Unidirectional cyclic resistance of Ticino and Toyoura sands from centrifuge cone penetration tests

The evaluation of the undrained cyclic resistance of sandy deposits is required to forecast the soil behaviour during an earthquake (liquefaction, cyclic mobility); due to the difficulties in obtaining undisturbed samples of most liquefiable soils, it is usually deduced from field test results such as cone penetration tests. This paper proposes a methodology to evaluate the undrained cyclic resistance from normalised cone resistance of two well-studied silica sands (Ticino and Toyoura), with different mineralogy, one mainly composed of feldspar, the other of quartz. The determination of the cyclic resistance of Ticino and Toyoura sands was achieved through undrained cyclic triaxial tests on reconstituted specimens. The tip resistance was deduced from CPTs performed in centrifuge with a miniaturised piezocone on homogeneous reconstituted models. Both the undrained cyclic and tip resistances were correlated with the state parameter ψ. Results of centrifuge and triaxial tests were combined through ψ to deduce the cyclic resistance ratio CRR directly from the normalised cone resistance. The shape of the curve relating CRR to the normalised cone resistance resulted unusual respect to all the recognised curves widespread in the geotechnical literature. The aim of the proposed correlations is to provide a useful instrument to improve the actual knowledge on liquefaction and to give a contribution based on the critical state soil mechanics framework to the development of refined correlations between the cyclic resistance of a sand and the results of cone penetration tests.     

Pilot-scale field investigation of ex situ solidification/stabilization of soils with inorganic contaminants using two novel binders

Recently, two novel binders, one by-product-based binder named as GM and one phosphate-based binder named as KMP, have emerged that can stabilize soils spiked with mixed lead and zinc contaminants. However, field evaluations of the stabilization of actual soils that contaminated with mixed zinc (Zn) and inorganic chloride (Cl^?) using GM and KMP have not been performed yet. This study presents a pilot-scale field test to evaluate the performance of GM and KMP to stabilize these inorganic contaminants in soils at two locations in an abandoned industrial plating plant site. The field soils were stabilized and cured for 1, 3, 7, and 28 days and tested for dry density, dynamic cone penetration, soil pH, and leachability. Laboratory unconfined compression tests were performed, and the relationship between unconfined compressive strength and dynamic cone penetrometer index was assessed. The results showed that the strength of both the GM- and KMP-stabilized soils after 28-day curing improved significantly, and the leached Zn and Cl^? concentrations were well below the corresponding remediation limits. In general, the KMP-stabilized soil demonstrated superior performance in terms of higher dry density, unconfined compressive strength, average dynamic cone penetration resistance, lower dynamic cone penetrometer index in the early curing stage (7 days), and lower leached Zn concentration under all curing times. In contrast, the GM exhibited superior immobilization of Cl^? in the contaminated soil irrespective of the curing time. The results demonstrate that GM and KMP are promising binders for treating Zn- and Cl^?-contaminated soils at plating and other industry sites with similar contaminants.

     

Correlations between Compression Index and Index Properties of Undisturbed and Disturbed Tehran clay

The compression index (Cc), which is used to calculate the consolidation settlement of fine-grained soils, can be determined through consolidation testing. Given that exploring the soil in a local region is highly important to determine the correlation between the Cc and other soil indices, the present study investigated these correlations in undisturbed and disturbed samples through 130 consolidation tests and determining the Cc of Tehran clay. The results are suggestive of the validity of the linear correlation between the Cc and the unit weight and initial void ratio of the soil, with several relations presented to estimate the Cc of Tehran clay soil. In contrast, the initial water content, liquid limit and the plasticity index do not produce reliable correlations with the Cc of the local clay soil, and a relation based on these index parameters cannot be recommended in this area. Further, the presented empirical correlations were compared with the existing ones. More over time-displacement and e-log σ’ graphs for undisturbed and disturbed samples are compared and stress history of the site are presented. The results are significant in terms of engineering applications, saving time and money and provides an initial estimation of compression index.

