Analysis of Soil Disturbance Associated with Mandrel-Driven Prefabricated Vertical Drains Using an Elliptical Cavity Expansion Theory

The installation of mandrel-driven prefabricated vertical drains and resulting disturbance of soft saturated clays are analyzed with a new elliptical cavity expansion theory. This formulated theory accounts for a concentric progression of elliptical cavities in an undrained condition and the large-strain effects in the plastic zone incorporating the modified Cam clay parameters. The total and effective stresses and excess pore water pressure in the soils surrounding the mandrel are predicted taking into account the mandrel installation rate, mandrel dimensions and the time factor. The theoretical variation of excess pore pressure is then compared with the results of large-scale consolidometer tests, which show that the estimated and measured pore pressures are almost the same. The plastic shear strain normalized by the rigidity index is then used to identify the zone of disturbance around the vertical drains. This formulation has been applied to a case history from the Muar clay region in Malaysia, and the results verify the usefulness of the method for determining the extent of the smear zone.     

Simplified Plane-Strain Modeling of Stone-Column Reinforced Ground

The acceleration of consolidation rate by stone columns was mostly analyzed within the framework of a basic unit cell (i.e., a cylindrical soil body around a column). A method of converting the axisymmetric unit cell into the equivalent plane-strain model would be required for two-dimensional numerical modeling of multicolumn field applications. This paper proposes two simplified conversion methods to obtain the equivalent plane-strain model of the unit cell, and investigates their applicability to multicolumn reinforced ground. In the first conversion method, the soil permeability is matched according to an analytical equation, whereas in the second method, the column width is matched based on the equivalence of column area. The validity of these methods is tested by comparison with the numerical results of unit-cell simulations and with the field data from an embankment case history. The results show that for the case of linear-elastic material modeling, both methods produce reasonably accurate long-term consolidation settlements, whereas for the case of elastoplastic material modeling, the second method is preferable as the first one gives erroneously lower long-term settlements.     

不同桩体材料复合地基承载及变形性状对比试验研究

通过室内模型试验,实测得到碎石桩、夯实水泥土桩和CFG桩复合地基桩土荷载分担比、桩土应力比和桩间土深层变形,并对三类不同桩体材料复合地基的承载及变形性状进行了对比分析.认为碎石桩复合地基和夯实水泥土桩复合地基均存在有效桩长或有效复合土层厚度;碎石桩桩长超过有效桩长,对提高复合地基承载力和压缩模量、减小变形效果不明显,除一些特别情况如为处理可液化地基外,设计桩长可适当超过有效桩长,但不宜过长;夯实水泥土桩复合地基的有效桩长与桩身强度相关性显著,应以桩身强度控制进行夯实水泥土桩桩体设计,使按桩身强度确定的单桩承载力大于或等于由桩周土及桩端土的抗力所提供的单桩承载力;CFG桩复合地基桩身强度高,桩体自身压缩性小,可全桩长发挥侧阻作用,当桩端落在好的持力层时,能很好地发挥端阻,提高承载力,减小变形,设计时应优先选择好的桩端持力层进行设计.

     

复合振冲碎石桩加固机理及施工过程数值模拟

采用振动荷载作用下超孔隙水压力产生的能量模型,考虑耗散能量和孔径扩张的影响及相互作用,并进行合理的简化,对复合振冲碎石桩施工过程做了数值模拟.采用有限差分离散求解复合振冲碎石桩边值问题,编制相应的数值程序,基于模拟结果详细讨论了排水井的存在对复合振冲碎石桩孔隙水压力发展变化的影响.最后对普通碎石桩和复合碎石桩的地基加固效果进行对比分析.数值模拟结果表明,复合振冲碎石桩由于排水井的存在,使地基加固范围明显增大,并且可以应用于渗透系数较小的粉土和砂质粉土地基中.     

