Brent Robinson

Inverse Analysis of Plate Load Tests to Assess Subgrade Resilient Modulus

Cyclic plate load testing is commonly used to investigate subgrade response under repetitive loads. Two frameworks for performing inverse analysis are described for backcalculating resilient moduli on the basis of measured key outputs. In the first approach, an elastic modulus is back-calculated in each selected domain; in the second, selected parameters in the resilient modulus model are estimated. The axisymmetric finite element model analysis results suggest that the second approach is more robust because it allows the modulus to be distributed in the selected domain. A series of sensitivity analyses was conducted with the second approach to illustrate how the assumed properties or model geometry affects the backcalculated parameters. Discrepancies between the back-calculated parameters and their known values were observed when the distance to the boundary–-that is, the radial distance from centerline to sidewall–-was not properly assigned. When backcalculating only selected parameters in the resilient modulus equation, it is necessary to assign the other parameters carefully (i.e., from laboratory tests or references). An example analysis shows the application of the proposed approach to an actual plate load test.

Laboratory Performance Comparison of Stabilized Undercut Subgrade Under Cyclic Loading

This study evaluated the performance of undercut subgrade stabilization measures during construction traffic loading prior to final paving. Twenty-two simulated undercut sections with different stabilization configurations over a typically undercut Coastal Plain clay subgrade were built in a large-scale test pit. The subgrade was placed at a California Bearing Ratio of ∼2–3 % and stabilized with granular layers, granular layers reinforced with geosynthetics, and lime. Granular layers consisted of either aggregate base course (ABC), sandy select fill, or a multi-layer system with both soil types. The four geosynthetics tested were a woven reinforcement geotextile, a woven separation geotextile, and two biaxial polypropylene geogrids. The soft nature of the subgrade and its consequences on the ability to compact the ABC layer show the importance of carefully analyzing the results when viewed on a comparative basis. Cyclic plate loading simulating construction traffic showed that thicker granular layers produced less surface displacement, barring subgrade strength differences from remolding effects. Tests with lime stabilized subgrade showed the least magnitude of deformation over initial and post-rut repair cycles. ABC tests with geotextile showed improvement over unreinforced sections but only when placed at depths approximately equal to the loading plate diameter and after initial displacements mobilized the geosynthetic strength.

Evaluation of Rotational Stiffness of Elastomeric Bearing Pad-Anchor Bolt Connections on Deep Foundation Bents

Experimental tests are performed on a bearing pad-anchor bolt connection to study rotational stiffness and moment transfer capabilities of a typical bridge configuration. The experimental program is divided in two phases. The first phase consisted of shear and compression properties of two types of bearing pads. The second phase consisted of a total of 42 full-scale tests of a bearing pad-anchor bolt connection. The tested bridge-bent configuration includes two AASHTO Type II girders made continuous with a slab and diaphragm, bearing pads, pile caps, and piles. Variables included axial loads applied to the piles and bearing pads, two different sets of bearing pads, and three different pile types. The bridge connection is subjected to lateral cyclic reversed loading in one-cycle displacement increments. Test results show the potential for this type of connection to sustain lateral loads and flexural moments, and to develop the full strength of the pile elements. Shear and compression modulus are also obtained for the bearing pad types used in this study. Rotational stiffness values for the connection are determined as a function of varying axial loads.

Static and Dynamic Load Tests on Driven Polymeric Piles

Repair and replacement of deteriorating piling systems cost the United States up to

A Point of Fixity Model for Pile and Shaft Bents

1 billion per year. In the case of marine piling, actions required by the Clean Water Act rejuvenated many of the nations waterways, but also allowed the return of marine borers, which attack timber piles. At the same time, less than 10% of the 13.7 million tons (122 GN) of plastic containers and packaging produced annually in the U.S. are recovered by recycling. Using recycled plastics to manufacture piles utilizes material which (1) would have been otherwise landfilled and (2) can be more economical in aggressive environments when life-cycle costs are considered. A series of polymer piles were driven in Elizabeth, New Jersey. Three concrete filled fiberglass shell piles, three polyethylene piles reinforced with steel bars, three polyethylene piles reinforced with fiberglass bars, and two solid polyethylene piles were installed. One closed end steel pipe pile was also driven for reference purposes. Three static load tests were performed on one of the concrete filled fiberglass shell piles, and one of each of the reinforced polyethylene piles. High strain dynamic pile tests were performed on all piles during initial driving and restrike after load testing. This study describes the adjustments to assumed material properties required during installation testing and the correlation between static and dynamic load tests.

