Design chart for the modified hyperbolic method

The modified hyperbolic method is used in practice to predict the ultimate primary consolidation settlement of compressible ground. This paper presents a new relationship between the average degree of consolidation for combined vertical and horizontal consolidation U_vh and the non-dimensional vertical time factor T_v, with a new parameter, ν_hv, introduced. ν_hv is defined as the ratio of the time factor in the horizontal direction to that in the vertical direction. This relationship is then adopted to calculate the slope of the linear segment of the theoretical hyperbolic plot (T_v/U_vh vs T_v), λ, which is a key factor in the modified hyperbolic observational method presented by Tan (1995). A design chart for λ as a function of ν_hv is proposed. Using this design chart, the determination for λ can be simplified from a procedure involving three parameters to only one, ν_hv. A new procedure for the use of the modified hyperbolic method is proposed. The new procedure is verified using a well-documented case history.     

Observational method for field performance of prefabricated vertical drains

An exponential formula is used to best-fit theoretical and measured time–settlement (or excess pore pressure) data over the full range of consolidation. The formula fits well theoretical consolidation solutions and measured data regardless of using the incompletely consolidated data, and it is possible to reliably predict the ultimate values. This result has a different trend from those of the hyperbolic and Asaoka (1978) methods. Thus the coefficients of horizontal consolidation and the mobilised discharge capacity q_w(mob) can be expressed in terms of parameters of the exponential formula corresponding to the measured data and the theoretical solutions. The application of the proposed method to six case records on three construction sites (with a maximum drainage path l_m of 7−50 m) indicates that the coefficient of horizontal consolidation for the ideal condition are likely to be used to reconstruct the monitored time–settlement curve and also to adjust the hydraulic and consolidation properties of each monitored point. Based on back-analysis, the mobilised and required discharge capacity for a preliminary design guideline are recommended as: q_w(mob) = (1–5)k_hl_m² and q_w(req) = 19.63k_hl_m², where k_h is the horizontal permeability of soil.     

推算最终沉降中工后次固结沉降比例研究

为了更准确地预测工后沉降,利用室内常规压缩试验、长期压缩试验和现场试验研究了三点法、双曲线法、星野法等最终沉降推算方法推算的黏性土地基最终沉降与工后次固结沉降的关系。研究表明,采用三点法推算的黏性土地基最终沉降包含的工后次固结沉降较少,采用双曲线法推算的黏性土地基最终沉降包括约23%～52%的工后次固结沉降,采用星野法推算的黏性土地基最终沉降基本包含了绝大部分工后次固结沉降。故建议采用三点法和双曲线法推算黏性土地基最终沉降并预测工后沉降时还应考虑工后次固结沉降。     

Design and analyses of floating stone columns

Two-dimensional (2D) finite element analyses were performed on floating stone columns using the unit cell concept to investigate the settlement and the consolidation characteristics of an improved foundation system. Undrained analyses, followed by consolidation analyses, were conducted throughout the study. The computed values for settlement and excess pore pressure distribution over time are compared for different area replacement ratios. Based on these coupled consolidation analyses, a simple approximate method is developed to predict the degree of consolidation for floating stone columns. In addition, a simple method to calculate the settlement improvement factor for floating columns is proposed. The proposed method may provide more realistic answers than other design methods in view of the yielding characteristics and the influence of key parameters that are considered in the analyses. The key parameters relevant to the design of floating stone columns include the area replacement ratio, the friction angle of the column material, the loading intensity, and the post-installation earth pressure. Closer agreements are obtained with the proposed method than with the established Priebe׳s method or α–β method.     

Asaoka法在软土地基中的应用

对Asaoka从理论基础和沉降递推结果进行了分析，利用三次样条插值曲线来处理非等时间间隔的沉降观测资料，从而求得最终沉降，推算出固结系数。     

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.     

基于动态压缩模量的路基工后沉降计算研究

基于太沙基单向固结理论,结合土体变形计算公式,推求饱和土压缩模量的表达式.研究饱和土体压缩模量随固结度、固结系数和排水路径的变化规律,并分析了压缩模量的时间效应及其在路基工后沉降预估中的应用研究.结论表明：1）固结初期,压缩模量随固结度增加呈缓慢线性增长;固结度U ＞60％时,压缩模量增长速度加快;固结后期,压缩模量增加很少;2）固结时间相同,压缩模量随固结系数增加而增加;固结系数Cv＜5000 cm2/a时,压缩模量几乎不变;3）双面排水时的压缩模量增长速度远大于单面排水;4）压缩模量随时间呈近似线性增长,且存在上限值;利用动态压缩模量可有效预估路基工后沉降.     

