Aggregate Piers Research Articles

Performance Monitoring of a Rammed Aggregate Pier Foundation Supporting a Mechanically Stabilized Earth Wall

The use of rammed aggregate pier (RAP) foundations for support of retaining walls and earth fill embankments has increased in recent years to become a geotechnical solution for rapid construction of earth structures in soft ground conditions. A nominal 6-m mechanically stabilized earth wall was constructed over piers installed in relatively compressible soil to investigate the performance of RAP foundation elements in terms of stress-deformation and settlement behaviors for such applications. Geotechnical instrumentation consisting of total earth pressure cells, settlement plates, and vibrating wire piezometers was installed within the pier elements and at the foundation surface for short- and long-term monitoring of pier response. Monitoring data indicate: (1) mobilization and concentration of vertical stress on pier elements and matrix soil; and (2) load transfer response for the boundary condition associated with support of geogrid-reinforced earth fill. The practical implications of the experimental research findings are briefly discussed.

Behavior of single rammed aggregate piers considering installation effects

A three-dimensional numerical method was used to simulate cavity expansion during installation and an in situ load test conducted on a single rammed aggregate pier (RAP). Verification of the numerical model was carried out by comparing the computed settlements at top and tip of the pier with the measured values. An un-rammed aggregate pier (UAP) of the same diameter and length was also simulated using the numerical method to demonstrate the effect of installation. Interpretations of the numerical results were focused on the load–settlement behavior, the bulging of the pier, the axial load distributions along the pier, and the stress paths of the soil around the pier. A parametric study was conducted to investigate the influence of the pier length/diameter ratio and the soil properties on the behavior of rammed and un-rammed aggregate piers.

Discussion of “Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses” by Ha T. V. Pham and David J. White

Discussion of “Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses” by Ha T. V. Pham and David J. White

Discussion of “Support Mechanisms of Rammed Aggregate Piers. I: Experimental Results” by David J. White, Ha T. V. Pham, and Kenneth K. Hoevelkamp

Discussion of “Support Mechanisms of Rammed Aggregate Piers. I: Experimental Results” by David J. White, Ha T. V. Pham, and Kenneth K. Hoevelkamp

Closure to “Load Transfer in Rammed Aggregate Piers” by Muhannad T. Suleiman and David J. White

Closure to “Load Transfer in Rammed Aggregate Piers” by Muhannad T. Suleiman and David J. White

Discussion of “Load Transfer in Rammed Aggregate Piers” by Muhannad T. Suleiman and David J. White

Discussion of “Load Transfer in Rammed Aggregate Piers” by Muhannad T. Suleiman and David J. White

Support Mechanisms of Rammed Aggregate Piers. I: Experimental Results

This paper is the first of a two-part series investigating the mechanical behavior of rammed aggregate pier (RAP) groups supporting isolated rigid footings. The first paper presents the experimental test results from instrumented load tests performed on two different 2.3 m square reinforced concrete footings supported by four 0.76 m diameter RAPs of two different pier lengths—2.8 and 5.1 m . Comparisons are made to load tests performed on three isolated RAPs of the same diameter and lengths. Instrumentation consisted of total stress cells, inclinometers, and tell-tale reference plates. Soil conditions at the test site were evaluated using various in situ testing techniques and consist of relatively uniform soft alluvial clay overlain by a 1-m-thick desiccated layer. Interpretations of the test results focused on load-deformation behaviors of the isolated piers and pier groups, group efficiencies in terms of settlement and bearing capacity, stress concentrations as a function of applied load at the top of ...

Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses

This paper is the second of a two-part series describing an investigation of the mechanical behavior of rammed aggregate piers in supporting rigid square footings. In this paper, the performances of two pier-supported footings and three isolated piers during compressive load tests were simulated using an axisymmetric finite element model and compared to experimental data. A hardening-soil constitutive model with parameters estimated from in situ and laboratory tests was used to characterize the constitutive behaviors of the pier material and the matrix soil. Pier groups were modeled as unit cells with the tributary area determined from the center-to-center spacing. Cavity expansion modeling was used to simulate the pier installation process. Verifications of the numerical model were carried out by comparing the numerical results with the data obtained from full-scale, instrumented load tests. Interpretation of the numerical results focused on the load-deformation behavior, group effect, stress concentration ratio, and the development of stresses in the matrix soil. The distributions of vertical stress underneath the pier-supported footings are also characterized.