Effect of Installation Method on External Shaft Friction of Caissons in Soft Clay

The influence of the installation method on the soil flow pattern, resulting external radial total stress changes, and final external shaft friction after consolidation has been investigated for caissons in soft clay by means of centrifuge model tests, large deformation finite-element (FE) analysis, and a simple cavity expansion approach. Both the centrifuge measurements and the FE results show that more soil is forced into the caisson under suction than under jacking. However, the difference in the resulting external radial total stress changes or penetration-induced excess pore-water pressure is much less significant, since the expansion-induced excess pore pressure is smaller for thin-walled caissons than for driven piles. After subsequent consolidation, the influence of the installation method reduces further, and the final shaft friction ratios are close for the two installation methods. Based on the magnitude of heave ratios derived from the centrifuge measurements and the FE analysis, a simple form of cavity expansion approach can reasonably estimate external radial stress changes during installation and after consolidation, and final shaft friction ratios for the caissons. An approach for estimating the external shaft friction ratios for vertical pullout of sealed caissons is proposed.     

Effects of Disturbance on Undrained Strengths Interpreted from Pressuremeter Tests

Although the cylindrical cavity expansion theory should provide a sound basis for obtaining the undrained shear strength of clays from pressuremeter tests, the interpreted strengths are often inconsistent with data measured in high-quality laboratory tests. This paper investigates how the pressuremeter results are affected by disturbances that inevitably occur during device installation. The installation of self-boring and displacement-type pressuremeters is simulated using strain path analyses, with realistic effective stress-strain-strength properties described by the MIT-E3 model. Derived strengths obtained from the simulated expansion of displacement-type pressuremeters tend to underestimate the in situ∕cavity expansion strength by amounts that depend on the relative volume of soil displaced, the time delay prior to testing, and the initial overconsolidation ratio of the clay. Interpretation procedures using the simulated contraction curves give much more reliable estimates of the true undrained shear strength. The simulated disturbance effects of self boring lead to derived peak shear stresses that are significantly higher than the reference undrained shear strengths. This overestimate depends on the volume of soil removed during installation and is enhanced when the finite membrane length is included in the analyses. Self-boring pressuremeter data from a well-documented test site in Boston confirm the general character of the predicted pressuremeter stress-strain behavior. The theoretical analyses underestimate the peak strengths derived from self-boring pressuremeter (SBPM) expansion tests, but match closely the measured postpeak resistance in the strain range of 3–6% (saddle point condition). Saddle point strengths are similar in magnitude to the shear strengths measured in laboratory undrained triaxial compression tests at this site. The current predictions are not able to explain the very high shear strengths derived from the SBPM contraction curves.     

Use of Deep Cement Mixing to Reduce Settlements at Bridge Approaches

Differential settlements between a bridge and the backfill behind the abutment have been a major problem in the construction of expressway embankments over a soft clay foundation. Deep cement mixing (DCM) columns with varying lengths were used to reduce such differential settlements along the Fu-Xia Expressway, Fujian Province, China. The performance and the feasibility of the DCM method were investigated in a trial embankment constructed prior to the actual construction. This paper presents the results of the instrumentation including total settlements, multipoint settlements, soil pressures on both the DCM columns and soil surface, pore-water pressures, and lateral movements obtained from in situ monitoring. The strength of the soil-cement from laboratory mix tests and from in situ quality control tests on DCM columns is presented in the paper. Study results indicate that DCM columns with varying lengths were a simple and effective method to reduce the total and differential settlements from a soft clay foundation at a bridge approach.     

Prediction of Ground Movements due to Pile Driving in Clay

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements δL∕R2 are functions of their dimensionless position x∕L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x∕L depend only on the wall thickness w; i.e., δ∕w = f[x∕L]. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of (1) building movements caused by installation of large pile groups; (2) uplift of a pile caused by driving of an adjacent pile within a group; and (3) spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the shallow strain path method analyses.     