从实测沉降过程推算固结系数

按照Terzaghi固结理论,直接从土体压缩时间过程定出控制孔压消散速率的固结系数,暗含固结度U_σ和压缩度U_ε等同的假设。由于土的应力应变关系是非线性的,所以U_σ和U_ε并不等同。在应力增量（△p/p）较大时,特别是对于高压缩性的软粘土来说,上述假设可能引起相当大的误差。本文采用双曲线型和半对数型的压缩曲线,推导出固结度U_σ和压缩度U_ε之间的两种解析关系,并提出从实测沉降过程推算现场土层平均固结系数的方法。经工程检验,证明从双曲线型压缩曲线求出的U_σ和U_ε之间的关系优于从半对数型压缩曲线求出者。最后指出,只须对古典固结理论中的线性化假设作出适当修改,从假设固结过程中土的压缩性m_υ和透水性k不变,改为假设固结系数C_υ或C_h不变（k/m_υ=常数）,则在采用非线性应力应变关系时,仍能运用古典理论进行计算。这样,就可避免那种明显不合理的U_σ和U_ε等同的假设,使实用时的误差大大减小。     

渗透特性及填筑速率对路基固结的影响

计算软土路基沉降的一般方法没有反映路基渗透系数的变化以及填筑过程的时间效应影响,而大多采用同一渗透系数和一次加载的方式分析路基沉降。笔者采用ABAQUS有限元软件在考虑软粘土渗透系数随孔隙比变化经验公式以及不同填筑速率的基础上分析了软土路基的固结沉降,并进一步研究了填筑阶段路基内竖向有效应力随时间的变化。结果表明:(1)固结随时间的变化跟填筑过程有相关性;(2)渗透系数随深度非线性分布对路基的固结影响不可忽略;(3)加快填筑速率使得竖向有效应力增大的速率加快,沉降也加快;(4)填筑速率对固结沉降的影响主要集中在填筑阶段,对工后沉降影响不大。     

Effect of surcharge loading rate and mobilized load ratio on the performance of vacuum–surcharge preloading with PVDs

The results from three laboratory model tests performed under various vacuum and surcharge loads with PVDs are reported. Different SLRs were adopted to investigate the effect on the consolidation of dredged soil. To measure the lateral displacement, a refitted inclinometer was developed and tested. In the tests, the settlement, lateral displacement, and vane shear strength were measured, and the degree of consolidation (DOC), horizontal coefficient of consolidation (C_h), and bearing capacity were calculated. The results indicate that larger SLR values promote consolidation. The largest vane shear strength, settlement, and C_h values were all obtained under the highest SLR, and the bearing capacity under this SLR was more than double that under the lowest SLR. The DOC was found to increase with the growth of the SLR. However, considering the vacuum pressure was higher in Case-III, the influence of SLR on reinforcement effect may not be so significant.     

吹填软土一维大变形自重固结的有限差分数值解

吹填软土发生自重固结的物理本质在于超孔隙水压力的消散与有效应力的增长,以往有关土体大变形自重固结问题的求解方法多基于Gibson大变形固结理论,未从物理本质上反映土体的固结过程。文章基于Gibson大变形固结理论的有关假定,推导建立以超孔隙水压力为变量的一维大变形自重固结控制方程,该方程的形式简单,系数的物理意义明确。结合吹填软土自重固结的边界条件及初始条件,采用修正隐式差分格式的有限差分法求解得到方程的有限差分数值解,该数值解能够求解任意e-σ′和e-k函数关系下吹填软土的自重固结过程。通过与Lee和Sills求得的解析解及SWC模型解对比,从超孔隙水压力、孔隙比及沉降3个方面对数值解的正确性进行验证。利用文章所提方法分析初始厚度、初始孔隙比及土粒比重对自重固结性状的影响。分析表明,土体自重固结过程中的沉降固结度始终大于孔压固结度,初始厚度对孔压固结度的影响更大,初始孔隙比及土粒比重对沉降固结度的影响更大。     

考虑桩体径竖向渗流的碎石桩复合地基固结解析解

提出一种新的复合地基固结度求解方法及表达形式,推导得到了考虑桩体及土体的径竖向渗流,上部荷载分级逐渐施加,扰动区渗透系数呈线性变化等因素的一个较全面的散体材料桩复合地基固结解析解。通过退化、与已有解的比较等方法对解进行了分析验证。结果表明,现有的单级荷载及瞬时荷载下考虑桩体径竖向渗流的解、考虑桩体竖向渗流与变形协调的解、太沙基一维固结解等都是解的特例,这说明了解的正确合理性,发展和完善了现有复合地基固结理论。不考虑桩体径向渗流会高估地基固结速度;地基加载过程越缓,地基固结越慢。提出的求解方法,为复合地基固结度的求解提供了一种新的思路。     

Modelling the Effects of Surcharge to Reduce Long Term Settlement of Reclamations over Soft Clays: A Numerical Case Study

Long-term settlement of reclamations constructed over soft soils may be reduced by the use of temporary pre-loading, although there is often uncertainty over how much surcharge to use, and how long it should be maintained. An elastic visco-plastic constitutive model is described and its application in assessing surcharging strategy for reclamation schemes is discussed. The model is incorporated in a one-dimensional finite difference consolidation analysis with vertical and radial drainage of a multi-layer soil profile. The analysis allows the inclusion of vertical drain resistance, a zone of peripheral smear around the drain, permeabilities that vary with void ratio, and creep both during and after primary consolidation. Drawing on data from the Bothkennar soft clay research site, the model is used to predict settlements beneath a hypothetical test fill, including the creep settlement behaviour when a surcharge is removed after different time periods. The behaviour during primary consolidation is back-analysed using Asaoka’s and the hyperbolic observational methods, and both are found to under-predict the magnitude of the settlement.     

