Liya Gao

Field Evaluation of Dynamic Compaction on Granular Deposits

The dynamic compaction (DC) method is a versatile ground treatment technique with growing popularity. It is applicable to a wide variety of soil types and conditions, particularly sandy materials and granular fills. This study presents a case history of the dynamic compaction with a high energy level of 6,000 kN·m on granular deposits at a site in China. The reclaimed site featured loose backfill with heterogeneity and saturated silt. In order to properly deal with such soil conditions and to optimize the DC design, field tests were conducted to determine the influencing factors in DC. Deformation tests were performed to ascertain the rational spacing of impacts and the optimal number of drops and to provide proofs to the adjustment of the original DC procedure. Monitoring of the pore water pressure helped obtain the time delay between passes. The approach to assess the depth of improvement was discussed based on interpretations of the spectral analysis of surface waves (SASW) test. Analysis of the SASW and plate-load tests demonstrated significant improvement in the soils at the site, with no obvious weak layers. Following dynamic compaction, the allowable ground-bearing capacity and the depth of improvement at the site were no less than 270 kPa and 7.4 m, respectively.

Seismic analysis for translational failure of landfills with retaining walls

In the seismic impact zone, seismic force can be a major triggering mechanism for translational failures of landfills. The scope of this paper is to develop a three-part wedge method for seismic analysis of translational failures of landfills with retaining walls. The approximate solution of the factor of safety can be calculated. Unlike previous conventional limit equilibrium methods, the new method is capable of revealing the effects of both the solid waste shear strength and the retaining wall on the translational failures of landfills during earthquake. Parameter studies of the developed method show that the factor of safety decreases with the increase of the seismic coefficient, while it increases quickly with the increase of the minimum friction angle beneath waste mass for various horizontal seismic coefficients. Increasing the minimum friction angle beneath the waste mass appears to be more effective than any other parameters for increasing the factor of safety under the considered condition. Thus, selecting liner materials with higher friction angle will considerably reduce the potential for translational failures of landfills during earthquake. The factor of safety gradually increases with the increase of the height of retaining wall for various horizontal seismic coefficients. A higher retaining wall is beneficial to the seismic stability of the landfill. Simply ignoring the retaining wall will lead to serious underestimation of the factor of safety. Besides, the approximate solution of the yield acceleration coefficient of the landfill is also presented based on the calculated method.

Seismic stability analyses for landfill cover systems under different seepage buildup conditions

Drainage of landfill cover systems is often inadequate and buildup of seepage forces can occur over time. However, in most design analyses the seepage buildup is neglected and assumed to have only a minor impact on the seismic performance of landfill cover systems. Actually, simply ignoring the seepage buildup will lead to serious overestimation of the factor of safety. A new two-wedge method was developed to analyze the seismic stability of landfill cover systems under different seepage buildup conditions. The solutions of the factor of safety and the yield acceleration coefficient can be obtained, and the permanent displacement of landfill cover systems can also be calculated using the Newmark method. Based on the developed seismic analysis method, the effects of a parametric variation on the seismic stability and permanent displacement of landfill cover systems are presented.

Application of High Energy Dynamic Compaction in Coastal Reclamation Areas

High energy dynamic compaction (HEDC) is adopted in a coastal reclamation area because the grain size of backfilled soil mostly ranges between 20 cm and 100 cm. The in situ tests for evaluating the effectiveness of HEDC were performed on the backfilled soil ground. The crater depth per drop and the whole test zone elevations before and after HEDC were measured and analyzed. Dynamic penetration tests and spectral analysis of surface wave (SASW) tests were used for investigating the improvement depth. Furthermore, the allowable bearing capacity of HEDC treated ground was determined based on the results of plate-load tests. It was found that HEDC did not cause the ground surface heave during construction, and was more effective than low energy dynamic compaction (LEDC) in terms of applied energy utilization. Based on the test results, the improvement depth of HEDC at this site was not less than 14 m, and there was no obvious weak layer within the range of improvement depth. The allowable bearing capacities were larger than 160 kPa. The investigation results indicate that the HEDC technique is an effective way for improving backfilled coarse-grained soil in coastal reclamation areas. This technique helps to achieve both greater improvement depths and higher ground bearing capacities as compared with LEDC.

Estimation of maximum saturated depth in two-layered drainage blankets over the barrier in landfill cover system

For accurately calculating the maximum saturated depth over the landfill barrier and analyzing the variation of liquid head with inflow time, a new method is presented that considers the unsteady-state condition. Based on the principle of water balance and the extended Dupuit assumption, the solution of the new method can be calculated. Also, this method is capable of revealing the effect of the multilayered drainage media. One of the advantages of the method presented in this paper is that it can be used to draw a phreatic surface in layered drainage media under unsteady-state condition and estimate the location of the maximum liquid depth, for both homogeneous conditions and two-layered drainage conditions. Based on the developed analysis method, the effects of a parametric variation on the maximum saturated depth and the variation of saturated depth with time are presented. The calculated results show that with the increase of slope gradients of cover system, the saturated depth of cover system is easy to reach a stable value. Moreover, when the drainage layer consists of two layers with different thickness and hydraulic conductivity, the increase of hydraulic conductivity of the upper drainage layer will make it easier for the saturated depth of cover system to reach a stable and lower value.

Analysis of Tension of Geomembranes Placed on Landfill Slopes

In engineered landfills, geomembranes are commonly used to isolate waste from the surrounding ground and groundwater in order to minimize the potential groundwater contamination. On the sides of a landfill, geomembranes are placed on prepared sloped surfaces and anchored at the crest level. Subsequent construction of a landfill includes the placement of soil cover and waste layers up various heights over these liners. This can result in application of substantial down-slope shear stresses on the geomembranes leading to development of significant geomembrane tension which is necessary to be studied so as to ensure the safety use. In this paper, three-states elastic-plastic model is presented to simulate the shear deformation property for the geomembrane-clay interface, and the governing differential equations of three states are got. Because the demarcation points among three states are unknown, the iterative computation method should be used to compute the tension of geomembrane. Finally, the important factors, including overburden height, slope gradient and geomembrane parameters, which influence the tension of geomembrane, are studied.