Dan A. Brown

Some numerical experiments with a three dimensional finite element model of a laterally loaded pile

Abstract This paper describes the results of several numerical experiments performed with a three dimensional finite element model of a laterally loaded pile. The experiments are used to investigate the effect of several factors on the p-y curves derived from the model. The three dimensional finite element model utilizes a plasticity model for soil with interface elements which allow gap formation; the model has been described by Brown and Shie (1). P-y curves are widely used in design because of the relative simplicity of the beam on elastic foundation approach and the ability of design p-y curves to represent nonlinear soil behavior. However, it is recognized that this procedure does not represent the soil as a continuum and that the relative importance of some possibly significant factors may be obscured. The procedures used to develop p-y curves are largely empirical. P-y curves derived from the three dimensional finite element model are used in this study to investigate the effect on the soil response of pilehead fixity, in-situ soil stresses, pile/soil interface friction, and sloping ground. The results provide the type of information needed to intelligently utilize the p-y method in the design process.

Three dimensional finite element model of laterally loaded piles

Abstract The behavior of a pile subjected to lateral loading has been analyzed using a three dimensional finite element model. The study represents an attempt to develop a reasonably realistic model of the problem, including gap formation and plastic deformations in the soil around the pile, so as to provide 3 basis for parametric studies of the effects of pile spacing, pile head fixity, and soil stiffness on pile response. Constitutive models for soil include a simple elastic-plastic model with a Mises yield surface and associated flow and an extended Drucker-Prager model with nonassociated flow. Frictional interface elements were used to provide for slippage at the pile/soil interface and to allow gapping in the space behind the pile. The results of the analyses provide insight into the deformation patterns and development of areas of plastic deformation around the pile. Such data also provide a basis for evaluation of more simple two dimensional approximations of the problem, such as described by Kooijman (1). In addition, be bending moment data from the pile were reduced to obtain p−y curves in a manner similar to that used to produce p−y curves from physical experiments. These data provide another level of comparison of the finite element results with the empirical design procedures currently in use.

NUMERICAL EXPERIMENTS INTO GROUP EFFECTS ON THE RESPONSE OF PILES TO LATERAL LOADING

Abstract The response of closely spaced piles subjected to lateral loading in either one or two rows has been analyzed using a three dimensional finite element model. This research is an extension of the work described in reference (7), which describes the finite element model and the response of a single pile to lateral loading. The model utilizes two types of plasticity models for soil to represent either undrained loading of saturated clay or drained loading of sands. Additionnally, frictional interface elements are used to provide for slippage and gapping at the pile-soil interface. The model is used to evaluate the effect of spacing within a row and between rows of piles on the p-y curves derived from the bending stresses within the piles, so as to provide information useful in developing design guidelines. The results of the analyses provide insight into the deformation patterns and development of areas of plastic deformation around the piles. The effect of pile spacing within a single row of piles (or the front row of a group) is seen to be relatively small for piles spaced at 3 diameters on center or more in undrained clay soil. The influence of pile spacing in a single row in sand is somewhat larger, but still relatively small at spacings of 3 diameters on center or more. Where multiple rows of piles are loaded such that one row trails another, the trailing row of piles is subject to a significant reduction in stiffness. Such patterns are in general agreement with the observations of field experiments. The p-y curves derived from the piles in each case are compared so as to provide guidelines for modification of design p-y curves to account for group effects.

GEORGIA'S EXPERIENCE WITH CRUMB RUBBER IN HOT-MIX ASPHALT

In 1991, the Georgia Department of Transportation (GDOT) began to evaluate the production and placement of crumb rubber hot-mix asphalt. The crumb rubber mix (CRM) used by GDOT was produced by adding ground tire rubber to hot-mix asphalt using the wet process. A test section of CRM was placed on I-75 in Henry County, just south of Atlanta, consisting of a surface mix containing 6 percent crumb rubber by weight of asphalt cement (AC). The test section was evaluated from 1991 to 1995. The test section indicated that the CRM became very brittle over time, as indicated by a large increase in viscosity and decrease in penetration, and by a large amount of transverse reflective cracking. Compared with the control mix, the CRM did not reduce rutting and was more than twice as expensive to place. In addition to the test section, two contract projects were initiated using CRM. These two projects indicated that CRM could be produced and placed using conventional equipment requiring only a few modifications. On-site blending units were used to combine the crumb rubber at a dosage rate of 16 percent by weight of AC. Pump and metering equipment was modified to accurately meter the stiff asphalt material, and correction factors were established for determining the AC content by vacuum extraction, since some of the rubber particles were retained in the aggregate portion of the sample.

