Aggregate Piers Research Articles

Estimation of Resistance Bias Factors for the Ultimate Limit State of Aggregate Pier Reinforced Soil

Estimation of Resistance Bias Factors for the Ultimate Limit State of Aggregate Pier Reinforced Soil

Monitoring Ground Improvement Using the Seismic Dilatometer in Christchurch, New Zealand

This article illustrates the use of the seismic dilatometer test (SDMT) in combination with the piezocone test (CPTu) to monitor the effects of ground improvement by different techniques at seven test sites in Christchurch, New Zealand. The results of SDMTs carried out in treated soils and in adjacent natural soils are compared in order to assess the effectiveness of different ground improvement techniques (Rammed Aggregate Piers, Low Mobility Grout, Stone Columns, Deep Soil Mixing) aimed at reducing the liquefaction susceptibility. Moreover, the article presents the variation of significant parameters obtained from parallel SDMT and CPTu soundings in treated versus natural soils. SDMT and CPTu results are also used in combination to estimate the overconsolidation ratio and the at-rest lateral earth pressure coefficient (K0) before and after the treatment. The effectiveness of the different ground improvement techniques is also evaluated in terms of the variation of the before and after integral liquefaction vulnerability indicators, such as the liquefaction potential index and the liquefaction severity number, which are calculated using different methods based on CPTu, the DMT horizontal stress index (KD), and the shear wave velocity (VS) for a design earthquake. The results confirm that the DMT, as well as the CPTu, is sensitive to changes in stress/density in sands and silty sands and is, therefore, well suited to detecting improvements in these soils, while VS provides less evident results.

3D FE Analysis on Settlement of Footing Supported with Rammed Aggregate Pier Group

3D FE Analysis on Settlement of Footing Supported with Rammed Aggregate Pier Group

Performance monitoring of rammed aggregate piers: a stockyard case history site

AbstractThis paper investigates the design aspects of Rammed Aggregate Pier® (RAP) treatment as a composite treatment for the unfavourable ground conditions, the construction, QA/QC measures and the verification of the performance via a comprehensive instrumentation scheme. For this purpose, instrumented full scale load test was performed for the stockyard project site in Izmir, Turkey. The soil profile is comprised of fill and soft sandy clay layers overlain by the pyroclastic bedrock. 6 to 18 m long RAP elements were installed in a square grid spaced between 1.5 m and 2.0 m to improve bearing capacity and settlement responses, to accelerate the time rate of settlement (i.e.: consolidation) and to mitigate the liquefaction potential at the project site. The foundation settlement was estimated by RocScience‐Settle 3D software, which was observed to be 30 % smaller than the measured values. After the ground improvement with RAP elements, the foundation settlements were measured as less than 10 cm as compared to 40 ‐ 60 cm settlement predictions of the untreated system.

Rammed aggregate pier installation effect on soil properties

This paper describes the changes in in situ index properties following installation of rammed aggregate piers in a loose sand deposit in New Madrid, Missouri, with a particular focus on the time-dependent strength gain, commonly called sand ageing, as indicated by increased cone penetration test (CPT) tip resistance and dilatometer test (DMT) indices. The method of disturbing the soil and the magnitude of disturbance significantly influence the magnitude of sand-ageing effects. This paper provides the first data on sand ageing following the installation of displacement rammed aggregate piers. In this study, CPT and DMT were performed both immediately before and after the installation of piers. Additionally, CPTs were performed 1 month following the installation of piers. The DMT horizontal stress index increased roughly 100% and the dilatometer modulus increased ∼200% in a loose clean sand layer immediately following the installation of the piers. The CPT tip resistance increased roughly 100% in this layer immediately following the pier installation and a further 33% in the month following the installation.