     

Geotechnical reconnaissance and liquefaction analyses of a liquefaction site with silty fine sand in Southern Taiwan

Geotechnical reconnaissance of a recurrent liquefaction site at a Quaternary alluvial deposit in southern Taiwan was conducted to establish a comprehensive case history for liquefaction on silty fine sand with high fines content. The liquefaction occurred at a silty fine sand layer with D₅₀ = 0.09 mm and fines content greater than 35% and was triggered by a M_w = 6.4 earthquake on March 4, 2010, which induced maximum horizontal acceleration up to 0.189 g at the site. In situ subsurface characterizations, including standard penetration test, cone penetration test, and shear wave velocity measurement, were performed as well as cyclic simple shear tests on undisturbed specimens retrieved by a modified hydraulic piston sampler. Comparisons of cyclic resistance ratios (CRRs) indicate that CPT sounding with standard penetration rate could overestimate the resistance ratio and drainage conditions during penetration should be considered for high fines content soil in the liquefaction analysis. Additionally, variations of CRRs from different in situ tests indicate that correlations among in situ tests and CRR could be soil specific and precautions should be taken when using these curves on silty fine sands.     

上海地区土体抗剪强度与静力触探比贯入阻力相关关系研究

基于上海地区19个工程共201组数据,通过黏土、粉质黏土和粉砂在上海地区土层中的分布,比贯入阻力Ps,室内固结快剪试验黏聚力cu和摩擦角?u,孔隙比e,塑性指数Ip沿土层深度的变化,分析得到3种土类的抗剪强度与比贯入阻力的相关关系,并总结了国内软土地区土体抗剪强度与静力触探比贯入阻力之间的相关关系。本文得到黏土的相关关系与以往的经验公式相近,同时给出了原位土层粉质黏土和粉砂的相关关系。区别不同土类使得相关关系可用性提高,在工程应用中更加合理。根据本文相关关系,通过静力触探比贯入阻力可以得到不同土层的抗剪强度值,进而应用于基坑稳定计算。本文研究的结果可以应用于海相沉积土的勘察和工程设计工作。     

Field study on the behavior of pre-bored grouted planted pile with enlarged grout base

This paper presents the results of field tests performed to investigate the compressive bearing capacity of pre-bored grouted planted (PGP) pile with enlarged grout base focusing on its base bearing capacity. The bi-directional O-cell load test was conducted to evaluate the behavior of full scale PGP piles. The test results show that the pile head displacements needed to fully mobilize the shaft resistance were 5.9% and 6.4% D (D is pile diameter), respectively, of two test piles, owing to the large elastic shortening of pile shaft. Furthermore, the results demonstrated that the PHC nodular pile base and grout body at the enlarged base could act as a unit in the loading process, and the enlarged grout base could effectively promote the base bearing capacity of PGP pile through increasing the base area. The normalized base resistances (unit base resistance/average cone base resistance) of two test piles were 0.17 and 0.19, respectively, when the base displacement reached 5% D_b (D_b is pile base diameter). The permeation of grout into the silty sand layer under pile base increased the elastic modulus of silty sand, which could help to decrease pile head displacement under working load.

     

Simplified DMT-based methods for evaluating liquefaction resistance of soils

This paper presents simplified dilatometer test (DMT)-based methods for evaluation of liquefaction resistance of soils, which is expressed in terms of cyclic resistance ratio (CRR). Two DMT parameters, horizontal stress index (K_D) and dilatometer modulus (E_D), are used as an index for assessing liquefaction resistance of soils. Specifically, CRR–K_D and CRR–E_D boundary curves are established based on the existing boundary curves that have already been developed based on standard penetration test (SPT) and cone penetration test (CPT). One key element in the development of CRR–K_D and CRR–E_D boundary curves is the correlations between K_D (or E_D) and the blow count (N) in the SPT or cone tip resistance (q_c) from the CPT. In this study, these correlations are established through regression analysis of the test results of SPT, CPT, and DMT conducted side-by-side at each of five sites selected. The validity of the developed CRR–K_D and CRR–E_D curves for evaluating liquefaction resistance is examined with published liquefaction case histories. The results of the study show that the developed DMT-based models are quite promising as a tool for evaluating liquefaction resistance of soils.     

