Behaviour of Gravel Compaction Piles in Soft Clay

Most of the field problems in geotechnical engineering are in three dimensional states or close to a plane strain condition. Strength and deformation properties of soils in plane strain condition are considerably different from those in an axisymmetric condition. Many researchers have investigated the behaviour of soils under a plane strain condition. However, most of the previous studies have concentrated on sedimentary type of soils like sand and clay. Our understanding on the plane strain behaviour for residual soils is less than that for sedimentary soils. A true triaxial system with four sliding rigid-plates and real time feedback control has been used to test specimens of a completely decomposed granite (CDG) soil (a residual soil) under plane strain condition. The setup of the true-triaxial rigid plates is briefly introduced first. The preparation of soil specimens and testing procedures are described. The basic properties of the CDG are presented. The stress-strain and strength behaviour of the soil obtained under plane strain condition was investigated, and compared to the behaviour obtained under axisymmetric loading conditions. The results reveal that the critical state line in q-p′ space obtained under plane strain condition is the same as that under axisymmetric condition. However, the critical state line in e- ln p′ space obtained under undrained plane strain condition is different from that under axisymmetric condition. The peak friction angle for plane strain tests is higher than that from axisymmetric loadings. It is also found that shear bands occur only in drained plane strain compression. Defuse bulging is the mode of failure for undrained plane strain as well as triaxial loading.

Behaviour of friction piles in soft sensitive clays : Blanchet, R; Tavenas, F; Garneau, R Can Geotech J, V17, N2, May 1980, P203–224

During the construction of heavy structures, such as bridges and overpasses, on soft clays on the north shore of the St. Lawrence Valley, a detailed load test program on friction piles was performed to establish the characteristics of the most suitable type of pile and to study its long-term behaviour. Three types of piles, timber, steel pipe with closed end, and precast concrete Herkules H-420 piles, were tested. Four timber piles driven in a group and submitted to a 712 kN load served to study the long-term settlement of a small group of piles. Three deep settlement gauges were installed in the centre of this group for measuring settlements in clay at various depths.This test program was completed by the instrumentation of two bridge piers in order to verify the behaviour of larger groups of piles.The paper presents the results of the test piles, the long-term behaviour (4 years) of the bridge pier foundations resting on friction piles in soft clay, and the interpretation of the results.This study shows that the pore pressures induced by pile driving are related to the pre-consolidation of the clay and that they are much larger for tapered piles. It is demonstrated that the effective stress analysis method proposed in 1976 by Meyerhof determines adequately the ultimate pile bearing capacity, but that the effect of the timber pile taper doubles the skin friction.The settlement analysis of pile groups shows that settlements are due to the reconsolidation of the clay and shear creep deformations in the clay close to the pile wall.

The results of an experimental study of the undrained behaviour of Changi sand under axisymmetric and plane-strain conditions are presented. K0 consolidated undrained plane-strain tests and K0 or isotropically consolidated triaxial tests on very loose and medium dense specimens were conducted. The undrained behaviour of sand at very loose and medium dense states under plane-strain conditions was characterised and compared with that under axisymmetric conditions. It was observed that the undrained behaviour of very loose and medium dense sand under plane strain is similar to that under axisymmetric conditions. However, because of the formation of shear bands in plane-strain tests, the post-peak behaviour of medium dense sand in plane strain is different from that in triaxial tests. It was also established that an instability line for plane-strain conditions can be defined in a way similar to that for axisymmetric conditions. Using the state parameter, a unified relationship between the normalised slope of instability line and the state parameters can be established for both axisymmetric and plane-strain conditions. Using this relationship, the instability conditions established under axisymmetric conditions can be used for plane-strain conditions.