Behaviour of geogrid-reinforced load transfer platforms for embankment on rammed aggregate piers

A test embankment was constructed over soft ground to evaluate the performance of different types of load transfer platform (LTP) supported on rammed aggregate piers (called ‘geopiers’). Three types of LTP were constructed in accordance with the recommended design for each: a geosynthetic-reinforced LTP with two layers of geogrid (catenary LTP), a geosynthetic-reinforced LTP with three or more layers of geogrid (beam LTP), and a reinforced concrete LTP. The results indicate that the differential settlement between the geopiers and the matrix soil is relatively small in all LTP sections, with the smallest in the beam LTP. The tensile strain in the geogrid was approximately 60% of the allowable design strain of 5% in the beam LTP. In the catenary LTP, the tensile strain in the geogrid was approximately 20% of the allowable design strain of 6%, although the differential settlement was larger than the beam LTP. The total and differential settlements observed indicate that the use of less stiff geopier columns together with LTPs provides an attractive option for supporting low embankments on soft ground with tolerable total and differential settlements. The presence of LTPs and the supporting columns also tend to reduce the lateral movement of the foundation soil. The cost analysis of the different LTPs indicates that construction costs for geosynthetic-reinforced LTPs are likely to vary with locality; however, it appears that beam LTPs offer a less costly approach, with enhanced performance.

Numerical analysis of foundation columns to support widening of embankments

Increased traffic volume has made it necessary to increase highway capacities by widening embankments and pavements. Adding a new embankment to an existing embankment induces additional stresses and deformations beneath the widened and existing portions of the embankment. Differential settlement may develop between and within the new and existing portions of the embankment, especially over soft soils. This differential settlement often causes pavement distress, such as longitudinal cracks or the drop-off (or sinking) of pavement sections. Different techniques have been adopted to remedy these problems, including the use of foundation columns, such as deep mixed columns, vibro-concrete columns, stone columns, and aggregate piers. However, design procedures for foundation columns constructed for this purpose are not well developed. The analyses of eight cases of column-supported widened embankments and two untreated foundations are presented in this paper. The factors considered include the consolidation of foundation soils under existing embankments and the spacing, region, and modulus of foundation columns. Two-dimensional finite difference software was used after the calibration of the model against a field case study and numerical analyses were conducted to investigate stresses and deformations of the widened embankments over soft soil with or without the remediation of foundation columns. The results presented in this paper include the vertical and the horizontal displacements, the maximum settlements, the transverse gradient change, and the distribution of the additional stresses induced by the widening. Recommendations are made for the design of foundation columns to remedy roadway pavement failure due to widening of embankments.

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...

Stress Zones near Displacement Piers: II. Radial Cracking and Wedging

Transient liquefaction of saturated soils near Rammed Aggregate Piers is described in Part I on the basis of radial stress measurements. This is supported by dynamic pore-water pressure measurements, as peak pore pressures approximately equal radial stresses imposed at the pier surface by ramming. Stress measurements outside of the liquefied/plastic zone indicate radial tension cracking in the elastic zone, which is consistent with the observation that pore pressures abruptly drop and momentarily can even become negative as soon as ramming stops. The drainage field created by extended radial cracking and hydraulic fracturing allows Rammed Aggregate Piers to be effective in saturated, fine-grained soils where other dynamic methods are reported to be less effective. Stress measurements indicate that liquefied soil injected into open tension cracks causes stress to be retained in the elastic zone through arching action. A stress path analysis indicates that lateral stress may play an important role in control of foundation settlement, by simulating an increase in the preconsolidation pressure without vertically surcharging the soil or waiting for it to consolidate.