Thermal volume change of poorly draining soils II: Model development and experimental validation

This study presents the development of an alternative constitutive model for predicting the thermal volume change of saturated and unsaturated soils using the concept of thermally-accelerated secondary compression. The model requires observation of the secondary compression response prior to drained heating to estimate the coefficient of secondary compression. The model was found to successfully capture the trends in thermal volume change arising from accelerated secondary compression resulting from prior mechanical loading for unsaturated, compacted Bonny silt. The model was also used successfully to represent data from the literature for normally consolidated and heavily overconsolidated soils prepared with different loading paths.     

Analysis of the compression of structured soils using the disturbed state concept

The aim of this note is to quantify the influence of soil structure on the compression behaviour of natural soils using the disturbed state concept (DSC). The behaviour of the fully adjusted state is chosen to be that of the corresponding soil in a reconstituted condition so that the disturbance function is a direct measure of the effects of soil structure. A new DSC compression model is proposed. This model is able to describe the compression behaviour of structured soils under loading, swelling and reloading. Special versions of the proposed model are also described for situations (a) where the compression behaviour of the corresponding reconstituted soils is linear in the e–ln p′ space and (b) where the compression is one‐dimensional. The ability of the proposed model and its various versions to describe the compression behaviour of structured soils has been verified. Copyright © 2000 John Wiley & sons, Ltd.     

Numerical study of partially drained penetration and pore pressure dissipation in piezocone test

The piezocone penetration test (CPTU) is commonly used as a fast and economical tool to identify soil profile and to estimate relevant material properties in soils ranging from fine to coarse-grained. Moreover, in the case of fine-grained soils (clays and silts), the consolidation coefficient and the permeability can be estimated through the dissipation test. Undrained conditions are commonly assumed for the interpretation of CPTU in fine-grained soils, but in soils such as silts, penetration may occur in partially drained conditions. This aspect is often neglected in data interpretation thus leading to an inaccurate estimate of soil properties. This paper investigates numerically the effect of partial drainage during penetration on the measured tip resistance and the subsequent pore pressure dissipation response contributing to a more accurate interpretation of field data. A realistic simulation of the cone penetration is achieved with the two-phase Material Point Method, modelling the soil response with the modified Cam-Clay model. The approach takes into account large soil deformations induced by the advancing cone, soil–water, and soil–structure interactions, as well as nonlinear soil behavior.     

Experimental evaluation of shear strength behaviour of plastic and non-plastic silts

Silt is available in many parts of the world in combination with sands and clays. However, due to lack of clear understanding of its engineering behaviour, most of the time it is interpreted in terms of either sands or clays. Structures that are usually built on silty soils are designed to take into account design procedures developed for sandy or clayey soils. Presence of silts in combination with varying amount of sand and clays produces silt that is either plastic or non-plastic in nature. Silt is available in and around the Delhi region, in a majority mixture along with fine sands, which is non-plastic in nature. On the other hand silty deposits found in offshore Bombay High region are found in abundance along with significant amounts of clays and are termed as plastic silts. In this paper a comparison of the stress-strain behaviour of plastic and non-plastic silts is carried out under triaxial compression loading during both drained and undrained conditions. Two representative samples each from Delhi and Bombay High regions were considered for this comparison and results of stress-strain under four sets of confining pressure are discussed in detail. It is observed from this study that behaviour of silts is mainly dependent on the composition and structure of the resultant soil matrix. It is concluded from the results that shear strength parameters as well as volume change/pore pressure response of silty soils is dominated by the constituent soil present along with the silt. It is seen from the comparative behaviour of non-plastic and plastic silts that the presence of sand and clays has a governing effect on pore pressure development and the resultant friction angle. The study also corroborated that the nature of silt is transitional both in the case of plastic and non-plastic forms.     

