Increase in Undrained Shear Strength of Clay with Respect to Rate of Consolidation

Estimating increase in undrained shear strength s_u of clay is an important purpose of consolidation analysis as well as settlement prediction when multi stage loading for construction of earth structures on soft clay is concerned. The present paper investigates the increase in s_u with regard to rate of consolidation. Since values of s_u are usually evaluated as a function of effective vertical stress σˊ_v, rate effect on shear strength increment ratio in normally consolidated state s_un/σˊ_v is first discussed considering the fundamental concept expressed by the equation of s_un/σˊ_v=s_uf/σˊ_y, in which s_uf is in-situ undrained shear strength and σˊ_y is consolidation yielding stress. The paper also describes two case histories where actual increases in s_u were observed in soft clay deposits. The first site is located offshore Osaka-bay where a large-scale seawall was constructed, and soft clay in the site was improved by sand drains. The second is a reclaimed land in Yanai City where a sand fill and a subsequent test embankment were conducted on a soft clay deposit without improvement by vertical drains. It is found from the study that: (1) rate effect on s_un/σ′_v cancels rate effect on σˊ_y, and the equation mentioned above is valid regardless of the rate of consolidation, and (2) the in-situ values of s_un/σˊ_v observed in the two sites vary from 0.27 to 0.37 as consolidation progresses, and they are well related to rate of consolidation. According to the experience at the two construction sites, values of s_un/σˊ_v with regard to rate of consolidation are proposed for design use at field/construction sites.     

Laboratory Investigation on Electro-Osmosis Properties of Singapore Marine Clay

In order to be able to design the electro-osmosis scheme, two major properties of clay, electro-osmosis permeability and coefficient of consolidation, need to be known. These properties can be measured in the laboratory by running the electro-osmosis consolidation test. This type of test is carried out with the help of modified triaxial apparatus in which the soil sample has an electric current applied, and its volumetric change accurately measured. It was found that the electro-osmosis permeability (k_e) of Singapore marine clay ranges between 10^–8 and 10^–9 m/v-s, whereas the electro-osmosis coefficient of consolidation (c_ve) varied between 0.17 and 2.81 m²/yr for tests under an electric potential gradient between 2 to 12 volts; c_ve increased with voltage applied. Physical and compressibility properties changes were noted after the electro-osmosis process. Gain in effective stress due to electro-osmosis was significant and magnitude of effective stress gain was directly proportional to the voltage applied. However vertical displacement was found to be smaller and significant magnitude of volume change was contributed from lateral deformation. This paper describes the test apparatus, and procedures and discusses the results from electro-osmosis tests, carried out on Singapore Marine Clay.     

Back-analyses of flow parameters of PVD improved soft Bangkok clay with and without vacuum preloading from settlement data and numerical simulations

Prefabricated vertical drains (PVDs) with embankment preloading (conventional PVDs) and with embankment combined with vacuum preloading (Vacuum-PVDs) are examined using the field data obtained from the site of the Suvarnabhumi Airport, Thailand. The flow parameters were back-analyzed by comparison of measured and predicted or simulated data. The flow parameters were illustrated in terms of the horizontal coefficient of consolidation (C_h) and the ratio between the horizontal hydraulic conductivity in undisturbed zone (k_h) and the horizontal hydraulic conductivity in smear zone (k_s) or (k_h/k_s). Numerical simulations using one-dimensional FEM PVDCON software with equivalent vertical permeability, k_ev, to determine the appropriate C_h and k_h/k_s of PVDs with conventional embankment preloading and with embankment combined with vacuum preloading schemes were made. Furthermore, numerical simulations using axisymmetric FEM by ABAQUS software, incorporating horizontal (k_h) and vertical (k_v) permeabilities, to determine the appropriate k_h/k_s based on back-calculated C_h of conventional PVD and Vacuum-PVD schemes were also done. The Vacuum-PVD scheme indicated faster rate of settlement than conventional PVD scheme by about 1.7–1.8 times with slight reduction of the k_h/k_s ratios. For conventional PVD, it was demonstrated that the increase in k_h/k_s ratios reduced the simulated rate of settlement.     