This dissertation is the first Geotechnical Engineering doctoral thesis in Griffith University, and a detail study of the soft clay as encountered in Southeast Queensland is carried out. In the study process, due to the insufficient laboratory equipments and access to Geotechnical softwares, the dissertation has to be presented in a practical format. In addition, laboratory tests were conducted to investigate the application of chemical (cement and lime) treatments. Three case histories (Sunshine Motorway, Port of Brisbane Motorway, and Gold Coast Highway) are presented in this thesis. The main focuses are on the following aspects: i) soil parameters needed in engineering design from laboratory tests and field measurement, ii) the behaviour of constructed embankment on soft ground with and without ground improvement, iii) the performance of ground improvement techniques. The methods which have been employed to achieve the main aims were conventional methods. Laboratory test data and field measurement data, which were utilised for back-analyses and prediction of constructed embankment of soft ground behaviours, were obtained from QDMR. Thick layer of soft sensitive marine clay were found in the studied areas with up to 13m depth. The performance behaviour of constructed embankment on Southeast Queensland soft clay deposit has been evaluated based on the interpretation of test data, the theoretical analyses and conventional methods for settlement, lateral displacement, and excess pore pressure dissipation. Detail study of the estuarine soft clay as encountered in the Sunshine Motorway is carried out. 33 borehole data were examined to delineate the soft clay profile, which is about 10.5m thickness and varies substantially along the longitudinal section of the motorway. The laboratory value of the coefficient of volume decrease ranged from 1 to 5 MN/m2 and the laboratory values of the coefficient of consolidation are in the range of 0.25 to 0.5 m2/year. The Port of Brisbane Motorway embankments are installed with vertical drains and consist of three sections. Embankment A had drains at 3 meter spacing, and Embankments B and C had drains at 1.5 meter spacings. The maximum settlement obtained after 226 days of monitoring is shown. It can be seen that vertical drain treatment significantly increased final settlement. This increase varied from 70 to 80%. It can be concluded from the settlement results that vertical drains would have increased rate of consolidation. A trial embankment was constructed along the Gold Coast Highway. This embankment was divided into three sections, one section contained no ground improvement, and the other two sections had stone columns at 2m spacing and 3m spacing. For embankment with 3m spacing, the maximum settlement was 490 mm. For embankment with 2m spacing, the maximum settlement was 386 mm. For embankment without stone column, the maximum settlement was 522 mm. Based on the laboratory tests, for cement treated samples with 5 percent to 15 percent cement content, the maximum unconfined compressive strength increased from 132 kPa to 370 kPa for 7 days curing period; these values for 28days curing increased from 170 kPa to 405 kPa. For lime treated samples with lime contents from 2 percent to 15 percent, the maximum unconfined compressive strength increases from 47 kPa to 199 kPa (for 7 days curing period). Results indicated that, 2 percent lime has little effect on peak unconfined compressive strength. This thesis summarises some ground improvement techniques used in Southeast Queensland, and demonstrated the applicable of chemical stabilisation. Overall it was concluded that the addition of cement and lime has favourable effects on the strength characteristics of Southeast Queensland soft clays.

The inclusion of soil–cement columns in soft soil is a ground improvement technique that is used to reduce the settlement and improve the bearing capacity of the soft soil ground. In the present study, model tests have been carried out on soft soil improved with groups of end-bearing and floating columns under the axisymmetric condition to evaluate the relative improvement in the stiffness and failure stress of the soft ground due to the installation of soil–cement columns. The effect of various group foundation parameters, such as area ratio, length and diameter of columns and binder content are investigated. The failure mode of end-bearing and floating columns after exhumation are presented. For the same area ratio, of the improved soil–cement columns to the present soft clay deposit, the usage of the smaller sized soil–cement columns rather than large size soil–cement columns is found to be relatively more beneficial in case of both end-bearing and floating columns. The stiffness and failure stress of the composite ground increase with an increase in the length of the columns. However, the increase in strength is only marginal for the increase in column length beyond 10 times the column diameter. In the case of end-bearing columns, area ratio has a significant effect on failure pattern. At an area ratio of 25%, the column failed by outward displacement and bending. On the other hand, at an area ratio of 32%, the columns failed due to bending at approximately one-half to two-third of the column length from the base of the footing. In the case of floating column, the relative strength of columns to the soil $$( {c_{\text{uc}}}/{c_{\text{us}}})$$ appears to be the major governing factor. For the high value of $$( {c_{\text{uc}}}/{c_{\text{us}}})$$ used in the present study, punching failure was observed with slight outward displacement and some horizontal cracks for area ratios of 25% and 32%. For validation, the failure stresses of model ground were compared with the results obtained using numerical analysis. The results show a good agreement between the bearing capacity value obtained from the experiment and the numerical software.

