Robert D. Holtz

Geotechnical Characterization and Random Field Modeling of Desiccated Clay

AbstractAn extensive set of in situ and laboratory test data is presented for a footing load test site east of Houston, Texas, in desiccated Beaumont clay. The in situ test program included standard and cone penetration tests (CPTs), the latter of which was selected for statistical analysis to produce vertical and horizontal random field model parameters for corrected cone tip resistance. Given the relatively high sampling frequency of the cone tip resistance in the vertical direction, the vertical random field model parameters were determined using the modified Bartlett’s test statistic with fitted autocorrelation models subject to a strict fitting criterion. Horizontal random field model parameters were generated by collapsing the two-dimensional distribution of the CPTs to a one-dimensional representation and by using less stringent evaluation of the autocorrelation. The results of this study indicate that Beaumont clay exhibits greater inherent spatial variability than previously reported at other cla...

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 ...

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...

Assessment of Reinforcement Strains in Very Tall Mechanically Stabilized Earth Walls

The grade raising associated with the Third Runway Project at Seattle-Tacoma International Airport included construction of tall mechanically stabilized earth (MSE) walls, including the near-vertical, two-tier, 26-m North MSE wall and the near-vertical, four-tier, 46-m tall west MSE wall. Twenty reinforcement strips at critical wall cross sections were instrumented with over 500 strain gauges to monitor strains during and following construction. The reinforcement loads inferred from observed strains are of interest because of their great height and global reinforcement stiffness, which place these walls outside the range in height and stiffness used to calibrate commonly used design methods. This paper presents the development and distribution of reinforcement strains measured during and following the construction of these walls. The reinforcement stresses calculated using the original reinforcement load design methods and design friction angle agreed with those inferred from the measured strains. The accuracy of two standard-of-practice and two alternate design methods is evaluated by compar- ing the reinforcement loads inferred from measured strains to those calculated using the actual friction angle of the reinforced fill material. Advantages and limitations in these design methods are identified, and recommendations for the improvement of some of these methods are provided. DOI: 10.1061/(ASCE)GT.1943-5606.0000586.

Reliability of Spread Footing Performance in Desiccated Clay

AbstractTo advance the use of reliability-based design procedures, it is necessary to evaluate the sources of the design parameter uncertainty including inherent variability, measurement error, and transformation uncertainty. The results of a probabilistic evaluation of undrained footing bearing performance are discussed in the context of an extensive test site characterization described in a companion paper. Kriged cone tip resistance values are transformed into design parameters using a second-moment probabilistic approach and compared with the parameters obtained from the laboratory test analyses on specimens retrieved from the test site. The spatial, measurement, and transformation uncertainty are incorporated into probabilistic finite-element and bearing capacity analyses in which the results are compared against a full-scale load test performed at the test site. The results indicate that the reliable assessment of the spread footing response depends to a large degree on the assumed strength anisotro...

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 methods available to predict the displacement of shallow foundations resting on aggregate pier reinforced clay, the accuracy of these models remains uncertain. After a brief review of existing load-displacement estimation methods, the accuracy of those models valid for isolated spread footings was investigated using a database of high-quality footing load test data. The methods were compared using the bias (i.e., the ratio of measured bearing capacity to that calculated) and were shown to exhibit a high degree of variability in the prediction of load and displacement. Then, the database was used to develop a multiple linear regression model for the prediction of footing displacements for aggregate pier reinforced clay under a wide range of pier configurations and soil conditions. The proposed statistical model is sho...

Evaluating Geotextile Pore Structure

There are many test methods available to determine the pore sizes of geotextile filters. Often overlooked is the fact that each method provides a different interpretation of a geotextiles pore structure. A survey of the available test methods, indicates that four categories of pores can be measured. Pore characteristics are identified by one of the following methodologies: sieving, theoretical, volumetric, and numerical. A critical review of the data produced by these methods shows that the actual pore structure as related to flow-through behavior can only be determined by the bubble point method, which provides a volumetric pore size. This test identifies the constriction size of each pore channel in a geotextile, where retention and clogging are most critical. The bubble point test method is advantageous because it provides for the complete pore size distribution of the geotextile, can be performed more efficiently than sieving methods, and provides an accurate estimate of the permeability of the geotextile.

Undrained Displacement Behavior of Spread Footings in Clay

The analytical and experimental undrained displacement behavior of spread footings in clay is investigated in this study. Three spread footings ranging in size from 0.76 to 2.74 m were loaded and displaced up to 130 mm into medium stiff to stiff Beaumont clay. Load-displacement curves were determined, and soil displacements below the edge of the largest instrumented footing were measured. The footing displacements were modeled using linear elastic stress distributions and a nonlinear constitutive model calibrated with common in-situ and laboratory tests results. The effect of soil nonlinearity and footing rigidity on soil displacement behavior was also investigated. Results show that the linear elastic stress distributions used in a nonlinear constitutive setting resulted in very good prediction of undrained displacements at the footing center. Displacements at the edge of the footing were less well predicted. Regardless of the assumption of footing rigidity, when nonlinear, in-situ stress-dependent behavior is modeled, realistic concentrations of displacement towards the footing boundaries are observed.

Factors Affecting the Development of MSE Wall Reinforcement Strain

The grade raising associated with the construction of a new runway at Seattle-Tacoma International Airport required construction of two near vertical tall reinforced earth walls that included the two tier 26 m North wall and the four tier 46 m tall West wall. Twenty reinforcement strips at critical wall cross-sections were instrumented with approximately 550 strain gages, and performance data were obtained during initial installation and backfill placement. The performance data facilitated the assessment of factors affecting the development of strains within the MSE walls. A simplified method to determine the amount of reinforcement strain due to installation of the reinforcement strip and subsequent compaction of fill is presented. Estimates of the initial reinforcement strain, fill thickness up to which compaction effects were noted, and the initial and geostatic reinforcement strain rate in terms of microstrain per meter of fill placed are tabulated and discussed. The local reinforcement stiffness contributes to the accumulation of initial reinforcement strain, but is found to play a significant role in controlling the rate of strain due to normal geostatic stresses. Implications for the design of very tall MSE walls are also given.

Discussion of the Paper: A State-of-the-Art Review of Stone/Sand-Column Reinforced Clay Systems by Shadi S. Najjar

The author has prepared a comprehensive review of the use of stone columns, or aggregate piers, for the improvement of soft, compressible clayey soils, which should prove convenient to practitioners or researchers beginning to study this ground improvement alternative. We note that although the author made rather extensive use of some of our research, the work was not referenced properly. This discussion therefore is to correct the record so that interested readers may easily find recent relevant work in the literature. Additionally, we wish to point to work that was not yet published and therefore unavailable to the author at the time of the manuscript submission, as well as other work that may have been overlooked by the author.