Deep Mixing Induced Property Changes in Surrounding Sensitive Marine Clays

This paper presents a field study of installation effects of deep mixed columns on properties of the sensitive Ariake marine clay. Cone penetration tests were performed in the field to evaluate the change in the strength of the surrounding clay with time. Soil samples were taken before and after column installation to evaluate variations of physical, mechanical, and chemical properties of the surrounding clay. Test results indicated that the water content of the surrounding clay decreased while the concentration of cations increased as sampling locations approached the columns. Shear strength of the surrounding clay decreased during the installation but recovered after a short period of curing. Shear strength continued to increase with time over a period of 70?days. Based on the regression results, the surrounding soil after the installation of the columns took approximately 10?days to recover to the strength value before installation. On average, the shear strength of the surrounding clay increased over the original strength by approximately 23% after 40?days and 50% after 70?days, respectively. Discussion is presented on strength changes and key influence factors including soil disturbance and fracturing, thixotropy, consolidation, and diffusion of cations from deep mixed columns to the surrounding clay.     

Unsaturated Constitutive Surfaces from Pressuremeter Tests

A methodology to identify the collapse potential of unsaturated soils is proposed in this paper on the basis of pressuremeter test results associated with independent measurements of the in situ matric suction. A solution combining the expansion of a cylindrical cavity to a modified Cam clay critical state model has been introduced and accommodated to the framework of unsaturated soil behavior. This accounts for changes in soil properties induced by suction changes. Interpretation of pressuremeter tests performed under unsaturated and soaked conditions links the amount of collapse to strength and stiffness changes and provides assessment to the constitutive soil parameters that are necessary to define the yield envelopes of the soil. A comprehensive site investigation program comprising field and laboratory tests carried out in two residual soil sites is discussed in order to validate the proposed methodology. Values of shear strength, in situ stress, and yield pressure derived from both field and laboratory data are used as input parameters of a constitutive model adopted for describing the yield envelopes of these unsaturated residual soil sites.     

Predicting RC Frame Response to Excavation-Induced Settlement

In many tunneling and excavation projects, free-field vertical ground movements have been used to predict subsidence, and empirical limits have been employed to evaluate risk. Validity of such approaches is largely unknown given that ground movements are in fact not one-dimensional and that adjacent applied loads are known to have an impact. This paper employed analytical and large-scale experimental efforts to quantify these issues, in the case of excavation adjacent to a reinforced concrete frame with tieback anchors and a sheetpile wall in dry sand. With this flexible system, a disproportionate amount of the soil and building movements occurred prior to installation of the first tieback, even when conservative construction practices were applied. Furthermore, free-field data generated a trough as little as one-half the size of that recorded near the building frames. Empirically based relative gradient limits generally matched the extent and distribution of the damage, while the application of various structural limits did not fully identify local damage distribution but did generally reflect global response. The use of fully free-field data or a failure to include lateral soil displacements both underpredicted building displacements by as much as 50% for low-rise concrete frames without grade beams on sand.     

Lessons Learned for Ground Movements and Soil Stabilization from the Boston Central Artery

This paper summarizes lessons learned about soil stabilization with the deep mixing method (DMM) as it was developed and applied over 10 years during construction of the Boston Central Artery and Tunnel (CA/T). It also summarizes lessons about the control of excavation-induced ground movements and their characteristics. Deep deposits of marine clay were stabilized with DMM for large open cuts at Bird Island Flats and Fort Point Channel, both of which are described with respect to site conditions, soil properties, DMM installation and characteristics, and measured field performance. Topics addressed in this paper include water pressure distribution behind DMM walls, statistical characterization of soil cement properties, quality control/quality assurance procedures, comparison of measured and numerically simulated deformation in clay stabilized with various configurations of soil cement elements, shear modulus degradation characteristics of in situ soil cement, and ground movement patterns. Recommendations are made for soil cement properties, installation procedures, analytical modeling, design, and inspection.     

