Robert L. Parsons

Engineering Behavior of Stabilized Soils

Stabilization of soils is an effective method for improving soil properties and pavement system performance. For many soils, more than one stabilization agent may be effective, and financial considerations or availability may be the determining factor on which to use. A series of tests was conducted to evaluate the relative performance of lime, cement, Class C fly ash, and an enzymatic stabilizer. These products were combined with a total of seven different soils with Unified Soil Classification System classifications of CH, CL, ML, and SM. Durability testing procedures included freeze–thaw, wet–dry, and leach testing. Atterberg limits and strength tests also were conducted before and after selected durability tests. Changes in pH were monitored during leaching. Relative values of soil stiffness were tracked over a 28-day curing period using the soil stiffness gauge. Lime- and cement-stabilized soils showed the most improvement in soil performance for multiple soils, with fly ash–treated soils showing substantial improvement. The results showed that for many soils, more than one stabilization option may be effective for the construction of durable subgrades. The enzymatic stabilizer did not perform as well as the other stabilization alternatives.

USE OF CEMENT KILN DUST FOR THE STABILIZATION OF SOILS

Stabilization of soils is an effective method for improving soil properties and pavement system performance. Cement kiln dust (CKD) represents an alternative to the tradition stabilization agents of lime, cement, and Class C fly ash. As with the other additives, cement kiln dust can be expected to vary in effectiveness depending on the soil type. A total of eight different soils with classifications of CH, CL, ML, SM, and SP were subjected to a series of durability tests, including freeze-thaw, wet-dry, and leach testing, to evaluate the relative performance of CKD as a stabilization agent. Results were compared with previous findings for the same soils stabilized with lime, cement, and fly ash. Atterberg limits and strength tests were conducted before and after selected durability tests. Relative values of soil stiffness were also tracked over a 28-day curing period using the soil stiffness gauge to evaluate ongoing reactions. The results provide guidance on which soil types can be expected to have significant improvements in performance wit the addition of CKD.

Performance of Geocell-Reinforced RAP Bases over Weak Subgrade under Full-Scale Moving Wheel Loads

Recycled asphalt pavement (RAP) has been increasingly used as an energy efficient and environmentally friendly paving material and is currently the most reused and recycled material in the United States. RAP has been used in new hot mix asphalt (HMA) mixtures and in base courses for pavement construction. When RAP is used as a base course material, the presence of asphalt in RAP may cause excessive deformation under traffic loading. Geocell, three-dimensional (3D) polymeric geosynthetic cells, was proposed in this study to minimize the deformation by confining the RAP material. Full-scale accelerated pavement tests were conducted to evaluate the effect of geocell reinforcement on RAP base courses over weak subgrade. Two types of RAP were used and a total of seven geocell-reinforced and unreinforced RAP sections were tested under full-scale traffic loads. The road sections were excavated and examined after each moving wheel test. The benefits of geocell reinforcement were evaluated in rut depths for a specific number of passes of the wheel load and the angle of stress distribution from the surface to the base course-subgrade interface. The test results demonstrated that the novel polymeric alloy geocell reinforcement improved the performance of unpaved RAP sections by widening the stress distribution angle and reducing the rut depth if the base courses were equally compacted in unreinforced and reinforced sections.

Behavior of Geocell-Reinforced Sand under a Vertical Load:

Geocells have a three-dimensional cellular structure, which can be used to stabilize foundations by increasing bearing capacity and reducing settlements. However, a considerable gap exists between the applications and the theories for the mechanisms of geocell-reinforced foundations. An experimental and numerical study on the behavior of geocell-reinforced sand under a vertical load is presented. A single geocell was filled with sand and subjected to a vertical load to failure. This test process was modeled by using the FLAC3D numerical software to investigate the mechanisms of geocell and sand interactions. Experimental and numerical results both demonstrated that the geocell increased the ultimate bearing capacity and the modulus of the sand. The numerical results include the distributions of displacements in the sand and geocell walls and the distributions of tensile stresses and shear stresses acting on the geocell walls. The numerical results for geocell-reinforced sand are compared to those for sand without geocell.

Performance of Triangular Aperture Geogrid-Reinforced Base Courses over Weak Subgrade under Cyclic Loading

AbstractGeogrid (uniaxial or biaxial) is one type of geosynthetics that has been successfully used in slopes, walls, roads, and other applications. The main application of biaxial geogrid is to stabilize soft subgrade and reinforce weak base courses by providing lateral confinement. The confinement due to the interaction between aggregates and the ribs of biaxial geogrid depends on the geometry properties of the geogrid, such as rib shape and apertures size, the stiffness of the ribs, and the properties of aggregates. Research has shown that biaxial geogrid cannot provide uniform tensile resistance in all directions. To overcome this problem, a geogrid product with triangular apertures was developed and introduced into the market. Recent studies showed that the triangular aperture geogrid can provide nearly uniform tensile resistance in all directions and is more efficient in improving the performance of reinforced bases as compared with biaxial geogrid. However, the performance of triangular aperture geo...

