil Using Manually Operated Miniature Full-Flow Penetrometer

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles.     

New Structure-Based Model for Estimating Undrained Shear Strength

This paper presents an experimental study of the strength in anisotropic clays by means of centrifuge model, cone penetration, and vane shear tests. To understand the effects of void ratio, overconsolidation ratio, and testing rate on the undrained shear strength (Su) of anisotropic Speswhite clay, a new centrifugal testing technique is designed to obtain constant overconsolidation ratio (OCR) profiles with varying void ratios (e), called the “descending gravity test.” The parameters controlling the generation of peak shear strength are quantified. As a result of this function, a new material and rate-dependent surface is defined in the e-OCR-Su space, which is identified as a “structural state capacity surface” since it relates the anisotropic structure to structure inherent capacity and properties. A new function for the estimation of excess pore pressure (uex) generated by cone penetration is found. By combining the strength and pore pressure functions a new model is proposed, called the “CU model.” The CU model is a structure-based model that provides reliable estimates of shear strength for in situ saturated clays using the knowledge of void and overconsolidation ratios. Finally, by combining Su-e-OCR and uex-e-OCR relationships, it estimates the void ratio and OCR profiles of anisotropic clays from piezocone penetration test results.     

Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.     

Influence of Shear Rate on Undrained Vane Shear Strength of Organic Harbor Mud

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors μ are proposed using the test results and the observed time to failure in the harbor basin. The proposed correction leads to significant lower design undrained shear strengths than the classical Bjerrum correction and would have predicted the failure during the construction.     

Estimating Liquefaction-Induced Lateral Displacements Using the Standard Penetration Test or Cone Penetration Test

A semiempirical approach to estimate liquefaction-induced lateral displacements using standard penetration test (SPT) or cone penetration test (CPT) data is presented. The approach combines available SPT- and CPT-based methods to evaluate liquefaction potential with laboratory test results for clean sands to estimate the potential maximum cyclic shear strains for saturated sandy soils under seismic loading. A lateral displacement index is then introduced, which is obtained by integrating the maximum cyclic shear strains with depth. Empirical correlations from case history data are proposed between actual lateral displacement, the lateral displacement index, and geometric parameters characterizing ground geometry for gently sloping ground without a free face, level ground with a free face, and gently sloping ground with a free face. The proposed approach can be applied to obtain preliminary estimates of the magnitude of lateral displacements associated with a liquefaction-induced lateral spread.     

Effect of Sampling Disturbance on Properties of Singapore Clay

This paper examines results from triaxial unconfined compression tests and undrained compression tests on reconsolidated samples of a Singapore marine clay retrieved using two sampling methods that offer differing quality of samples. Both local internal strain measurements using a Hall-effect transducer and external strain measurements using LVDTs were employed in the triaxial tests. Bender elements were embedded in some of the samples to establish the maximum shear modulus. If the samples are not reconsolidated, the shear strength and stiffness determined from triaxial tests are found to be sensitive to the quality of the samples, and generally lower than that determined by in situ tests. However, if the samples are subjected to isotropic or K0 consolidation to the estimated in situ condition, there is little difference between the shear strengths of samples retrieved using different samplers, and also consistent with results from vane shear tests. However, for the maximum shear modulus, even with reconsolidation, there is still a 10% difference between the results from samples retrieved using different samplers. Further, the laboratory determined maximum shear moduli are about 10% lower than the value determined in an in situ seismic cone test.     

Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers

Full-flow penetrometers (the T-bar and ball) are increasingly used on sites with thick deposits of soft clays, particularly prevalent offshore. Full-flow penetration tests were performed at five international well-characterized soft clay test sites to assess the use of full-flow penetrometers to estimate undrained shear strength. Field vane shear data were used as the reference undrained strength. Statistical analyses of strength factors indicates that full-flow penetrometers provide an estimate of undrained shear strength at a similar level of reliability compared to the piezocone. Relationships for estimating the strength factor and soil sensitivity using only full-flow penetrometer data obtained during initial penetration and extraction are developed. A strong dependence of the strength factor on sensitivity was identified and can be used for the estimation of undrained strength. The effectiveness and use of the developed correlations are demonstrated through their application at an additional test site.     