Experimental and Numerical Investigations of Behavior of Rammed Aggregate Piers

Abstract This study presents the results from a series of laboratory tests carried out on rammed aggregate piers (RAPs) installed in different clayey sand beds formed in a large scale model test tank. The clayey sand beds were prepared by mixing 60 % sand, 40 % clay at a water content of 1.5 times the liquid limit of the clayey sand. The slurry mixtures were allowed to consolidate one-dimensionally until they reach different undrained strength values (cu = 2.5, 3, and 10 kPa). The diameter and length of RAPs were in the order of 10 cm and 60 cm, respectively for all the model tests carried out. The rammed aggregate piers were subjected to vertical loading at the top through a loading plate which was displaced by jacking of the installation system. Soil pressure cells were installed at different depths of the clayey sand bed to evaluate stress changes in them. Behavior of the single and group rammed aggregate pier and the surrounding soil were investigated by evaluating the measurements obtained from load tests and pressure cells. As a result of the laboratory tests, noteworthy results were obtained regarding the working mechanism of the RAPs in soils having different soil strengths, stress increases in the surrounding soil, and load sharing between the RAPs and the surrounding soil. A model test was also analyzed numerically and the results of laboratory measurements were compared with the numerical simulations. The results of the numerical analysis yielded reasonably close agreement with the experimental result.

Reliability-based design of spread footings on clays reinforced with aggregate piers

ABSTRACTThe economical and safe design of footings supported on aggregate-pier-reinforced clay could benefit from the implementation of a reliability-based approach that incorporates the different sources of uncertainty. Monte Carlo simulations are conducted to quantify the probability distribution of the ultimate bearing capacity for practical design scenarios. A reliability analysis is then conducted to propose design charts that yield the required factor of safety as a function of the major input parameters. The novelty in the proposed methodology is the incorporation of a lower bound shear strength that is based on the remoulded undrained shear strength in the reliability analysis.

A new approach for studying the load–displacement characteristics of rammed aggregate piers

This paper investigates the load–displacement characteristics of rammed aggregate piers (RAPs) by applying the innovative idea of simulating the ramming procedure of the aggregates as a fully dynamic process. High radial stresses, induced by ramming, play an important role in the load–settlement behaviour of RAPs. Previously, two simplifying assumptions have been used for considering the effects of ramming in the numerical simulations of RAPs by researchers: (a) prescribed displacement of the cavity wall, and (b) prescribed volumetric strain of the cavity. The dynamic method in this paper, which simulates the cavity expansion in separate lifts, offers a useful option for the analysis of the cases in which the exact amount of the increase of the cavity diameter is not known earlier such as for RAP design purposes or evaluation of a completed project installed on RAPs. A parametric study is conducted using this method to investigate the effects of various parameters such as embedment depth, decreased compaction energy and soil friction angle on the behaviour of RAPs. A simple design method is then provided along with the design charts based on the results of the parametric study.

Reliability-based ultimate limit state design of spread footings on aggregate-pier-reinforced clay

This paper summarises the evaluation of previously existing and recently developed bearing capacity estimation methods for spread footings resting on aggregate pier reinforced clay. A load test database of full-scale footings resting on stone columns was used to generate an accurate ultimate limit state design model for bearing capacity and characterise the associated model uncertainty. Different levels of variability in area replacement ratio, slenderness ratio, length of pier, depth of footing embedment, and undrained shear strength, were incorporated in the resistance factor calibration in order to account for possible uncertainty in the load test database and future construction. Resistance factors were calibrated as a function of the ratio of dead to live load, reliability index, and total variability in the nominal bearing capacity. The operational factor of safety was computed for comparison to allowable stress design procedures and indicated that typical factors of safety associated with current practices can continue to be used for soil deposits with moderate variability.

쇄석다짐말뚝의 극한지지력 데이터베이스 구축 및 통계학적 분석

쇄석다짐말뚝의 극한지지력 데이터베이스 구축 및 통계학적 분석

Numerical Modeling of the RAP Construction Process and Its Effects on RAP Behavior

Although rammed aggregate piers (RAPs) have attracted much attention as a cost-beneficial ground improvement technique, their construction process and the other affecting parameters on the behavior of these geotechnical structures during the construction and loading phases have not been properly investigated and only some predictions for downward and radial expansion resulting from the construction process have been proposed to be used in numerical simulations. The goal of this paper is to numerically simulate the RAP construction process to investigate the effect of construction sequences on RAP behavior. Keeping this aim in mind, a hybrid distinct element-finite difference algorithm is developed to simulate the placing and ramming of the RAP’s clean crushed stone layers in detail. Introducing several numerical models, a parametric study was then conducted to compare the behavior of RAPs during the construction process for different pier geometries and soil properties. The results of this research not only help to understand the RAP behavior during the ramming process but can also be used as a guide for predicting the amount of downward and lateral expansion of RAPs after the ramming process.