Accelerated Pavement Testing of Geocell-Reinforced Unpaved Roads over Weak Subgrade

Full-scale trafficking tests were conducted to evaluate the effect of novel polymeric-alloy geocell reinforcement on base courses for low-volume unpaved roads over weak subgrade. Three types of in-fill materials—crushed limestone (AB-3) aggregate, quarry waste (QW), and recycled asphalt pavement (RAP)—were used for the base courses over a weak subgrade layer consisting of A-7-6 clay. Four unpaved sections that included one unreinforced control section of AB-3 aggregate 30 cm thick and three 17-cm novel polymeric-alloy geocell-reinforced sections were tested under a single-axle dual-tire wheel loading. The road sections were exhumed and examined after the moving-wheel test. The benefits of novel polymeric-alloy geocell reinforcement were evaluated in relation to rut depths for a specific number of passes of the wheel load and the angle of stress distribution from the surface to the base course–subgrade interface. The test results demonstrated that the novel polymeric-alloy geocell reinforcement improved the performance of unpaved AB-3 and RAP sections. The QW section also showed better performance in relation to stress distribution angle.

Stress Analysis on Triangular-Aperture Geogrid-Reinforced Bases over Weak Subgrade Under Cyclic Loading: An Experimental Study

Geogrids have been successfully used to improve soft subgrade and reinforce weak base courses for low-volume roads by providing lateral confinement. However, uniaxial and biaxial geogrids with rectangular or square apertures cannot provide uniform resistance in all directions. A new geogrid product with triangular apertures was developed and introduced into the market to overcome this problem. The triangular-aperture geogrid has a more stable grid structure and can provide uniform resistance in all directions compared with uniaxial and biaxial geogrids. However, the performance of triangular-aperture geogrid-reinforced bases has not been well evaluated. In this study, unreinforced and triangular-aperture geogrid-reinforced bases over a weak subgrade were constructed in a large geotechnical testing box (2 × 2.2 × 2 m) at the University of Kansas and tested under cyclic loading. During the tests, surface deformations and vertical stresses at the interface between the base and the subgrade were monitored. The test results showed that triangular-aperture geogrids reduced permanent surface deformations and vertical stresses at the interface compared with an unreinforced base. The benefit became more pronounced when a heavier-duty geogrid was used. The backcalculations from the measured vertical stresses at the interface between base and subgrade showed that the stress distribution angle and the modulus ratio of base course to subgrade decreased with an increase in the number of cycles. The rates of reduction in the stress distribution angle and the modulus ratio for the unreinforced base were faster than those for the reinforced bases. This paper focuses on the stress analysis of the test sections under cyclic loading.

Laboratory Study on Geosynthetic Protection of Buried Steel-Reinforced HDPE Pipes from Static Loading

AbstractGeosynthetic layers above a pipe can potentially reduce the deflection and strain in the pipe attributable to static loads. This paper discusses the laboratory results of shallowly buried steel-reinforced high-density polyethylene (HDPE) pipes subjected to static loads with or without geogrid. In the testing, static loads were applied to a steel plate seated on the ground with a 0.61-m-diameter steel-reinforced HDPE pipe buried in a compacted-sand trench. Four static loading tests were run with two different base courses and geogrids inside and above the trench. The test section was instrumented to record pipe deflections, earth pressures, and strains in the pipe wall and geogrid. Installation deflections were monitored and compared with a theoretical model. The measured earth pressures were compared with those estimated by the current AASHTO live-load distribution method. Reduced deflections and strains of the pipe were recorded as a result of the geogrid reinforcement. The type of base course al...

Resistance Factors for Drilled Shafts in Weak Rock Based on O-Cell Test Data

Load and resistance factor design (LRFD) has been mandatory for all FHWA-funded bridges since October 2007. The resistance factors included in the current AASHTO specifications for foundation design are not all calibrated by using field data. A calibration of resistance factors for side resistance of drilled shafts in weak rock is based on the statistical data collected from 19 O-cell tests in the midwestern United States. The field test data were used to determine the measured resistance, and the in situ rock properties and the dimensions of drilled shafts were used to calculate the predicted resistance by using the FHWA method. The Monte Carlo method was selected to perform the calibration. On the basis of the normally distributed loads and log normal distributed resistance from the test data, side resistance factors were determined at a target reliability index of 3.0. The calibrated resistance factors were compared with those in the current AASHTO LRFD Bridge Design Specifications.

Analysis of Laterally Loaded Piles in Sand Considering Scour Hole Dimensions

AbstractScour is a process of soil erosion, which can occur around the foundations of bridges or offshore structures through the action of flowing water; it can, therefore, reduce the capacity of the foundations and sometimes lead to the failure of structures. During an analysis of scour effects on laterally loaded piles, scour hole geometry is often ignored. Instead, scour effects are considered by simply removing the whole soil layer to the scour depth. However, a scour hole has not only depth, but also width and a slope. At present, the widely used p-y method for analyzing laterally loaded piles cannot consider three-dimensional scour hole dimensions. For this reason, a simplified method was developed herein for the analysis of laterally loaded piles in sand under a scour condition. The p-y curves in this method were based on a wedge type of failure. The effects of the scour hole dimensions on the response of laterally loaded piles in sand were evaluated using this simplified method. The computed resul...