十字板优化轴试验研究

文章设计了十字板剪切仪的静态标定方案,按深海底沉积物剪切强度力学特性分层配制试验样品,并在室内分别采用便携式剪力仪对样品进行手剪和十字板剪切仪采用优化杆和原杆对样品进行原位测量,试验结果说明试验室十字板剪切仪原位测量剪切强度值接近且均小于便携式剪力仪剪切强度测量值、采用优化杆原位剪切强度测量的精度优于采用原杆原位剪切强度测量精度。     

Influence of Strength Anisotropy on Piezocone Resistance in Clay

Since different undrained shear strengths may be obtained from different laboratory or field tests, different cone factors are needed to calculate the undrained shear strengths from the results of a piezocone test in clay. Based on the anisotropic undrained shear strength criterion, a set of cone factors for calculating undrained shear strength is proposed in this paper. These cone factors are interrelated with each other in terms of the strength anisotropy ratio of the clay. The suitability of the proposed cone factors has been verified using the test results generated from different tests and from clays with different origins. By comparing with the cone factor for isotropic clay, it was found that the effect of strength anisotropy on the cone factor is not always significant. In fact, the effect of strength anisotropy becomes significant only in lightly overconsolidated to normally consolidated clay with moderate to high strength anisotropy. Furthermore, the maximum difference in cone factor is not expected to exceed 20% with or without taking into account the strength anisotropy of clay.     

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.     

Evaluation of Flow Liquefaction and Liquefied Strength Using the Cone Penetration Test

Flow liquefaction is a major design issue for large soil structures such as mine tailings impoundments and earth dams. If a soil is strain softening in undrained shear and, hence, susceptible to flow liquefaction, an estimate of the resulting liquefied shear strength is required for stability analyses. Many procedures have been published for estimating the residual or liquefied shear strength of cohesionless soils. This paper presents cone penetration test-based relationships to evaluate the susceptibility to strength loss and liquefied shear strength for a wide range of soils. Case-history analyses by a number of investigators are reviewed and used with some additional case histories. Extrapolations beyond the case-history data are guided by laboratory studies and theory.     

Using a Small Ring and a Fall-Cone to Determine the Plastic Limit

The plastic limit is an important property of fine-grained soils. The standard thread-rolling method for determining the plastic limit has long been criticized for requiring considerable judgements from the operator. This paper presents the design of a small specimen ring for fall-cone tests for determining the plastic limit. A small specimen ring of 20 mm in diameter and 20 mm in depth is designed and used for cone penetrations between 10 and 3 mm. For each soil tested, a linear relationship between the logarithmic cone penetration and the logarithmic water content is obtained by a regression analysis on four data points. This linear relationship is extended to determine the plastic limit at 2 mm of cone penetration. The fall-cone plastic limits of 21 soils are in good agreement with their thread-rolling plastic limits. In addition, a one-point fall-cone method is also recommended for determining the plastic limit. As the fall-cone has been recommended in several standards for determining the liquid limit, it is preferable that it is also used for determining the plastic limit.     

Numerical Analysis of T-Bar Penetration in Soft Clay

The penetration resistance of a cylindrical T-bar penetrometer in soft clay is affected by features such as anisotropy, high strain rates, and gradual strain-softening during passage of the T-bar. In order to evaluate these effects, a detailed numerical study has been undertaken, comprising: (1) finite-element analysis; and (2) a strain path approach within the upper bound plasticity mechanism. These studies showed that the T-bar factor is relatively insensitive to the degree of strength anisotropy, provided the penetration resistance is normalized by the average shear strength. Strain rates were found to be six or seven orders of magnitude greater than typical laboratory testing rates, and about three orders of magnitude higher than in a standard vane test. However, the effect of high strain rates is partly compensated by remolding of the soil, where average strains of 400% are imposed on the soil. Charts are presented showing how the separate effects of high strain rates and partial softening may be combined to derive a T-bar factor for a given soil. The paper concludes with a discussion of the measurement of remolded shear strength using cyclic T-bar tests, and interpretation of the T-bar resistance in fully remolded soil.     

