Marolo C. Alfaro

Soil-geogrid reinforcement interaction by pullout and direct shear tests

Pullout and direct shear tests have been conducted to investigate the soil-reinforcement interaction behavior in pullout and direct shear mechanisms. An apparatus—discussed in this paper—has been developed that is capable of performing both pullout and direct shear tests. Displacement measurements to monitor dilatancy on the backfill soil during pullout tests are incorporated in the apparatus. Test results for the geogrid type of reinforcement embedded in dense granular soil are discussed. The devised testing program provides valuable information on the appropriate interaction models and parameters that will be employed in the design and analysis of reinforced soil structures. The dilatancy measured in the laboratory may also provide useful information on the increase of interface shear resistance when dilatancy is restrained under field conditions.

INTERACTION BETWEEN COHESIVE-FRICTIONAL SOIL AND VARIOUS GRID REINFORCEMENTS

Abstract A total of 52 large-scale laboratory pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the pullout test, and the maximum pullout resistance was reached within a relatively small pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of resistance mobilization along the reinforcement varies, and the pullout-resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction resistance from the longitudinal member contributed only to about 10% of the total pullout resistance of the grids. The pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the pullout resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a pullout test because of the complicated pullout-resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear resistance can exceed the direct-shear resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.

IMPROVEMENT OF SOFT BANGKOK CLAY USING VERTICAL DRAINS

Abstract This paper presents case records demonstrating the use of vertical drains on soft Bangkok clay. The documented cases include four sand drains and two prefabricated vertical drains. The observed performances of the vertical drains are evaluated. The effects of soil flow parameters, including smear effects due to the installation of the drains, in particular the prefabricated vertical drains, are also evaluated in the light of the current practice of using vertical drains for ground improvement. It is concluded that the use of vertical drains is a viable ground improvement method for soft Bangkok clay.

Prediction of vertical-band-drain performance by the finite-element method

Abstract This paper presents the predictions of the performance of vertical-band drains (i.e. Alidrains) on soft Bangkok clay under full-scale embankment loading in the field and during large-scale consolidation with a drain by using a reconstituted sample in the laboratory. A finite-element model of transient- and axisymmetric-flow problems considering equal strain was used. The numerical model also included the smear effects. The predicted data were compared with the laboratory- and field-test results. Laboratory consolidation tests using a Rowe cell and an oedometer apparatus were performed to obtain the compressibility parameters. By using a value of kh/ks of together with ds=2dm, the predicted results agreed with the field performance of the vertical drains. In the laboratory, by using a reconstituted clay specimen, better predictions were obtained with kh/ks=2 and ds=2ds. Thus, the prediction by means of an equal-strain finite-element model was found to be satisfactory for the performance of vertical-band drains on soft Bangkok clay provided that the disturbance due to its installation was taken into account.

Adaptation Strategies for Road Embankments on Permafrost Affected by Climate Warming

Road embankments in Northern Manitoba experience lateral spreading and settlements that result in longitudinal cracking of road surfaces. Highways and road embankments can degrade permafrost because their construction and use increase the ground thermal regime beneath the embankments. In addition climate warming trend is likely to magnify this problem. When thawed, the permafrost beneath the road embankments results in differential settlements and dips, which contribute to a concern of public safety. A thermal modeling to reproduce the conditions and trends in the ground beneath road embankments in areas of discontinuous permafrost in Northern Manitoba was used as basis for assessing the impacts of climate change. Based on the simulated impact of climate change to the permafrost, a stress-deformation analysis was performed to determine the deformation and stability of the embankments with and without mechanical stabilization of the degrading permafrost.

Highway Embankment on Degrading Permafrost

The highway embankment is about 800 km (500 miles) north of Winnipeg, Manitoba, Canada in an area where permafrost is degrading rapidly. Soil sampling and installation of instrumentation (temperatures, settlements, lateral deformations and pore water pressures) was undertaken in October 2008. The paper describes the past performance of the embankment, the instrumentation that has been installed, data from the instruments during the first winter of operation, and initial results from the laboratory testing program.