Subgrade Undercut Criteria Based on Modeling of Rutting and Pumping Response

Pile bents are often used in bridge foundation systems. These sub- structural elements are constructed by installing a row of piles and connecting them with a concrete cap. A common design practice is to use a point of fixity approach which idealizes the soil-pile system as a cantilever of a particular length, forming a single column in an elastic frame. In this paper, current design practices are reviewed, and a new method for calculating point of fixity is developed for engineers who must consider separate single pile lateral analyses and elastic frame analyses. The proposed approach better matches the maximum moments and displacements obtained from non-linear analysis for a pile in the foundation system that is subjected to the lateral loading applied to the bridge. An example that compares a pile bent designed as an equivalent elastic frame with a nonlinear analysis is presented. Comparative results show the elastic frame model with two equivalent lengths (based on transverse and longitudinal loading conditions) satisfactorily matches results from nonlinear analysis, while a frame with a single point of fixity, based on deepest equivalent length, provides a conservative approximation of the nonlinear model.

Energy Efficiency and Rod Length Effect in Standard Penetration Test Hammers

AbstractThe stability of subgrade soils is a major concern during roadway construction with inappropriately soft layers often undercut and replaced by competent or stabilized materials. Systematic undercut criteria are established using numerical modeling with varying the strength and stiffness parameters of the subgrade and representing the mechanistic behavior as an elastic-perfectly plastic medium. Two modes of domain configurations were considered: the plane strain and axisymmetric conditions. The plane strain mode is assumed to simulate proof roller loading with four parallel tires and mainly provides information about excessive pumping response as materials at deeper layers are affected. The axisymmetric mode provides information related to excessive rutting and is used to simulate the effect of single or dual tires representing construction traffic, rather than a series of closely spaced axle loads. Undercut criteria are proposed for meeting a deformation limit state of 25 mm for both pumping and r...

Configuration Optimization of Drilled Shafts Supporting Bridge Structures: Three Case Studies

Twenty-eight standard penetration test (SPT) hammers owned by the North Carolina Department of Transportation and private consultants were used to investigate the average energy efficiency and variability of manual versus automatic hammers, as well as the effect of SPT rod length on hammer efficiency. The results agree with published data in several regards. Automatic hammers in the study were found to have an average transferred efficiency of 80.9%. This finding agrees very well with the 80% efficiency assumed in geotechnical engineering practice for automatic hammers. Manual hammers in the study averaged 63.9%, close to the 60% efficiency assumed for manual hammers. Manual hammers were found to be twice as variable as automatic hammers in transferred energy from blow to blow within an SPT blow count. The study demonstrated that the measured transferred energy appeared to be affected by rod length. Lengths shorter than approximately 40 ft caused reduced energy to be transferred into the rod. An empirical formula is presented for correcting short rod length energy losses. The data did not demonstrate a strong dependence on SPT N-value, although the data set lacked observations where the N-value was less than 6 blows per ft.

Simplified Lateral Analysis of Deep Foundation Supported Bridge Bents: Driven Pile Case Studies

A common approach to estimating the point of fixity is to utilize the results of a single pile lateral analysis. Although no universal agreement exists as to the definition of the location of the point of fixity, it is generally accepted that its location will affect the computed stresses and displacements of a bridge structure. This study summarizes a method to determine a cantilever’s equivalent length of drilled shaft foundation elements supporting a bridge. Results from an equivalent frame model are compared to those for bents modeled using the finite element method and nonlinear soil models for three bridges in North Carolina. Results indicated that the equivalent frame model provides responses that are comparable to those obtained from more rigorous finite element analyses. The study presents the results of the optimization of the support system by reducing the number, or size, of the shafts while maintaining an acceptable level of safety.

Pile Bent Design Criteria

A simplified approach for modeling soil and foundation system supported bridge bents is applied to three bridges that represent three pile types and three superstructures. This point-of-fixity approach is applied by modeling the bridge bent substructure as an elastic frame. The models are compared with more refined analyses in FB-MultiPier, with SAP as an independent verification tool, using pile sections with nonlinear soil, pile, and pile cap material properties. The results for simple pile bents show that an equivalent frame model provides similar moment, shear, and displacement values as those obtained from both the SAP and MultiPier nonlinear analyses. Analysis results also indicated that the equivalent frame model parameters are particularly sensitive to the comparable selection of both axial and lateral loads. If lateral loads used to develop the equivalent model are higher than experienced, the axial and lateral deflections and moments will also be higher. For design purposes, this is conservative.