Arbitrary Lagrangian Eulerian based finite element analysis of cone penetration in soft clay

This paper presents a numerical model for the analysis of cone penetration in soft clay based on the finite element method. The constitutive behaviour of the soil is modelled by modifying an elastic, perfectly-plastic soil model obeying Von-Mises yield criterion to take into account the strain-softening, rate dependent behaviour of soft clay. Since this is a problem involving large soil deformations, the analysis is carried out using an Arbitrary Lagrangian Eulerian method where the quality of the mesh is preserved during penetration. The variation of cone resistance is examined with various parameters such as rigidity index of the soil, in situ stress anisotropy and roughness at the cone–soil interface, which influence the penetration resistance of the cone. A theoretical correlation has been developed incorporating these parameters and the results have been compared with previous correlations based on the cavity expansion theory, finite element method and strain path method. With the increase in strain-softening, relative brittleness of the soil increases and the penetration resistance is significantly reduced. With the rising strain-rate dependency, penetration resistance increases but this increase is independent of the degree of brittleness of the soil.     

Performance of a compaction-grouted soil nail in laboratory tests

This study proposed a new soil nail known as the compaction-grouted soil nail, and a physical model was established to investigate its pull-out behaviour with different grouting pressures. The study on scale effect of the physical model was performed subsequently via numerical modelling. Additionally, interface shear tests were performed using the same boundary conditions as the physical model test. The strength parameters obtained were used to estimate the pull-out resistance of a conventional soil nail. The merits of these two soil nail types were compared based on their pull-out resistances. The physical model test results showed that the pull-out resistance of the compaction-grouted soil nail increases with increasing grouting pressure. In addition, the pull-out resistance exhibits hardening behaviour without a yield point, indicating that the compaction-grouted soil nail enables soils to remain stable against a relatively large deformation before ultimate failure. Furthermore, a higher grouting pressure results in a higher rate of increase for pull-out resistance versus pull-out displacement, which improves the performance of the compaction-grouted soil nail in the stabilization of large deformation problems. A comparison of the two types of soil nails suggests that the new compaction-grouted soil nail is more sensitive to grouting pressure than the conventional soil nail in terms of pull-out resistance improvement. In other words, the performance (pull-out resistance) of the compaction-grouted soil nail can be markedly improved by increasing the grouting pressure without inducing any accidental or undesired cracking or soil displacement.

     

天然沉积中间土的力学特性研究

长江口北岸天然沉积土主要由粉粒和细砂粒组成,其渗透系数介于10-7～10-3 cm/s之间,既不属于完全排水的砂性土,也不属于完全不排水的粘性土,针对这样的天然沉积中间土的我国研究并不多见。通过大量的物理力学试验,探讨了天然沉积中间土的物理力学性状,结果表明天然沉积中间土具有类似于软粘土的物理性质,但原位静力触探试验显示其天然沉积中间土的原位强度却与天然含水量分布在塑限附近的硬粘土差不多。研究结果亦表明颗粒分布对天然沉积中间土的原位强度有较大的影响。同时采用灰理论中的关联度分析,对这类中间土物理状态指标与力学特征指标间相关性进行了分析,认为孔隙比用于亚砂土稠度划分,含水比用于亚粘土、粘土稠度划分更加合理     

A new approach for stabilization of lateritic soil with Portland cement and sand: strength and durability

