An analytical study of the effect of penetration rate on piezocone tests in clay

Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post‐penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi‐rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd.     

孔压静探模型槽试验及成果分析

本文介绍了对以饱和粘性土为介质的模型槽中所进行的孔压静力触探(CPTU)试验,通过对试验数据的分析,得出探头贯入时周围土体的轴向附加应力和径向附加应力的变化及分布,锥尖、侧壁摩阻力以及超孔隙水压力的变化,以及停止贯入后孔压消散过程中探头周围土体的超孔压变化,从这些方面来探讨土中应力场的变化,附加应力的影响范围,临界深度现象以及超孔隙水压力的变化规律。     

Installation effects of the post-grouted micropile in marine soft clay

The grouted steel pipe micropile is widely used as structural support and in situ improvement in China. This paper presents measurement of the radial soil stress and excess pore water pressure during the construction processes of the grouted steel pipe micropile (with an enlarged driving shoe) embedded in marine soft clay. Comparative analysis was conducted between the predictions by cavity expansion method (CEM) and maximum stress values in situ. The results show that the existence of the enlarged driving shoe has an effect on the stress change in the surrounding soils during penetration. The maximum radial total stress and excess pore water pressure generated during micropile penetration are approximately 4–6σ_v0′ and 1.5–2.5σ_v0′, respectively. The maximum radial total stress and excess pore water pressure, which appeared near the pile wall during the process of post-grouting, are approximately 5–7c_u and 4–6c_u, respectively. The predictions of CEM for pore water pressure during micropile penetration and post-grouting are in reasonable agreement with the field test data.

     

Consolidation behavior of soil–cement column improved ground

Columnar inclusion is one of the effective and widely used methods for improving the engineering properties of soft clay ground. This article investigates the consolidation behavior of composite soft clay ground using both physical model tests under an axial-symmetry condition and finite element simulations using the PLAXIS 2D program. It was determined that the final settlement and the rate of consolidation of the composite ground depended on the stress state. For an applied stress that is much lower than the failure stress, the final settlement of the composite ground was lower, and the consolidation was rapid. When the soil–cement column failed, the stress on the column suddenly decreased (due to strain-softening); meanwhile, the stress on the soil increased to maintain the force equilibrium. Consequently, the excess pore pressure in the surrounding clay increased immediately. The cracked soil–cement column acted as a drain, which accelerated the dissipation of the excess pore pressure. The consolidation of the composite ground was mainly observed in the vertical direction and was controlled by the area ratio, which is the ratio of the diameter of the soil–cement column to the diameter of the composite ground, a. The stress on the column was shown to be low for a composite ground with a high value of a, which resulted in less settlement and fast consolidation. For a long soil–cement column, the excess pore pressures in the surrounding clay and the column were essentially the same at a given consolidation time throughout the improvement depth. It is proposed that the soil–cement column and surrounding clay form a compressible ground, and the consolidation occurs in the vertical direction. The composite coefficient of consolidation (c_v(com₎) that was obtained from the physical model test on the composite ground can be used to approximate the rate of consolidation. This approximation was validated via a finite element simulation. The proposed method is highly useful to geotechnical engineers because of its simplicity and reliable prediction.     

基于孔压静力触探(CPTU)测试的连云港海相粘土前期固结压力确定方法研究

首先回顾了基于孔压静力触探(CPTU)测试确定前期固结压力的方法,通过连云港海相粘土场地进行的CPTU试验资料,以室内固结试验得到的前期固结压力作为参考值评估了经验方法预测前期固结压力的有效性。最简单的方法是直接建立前期固结压力和净锥尖阻力的关系,同时也是最有效的方法。     

Developing a Damage Model to Simulate Multiple-Step Loading Triaxial Compression Tests in Rocks