Stress Zones Near Displacement Piers: I. Plastic and Liquefied Behavior

Radial stresses measured with the K0 stepped blade in saturated soils near Rammed Aggregate Piers indicate temporary liquefaction of soil in the vicinity of the rammer. Drainage and compaction proceed during ramming of succeeding layers of the pier, such that the final distribution of radial effective stresses follows plastic cavity expansion theory. A plastic zone and liquefaction can develop only if the radial stress from ramming exceeds the soil compressive strength. If that condition is met, the transient hydrostatic condition allows the zone to expand with additional ramming. A passive condition occurs where radial stresses cannot be fully contained by the overburden pressure, in which case they approximate Rankine passive pressures. The tangential intermediate principal stress then can be modified by ramming of adjacent piers. Stress-induced cracking in the elastic zone contributes to rapid drainage and is discussed in Part II of this two-part series.

Slope Rehabilitation at the Baltimore-Washington Parkway with Rammed Aggregate Piers

The Baltimore-Washington Parkway is a major traffic artery traveled by daily commuters in suburban Washington, D.C. Much of the parkway roadbed lies on a raised embankment constructed to allow for grade separation between the parkway and surface streets, facilitating access to the parkway. After a series of rainstorms in the fall of 2002, the fill embankment at the recently reconstructed Route 197 interchange failed, with resulting settlement and lateral movement of a retaining wall that supported the southbound lanes of the parkway. The slide encroached on the traffic lanes and jeopardized the integrity and safety of the parkway. After analyzing a variety of possible solutions, FHWA and the National Park Service opted to stabilize the slide by using Geopier rammed aggregate piers, acting in concert with a toe berm stabilized with Tensar highstrength structural geogrid. The rammed aggregate piers improved the stability of the slope by providing significant increases in the composite shear resistance because of their high angle of internal friction (44° to 52°). A case history is presented of the use of rammed aggregate piers to stabilize a landslide. The analytical methods used in the design solution are presented along with a description of the construction sequence. This study is significant because it describes how a simple and cost-effective solution may be implemented to stabilize landslides.

Design of Short Aggregate Piers to Support Highway Embankments

When new embankments are constructed over soft soils and existing roadways and embankments are widened rapidly, stability and differential settlement problems often result. Past approaches to mitigate these problems include overexcavation and replacement, preloading, lightweight fill, piles and concrete caps, geosynthetic-reinforced soil and pile-supported embankment systems, stone columns, lime-cement columns, and deep soil mixing. The advantages and limitations of these approaches are well documented in the literature. An alternative approach to support highway embankments that is less well documented, but that has seen increased use in recent years, is short aggregate piers. Although the technique has been used to support shallow spread footings, design approaches and parameter values have not been widely available for embankment support applications and are not well known in the field. The engineering properties and an approach used to design short aggregate piers for the support of highway embankments are summarized, and the mechanisms of load-settlement behavior are described. Techniques for evaluating overall stability, control and time rate of settlement, and bearing capacity are summarized. Performance measurements are referenced for embankment support projects recently constructed in Iowa.

Does Lateral Stress Really Influence Settlement?

Construction innovations such as compaction grouting, the use of a reverse pitch section for installing augercast piles, and the use of beveled rammers to compact rammed aggregate piers, all emphasize the development of lateral in situ soil stress. The question posed herein is whether lateral stress in itself may reduce settlement, aside from any influences through densification or increased skin friction. Stress paths for a normally consolidated soil that is laterally stressed to the passive limit, then vertically loaded, gives a sequence of Mohr circles that indicates a shift from consolidating to near-linear-elastic behavior. The change requires and appears to depend on reversals in the directions of intergranular friction. As these reversals should occur with very little strain, they do not require that the soil be significantly compacted. Excess pore pressures that may develop should be relieved with the aggregate pier method of soil reinforcement. The reinforced soil layer should distribute stresses to the underlying soil in a manner that is similar to the action of a highway base course.

Short aggregate piers defeat poor soils: E. C. Lawton, N. S. Fox & R. L. Handy, Civil Engineering - ASCE, 64(12), 1994, pp 52–55

A short aggregate pier system, developed to provide an economical alternative to the overexcavation/replacement technique, has been used used since 1988 to control settlement of structures located at sites with marginal or inadequate soils. Highly densified aggregate piers, composed of stone as used in highway construction, are formed within excavations. They have been shown to substantially reduce settlements of shallow foundations, and also to be suitable for use as anchors to prevent uplift at an airplane hangar. Case histories include a site subject to seismic codes.