Evaluation of Self-Consolidating Concrete for Drilled Shaft Applications at Lumber River Bridge Project, South Carolina

Case studies have shown that when conventional concrete mixtures are used in congested drilled shafts, lack of adequate workability or flow between reinforcing bars may lead to trapped laitance or segregation between the inside and outside of the reinforcing cage. Due to its flow-ability and resistance to segregation, self-consolidating concrete (SCC) was evaluated as a viable material to overcome this problem. Several 1.8-m (6-ft) diameter drilled shafts were constructed using SCC as part of a field trial during the Lumber River Bridge Project, South Carolina. Identical shafts were constructed with SCC and a high slump gravel-aggregate concrete mixture typically used in coastal South Carolina. Both mixtures were observed to have excellent workability characteristics. Observations of the hardened concrete from exhumed drilled shafts indicate that generally good performance can be achieved in difficult construction conditions (congested cage, tremie placement, and lengthy placement times) if highly workable concrete is used. Some imperfections in the concrete were observed, even under these closely monitored conditions, and some degree of imperfection in this type of construction appears to be practically unavoidable. The imperfections observed in these field trials were detected by crosshole sonic logging, but they do not appear to have significant adverse consequences for foundation performance. On the basis of results of this project, it is concluded that SCC may be feasible for use in congested drilled shaft applications.

Jet Grouting and Soil Mixing for Increased Lateral Pile Group Resistance

Lateral load tests were performed on a full-scale pile cap in clay before and after construction of shallow soilcrete walls produced by soil mixing and jet grouting on either side of the pile cap. This relatively simple approach increased the lateral resistance of the pile cap by 60% for soil mixing and 160% for jet grouting. For the soil mixed wall, essentially all of the increased resistance was due to passive pressure and side/base shear against the soil mixed wall as the pile cap pushed the wall laterally. However, for the jet grout wall, about 25% of the increased resistance came from soil-pile interaction because the jet grout wall extended under the cap and against the piles. Soil mixing and jet grouting provide a means to significantly increase the lateral resistance of existing pile group foundations with relatively little investment of time, effort, and expense relative to adding more piles.

Characterization of Loess for Deep Foundations

Abstract This paper describes the results of a detailed analysis of a loess deposit at the deep foundations test site established by the Kansas Department of Transportation. Multiple CPT, SPT, and pressuremeter tests were conducted as part of the investigation. This paper includes an evaluation of the effectiveness of common correlations used with the CPT, SPT when used for classification of loess and estimating its properties. Results show that common correlations must be used with caution as significant errors in estimation may result from using methods developed for other soil types. Laboratory testing included direct shear, triaxial, collapse, and consolidation testing, with many of the tests conducted on both vertically and horizontally oriented samples to evaluate anisotropic behavior. The results showed that strength properties could be considered to be isotropic.

Jet Grouting to Increase Lateral Resistance of Pile Group in Soft Clay

Lateral load tests were performed on a full-scale pile cap in clay before and after construction of eight 1.5 m diameter jet grout columns to a depth of 3 m around the pile group. Jet grouting with a cement content of about 400 kg/m3 (20% by weight) increased the average compressive strength of a soft, plastic clay from 40 to 60 kPa to an average of 4500 kPa. The lateral resistance was increased by 2200 kN or 177% and the initial stiffness was increased by 400%. About 65% of the increased resistance could be accounted for by passive pressure and side/base shear on the jet grout mass; however, the remaining 35% increase must be due to the interaction between the piles and the strengthened soil. Jet grouting provides a method to significantly increase the lateral resistance of pile group foundations at costs much lower than typical structural approaches.

Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils

This report presents design guidance for strengthening of soils to resist lateral forces on bridge pile foundations. Lateral loads may be produced by wave action, wind, seismic events, ship impact, or traffic. Strengthening of soil surrounding the upper portions of piles and pile groups—for example by compaction, replacement of native soil with granular material, or mixing of cement with soil—may be more cost-effective than driving additional piles and extending pile caps as ways to increase the bridge foundations capacity to resist lateral forces associated with these loads. This report presents computational methods for assessing soil-strengthening options using finite-element analysis of single piles and pile groups and a simplified approach employing commercially available software. The analysis methodology and design guidelines will be helpful to designers responsible for bridge foundations likely to be exposed to significant lateral loads.

Lateral Load Capacity of Cast-in-Place Shafts behind an MSE Wall

Current practice for designing laterally loaded cast-in-place shafts that pass through an MSE Wall involves isolating the shafts from the MSE mass and anchoring the shafts into the underlying foundation material. Sizeable cost and time savings could be realized, while still maintaining stability and reliability, if a method were available to evaluate the lateral load capacity of a shaft that is supported by the MSE mass alone with no rock socket. Construction, instrumentation, and testing of multiple 0.9m (36in.) diameter shafts solely supported by a 6m (20 ft) MSE block wall was conducted for the Kansas Department of Transportation (KDOT). This paper describes the design and construction of the wall and shafts, and the results from the lateral load tests of two of the shafts. These shafts had lengths that were equal to the full height of the wall and 75 percent of the full height of the wall to evaluate the reduction in capacity if shorter foundation elements suspended in the MSE mass were used. Results for both load and deflection of the shafts and the relative deflections of the shafts and wall facing during loading are presented.