Evaluating shear mobilization in rockfill columns used for riverbank stabilization

The major rivers within the City of Winnipeg are founded in glacial Lake Agassiz clay and silt sediments that have low shear strength. As such, riverbank instabilities are a common issue along many stretches of the rivers. The use of rockfill columns has become an increasingly utilized approach for stabilizing failing banks. Recent cases in Winnipeg have shown that movements can occur following installation of rockfill columns. Uncertainty regarding the magnitude of these movements that is required to mobilize shearing resistance in the rockfill columns has resulted in situations where the stability of riverbanks following remediation has been questioned. This has provided a need to improve our understanding about how much movement a stabilized slope must undergo before sufficient shear resistance of the rockfill column will be mobilized. The results of experimental testing conducted to assess the shear mobilization of rockfill column materials using a large-scale direct shear test apparatus are presented...

Prediction of Embankment Settlements by In-Situ Tests

The compressibility of soft Bangkok clay preloaded with an instrumented test embankment was studied using the results of a series of in-situ and laboratory tests. In the field, eight screw plate and eight pressuremeter tests were performed at four test levels in the subsoil. In the laboratory, twelve Rowe cell consolidation tests were carried out. The method of Asaoka (1978) was used in determining the coefficient of consolidation from the screw plate test. It was found that Cv values obtained from the Rowe cell consolidation tests were twelve times smaller compared to those derived from the screw plate tests. The soil parameters obtained from the two types of in-situ tests conducted in this study were then used to predict the settlement behavior of the two test embankments, one improved by prefabricated vertical drains (PVD), and the other, on unimproved ground. The difficulty associated with the proper choice of the drained modulus values are highlighted, and it was found that the predicted settlement magnitudes using elastic theory can vary considerably for the ratio of drained and undrained soil moduli, E′/Eu, ranging between 0.25 to 0.50. The undrained modulus from the screw plate test corresponding to the fitted data was found to be 1.38 times the undrained modulus from the pressuremeter test. General agreement with the observed settlement was found using the graphical procedure of Asaoka (1978) and adopting the coefficient of consolidation from the screw plate test.

Laboratory Performance of Geogrid-reinforced Soils Subjected to Freezing and Thawing

This paper reports results from bench-scale plane-strain laboratory tests conducted in order to investigate the behavior of geogrid-reinforced silt specimens during freezing and thawing cycles. Soil–geogrid interaction was analyzed through comparison of the soil and geogrid strains. Vertical and lateral pressures were applied to the specimens to simulate anisotropic loading conditions in the field. Reinforced and unreinforced specimens were subjected to cycles of freezing (−25°C) and thawing (+23°C) temperatures inside a walk-in temperature-controlled environmental chamber. Measured geogrid strains at the end of 12 freezing-thawing cycles were on the order of 0.57 %. An additional strain of this magnitude in the reinforcement due to the freezing-thawing cycles would have only minimal effect on working strain levels in the design, which range from 1 % to 2 %. However, the strains induced by freezing and thawing can approach working design strains as the number of cycles increases. This could have significant long-term implications if accumulated strains were to overstrain the geogrid. The soil deformations were observed to be mostly horizontal. This pattern of deformations during the freezing and thawing of silt could result in shallow sliding at the face of slopes and embankments. The soil strains were higher than the geogrid strains, indicating relative movements between soil and reinforcement, mostly during thawing.

Segregation Potential from a Highway Embankment on Thawed Permafrost

Frost heave occurs in soils where there is seasonal soil freezing with a supply of water; and in the active layers of permafrost soils in cold regions. In a series of papers, Konrad and Morgenstern developed their theory for frost heave prediction using the concept of segregation potential (SP) in fine grained soils. Most research on segregation potential has been done in the laboratory under controlled laboratory conditions. Fewer projects have studied frost heaving of natural ground in the field. This paper presents estimates of segregation potential from data collected from an instrumented full-scale, relatively low, engineered embankment.