The engineering properties of soil can be enhanced through artificial cementation which is realized by premixing or injecting chemical agents such as cement, lime and gypsum in soil. Biocementation of soil using biogenic cements has also been developed in recent years. In this thesis, the engineering behaviour of sand cemented using either cement or biocement was investigated. Cement-treated sand was prepared by mixing uncemented sand with Portland cement, while biocement-treated sand was made using the low-pH-one-phase-injection method via the microbially induced calcite precipitation (MICP) process. The differences and similarities in the cementation effects of the two different cementation materials and cementation processes were analysed. Drained and undrained shear behaviour of cemented sand were studied under both axisymmetric and plane-strain conditions using either triaxial or plane-strain apparatus. Under a drained axisymmetric condition, the stress-strain behaviour of cemented sand becomes brittle, and its volumetric deformation becomes dilative as compared with the ductile and compressive behaviour of the loose parent sand under the same conditions. The cementation effect was also affected by the effective confining stress. The lower the stress level, the more significant the cementation effect. Compared with cement-treated sand, biocement-treated sand with similar biocement content is more brittle with higher tangent modulus, but less dilation in general. The cementation effect and its mobilization could be analysed using an energy equation. The effect of cementation due to cement or biocement was dominated only at the early phase of shearing. During a drained triaxial test, the cementation effect was mobilized first while the dilation was restricted by bonding. With further shearing, bonding due to cementation began to break and the contribution of dilation increased. When dilation ceased to develop further, the shear strength would be controlled mainly by friction. For both cement or biocement, cementation contributed mainly to the increase in the effective cohesion, but not much to the effective friction angle. The bonding or yielding stress increases with the increase in cement/biocement content. The bonding provided by biocement is stronger than that by cement. Under drained plane strain conditions, the bonding provided by cementation could be degraded during K0 consolidation when the consolidation stress was higher than the yielding stress of the cemented soil. As a result, the stress-strain behaviour of cemented soil was affected by the consolidation state. When the mean effective consolidation stress was smaller than the mean yield stress, the shear stress versus shear strain curve of cemented sand could still be affected significantly by cementation as indicated by a steep peak in the stress-strain curve and the contractive volumetric strain. On the other hand, when the mean effective consolidation stress was greater than the mean yield stress, the stress-strain behaviour of cemented sand would be less affected by cementation. Similar trends were observed in undrained plane strain tests where the stress-strain curves and pore water pressure changes were also affected by the consolidation state. The behaviour of cemented sand in plane-strain conditions was different from that in axisymmetric conditions. The stress-strain curve in drained plane-strain was steeper and the volumetric strain was less dilative as compared with those in axisymmetric conditions. The effective stress path in plane-strain might not be a straight line due to the influence of σ′2. Despite the differences, the failure conditions of cemented sand under both axisymmetric and plane-strain conditions can be described by Lade’s 3D failure criterion considering tensile stress.

Ground improvement technique by prefabricated vertical drain (PVD) in combination with vacuum preloading is widely used to facilitate consolidation process and reduce residual settlement. However, this technique seem hardly be estimated by both analytical method and numerical method because it has complex boundary conditions (such as vacuum pressure changing with time). Moreover, lateral displacements caused by this technique are also significant problem. Numerical modelling may be an effective design tool to estimate behavior of soft soil treated by this method, however it needs to have a proper calibration of input parameters. This paper introduces a matching scheme for selection of soil/drain properties in analytical solution and numerical modelling (axisymmetric and plane strain conditions) of a ground improvement project by using Prefabricated Vertical Drains (PVD) in combination with vacuum and surcharge preloading. In-situ monitoring data from a case history of a road construction project in Vietnam was adopted in the back-analysis. Analytical solution and axisymmetric analysis can approximate well the field data meanwhile the horizontal permeability need to be adjusted in plane strain scenario to achieve good agreement. In addition, the influence zone of the ground treatment was examined. The residual settlement was investigated to justify the long-term settlement in compliance with the design code. Moreover, the degree of consolidation of non-PVD sub-layers was also studied by means of two different approaches.

In this paper, an in-situ experiment under lateral static, cyclic, and dynamic loadings, on a large sized steel pipe pile in soft silty clay on the downstream of the Yangtze River and static and dynamic triaxial tests on soil samples in the laboratory are described, and the pile-soil interaction behavior is analyzed and discussed. In the analysis, the finite-element method and other simplified methods are compared using different models of lateral soil stiffness. From the results of the tests as well as the analysis, a possible formulation of the p-y curves of the pile in soft clay under lateral loadings is suggested, and a simplified method of dynamic analysis taking into account the pile-soil interaction is also proposed.

In this paper, the pile-soil interaction behavior of a large-sized steel pipe pile in soft silty clay along the downstream of the Yangtze River is analyzed and discussed through an in-situ experiment under lateral static, cyclic and dynamic loadings, along with the static and dynamic triaxial tests on soil samples in laboratory. In the analysis, the finite element method and other simplified methods are compared using different models of lateral soil stiffness. From the results of the tests as well as the analysis, a possible formulation of the p-y curves of the pile in soft clay under lateral loadings is suggested, and a simplified method of dynamic analysis taking into account of the pile-soil interaction is also proposed.