Plastic Limit, Liquid Limit and Undrained Shear Strength of Soil—Reappraisal

The concept that plasticity index of soils can be defined as a range of water contents producing a 100-fold variation in undrained shear strength has been experimentally verified with the help of a large number of tests on soils of diverse nature. This has led to the redefinition of the plastic limit as the water content at which undrained shear strength is around 170 kN/m2. Undrained shear strength of a soil at the liquid limit can be considered to be around 1.7 kN/m2. Accordingly, both the liquid limit and the plastic limit have been determined in the present work by a single consistent method, i.e., the Swedish fall cone method. The undrained shear strength-water content relationship has been found to be log-linear for a wide range of water contents beginning from lower than the plastic limit to higher than the liquid limit. This resulted in the formulation of an expression for predicting undrained shear strength of a remolded soil at any water content based solely on its liquid limit and plastic limit.     

含水率对重塑红黏土反复抗剪强度影响试验研究

红黏土性质的稳定与安全生产息息相关,抗剪强度作为红黏土强度特性之一,与红黏土的稳定性密切相关。为探究含水率对红黏土反复抗剪强度的影响,以山西长治地区的重塑红黏土为研究对象进行反复直剪试验。试验结果表明：无竖向压力时,随着剪切次数的增加,抗剪强度减小并趋于稳定值,第2次剪切达到抗剪强度时对应的剪切位移小于第1次剪切时的位移,稳定剪切时对应的剪切位移大于第1次剪切时的位移;抗剪强度与含水率呈负线性关系,且抗剪强度为土体的真黏聚力,随着含水率的增加先增大后减小。当施加竖向压力后,抗剪强度随着含水率的增加而减小;利用抗剪强度表达式拟合得到内摩擦角和黏聚力,其中黏聚力随着含水率的增加先减小后增大;通过真黏聚力计算得出新的内摩擦角,2种分析方法得出的内摩擦角基本接近且均随着含水率的增加而减小,减小幅度逐渐增大。定义黏聚力差异指数比来研究真黏聚力与黏聚力之间的差异,分析得到黏聚力与真黏聚力的差异指数比在0.75~9.96之间,当含水率为19.5%时黏聚力差异指数比达到最小。利用所建立的相关经验公式,能够为红黏土地区矿山岩土勘察、设计和开挖过程中土性参数的合理选取提供一定参考。     

Interpretation of Impact Penetration Measurements in Soft Clays

The need for obtaining estimates of undrained shear strength of shallow seafloor sediments often arises in offshore engineering practice. Impact penetrometers offer a promising means of obtaining strength estimates in such sediments. However, variable conditions of embedment and velocity require careful consideration in the interpretation of impact penetration tests. This paper presents an analysis of the expendable bottom penetrometer (XBP), a device that measures acceleration during impact penetration. The analyses indicate that acceleration measurements can be reasonably related to undrained shear strength of soft clays. Further, acceleration measurements can be integrated to obtain velocity and embedment depth data at any point during the penetration analysis, thereby providing a basis for accounting for rate and embedment effects. Applying the proposed analysis to data from a series of test sites in the Gulf of Mexico indicate satisfactory agreement between XBP and reference strength profiles in soft clays.     

Evaluation of a Dike Damaged by Pile Driving in Soft Clay

This paper presents a case study detailing a riverbank dike damaged by pile driving in very soft clay in Shanghai, P.?R. China. Driven piles were designed to support the existing dike to be raised to a higher elevation. The subsoil mainly consisted of very soft clay with its natural moisture content greater than its liquid limit. This paper describes the phenomena of dike movement and crack development during pile driving based on field observations and instrumentation data. Cone penetration tests and vane shear tests were conducted after pile driving to investigate the slip surfaces. Degradation of soil strength was identified as the main cause for the failure of this dike. Slope stability analysis was conducted to back-calculate the degraded undrained shear strength of the clay. The results indicate that the soil strength in the disturbed area due to pile driving approached the level of its remolded strength.     

Yield Stress of Super Soft Clays

Super soft clays can be defined as insensitive cohesive soils that have a water content higher than the liquid limit. It is difficult to define and measure the strength of these soils using conventional soil mechanics apparatus. It is proposed that the shear strength be determined using a rotary viscometer and be defined as the shear stress at zero strain (shear strain) rate in this test of viscosity. In this paper a number of potential methods to determine the shear strength or yield stress of super soft clays is considered. The yield stress of four super soft soils, each with varying water contents, have been measured using a rotary viscometer. The results of these tests together with published data are used to develop a relationship between the yield stress and the index properties of super soft clays.