Finite-Element Analysis of Helical Piers in Frozen Ground

This paper presents a study of the behavior of helical pier foundations in frozen ground. The scope of the work presented includes developing finite-element analysis (FEA) models that simulate the stress-strain relationships in the pier and in the surrounding frozen soil. The paper also presents the results of the analysis models developed. The FEA computer program ANSYS is used to perform the computations that include three phases: The first phase is related to the instantaneous stability of piers in frozen or thawed soil under applied design loads. The second phase is related to the analysis of the pier strength during installation into frozen ground when the piers need to withstand installation torque and axial loads. The third phase is related to the long-term stability and critical loads of piers owing to creep in frozen soil. The most important finding of the analysis is that the load is not distributed into the frozen ground equally by the helixes of a multi-helix pier; instead, the bottom helix transfers most of the load.     

Interpretation of Pressuremeter Tests in Sand using Advanced Soil Model

This paper describes a numerical study of drained pressuremeter tests in sand using a one-dimensional finite-element method in conjunction with an advanced soil model MIT-S1, and input parameters corresponding to Toyoura sand. This soil model is capable of describing realistically the transitions in peak shear strength parameters of cohesionless soils that occur due to changes void ratio and confining pressure. The predicted peak shear strengths can be normalized, at least approximately, by introducing a state parameter that references the initial (preshear) void ratio to the value occurring at large strain critical state conditions at the same mean effective stress. The numerical analyses idealize the pressuremeter test as the expansion of a cylindrical cavity and ignore disturbance effects caused by probe insertion. This idealization is relevant to self-boring pressuremeter tests. Results confirm that there is a linear correlation between the in situ (i.e., preshear) state parameter of the soil and the gradient of the log pressure-cavity strain expansion, as first suggested by Yu in 1994 using a much simpler soil model. Indeed, the linear coefficients derived for Toyoura sand differ only slightly from those obtained previously by Yu for six other sands.     

高炉冷却柱的设计优化与数值模拟

 在高炉炉役后期冷却壁完全损坏的情况下,一般采用冷却柱对其进行修复,而冷却柱的安装数量和安装位置几乎都是依靠现场技术人员的实践经验来确定。为了解决目前高炉冷却柱在一块炉壳上的安装数量和安装位置存在难以确定的问题,通过分析冷却柱安装数量和安装位置与冷却效果之间的关系,提出了一种充分利用冷却柱冷却性能的优化安装方法。首先以冷却柱的总热交换面积大于原冷却壁的总热交换面积为基本原理,通过计算冷却柱和原冷却壁的热交换面积,得到设定的一块炉壳上冷却柱安装数量为11个;其次以11个冷却柱安装位置的中心坐标为设计变量,利用格点法的基本原理建立计算最大冷却面积的优化数学模型,设置好约束条件后通过遗传算法在MATLAB软件中进行求解,得到了91.68%的冷却柱冷却覆盖面积以及11个冷却柱排列的中心坐标;最后通过11个冷却柱的中心坐标建立三维模型,导入Fluent软件进行模型分析,经过充分迭代得到高炉冷却柱的温度场,并将3种排列的炉壳表面温度场进行对比。数值模拟结果表明,通过本方法得到优化排列的炉壳表面最高温度为73.34 ℃,平均温度为54.29 ℃,相比另外两种排列,最高温度分别降低了14.69%和30.21%,平均温度分别降低了13.33%和17.42%,有效提高了高炉冷却柱的冷却性能和利用效率。     