The presence of lateritic soils occurs in tropical and subtropical regions. The improvement of lateritic soils that are not suitable for a particular purpose through techniques that combine modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction is seen as an alternative. In this context, a dosage method to use a local lateritic soil as construction material in a most rational way reducing the economic and environmental impacts related to this activity is still missing. Therefore, the current research aims to evaluate the performance of a lateritic soil via modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction. For this, unconfined compression, and durability (wetting and drying) tests were carried out on specimens of compacted clayey gravel lateritic soil, whose granulometry was modified by the insertion of distinct amounts (from zero to 45%) of weathered sand, treated with distinct Portland cement contents (from 4 to 10%), molded at different dry unit weights (from 16.8 to 20.1 kN/m³) and cured for 7 and 28 days. Results of the mechanical tests have shown the significant influence exerted by cement content and dry unit weight of the blend, followed by curing time and finally sand insertion. Satisfactory correlations between the response variables (q_u and ALM) and the adjusted porosity/cement index (η/C_v) were obtained. Furthermore, an innovative approach to replacing the laborious durability test is proposed.

     

Experimental Studies on Behaviour of Stone Columns in Layered Soils

Stone columns are found to be effective and economical ground improvement technique in soft grounds. Understanding its behaviour when they are installed in stratified soils, in particular when the upper layer consists of weak soil, will be of great practical significance. This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the behaviour of stone columns in layered soils, consisting of weak soft clay overlying a relatively stronger silty soil, for various thicknesses of the top layer. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. Laboratory tests were carried out on a column of 90 mm diameter surrounded by layered soil, for an area ratio of 15%. It is found that the depth of top weak layer thickness has a significant influence on the stiffness, load bearing capacity and bulging behavior of stone columns.     

Volume change response and fabric evolution of granular MX80 bentonite along different hydro-mechanical stress paths

Despite the increasing understanding of bentonite behaviour, there is still missing evidence on how different hydro-mechanical loadings, including sequences of hydration and compression, affect the fabric and the volume change behaviour of the material. It is generally assumed that the interplay between the behaviour of clay assemblages and the overall fabric of the material is the reason of having final states that are dependent on the stress path followed. Here the results of an experimental campaign aiming to study these factors are reported and discussed. Free swelling and swelling pressure tests were performed, both followed by compression to a relatively high stress. The experimental program involved various samples that were dismantled at intermediate states in order to perform microstructural observations by means of mercury intrusion porosimetry and electronic scanning microscopy. It was observed that while the void ratio at a given stress level depends on the stress path, subsequent compression led to a unique virgin compression line. The data obtained at the microscale gave further insight for an interpretation of the volume change behaviour observed at the macroscale, showing that at high stress the material tends to recover the same fabric regardless of the path to saturation.

     

Load-settlement behaviour of socketed piles in sandstone

Nine Static vertical pile loading tests were carried out in Abu-Dhabi, the United Arab Emirates in order to assess the end bearing capacity and settlement of piles bored in slightly silty sands overlying soft rocky horizons like sandstone and calcarenite. The paper is aimed at presenting the results of interpretation of the pile tests in order to estimate the end bearing capacity in correlation with the unconfined compressive strength R _c and the rock quality designation (RQD). Load-settlement behaviour is also studied through an analysis by the elastic methods commonly used in pile design.     

Directional response of a reconstituted fine‐grained soil—Part I: experimental investigation

This paper discusses the results of a large experimental program designed to investigate in a systematic manner the main features of the incremental response of fine‐grained soils. The results are obtained from triaxial stress probing experiments carried out on a French silty clay (Beaucaire Marl). All the tests have been performed on reconstituted specimens, normally consolidated to an initial state which is either isotropic or anisotropic. In the interpretation of the experimental results, extensive use is made of the concept of strain response envelope. The response envelopes obtained for different stress increment magnitudes are remarkably consistent with each other and indicate an inelastic and irreversible material response, i.e. a strong dependence on the stress increment direction, also at relatively small strain levels. A companion paper (Int. J. Numer. Anal. Meth. Geomech., this issue, 2006) assesses the performance of some advanced constitutive models in reproducing the behaviour of reconstituted Beaucaire Marl as observed in this experimental program. Copyright © 2006 John Wiley & Sons, Ltd.