This paper presents the details and preliminary results of full-scale lateral load tests on driven piles in soft Bangkok clay. Steel pipe piles with a diameter of 0.25 m and a depth of 12.5 m were instrumented with strain gauges at various depths. Load tests were performed to large pile displacements. Based on the framework of elastic beam on foundation, the strain gauge outputs were employed to determine pile bending moments, deflections and soil reactions. These calculated pile deflections were validated by means of the results of inclinometer measurements during the course of testing. The experimental pile responses were compared to the numerical predictions of the computer program LPILE. It was found that the numerical model predicted unrealistically stiff load-deflection behavior. The reason was likely the use of Matlock \( p - y \) model for soft clay which significantly overestimated the experimental \( p - y \) relationships of Bangkok clay particularly near the ground surface.

This engineering educational module on the behavior of piles in soft clays during earthquake loading sought to introduce engineering as a viable career to eighth graders and teach students how geotechnical engineers design foundations in marginal soils to minimize damage to infrastructure during earthquakes. This module could also be used at various educational levels, from elementary to middle and high school, as well as at the undergraduate level, with appropriate modifications. A 5-h module was created to simulate the real-world behavior of piles in soft clays during earthquake loading and visually show the improvement in how these same piles behave after being stabilized with a deep-soil-mixing technique. In this module, soft soil was simulated using Jell-O, piles were simulated using Slim Jims, and soil stabilization was simulated using peanut butter, marshmallows, or cheese. Each student group had to design a stabilization procedure to improve the behavior of the piles. The students competed to see who could design a stabilized pile with the least amount of deflection for the least amount of money. An abbreviated module was also administered to a group of middle-school science and mathematics teachers. The students’ and teachers’ learning and perceptions were assessed by administering pre- and post-assessment questions, which were matched.

Presented in this paper are results of two centrifuge tests on single piles installed in unimproved and improved soft clay (a total of 14 piles), with the relative pile–soil stiffness values varying nearly two orders of magnitude, and subjected to cyclic lateral loading and seismic loading. This research was motivated by the need for better understanding of lateral load behavior of piles in soft clays that are improved using cement deep soil mixing (CDSM). Cyclic test results showed that improving the ground around a pile foundation using CDSM is an effective way to improve the lateral load behavior of that foundation. Depending on the extent of ground improvement, elastic lateral stiffness and ultimate resistance of a pile foundation in improved soil increased by 2–8 times and 4–5 times, respectively, from those of a pile in the unimproved soil. While maximum bending moments and shear forces within piles in unimproved soil occurred at larger depths, those in improved soil occurred at much shallower depths and within the improved zone. The seismic tests revealed that, in general, ground improvement around a pile is an effective method to reduce accelerations and dynamic lateral displacements during earthquakes, provided that the ground is improved at least to a size of 13D × 13D × 9D (length × width × depth), where D is the outside diameter of the pile, for the pile–soil systems tested in this study. The smallest ground improvement used in these tests (9D × 9D × 6D), however, proved ineffective in improving the seismic behavior of the piles. The ground improvement around a pile reduces the fundamental period of the pile–soil system, and therefore, the improved system may produce larger pile top accelerations and/or displacements than the unimproved system depending on the frequency content of the earthquake motion.

This paper investigated the installation effects of a driven prestressed high-strength concrete (PHC) nodular pile in deep soft clay through full-scale tests, and the behavior of a driven PHC nodular pile is analyzed based on the field test results. The test results demonstrated that the driven PHC nodular pile installation process induced significant disturbance in the surrounding soil. The excess pore water pressures (Δu) that were induced by the installation of a driven PHC nodular pile were larger than the Δu that were induced by the installation of a driven PHC pipe pile. The driven PHC nodular pile installation caused significant Δu in the soil down to 2 m below the pile base, and the Δu became much smaller in the soil at 4 m below the pile base. The Δu dissipated slowly in the soft clay layers, and the Δu in the surficial soil layers dissipated faster than that in the deep soil layers. The nodules along the PHC nodular pile shaft could increase the pile shaft capacity. The measured shaft resistances of 350(400)-mm PHC nodular piles were 1.17–1.18 times the calculated shaft resistances of 350-mm PHC pipe piles in soft clay layers.

In this article, the degradation of the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading is studied numerically. A modified kinematic hardening constitutive model is employed to simulate the degradation of soft clay after cyclic loading. The modified model is verified by comparing the numerical simulation results with the results of centrifuge model tests. Furthermore, the modified model is applied to numerical simulations for evaluating the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading. The degradation of the lateral bearing capacity of piles in soft clay after different cyclic displacement levels and different numbers of cycles is investigated. The study reveals that the modified kinematic hardening constitutive model can effectively estimate the cyclic degradation behavior of piles in soft clay subjected to cyclic lateral loading. The degradation of the ultimate lateral bearing capacity progresses slowly with increasing cyclic displacement level for fewer cycles, and the degradation develops significantly at higher levels of cyclic displacement after applying a larger number of cycles.