Predicting deformation of PVD improved deposit under vacuum and surcharge loads

A series of modified triaxial tests was conducted to investigate the deformation characteristics of mini-prefabricated vertical drain (PVD) unit cells. The factors considered are the (1) magnitudes of surcharge load (p_s) and vacuum pressure (p_vac); (2) pre-vacuum consolidation period (t_va) before applying surcharge load; (3) surcharge loading rate (SLR); and (4) initial effective stress state in the specimens. Based on the test results, relationships between the coefficient of earth pressure (K_es) at the end of surcharge load application and the normalized horizontal and vertical specimen strains are established. Further, a method is proposed for estimating the value of K_es, and therefore the horizontal and vertical strains of the PVD improved soil layer subjected to combined vacuum pressure and surcharge load using loading conditions and basic soil properties. Finally, the proposed method was applied to a case history reported in the literature and good agreement between the field-measured and calculated lateral displacement and settlement was obtained, which suggesting that the proposed method can be a useful tool for designing preloading projects involving combined vacuum and surcharge loads.     

塑料排水板堆载预压法处理软土地基监测研究

采用塑料排水板联合堆载预压技术对深厚软地基进行处理,在堆载过程中对孔隙水压力、地表沉降、土层分层沉降和土层深部水平位移进行监测.并利用监测数据推求了最终沉降量和对地基进行了固结情况分析.结果表明,该法对处理深厚软地基是一种行之有效的处理方法.     

基于非Darcy渗流的饱和黏土一维非线性固结分析

在考虑土体变形非线性的基础上,引入描述非Darcy渗流的Hansbo方程,修正饱和黏土一维固结方程,并采用有限差分法对该方程进行求解。通过与Davis和Raymond一维非线性固结理论解析解的对比,证明本文数值方法的有效性。在此基础上,探讨土体非Darcy渗流参数、压缩指数以及地面荷载值对固结过程的影响。计算结果表明,非Darcy渗流延缓饱和黏土中孔隙水压的消散速率和地基沉降速率,且随着压缩指数与渗透指数之比c或非Darcy渗流参数m和i1的增大,孔压消散速率和地基沉降速率都变慢,即不考虑渗流非Darcy特性的固结分析将会高估孔压消散速率和地基沉降速率,而地面荷载对固结进程的影响则与固结时间和压缩指数与渗透指数的比值有关。     

塑料排水板处理软土地基的离心模型试验研究

采用大型土工离心机对某海上机场工程近海软土地基上堤坝施工期及运行期进行了模拟。原型中采用塑料排水板固结法处理软土地基,模型中则根据固结过程相似的原理,换算成等效圆截面排水体,在模型制作中采用等效透水滤芯来模拟。试验中采用停机加载法模拟分级施工加载过程。根据激光位移传感器试验数据可以计算得出相应原型软土地基的沉降特性。利用试验得到的沉降曲线,采用"经验双曲线法"推算出了地基最终沉降,然后得出按沉降推算的分层地基平均固结度随时间的变化。对比沉降推算的运行5 a时软土地基固结度和理论计算结果,二者基本接近,表明文中塑料排水板的模拟方法用于离心模型试验是可行的。通过与理论计算结果比较,表明试验采用的塑料排水板模拟方法是可行的。     

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.     

基于等效线性模型等价结点力法的高速公路软基震陷反应研究

一般认为 ,震陷发生的机理 ,震陷与地震作用强度、持续时间、土体性状、构筑物强度、位置及抗震措施等有关。从工程应用来看 ,为了预防或降低震陷的危害 ,除了加强抗震设施外 ,通过数值计算来估算震陷量 ,具有一定的社会效益与经济效益。通过有效应力等效线性动力反应分析 ,运用等价结点力法分析了某高速公路软粘土路基震陷随地震最大加速度的变化趋势 ,建立了震陷同路堤高度、软土动模量系数、饱和重度、遭遇的竖向与水平向地震加速度放大系数之比和水平向地震系数等因素的相关关系式。此外还认识了频谱对震陷的影响 ,对相关研究具有一定的启发性。     

真空联合堆载预压下竖井地基固结解析解

考虑真空度沿竖井的发展是一个深度的函数,同时堆载所引起的附加应力既随时间变化也随深度变化,还考虑了地基的径竖向渗流以及扰动区土体水平渗透系数的3种变化模式,推导了真空联合堆载预压下竖井地基固结度的一个较普遍的解析解,并分析了在真空度沿竖井线性下降,堆载线性施加和附加应力沿深度梯形分布等情况下的地基固结性状。结果表明,荷载线性施加时,真空度对地基固结度有较大影响,真空度越大,沿深度衰减越慢,固结越快;而在荷载瞬时施加时,真空度对固结度没有影响。在地基井径比和水平渗透系数与竖向渗透系数之比较小时,地基的竖向渗流对地基的固结度有较大的影响。