Multiple-step loading triaxial compression test (ML-TCT) method is a useful tool to evaluate strength parameters of rock samples using a single specimen applying several loading/unloading. However, because of accumulated damages in the specimen with repeated cycles of axial loading/unloading, the shear strength is prone to be underestimated. A multiple-step loading damage (MLD) model was proposed to simulate ML-TCT results. Two series of ML-TCTs were carried out on a sedimentary soft rock of mudstone. The first series was to determine the geotechnical parameters to describe the MLD model, and the second series was to verify the model. The results demonstrated that the proposed MLD model was powerful to simulate ML-TCTs on the mudstone and modify the results of carried out tests to generate more reliable results. Moreover, a generalized MLD model was constructed. This model allows prediction of peak deviator stresses and the relevant excess pore water pressures in a ML-TCT for rocks having different strength which generally are affected by the previous loading history. The generalized MLD model indicates that the margin between shear strength parameters obtained by single-step loading triaxial compression tests and ML-TCTs, increases with an increase in the rock strength. Moreover, upper bound values for effective cohesion, c′, and lower bound values for, effective friction angle, ?′, was obtained in a ML-TCT with increasing effective confining pressure, σ′_c. Whereas, upper bound values for ?′ and lower bound values for c′ predicted in a ML-TCT with decreasing σ′_c. It was concluded that, ML-TCT increasing σ′_c is preferable to ML-TCT decreasing σ′_c.     

Prediction of long-term pore pressure dissipation behavior by short-term piezocone dissipation test

The coefficient of consolidation determined from piezocone dissipation test and common interpretation methods makes the predicted dissipation curve match only at 50% degree of dissipation of the measured dissipation curve. It makes it difficult to predict the long-term pore pressure dissipation behavior of in-situ soft deposits. Therefore, based on the authors' previous result [Kim YS, Lee SR, Kim YT. Application of an optimization design technique for determining the coefficient of consolidation by using piezocone test data. Computers and Geotechnics 1997;21(4): 77–93] in which an optimized coefficient of consolidation reflects well the measured dissipation trend over the input degree of dissipation range, a systematic way of predicting a more realistic pore pressure dissipation behavior at high degree of dissipation with the optimized coefficient of consolidation is proposed. It was found that, relatively over the wide range of dissipation, the optimized coefficient of consolidation is more consistent than those determined by other researchers' methods. Applied to some real examples, it is also shown that the dissipation behavior of pore water pressure at a high degree of the dissipation range can be predicted well if the coefficient of consolidation is determined using the optimization technique with the data up to around 50% dissipation. Thus, it is expected that the proposed method saves time and expenses in conducting dissipation tests in the field.     

Flow slides run-out prediction using a sliding-consolidation model

The estimation of maximum travel distance of flow slides is an important topic to assess the consequence of natural disasters caused by landslides. During debris transportation, dissipation rules of pore-water pressure determine movement properties of flow slides. Based on 1-D Terzaghi consolidation theory, expressions of excess pore-water pressure with three cases of initial conditions are deduced and are programmed using Mathematica^® language. Furthermore, the factors affecting the distribution of pore-water pressure are studied using nondimensional method interactively, such as z/h, u _b /u _a , and T _v , which are fairly significant to investigate soil consolidation during the movement of flow slides. On the basis of the sliding-consolidation model first provided as reported by Hutchinson (Can. Geotech. J. 23(2):115-126, 1986), equations of pore-water pressure, velocity, and travel distance of flow slides are obtained and the physical quantities are coded as mathematical functions using Mathematica^® language characterized by its user-friendly interfaces to study run-out properties of flow slides very easily. The program can be used to compute velocity of flow slide, time, and pore-water pressure at a certain position, and thus judge automatically when and where flow slide will stop on slopes with different slope angles, solving the computing difficulties encountered during the Hutchinson's model application, especially in the last decades when computing technique with computers did not develop so rapidly as at present. At last, back analysis for properties of the 1966 flow slide at Aberfan, South Wales is done to test the model and the program, whose results are compared with those as reported by Hutchinson (Can. Geotech. J. 23(2):115-126, 1986). The results show that the program developed by the authors makes the application of Hutchinson's model more correct and easier.     