Simple Method for the Design of Jet Grouted Umbrellas in Tunneling

Tunnel excavation in cohesionless soils implies the use of a temporary supporting structure prior to lining installation. This temporary structure has to couple safety and economy, and can be conveniently realized using ground improvement techniques (for instance, by creating an arch of partially overlapped subhorizontal jet grouted columns). The adoption of ground improvement techniques results in structures far from having a perfect shape because they are intrinsically affected by defects (in both geometrical and mechanical characteristics), and therefore their design may hide unforeseen risks. As a consequence, this is the typical case in which sophisticated numerical analyses may just give the illusion of being refined, if possible defects are not correctly taken into account. In this paper a simple yet rational analytical method for the design of a nonclosed tunnel supporting structure that may be of some help to this aim is presented. It is done with reference to a simple two-dimensional scheme. In the first part of the paper, a design chart of optimal shape and minimum structural thickness of the cross section of the supporting structure is shown. In the second part, an iterative procedure to verify the stability or to design the minimum structural thickness of an existing supporting structure with a predefined shape is described. This method, coupled with the analysis of structural demand, allows one in principle to plot design charts. This approach can easily take into account structural defects with a semiprobabilistic approach and therefore with a chosen risk level, which is of great help to the designer at least in a preliminary design stage. The proposed semiprobabilistic procedure is applied to the case of a temporary supporting structure realized by partially overlapped subhorizontal jet grouted columns, intrinsically affected by defects in diameter and position. The variability of these geometrical parameters was considered based on the large quantity of experimental evidence collected in field trials by the writers and published elsewhere.     

圆锥圆柱齿轮减速器故障原因分析及技术改造

介绍了圆弧圆锥齿轮传动的特点，针对原圆锥圆柱齿轮减速器结构设计和安装中存在的问题，进行分析和优化设计、改进材质和热处理工艺，并对减速器结构进行改造，彻底解决了圆弧齿轮折齿失效等问题，成效显著。     

Analysis of Cavity Expansion in Sand

Cavity expansion analysis plays a significant role in modern soil mechanics. The analysis of many of the most important problems in the practice of geotechnical engineering (such as cone penetration testing, pile loading, or pressuremeter testing) rely to a large extent on cavity expansion analyses. Cavity expansion processes are of two basic types: expansion from a finite radius and expansion from zero initial radius. It is usual to use a different type of analysis for each of these problems. Analysis of the cavity creation problem yields only the limit pressure, but not necessarily information on the pressure‐strain relationship during expansion. Analysis of expansion from an initially finite cavity radius gives a pressure‐strain curve, but no information on the limit pressure. In this article, we present a simple numerical analysis that provides the solution to both problems simultaneously. The analysis takes full account of the flow rule and dependence of the friction angle on stress state, providing a rigorous solution for the cavity expansion problem throughout the plastic zone. The analysis can be used for both spherical and cyclindrical cavities. As illustration of the versatility of the analysis, plots of limit pressure versus soil state, cavity pressure versus strain for various soil states, and evolution of soil state within the plastic zone are provided.     

Ground Subsidence due to a Shallow Tunnel in Dense Sandy Gravel

Finite-element procedures have been applied to predict ground subsidence caused by tunneling; however, the calculated results are strongly dependent on the adequacy of the model used. The construction of a shallow tunnel in the typical gravel of Santiago (Chile) is herein back-analyzed using a finite-element method based on a modified version of the hyperbolic model. Observed ground subsidence as a function of distance to the face and to the tunnel axis is examined and related to the construction procedure. Although the ratio of overburden cover to excavation width was 0.52, the maximum settlement measured was only 9 mm. It was found that, as a result of the excavation process, a certain thickness of soil behind the excavation line experienced a significant alteration. The hyperbolic parameters of the disturbed soils were determined by fitting the calculated subsidence to the observed results. From the good agreement obtained, it is concluded that the model used represents reasonably well the construction stages of the tunnel and the stress–strain behavior of the materials involved.     

Multiflood and Multilayer Method for Scour Rate Prediction at Bridge Piers

The SRICOS method was proposed in 1999 to predict the scour depth versus time curve at a cylindrical bridge pier for a constant velocity flow, a uniform soil, and a deep-water condition. In this article, the method is extended to include a random velocity-time history and a multilayer soil stratigraphy; it is called the Extended-SRICOS or E-SRICOS. The algorithms to accumulate the effects of different velocities and to sequence through a series of soil layers are described. The procedure followed by the computer program to step into time is outlined. A simplified version of E-SRICOS called S-SRICOS is also presented; calculations for the S-SRICOS method can easily be done by hand. Eight bridges in Texas are used as case histories to compare predictions by the two new methods (E-SRICOS and S-SRICOS) with measurements at the bridge sites.