Generalized Model for Geosynthetic-Reinforced Granular Fill-Soft Soil with Stone Columns

This paper pertains to the development of a mechanical model to predict the behavior of a geosynthetic-reinforced granular fill over soft soil improved with stone columns. The saturated soft soil has been idealized by Kelvin–Voight model to represent its consolidation behavior. The stone columns are idealized by stiffer springs. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement layer, respectively. The nonlinear behavior of the granular fill and the soft soil is considered. Effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution, and results are presented in nondimensional form. Comparison between the results from the present study and the analytical solution using theory of elasticity shows reasonable agreement. The advantage of using geosynthetic reinforcement is highlighted. Results indicate that inclusion of the geosynthetic layer effectively reduces the settlement. Nonlinearity in the behavior of the soft soil and the granular fill is reduced due to the use of geosynthetic reinforcement layer.     

Studies on the Behavior of Single and Group of Geosynthetic Encased Stone Columns

The stone columns (or granular piles) are increasingly being used as ground reinforcement elements for supporting a wide variety of structures including buildings and flexible structures. The stone columns derive their load capacity from the confinement offered by the surrounding soil. In very soft soils this lateral confinement may not be adequate and the formation of the stone column itself may be doubtful. Wrapping the individual stone columns with suitable geosynthetic is one of the ideal forms of improving the performance of stone columns. This type of encasement by geosynthetic makes the stone columns stiffer and stronger. In addition, encasement prevents the lateral squeezing of stones in to the surrounding clay soil and vice versa, preserves drainage function of the stone column and frictional properties of the aggregates. In spite of many advantages, the behavior and the mechanism of the geosynthetic encased stone columns is not thoroughly understood. This paper investigates the qualitative and quantitative improvement of individual load capacity of stone column by encasement through laboratory model tests conducted on stone columns installed in clay bed prepared in controlled condition in a large scale testing tank. The load tests were performed on single as well as group of stone columns with and without encasement. Tests were performed with different geosynthetics for the encasement of stone column. The results from the load tests indicated a clear improvement in the load capacity of the stone column due to encasement. The increase in the axial load capacity depends very much upon the modulus of the encasement and the diameter of the stone column. The increase in the stress concentration on the stone columns due to encasement was also measured in the tests. The results from the tests were used to develop the design guidelines for the design of geosynthetic encasement for the given load and settlement.     

In Situ Pore Pressure Responses of Native Peat and Soil under Train Load: A Case Study

Settlement and formation of piping holes on surfaces were observed along a rail embankment subject to normal traffic load. Piezometers were installed in the native peat and soil underneath the embankment inside and outside problematic area to measure the pore pressure responses during train traffic. Peculiar pore pressure responses were observed. Cyclic pore pressures were only measured during the first 60–80?s of the 6-min train passage, and thereafter the pressures decayed rapidly to the initial values. The pore pressure changes in the shallow peat layer were lower than those in the deep soil layer. Possible mechanisms causing such peculiar pore pressure responses, surface settlement, and piping holes were explored and identified. It was found that the stiffness contrast between the stiff, upper granular fill and the soft, native peat material could lead to a redistribution of tensile stress in the granular fill layer to the peat layer due to the moving train load. This stress redistribution promotes the propensity of vertical piping in the peat layer.     

Two-Dimensional Physical and Numerical Modeling of a Pile-Supported Earth Platform over Soft Soil

This paper focuses on the mechanisms occurring in a granular earth platform over soft ground improved by rigid piles. Two-dimensional physical model experiments were performed using the Schneebeli’s analogical soil to investigate the load transfer mechanisms by arching and the settlement reduction and homogenization. Experimental outputs are compared to results obtained on a numerical model using a plane strain continuum approach. The impact of the constitutive model complexity to simulate the platform material behavior was first assessed, but no large difference was recorded. As far as the proposed model, which takes the main features of the observed behavior satisfactorily into account, the numerical procedure could be validated and the parametric studies extended numerically. Both approaches of this study underlined the main geometrical and geotechnical parameters which should inevitably be taken into account in a simplified design method, namely the capping ratio, the platform height, and the platform material shear strength.     