Estimation of undrained shear strength in moderately OC clays based on field vane test data

The undrained shear strength (s _u) of cohesive soils is a crucial parameter for many geotechnical engineering applications. Due to the complexities and uncertainties associated with laboratory and in situ tests, it is a challenging task to obtain the undrained shear strength in a reliable and economical manner. In this study, a probabilistic model for the s _u of moderately overconsolidated clays is developed using the Bayesian model class selection approach. The model is based on a comprehensive geotechnical database compiled for this study with field measurements of field vane strength (s _u), plastic limit (PL), natural water content (W _n), liquid limit (LL), vertical effective overburden stress (\(\sigma_{\nu }^{\prime }\)), preconsolidation pressure (\(\sigma_{\text{p}}^{\prime }\)) and overconsolidated ratio (OCR). Comparison study shows that the proposed model is superior to some well-known empirical relationships for OC clays. The proposed probabilistic model not only provides reliable and economical estimation of s _u but also facilitates reliability-based analysis and design for performance-based engineering applications.     

密度孔压静力触探的测试理论和工程应用综述

孔压静力触探试验(CPTU)是目前使用最为广泛的原位测试方法之一,但CPTU不能原位测量土体最基本的指标——密度。密度孔压静力触探试验(DCPTU)是把核子密度仪结合到孔压静力触探系统中,贯入过程中可以同时测量锥尖阻力、侧壁摩阻力、孔隙水压力和密度的一种新型原位测试方法。本文首先介绍了DCPTU的试验装置和密度测试理论;然后通过与室内试验结果对比评价了利用DCPTU测量砂土、黏土、填土地层以及海底淤泥的密度的可靠性,分析了非均质地层中DCPTU测量密度与真实密度的区别、并提出了真实密度剖面的推求方法;然后综述了DCPTU在判别软弱夹层、评价砂土的液化势、检验地基处理效果、检测块状填土中的空隙、调查浮泥层的厚度和密度分布等方面的工程应用;最后展望了DCPTU在工程勘察中的发展趋势和应用前景。     

多功能孔压静力触探在基坑工程降水设计中的应用分析

针对南京长江四桥漫滩沉积土,分别进行了室内试验和多功能地震波孔压静力触探（SCPTU）原位测试,从SCPTU应用于场地复杂地层 的精细划分、长江漫滩沉积黏性土透水性能的评价和预测渗透系数kh 3个方面进行了详细分析,试验结果表明：综合利用 多功能SCPTU测试qt、fs、u2 3个参数可以对具有高度分层性和不均匀性的长江漫滩沉积土进行精确分层,对不同性质土 层界面、同一土层内透水或不透水夹层进行精确探测;利用SCPTU钻进过程中的孔压消散试验功能,可以对复杂的混合土层的透水性能进行评价;基于SCPTU的渗透系数kh预测值均具有一定的离散性,一般比室内试验结果高1～2个数量级,Burns & Mayne方法预测值可以作为工程初设阶段的参考值;SCPTU的应用为基坑工程降水设计提供了一种简便、快捷的新型技术手段。     

Piles shaft capacity from CPT and CPTu data by polynomial neural networks and genetic algorithms

Cone penetration test (CPT) is one of the most common in situ tests which is used for pile design because it can be realized as a model pile. The measured cone resistance (q_c) and sleeve friction (f_s) usually are employed for estimation of pile unit toe and shaft resistances, respectively. Thirty three pile case histories have been compiled including static loading tests performed in uplift, or in push with separation of shaft and toe resistances at sites which comprise CPT or CPTu sounding. Group method of data handling (GMDH) type neural networks optimized using genetic algorithms (GAs) are used to model the effects of effective cone point resistance (q_E) and cone sleeve friction (f_s) as input parameters on pile unit shaft resistance, applying some experimentally obtained training and test data. Sensitivity analysis of the obtained model has been carried out to study the influence of input parameters on model output. Some graphs have been derived from sensitivity analysis to estimate pile unit shaft resistance based on q_E and f_s. The performance of the proposed method has been compared with the other CPT and CPTu direct methods and referenced to measured piles shaft capacity. The results demonstrate that appreciable improvement in prediction of pile shaft capacity has been achieved.     