Experimental study on the clogging effect of dredged fill surrounding the PVD under vacuum preloading

Clogging effect surrounding prefabricated vertical drains (PVDs) is a typical problem when vacuum preloading is applied to a dredged fill foundation. A large-scale model test was designed to clarify the cause and mechanism of the clogging effect, and the basic physical and mechanical parameters of the soil in the clogging zone were tracked during the test. The results demonstrated that a clogging zone was formed around the PVD in the early stage of improvement with conventional vacuum preloading, and the boundary of the clogging zone was approximately 0.2–0.4 of the boundary radius. The clogging zone surrounding the PVD was formed because of the overall movement of the soil toward the PVD under the high vacuum pressure gradient, rather than fine particle migration. The soil in the clogging zone exhibited permeability anisotropy and equivalent ‘smear’ effect. The permeability ratio (k_h/k_v) was less than 1, and the ratio of horizontal permeability coefficients at the test distances of 45 cm and 10 cm were 9.6 at a depth of 20 cm and 8.9 at a depth of 80 cm. An analysis of the microstructure of the soil in the clogging zone demonstrated that the clay particles tended to be vertically oriented. The re-orientation of the clay particles reduced the horizontal permeability coefficient and led to the permeability anisotropy of the soil in the clogging zone. Thus, decrease in the horizontal permeability coefficient and equivalent ‘smear’ effect of the soil in the clogging zone affect the consolidation of dredged fill, which leads to the clogging effect. The permeability anisotropy also slightly affects consolidation.     

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.     

Performance monitoring and numerical assessment of a test embankment with preloading and vertical drains on Texcoco lacustrine soft clays

This paper presents the performance monitoring results and long-term numerical analyses of a 2.8-m-high test embankment with vertical drains on soft highly compressible clays during a four years and two months observation period (1525 days). The peculiar study site is characterized by thick layers of lacustrine soft clay with water contents up to 300%, void ratios between 7 and 9 and ratios C_α/C_c range from 0.06 to 0.03. The loading applied by the test embankment was 43.4 kPa. The vertical drains installed were of two types: sand and prefabricated. The settlements that only take into account the effect of the preloading embankment at the end of the observation period were 2.62 m and 2.71 m, in the zones with sand and prefabricated vertical drains, respectively. The settlement measured by regional subsidence was 0.47 m. The ultimate primary settlement was approximately 2.0 m and was estimated by two observational methods based on field settlement records. The settlement developed by secondary consolidation in the embankment ranged from 0.62 m to 0.71 m at the end of the observation period. The test embankment behavior was simulated by 2D and 3D numerical analyses. The 2D analyses used a theory to convert the axisymmetric drainage into plane drainage. The long-term numerical results and the field measurements were compared and discussed.     

Coupled consolidation via vertical drains in unsaturated soils induced by time-varying loading based on continuous permeable boundary

Considering continuous permeable boundary and coupled radial-vertical flow in this paper, a generalized semi-analytical solution for the consolidation enhanced by vertical drains (VDs) in unsaturated soils under time-varying loading is proposed. Firstly, the continuous permeable boundary is introduced into the axisymmetric consolidation model. Afterwards, the coupled controlling governing equations of excess pore pressures (EPPs) are solved by mathematical methods such as the decoupling technology, the Laplace transformation and inversion. Then, the validity of the current solution is verified by special cases with double-permeable boundary conditions (BCs) and instantaneous loading. The case analyses eventually were carried out, and the results show that the smear effects slow down the consolidation process but that is not obvious when the smear parameter is great than five. The effect of vertical flow on consolidation becomes insignificant when the ratio of radial to vertical permeability coefficient is greater than five; conversely, when its value is less than five, the above effect increases with the enhancement of the permeability properties at the top (or bottom) boundary. Moreover, the new solution can be applied to time-varying loadings, and different distributions of top and bottom boundary permeability.     

Modified method for predicting lateral displacement of PVD-improved ground under combined vacuum and surcharge loading

A modified method is proposed to predict the lateral displacement (δ) of prefabricated vertical drains (PVDs) improved ground under combined vacuum and surcharge loads, which is derived based on a few modified triaxial tests and a series of finite element analyses of PVD unit cells. It is observed that reducing the surcharge load (p_s) and loading rate (LR) and increasing the vacuum pressure (p_v), pre-vacuum consolidation period (t_v), and initial undrained shear strength (s_u0) could be effective in controlling the outward δ. Variations of the effective stress ratio (K_e) that controlling the δ with p_s, p_v, LR, t_v, and s_u0 are then presented. A synthetic relationship between the normalized horizontal strain (ε_h) by a reference one-dimensional vertical strain (ε_v1) and the normalized K_e by the at-rest earth pressure coefficient (K₀) is proposed for cases with and without t_v. Further, a modified index parameter (β₁) is introduced for quantitatively considering the effect of p_s, p_v, LR, t_v, s_u0, and consolidation properties of the soil, a relationship between K_e and β₁ is then established for evaluating the value of K_e. Combinations of the ε_h/ε_v1–K_e/K₀ and K_e–β₁ relationships enable modified predictions of the δ from basic preloading conditions and soil parameters.     