Reliability-Based Serviceability Limit State Design of Spread Footings on Aggregate Pier Reinforced Clay

Despite the availability of numerous methods to predict the load-displacement response of aggregate pier reinforced clay, accurately modeling the nonlinear displacement response remains challenging. Moreover, the uncertainty in the bearing pressure-displacement prediction has not been satisfactorily estimated, preventing the generation of reliability-based design (RBD) procedures. This study proposes a simple RBD procedure for assessing the allowable bearing pressure for aggregate pier reinforced clay in consideration of the desired serviceability limit state. A recently established bearing-capacity model for aggregate pier reinforced clay and its uncertainty is incorporated into a bivariate bearing pressure-displacement model appropriate for a wide range in displacement and calibrated using a high-quality full-scale load-test database. Several copulas were then evaluated for goodness of fit to the measured dependence structure between the coefficients of the selected two-parameter bearing pressure-displacement model. Following the generation of an appropriate performance function, a combined load and resistance factor is calibrated in consideration of the uncertainty in the bearing pressure-displacement model, bearing capacity, applied bearing pressure, allowable displacement, and footing width using Monte Carlo simulations of the respective source distributions. An example is provided to illustrate the application of the proposed procedure to estimate the bearing pressure for an allowable displacement at the desired serviceability limit probability.

Displacement of Spread Footings on Aggregate Pier Reinforced Clay

AbstractAggregate piers, also known as stone columns, are commonly used to improve ground for the support of structure foundations. Despite the number of existing analytical and semiempirical metho...

Numerical analysis of vertically loaded rammed aggregate piers and pier groups

Predicting the effects of piers in settlement control and capacity enhancement of foundations supported by rammed aggregate piers (RAPs) is an important aspect in design of these foundations. To quantify these effects, employing the finite-element approach, a series of three-dimensional elasto-plastic numerical models of RAP foundations were developed. To take into account the influence of radial stress development in the matrix soil during the ramming process, which forces aggregate downwards and outwards into the borehole wall, the cavity was expanded both in radial direction and downward at the base of the pier. Using a parametric study, the effects of some parameters such as piers slenderness, the ratio of piers area to the foundation area and soil properties on static behaviour of such a foundation system were investigated. Using the results of the parametric study conducted on single piers, the research aimed also to provide new insights into the group behaviour of piers when they are subjected to vertical loads. The results demonstrated that the dominant deformation of piers in group is a tendency to tip-stress behaviour. Based on the results provided in this study, which were focused on the load–settlement behaviour of RAPs-supported foundations, a settlement-based design procedure was presented for RAP systems in which the values of slenderness and the area ratios are determined.

Bearing Capacity of Spread Footings on Aggregate Pier Reinforced Clay

AbstractAggregate piers, also known as stone columns, are a commonly used ground improvement technique to stiffen existing soils for the support of structure foundations. This paper presents an evaluation of existing analytical expressions for the bearing capacity of spread footings supported on aggregate pier reinforced clay. The accuracy of these models was investigated using a database of high-quality footing load test data. The existing models were compared using the bias (i.e., the ratio of measured and calculated bearing capacity), and they produced a wide range in predicted bearing capacities. Selected analytical models were empirically modified using the load test database, and this resulted in improved accuracy and reduced variability. Back-calculated aggregate pier bearing capacity and cavity expansion factors are shown to be inversely proportional to undrained shear strength, and therefore to the ultimate confining pressure available at failure. This finding is attributed to the curved failure ...

Numerical analysis of vertically loaded rammed aggregate piers and pier groups under dynamic loading

Rammed aggregate piers (RAPs) systems have recently received much attention as a cost beneficial technique for ground improvement. Most of the research efforts have focused on the static behavior of piers either in the form of experimental investigations or using numerical techniques. However, their dynamic response has received less attention, despite the fact that there is a tendency to use such systems for machine foundations. Predicting the response of RAP-supported foundations subjected to vertical dynamic loading is then of particular significance in design of structures and machines supported by such a foundation system. To numerically evaluate dynamic response of RAP-supported foundations, a series of 3D finite element models of isolated piers and pier groups were developed taking into account the installation effect and nonlinear behavior of both aggregates and the surrounding soil matrix. The dynamic response of RAP-supported foundations was investigated for different conditions of geometry and soil matrix properties. The paper newly presents the vertical dynamic impedances of isolated piers and pier groups providing new insights into the dynamic applications of aggregate pier systems. The results indicated the capability of rammed aggregate piers technique in reinforcing foundations under dynamic vertical loading.