Risk assessment and prediction of safety factor for circular failure slope using rock engineering systems

Circular failure is generally observed in the slope of soil, highly jointed rock mass, mine dump and weak rock. Accurate estimation of the safety factor (SF) of slopes and their performance is not an easy task. In this research, based on rock engineering systems (RES), a new approach for the estimation of the SF is presented. The introduced model involves six effective parameters on SF [unit weight (γ), pore pressure ratio (r _u), height (H), angle of internal friction (φ), cohesion (C) and slope angle (\(\beta\))], while retaining simplicity as well. In the case of SF prediction, all the datasets were divided randomly to training and testing datasets for proposing the RES model. For comparison purposes, nonlinear multiple regression models were also employed for estimating SF. The performances of the proposed predictive models were examined according to two performance indices, i.e., coefficient of determination (R ²) and mean square error. The obtained results of this study indicated that the RES is a reliable method to predict SF with a higher degree of accuracy in comparison with nonlinear multiple regression models.     

A proposed analysis of saturation-dependent anisotropy for US Department of Energy (DOE) Hanford site soils

Technetium (⁹⁹Tc) spreads predominantly laterally through US Department of Energy Hanford site sediments. Lateral transport implies that at higher tensions, h, in the unsaturated zone, the effective hydraulic conductivity, K, may be strongly anisotropic. A modeling procedure has been developed to predict characteristics of the Tc plumes. The procedure consists of: (1) Adapting existing numerical techniques based on critical path analysis to calculate K(h), (2) Statistically correlating predicted K at various h values with texture, (3) Seeking value of h, for which anisotropy and horizontal K values are both sufficiently large to accommodate multi-kilometer horizontal spreading, (4) Predicting the distribution of K values for vertical flow as a function of system support volume, (5) Comparing the largest likely K value in the vertical direction with the expected K in the horizontal direction, (6) Finding the length scale at which the two K values are roughly equal, and (7) Comparing that length scale with horizontal spreading of the plume. Predictions of the typical value of h at which spreading is occurring compares well with inference. However, the length scale at which Tc transport in the unsaturated zone changes from predominantly horizontal to more nearly vertical appears underestimated.     

Assessment of direct CPT and CPTU methods for predicting the ultimate bearing capacity of single piles

Twelve methods to determine axial pile capacity directly based on cone penetration test (CPT) and piezocone penetration test (CPTU) data are presented, compared and evaluated. Analyses and evaluation were conducted on three types of piles of different size and length. All the tested piles have failed at the end of static load test. Both the CPT methods and the CPTU methods were used to estimate the load bearing capacities of the investigated piles (Q_p). The static load test was performed to determine the measured load bearing capacities (Q_m). The pile capacities determined through different methods were compared with the measured values obtained from the static load tests. Two criteria were selected as bases of evaluation: the best fit line for Q_p versus Q_m and the arithmetic mean and standard deviation for the ratio Q_p/Q_m. Results of the analyses showed that the best methods for determining pile capacity are the two CPTU methods. Furthermore, the CPTU method is simple, easy to apply, and not influenced by the subjective judgements of operating staff. Therefore, it is quite suitable for the application in pile engineering practice.     

Application of an optimum design technique for determining the coefficient of consolidation by using piezocone test data

For normally consolidated clay, several researchers have developed a number of theoretical time factors to determine the coefficient of consolidation from piezocone test results. However, depending on assumptions and analytical techniques, it could vary considerably, even for a specific degree of consolidation. In this paper a method is proposed to determine a consistent coefficient of consolidation by applying the concept of an optimum design technique over all ranges of the degree of consolidation. Initial excess pore pressure distribution is assumed to be capable of being obtained by the successive spherical cavity expansion theory. The dissipation of pore pressure is simulated by means of a two-dimensional linear-uncoupled axi-symmetric consolidation analysis. The minimization of differences between measured and predicted excess pore pressure was carried out by the BFGS unconstrained optimum design algorithm with a one-dimensional golden section search technique. By analyzing numerical examples and in-situ test results, it was found that the adopted optimum design technique gives consistent and convergent results.     