Load Transfer Curves along Bored Piles Considering Modulus Degradation

The load-transfer (or t-z) curve, which reflects the interaction between a pile and the surrounding soil, is important for evaluating the load-settlement response of a pile subjected to an axial load using the load-transfer method. Preferably, the nonlinear stress-strain behavior of the soil should be incorporated into the t-z curve. This paper presents a practical approach for the estimation of t-z curves along bored piles by considering the nonlinear elastic properties and modulus degradation characteristics of the soil. A method for evaluating the modulus degradation curve from the results of a pressuremeter test is proposed. The results of load tests on one instrumented bored pile in Piedmont residual soil in Atlanta and another in the residual soil of the Jurong Formation in Singapore provide verification of the validity of the proposed approach.     

Numerical Analysis of Geosynthetic-Reinforced and Pile-Supported Earth Platforms over Soft Soil

Geotechnical engineers face several challenges when designing structures over soft soils. These include potential bearing failure, intolerable settlement, large lateral pressures and movement, and global or local instability. Geosynthetic-reinforced and pile-supported earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. The inclusion of geosynthetic(s) in the fill enhances the efficiency of load transfer, minimizes yielding of the soil above the pile head, and potentially reduces total and differential settlements. A numerical study has been conducted to investigate pile-soil-geosynthetic(s) interactions by considering three major influence factors: the height of the fill, the tensile stiffness of geosynthetic, and the elastic modulus of pile material. While current methods have not fully addressed important effects of the geosynthetic stiffness and pile modulus on the soil arching ratio, numerical results suggested that the stress concentration ratio and the maximum tension in geosynthetic increase with the height of the embankment fill, the tensile stiffness of geosynthetic, and the elastic modulus of the pile material. The distribution of tension force in the geosynthetic reinforcement indicated that the maximum tension occurs near the edge of the pile.     

Performance of Geosynthetic-Encased Stone Columns in Embankment Construction: Numerical Investigation

This paper presents the results of a numerical investigation into the performance of geosynthetic-encased stone columns (GESCs) installed in soft ground for embankment construction. A three-dimensional finite-element model was employed to carry out a parametric study on a number of governing factors such as the consistency of soft ground, the geosynthetic encasement length and stiffness, the embankment fill height, and the area replacement ratio. The results indicate among other things that additional confinement provided by the geosynthetic encasement increases the stiffness of the stone column and reduces the degree of embankment load transferred to the soft ground, thereby decreasing the overall settlement. It is also shown that the geosynthetic encasement has a greater impact for cases with larger stone column spacing and/or weaker soil. Also revealed is that unlike isolated column loading conditions, full encasement may be necessary to ensure maximum settlement reduction when implementing GESCs under an embankment loading condition. Practical implications of the findings are discussed in detail.     

Load Transfer in Rammed Aggregate Piers

Predicting the load–settlement and load–transfer behaviors of rammed aggregate piers are important aspects of design. Use of advanced engineering models, however, can be complex involving uncertainty in selection of nonlinear constitutive model parameters for the aggregate and surrounding matrix soils and in selection of in situ stress fields. For purposes of simpler design calculations, this paper uses the closed-form approximate solution and the boundary-element method using both elastic (i.e., neglecting interface slip) and elastic–plastic soil–pier interface (i.e., considering interface slip) to predict load–settlement and load–transfer for rammed aggregate piers. Unlike previous studies that evaluate load–settlement and load–transfer for stiff, slender piles (e.g., concrete and steel piles) or fully penetrating granular piles, this paper focuses on floating rammed aggregate piers having slenderness ratios (L/D) of 3–10 and pier–soil stiffness ratios (Ep/Es) of 5–80. Predictions of load–settlement and load–transfer as a function of depth are compared to three full-scale instrumented load tests. Based on the calibrated models, equations for predicting load–settlement response and load–transfer as functions of Ep/Es, and L/D are presented with example calculations.     