Consolidation behavior of dredged ultra-soft soil improved with prefabricated vertical drain at the Mae Moh mine,Thailand

The effectiveness of the prefabricated vertical drains (PVDs) in the consolidation of ultra-soft dredged soil with various soil water contents (W) in Mae Moh mine, Lampang, Thailand was researched via a series of large-scale model tests and numerical analysis. Large settlements with the delay of excess pore pressures is a distinct behavior of ultra-soft soil. The PVD dimensions were found to have a significant effect on the rate of consolidation and the delay of excess pore pressure at low total vertical stress (σ_v). The smaller PVD dimension resulted in the smaller rate of consolidation and longer delay of excess pore pressure. The undrained shear strength (S_u) of ultra-soft clay at various degrees of consolidation could be approximated by the vertical effective stress (σ′_v) based on the SHANSEP where the σ′_v was determined from the Asaoka's observational method. The finite element analysis with axisymmetric and plane strain models showed that the axisymmetric model produced an excellent settlement prediction. However, the excess pore pressures were not well predicted by the axisymmetric model, due to the delay of excess pore pressures at the early stages of consolidation. In practice, the plane strain models proposed by Chai et al. and Indraratna and Redana's methods are suggested to predict the consolidation settlement of the Mae Moh dredged soil improved with PVD. The outcome of this research will facilitate the geotechnical design of reclamation of ultra-soft dredged soil in Mae Moh mine and other similar soils.     

An observational method for consolidation analysis of the PVD-improved subsoil

Surcharge preloading together with prefabricated vertical drains (PVDs) have been widely used to improve properties of thick clay deposits. To assess the performance of soil improvement works, the average degree of consolidation needs to be estimated. A curve fitting formula is proposed in this paper to simulate the degree of consolidation versus a non-dimensional time factor relationship. The proposed formula fits the theoretical consolidation solutions well with a regression coefficient R² larger than 0.9996 and an error of less than 1.2%. Based on the formula, a modified Asaoka's observational method is proposed to predict the ultimate settlement and calculate the coefficient of consolidation using field settlement monitoring data. The effectiveness of the proposed observational method has been verified using some well-documented case histories. Comparisons between the proposed method and the Asaoka's method indicate that the proposed method will give a less than 1.0% higher ultimate settlement than that by the Asaoka's method and the proposed method is able to predict the c_h value with the consideration of both vertical and horizontal flow through the ratio of time factor in horizontal and vertical direction ν_hv.     

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.     

路堤下等应变复合地基的固结分析

 路堤下等应变复合地基固结沉降时,桩间土承担的荷载向桩体转移,桩土总应力不断变化,超静孔压增减量不等于有效应力增减量。考虑桩土总应力的变化、竖向排水体的压缩和孔隙水压力、桩体的排水性能、桩与排水体的距离等对复合地基的固结产生的影响。推导考虑桩体排水性能、桩土荷载转移、排水体压缩性和井阻的路堤下复合地基孔压、固结度、桩土应力比的解析解。固结分析表明,不透水桩可以加速地基固结,透水桩可能减缓固结,复合地基固结的快慢受桩身固结系数影响很大,高固结系数桩复合地基固结度大于小固结系数桩复合地基。     

基于Hansbo渗流的理想砂井地基固结分析

室内试验和现场观测表明,许多饱和黏性土中的渗流在小水力梯度时不能用Darcy定律描述,这应该是导致基于Darcy渗流的传统砂井固结理论有时不能很好地解释某些砂井地基固结特性的原因。引入同时考虑低速渗流幂函数曲线段和较高速渗流直线段的Hansbo渗流方程描述非Darcy渗流,在自由竖向应变假定下修正了Barron的理想砂井地基固结方程,并给出了有限差分法数值求解格式。据此探讨了Hansbo渗流参数、地基厚度等对砂井地基固结进程的影响。计算结果表明:和Darcy渗流相比,Hansbo渗流延缓了砂井地基内的孔压消散速度,导致固结速度变慢,并且径向排水对孔压消散的作用会更显著。如将Hansbo渗流方程简化为幂函数形式,则往往会高估砂井地基的固结程度,特别是在固结的初期。最后对比了自由竖向应变假定和等竖向应变假定对计算结果的影响。     

Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects,such as subgrades,engineered clay barriers,and buffers for radioactive waste disposal.These layers are exposed to a variety of stresses and wetting conditions during field serviceability.Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay.This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loadingeunloading (RLU) conditions.Two series of onedimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 k Pa using the axis translation technique (Fredlund soil-water characteristic curve device,SWC-150).The first test series was carried out at different levels of controlled matric suction for non-repeated loading eunloading (NRLU) cycles.RLU cycles were applied in the second test series at different repetitivestress levels and under different levels of matric suction.The results indicated increasing axial wetting strain ε_a(s),axial swell pressure s_s(s),compression index C_c(s),and swell index C_s(s) with suction reduction.The estimated loadecollapse (LC) curves obtained from NRLU series (LCN) and RLU series (LCR)indicated increasing yield stress s_y(s) with increasing suction.This is attributed to the developed apparent cohesion between soil particles,which in turn rigidifies the material response.Applying repetitive loading induced a notable reduction of compression index C_c(s) at the same level of suction,whereas swell index C_s(s) seems to be independent of repetitive loading.Finally,repetitive loading exceeding initial yield stresses results in plastic hardening and,hence,enlargement of yield stress locus(i.e.LC_R curve).     