Operational Coefficient of Consolidation Around a Pile Group Driven in Clay/Silt

Although finite element packages facilitating coupled consolidation analyses are increasingly in use, many practitioners still favour linear uncoupled analysis out of familiarity with the use of coefficients of consolidation. However, coefficients of consolidation measured by any single means tend to exhibit significant variation, with mean results from different laboratory and field tests also varying widely, leaving uncertainty over the correct values to apply to field problems. In this paper, a finite difference approach is used to back-calculate operational coefficients of consolidation from pore pressure measurements pertinent to a pile group driven in clay–silt. The research shows that this method is capable of successfully capturing the process of pore pressure dissipation, and that the operational coefficient of consolidation around the pile group is higher than that derived from piezocone dissipation tests in the same material.     

Numerical analysis of the miniature piezocone penetration tests (PCPT) in cohesive soils

An analytical model to simulate the penetration of the piezocone penetrometer in cohesive soils is presented here. The elasto-plastic coupled field equations of the saturated cohesive soils (given by Voyiadjis and Abu-Farsakh) is used in this analysis. The numerical simulation of the piezocone penetration is implemented into a finite element program. The analytical model is used to analyze the miniature piezocone penetration tests (PCPT) conducted at LSU calibration chambers. Simulation of the piezocone penetration is done for two cases. In the first case, the soil–penetrometer interface friction is neglected, while in the second case, the soil–penetrometer interface friction is taken into consideration. The constraint approach is used to model the soil–piezocone interface friction in which the Mohr–Coulomb frictional model is used to define the sliding potential. Analysis is done for three different soil specimens with different stress histories. The results of the numerical simulations are compared with the experimental measurements of the miniature piezocone penetration tests (PCPT) in cohesive soil specimens conducted in LSU calibration chambers. The resulting excess pore pressure distribution and its dissipation using the numerical model are compared with some available prediction methods. © 1998 John Wiley & Sons, Ltd.     

Non-convergent ordering and displacive phase transition in pigeonite: in situ HT XRD study

A natural Ca-rich pigeonite (En₄₇Fs₄₃Wo₁₀), free of augite exsolution products, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P2 ₁ /c (a=9.719(7) Å, b=8.947(9) Å, c=5.251(3) Å, β=108.49(5), V=433.0(6) ų), was annealed up to 1000 °C to induce a phase transition from P2 ₁ /c to C2/c symmetry. Complete single-crystal X-ray diffraction data collections were carried out in situ at 650, 750, 850 and 950 °C after the crystal had reached equilibrium for the Fe–Mg intracrystalline exchange reaction at each temperature. The variation, with increasing temperature, of lattice parameters, of intensity of hkl reflections with h + k=2n + 1 (which vanish at high temperature) and of some geometrical parameters from structure refinement, showed that the displacive phase transition P2 ₁ /c?C2/c was continuous in character. This contrasts with the first-order character for the HT phase transition in pigeonite containing significantly less calcium.     

ND-CPT测量非均质地层密度剖面的研究

同位素密度静力触探仪（ND-CPT）是一种把同位素密度仪并入到静力触探仪的探头中,同时测量锥尖阻力、侧壁摩阻力、孔隙水压力和密度的新型原位测试仪器。根据ND-CPT测量的密度为 射线源和检测器之间回转椭球体内土体的平均密度 的原理,首先建立了可以计算回转椭球体内土体的平均密度 的后向散射模型和平均值模型,然后调查了ND-CPT贯入各种非均质地层时的测量密度 剖面与实际密度 剖面的区别,结果表明在地层分界处附近的测量密度 与实际密度 有较大差别,最后对ND-CPT在实际工程中测量的有明海底泥的测量密度 剖面进行了理解,得到了海底泥的实际密度 剖面。研究结果可为利用ND-CPT准确划分地层提供依据。