Shear Load Transfer Characteristics of Drilled Shafts in Weathered Rocks

The load distribution and deformation of drilled shafts subjected to axial loads were evaluated by a load-transfer approach. The emphasis was on quantifying the load-transfer mechanism at the interface between the shafts and surrounding highly weathered rocks based on a numerical analysis and on small-scale tension load tests performed on nine instrumented piles. An analytical method that takes into account the soil coupling effect was developed using a modified Mindlin's point load solution. Based on the analysis, a single-modified hyperbolic model is proposed for the shear transfer function of drilled shafts in highly weathered rocks. Through comparisons with field case studies, it is found that the prediction by the present approach is in good agreement with the general trend observed by in situ measurements, and thus represents a significant improvement in the prediction of load deflections of drilled shafts.     

Performance of Long-Driven H-Piles in Granitic Saprolite

There has been much advancement using conceptual models and analytical methods to explain various aspects of pile performance. They are mainly based on the findings of model tests and full-scale pile tests in fine-grained and coarse-grained soils, and driven piles on land are normally less than 40?m. Design methods developed from this data bank of pile geometries and soil conditions for long piles should be treated with caution. In this paper, 13 H-piles of 34–60?m and 7,096?kN capacity founded on granitic saprolite are studied. Among them, two piles were restriked at different time intervals. All piles were axially load tested statically using a maintained load method. In contrast to the short rigid piles founded on weaker soil, their load-transfer mechanism varied with the magnitude of applied load and pile length. They deformed almost linearly at small loads and might have buckled when the loads were large and the creep settlements were found to be length dependent. Existing criteria might not be able to interpret failure loads sometimes, but a pile dynamic analyzer was found to give the best estimate on pile capacity.     

Kinematic Pile Response to Vertical P-wave Seismic Excitation

An analytical solution based on a rod-on-dynamic-Winkler-foundation model is developed for the response of piles in a soil layer subjected to vertical seismic excitation consisting of harmonic compressional waves. Closed-form solutions are derived for: (1) the motion of the pile head; (2) the peak normal strain in the pile, and (3) the group effect between neighboring piles. The solutions are expressed in terms of a dimensionless kinematic response factor Iv, relating pile-head motion and free-field soil surface motion, a dimensionless strain transmissibility factor Iε, relating pile and soil peak normal strains, and a pile-to-pile interaction factor α measuring group effects. It is shown that a pile foundation may significantly reduce the vertical seismic excitation transmitted to the base of a structure.     

Experimental Study of Eccentrically Loaded Raft with Connected and Unconnected Short Piles

Piled raft foundations are often used when the supporting soil has adequate bearing capacity but the raft settlements exceed allowable values. In traditional practice, long piles with high load capacity are usually used that may lead to two structural problems: the structural collapse of the pile and large strains mobilized in the raft leading to an uneconomic design. This paper presents an experimental study of the effectiveness of using short piles either connected or unconnected to the raft (instead of long piles) on the behavior of an eccentrically loaded raft. The load configuration was designed to simulate rafts under vertical loads and overturning moment. Several arrangements of piles with different lengths and numbers along with the effect of the relative density of the soil and the load eccentricity were studied. Test results indicate that the inclusion of short piles adjacent to the raft edges not only significantly improves the raft bearing pressures but also leads to a reduction in raft settlements and tilts leading to an economical design of the raft. However, the efficiency of the short piles-raft system is dependent on the load eccentricity ratio and pile arrangement. Also, connecting short piles to the raft gives greater improvement in the raft behavior than unconnected piles. Based on test results, the effects of different parameters are presented and discussed.     