쇄석다짐말뚝의 액상화 저감효과에 관한 연구

쇄석다짐말뚝(Rammed Aggregate Piers, RAP)은 시공 시 타격에 의한 반복다짐으로 횡방향 응력을 증가시켜 주변지반의 전단저항력을 증가시키고 배수를 용이하게 함으로써 지진발생시 발생되는 간극수압의 소산을 촉진시켜 액상화 가능성을 조기에 저감시킬 수 있는 공법이다. 따라서 본 연구에서는 느슨한 포화 사질토 지반에 쇄석다짐말뚝을 조성하여 진동대 시험(Shaking Table Test)을 통해 액상화 저감효과를 고찰하였다. 이 연구를 수행하기 위해서 지반가속도와 치환율 변화에 따른 쇄석다짐말뚝으로 보강되지 않은 미처리 지반과 쇄석다짐말뚝 보강지반의 침하특성 및 과잉간극수압의 저감효과를 분석하였다. 치환율 증가에 따른 지반의 침하는 크게 감소하는 것으로 나타나 지반가속도 0.1 g인 경우 10배, 0.2 g인 경우 2.5배, 0.4 g인 경우 1.5배 이상 액상화 저감효과가 있음을 알 수 있었다. The Rammed Aggregate Piers(RAP) is the method to reduce the possibility of liquefaction by accelerating drainage and increasing shear resistance, which promotes dissipation of pore pressure occurred by vibration. Thus, this study investigates the reduction effect of liquefaction performing Shaking Table Test on soft ground. The reduction effect of excess pore pressure and characteristics of settlement of untreated and reinforced ground conditions were studied by varying ground acceleration and area replacement ratio. In results, the reduction effect of liquefaction is increased 10, 2.5, and 1.5 times from ground acceleration 0.1, 0.2, and 0.4(g) respectively.

Analysis of Footing Load Tests on Aggregate Pier Reinforced Clay

AbstractDespite the increased use of aggregate piers for soil reinforcement, the role of typical construction variables on footing performance remains uncertain. This paper describes a series of full-scale footing load tests conducted to evaluate the effect of aggregate gradation, pier length, and compaction method. Tests were conducted on small (0.76 m) and large (2.74 m) instrumented spread footings supported on single aggregate piers and groups of aggregate piers, respectively. The bearing pressure-displacement response of these tests is presented and the statistical significance of the construction variables is quantified for single piers using a 23 factorial analysis within an ANOVA framework. The statistical significance of the effect of pier length, gradation, and compaction was determined by controlling for the spatial variability in matrix soil strength and stiffness across the test site, which indicated that these variables were largely insignificant at the treatment levels evaluated. Additional...

A field study of the behavior of small-scale single rammed aggregate piers, testing methodology and interpretation

This paper presents the measured behavior of small-scale single rammed aggregate piers as a function of the piers’ slenderness ratio. For this purpose, loading tests at the site were carried out on two groups of single rammed aggregate piers with a constant diameter of 135 mm and variable lengths of 350–1000 mm, and two groups with a constant length of 1000 mm and various diameters of 105–185 mm. The testing area consisted of relatively uniform saturated soft alluvial clay overlain by a 1-m-thick wet soft-to-stiff silt layer. Results show that when length and diameter change, the pier load and top settlement variations at the design limit, in terms of the slenderness ratio, are not in the same direction while other design limit parameters’ variations are. The variations of design limit parameters in the two modes of change to pier length and diameter including the applied load, top settlement, stiffness modulus and pier modulus, in terms of the slenderness ratio, make a linear function while the variations of load and settlement ratio show an exponential function. Interpretations of the test results are particularly focused on the load-settlement behavior and variations of design limit parameters as a function of pier slenderness ratios.

Discussion of “Performance Monitoring of a Rammed Aggregate Pier Foundation Supporting a Mechanically Stabilized Earth Wall” by Mark J. Thompson, Kord J. Wissmann, and Ha T. V. Pham

Discussion of “Performance Monitoring of a Rammed Aggregate Pier Foundation Supporting a Mechanically Stabilized Earth Wall” by Mark J. Thompson, Kord J. Wissmann, and Ha T. V. Pham