堆载预压径竖向固结等体积应变解答

现有的含竖向排水体地基固结问题的解析解答大都是在传统的等竖向应变假设基础上建立的,而且在考虑涂抹区土体的渗流和压缩作用方面,还没有真实地体现出涂抹区土体的竖向渗透系数和体积压缩系数,其对地基固结的影响也尚不明确。为此,针对含竖向排水体地基径竖向同时固结的轴对称问题,采用等体积应变假设,考虑井阻作用、涂抹作用、随时间线性堆载预压以及地基附加球应力任意分布,推导了完整的径竖向固结微分方程的解答,给出了超静孔隙水压力和固结度的显式表达式,分析了侧向变形即泊松比效应以及涂抹区土体的竖向渗透系数和三维体积压缩系数对地基整体平均固结度的影响。结果表明,泊松比效应较为明显,基于传统的等竖向应变假设的解答高估了地基的固结速率;不考虑涂抹区土体体积压缩的解答虽然也高估了地基的固结速率,但影响有限;而涂抹区土体的竖向渗透系数对于地基固结的影响则几乎可忽略不计。     

Verification of effectiveness and design procedure of gravel drains for liquefaction remediation

The current design practice of using gravel drains as a liquefaction countermeasure involves the selection of the drain spacing and drain diameter to keep the peak excess pore water pressure ratio low. As it has mostly been verified through small-scale 1 g shaking tests, its validity for the field-scale prototype has yet to be well investigated. In this study, a series of centrifuge tests was conducted to gain insight into the stress-dependent behavior of loose sand deposits with a level surface improved by gravel drains. Meanwhile, the current design procedure was validated with experimental data. The results revealed that the effects of gravel drains in suppressing the excess pore pressures depend significantly on the depth of the drains. The current design procedure has failed to elucidate the depth-dependent behavior of sand deposits. One of the important features of the mechanical properties of the soil used for the design of gravel drains was revealed through laboratory tests in which the coefficient of volumetric compressibility, m_v, was found to be highly dependent on the stress level, while m_v was assumed to be fixed in the design procedure. It was also found that the water flow regime in gravel drains can be a turbulent flow. The Reynolds number in drains increases from the bottom to the top, and the permeability coefficient decreases accordingly, resulting in more significant well resistance than expected based on the current design procedure. In the present study, when stress level-dependent m_v and Reynolds number-dependent k_w were used as input soil parameters, the axisymmetric diffusion equation, with consideration given to the well resistance, satisfactorily predicted the excess pore pressures in sand with gravel drains.     

成层未打穿砂井地基固结Lagrange插值解法

考虑井阻和涂抹区的影响,建立成层未打穿砂井地基固结的数值模型和算法。将未打穿砂井底面下的软土层视为虚拟排水井,同时将未打穿砂井转化为打穿的多段砂井,使得土层边界条件和数值计算过程简单化。对超孔隙水压力用Lagrange插值多项式等结点插值近似,把砂井打设区固结方程转化为结点的超孔隙水压力的简单微分方程组,砂井固结方程转化为一线性代数方程组,以4层土为例,导出详细的计算公式。如增加土层数,仅需改变边界结点系数公式。最后,把该解法与相关近似方法的计算结果进行分析比较,并给出一个多层软土路基预压加固工程案例。结果表明,将下卧层简化为一维固结与考虑其三维固结对计算结果影响很小。     

考虑涂抹作用的未打穿砂井地基固结理论分析

 在等应变假设和瞬时加载条件下,研究单面和双面排水情况下未打穿砂井地基的固结问题。通过设置虚拟砂井的方法考虑未打穿土层的径竖向组合渗流;同时,为考虑施工对土体的扰动随离砂井的距离增大而逐渐减小的事实,引入一个函数,将土体水平渗透系数统一表达,并在假设的3种模式下(涂抹区水平渗透系数不变、呈线性变化和呈抛物线变化),得到未打穿砂井地基固结解析解。编制计算程序,详细阐述编程过程中要注意的几个问题,对未打穿砂井地基的固结性状进行分析。结果表明,考虑涂抹区水平渗透系数呈抛物线变化时地基固结最快,呈线性变化时次之,不变时最慢;施工扰动范围、程度越大,固结越慢;砂井打入深度越深,固结越快;砂井渗透系数越大,固结越快。