Wedge Failure Analysis of Soil Resistance on Laterally Loaded Piles in Clay

A fundamental study of pile-soil systems subjected to lateral loads in clay soil was conducted by using experimental tests and a lateral load-transfer approach. The emphasis was on an improved wedge failure model developed by considering three-dimensional combination forces and a new hyperbolic p-y criterion. A framework for determining the p-y curve on the basis of both theoretical analysis and experimental load test results is proposed. The proposed p-y method is shown to be capable of predicting the behavior of a large-diameter pile under lateral loading. The proposed p-y curves with an improved wedge model are more appropriate and realistic for representing a pile-soil interaction for laterally loaded piles in clay than the existing p-y method.     

Centrifuge Modeling of Large-Diameter Bored Pile Groups with Defects

In this research, centrifuge model pile-load tests were carried out to failure to investigate the behavior of large-diameter bored pile groups with defects. The model piles represented cast-in-place concrete piles 2.0?m in diameter and 15?m in length. Two series of static loading tests were performed. The first series of tests simulated the performance of a pile founded on rock and a pile with a soft toe. The second series of tests simulated the performance of three 2×2 pile groups: One reference group without defects, one group containing soft toes, and one group with two shorter piles not founded on rock. The presence of soft toes and shorter piles in the defective pile groups considerably reduced the pile group stiffness and capacity. As the defective piles were less stiff than the piles without defects, the settlements of the individual piles in the two defective pile groups were different. As a result, the applied load was largely shared by the piles without defects, and the defective pile groups tilted significantly. The rotation of the defective pile groups caused large bending moments to develop in the group piles and the pile caps. When the applied load was large, bending failure mechanisms were induced even though the applied load was vertical and concentric. The test results confirm findings from numerical analyses in the literature.     

Performance of a Geogrid-Reinforced and Pile-Supported Highway Embankment over Soft Clay: Case Study

This paper describes a case history of a geogrid-reinforced and pile-supported (GRPS) highway embankment with a low area improvement ratio of 8.7%. Field monitored data from contact pressures acting on the pile and soil surfaces, pore-water pressures, settlements and lateral displacements are reported and discussed. The case history is backanalyzed by carrying out three-dimensional (3D) fully coupled finite-element analysis. The measured and computed results are compared and discussed. Based on the field observations of contact stresses and pore-water pressures and the numerical simulations of the embankment construction, it is clear that there was a significant load transfer from the soil to the piles due to soil arching. The measured contact pressure acting on the pile was about 14 times higher than that acting on the soil located between the piles. This transfer greatly reduced excess positive pore water pressures induced in the soft silty clay. The measured excess pore water pressure ratio max in the soft silty clay was only about 0.3. For embankment higher than 2.5?m, predictions of stress reduction ratio based on two common existing design methods are consistent with the measured values and the 3D numerical simulations. During the construction of the piled embankment, the measured lateral displacement–settlement ratio was only about 0.2. This suggests that the use of GRPS system can reduce lateral displacements and enhance the stability of an embankment significantly.     

Behavior of Driven Ultrahigh-Performance Concrete H-Piles Subjected to Vertical and Lateral Loadings

In the United States, an estimated $1 billion is spent annually on repair and replacement of deep foundations. In a recent study, the possibility of using ultrahigh-performance concrete (UHPC) for deep foundation applications was explored with the objectives of increasing the service life of deep foundations supporting bridges to 75 years and reducing maintenance costs. This paper focuses on field evaluation of two UHPC piles and references a steel H-pile. An UHPC pile with an H shape was designed to simplify the process of casting the pile and reduce the volume (i.e., cost) of the material needed to cast the pile. Two instrumented UHPC piles were driven in loess on top of a glacial till clay soil and load tested under vertical and lateral loads. This paper provides a complete set of results for the field investigation conducted on UHPC H-shaped piles. The results presented in this paper prove that the designed UHPC piles can be driven using the same equipment used to drive steel H-piles through hard soil layers without a pile cushion. The vertical load capacity of the UHPC pile was over 80% higher than that of the steel H-piles.     

Long-Term Reinforcement Load of Geosynthetic-Reinforced Soil Retaining Walls

As increasing number of geosynthetic-reinforced soil (GRS) retaining walls are built for permanent purpose, and their long-term behaviors have become one of the most critical issues in design. However, there has been very limited study on long-term reinforcement load and its relation to various parameters of GRS walls. A finite-element procedure for the long-term response of geosynthetic-reinforced soil structures with granular backfills was first validated against the long-term model test. Extensive finite-element analyses considering the viscous properties of geosynthetic reinforcements were then carried out to investigate the load distributions in geosynthetic reinforcements of GRS walls under operational condition. Construction sequence was simulated and a creep analysis of 10?years was subsequently conducted on each model wall. The effects of wall parameters, including backfill soil, reinforcement length, reinforcement spacing, reinforcement stiffness, and creep rate of reinforcement were investigated. It is found from the analyses that: (1) the maximum reinforcement load of GRS walls under working stress condition was generally smaller than that estimated using the FHwA design but it is dependent on the global reinforcement stiffness Sglobal; (2) the surface of maximum reinforcement load did not coincide with the Rankine’s surface suggested by FHwA design guidelines for vertical GRS walls and it was affected by the strength of backfill soil, reinforcement length, reinforcement spacing, and reinforcement stiffness; (3) for GRS walls under operational condition, reinforcement loads were closely related to the mobilized stiffness of backfill soil; (4) isochrone curves can be used to interpret the effects of reinforcement stiffness and creep rate on both short-term and long-term performances of GRS walls under operational condition, and with an increase in the reinforcement stiffness, the maximum reinforcement load increased; and (5) the global reinforcement stiffness Sglobal, which is related to the isochrones stiffness of reinforcement as well as reinforcement spacing was related to the total reinforcement load Ttotalmax and with an increase in the global stiffness, the total reinforcement load increased.     

Behavior of Model Rafts Resting on Pile-Reinforced Sand

Piles in a pile raft are sometimes considered as settlement reducers, not load-carrying members. In design, one often tries to minimize the number of piles. This often results in a high axial stress in the piles that may deter their use due to the limits on pile stress in practice. An alternative is to consider the pile as reinforcement in the base soil, and not as a structural member. Serving as a soil stiffener, the pile can tolerate a lower safety margin against structural failure without violating building codes. Previous numerical studies on the use of disconnected piles as settlement reducers have shown the effectiveness of such piles. This study aims to verify experimentally the effectiveness of such piles through load tests of model rafts resting on pile-reinforced sand. By varying factors such as raft stiffness, pile length, pile arrangement, and pile number, results of the investigation indicate that structurally disconnected piles are effective in reducing the settlement and bending moments in the model rafts.     

Practical Method for Settlement Analysis of Pile Groups

This paper presents a practical method that can be used to evaluate the settlement of pile groups. This method is developed based on variational solutions for pile groups embedded in soil modeled using a load-transfer curve model and elastic half-space model. Using this practical method, the settlement of pile groups can be estimated quickly without recourse to complex computer programs based on numerical models, resulting in savings in time and cost. The solutions obtained using this approach are in good agreement with rigorous variational solutions and are also in reasonable agreement with field measurements.     

Analytical Method for Load-Transfer Characteristics of Rock-Socketed Drilled Shafts

The load-settlement behavior of rock-socketed drilled shafts under axial loading is investigated by a load-transfer approach. Special attention is given to the shear load-transfer function and an analytical method for estimating load-transfer characteristics of rock-socketed drilled shafts. A nonlinear triple curve is employed to determine the shear load-transfer function of rock-socketed drilled shafts based on the constant normal stiffness direct shear tests and the Hoek-Brown failure criterion. An analytical method that takes into account the soil coupling effect was developed using a modified Mindlin’s point load solution. Through comparisons with field case studies, it is found that the proposed methodology in the present study is in good agreement with the general trend observed by in situ measurements and, thus, represents a significant improvement in the prediction of drilled shaft shear behavior.