Dennes T. Bergado

Pullout force/displacement relationship of extensible grid reinforcements

Abstract A model for predicting the pullout resistance of polymer-grid reinforcement has been proposed. The influence of bearing member rigidity and spacing ratio (S/D) are explicitly expressed in the hyperbolic model. A new bearing capacity equation is incorporated for calculating the maximum pullout force. The displacement along the reinforcement is calculated by using the proposed pullout bearing resistance model together with the elongation of the grid longitudinal member. The validity of the method is confirmed by good agreement between calculated values and actual test data. The analytically determined effective reinforcement embedment lengths (i.e. the length of the reinforcement in tension) and pullout displacement to mobilize the desired pullout resistance of polymeric grids under different backfill conditions and under different applied normal pressures, provide useful information for the design of reinforced earth structures against pullout failure.

A case study of geotextile-reinforced embankment on soft ground

Full-scale test embankments, with and without geotextile reinforcement, were constructed on soft Bangkok clay. The performances of these embankments are evaluated and compared with each other on the basis of field measurements and FEM analysis. The analyses of failure mechanisms and the investigations on the embankment stability using undrained conditions were also done to determine the critical embankment height and the corresponding geotextile strain. The high-strength geotextile can reduce the plastic deformation in the underlying foundation soil, increase the collapse height of the embankment on soft ground, and produce a two-step failure mechanism. In this case study, the critical strain in the geotextile corresponding to the primary failure of foundation soils may be taken as 2.5-3% irrespective of the geotextile reinforcement stiffness

ELECTRO-OSMOTIC CONSOLIDATION OF SOFT BANGKOK CLAY USING COPPER AND CARBON ELECTRODES WITH PVD

Electro-osmotic consolidation of Bangkok clay using copper and carbon electrodes with prefabricated vertical drain was studied. A laboratory testing program was conducted on undisturbed and reconstituted samples in a small cylinder cell and a large consolidometer in order to assess the probable effectiveness of electro-osmotic treatment. The tests were performed under the voltage gradients of 60 and 120 V/m with a polarity reversal of every 24 h. The time to achieve 90% degree of consolidation induced by electro-osmosis ranges from 1.4 to 2.1 and 1.2 to 2.2 times faster than the normal consolidation using PVD only for undisturbed and reconstituted samples, respectively. The faster rate of consolidation and higher magnitude of settlement were achieved at a higher voltage gradient. Higher reduction of water content up to 9% and increase in shear strength up to 144% were obtained using electro-osmotic consolidation with PVD compared to using PVD only, especially when using the carbon electrode. The liquid limit, plastic limit, and plasticity index were increased due to increased salinity during electro-osmotic consolidation. With its total dissolved salts of 4050 ppm well below the 6000 ppm limit, the soft Bangkok clay is considered to be suitable for electro-osmotic consolidation.

New and economical mixing method of cement-admixed clay for DMM application

Results of unconfined compression and one-dimensional compression tests of cement-admixed Bangkok clay confirmed the existence of an optimum mixing clay water content. That figure is defined as the total clay water content of the clay-cement paste that can give the highest possible improvement in strength of cured cement-admixed clay.Some related results of undefined compression tests of higher water content cement-admixed Ariake clay is also presented and discussed in the context of the existence of optimum mixing clay water content. Moreover, a concept supporting the possibility of the existence of optimum mixing clay water content in deep mixing applications has been explained, with a schematic diagram illustrating the effects of mixing clay water contents on the strength development of cement-admixed clay at a particular cement content. Significantly, at optimum mixing water contents, only 10 percent cement content by weight is needed instead of the corresponding 17 percent in conventional mixing methods, resulting in savings of 40 percent in cement content and cost.

Investigation and simulation of behavior of stiffened deep cement mixing (SDCM) piles

Abstract The low strength and stiffness of Deep Cement Mixing (DCM) pile causes unexpected failure that has been mitigated with the introduction of stiffened deep cement mixing (SDCM) pile. The SDCM is a new type of DCM pile reinforced by concrete core pile. In this paper, the interface behavior of SDCM pile and its strength have been studied by various laboratory tests. The cement content was varied from 10 to 20% by dry weight of clay and mixed at the water content corresponding to its liquid limit to obtain optimum strengths. The interface friction between the core concrete pile and the cement-admixed clay was studied by means of the direct shear interface tests and Ko pullout interface tests. The 15% cement content yielded optimum interface shear strength. The CIU triaxial compression test of model SDCM pile revealed that the concrete core pile length should be more than 75% of the DCM pile length in order to have significant improvement. The physical modeling together with numerical modeling of consolidation behavior of SDCM piles on soft Bangkok clay verified the possibility of further reduction on settlement than DCM only. The settlement reduction was directly proportional to the increase in length of the concrete core pile in the SDCM pile.

Reliability-based analysis of embankment on soft Bangkok clay☆

Abstract Six slope failures occurred at random locations along a 10 km embankment adjacent to an irrigation canal. The slope failures occurred when the embankment was raised to 2.05 m above MSL from an average elevation of 1.7 m above MSL coinciding with the lowering of the canal water level at the end of the dry season. Slope stability analysis was carried out using both conventional and reliability-based procedures. The spatial variability of undrained strength, the actual variation in embankment geometry, and the varying water level in the canal were considered in the analysis. Both idealized and empirical autocorrelation functions (ACF) of the undrained shear strengths were used in the analyses. An analysis using a factor of safety based on the deterministic soil profile defined by the mean undrained strength resulted in a prediction favoring a reverse failure pattern along the embankment. Using the probability of failure which incorporates spatial variation of undrained strength and uncertainties associated with stability prediction yielded a result conforming to the actual failure pattern along the embankment. The use of empirical autocorrelation function (ACF) seems to confirm and explain better the occurrence of the failure zones than utilizing the idealized ACF.

Large-scale soil erosion performance test of water hyacinth limited life geosynthetics combined with Ruzi grasses

Abstract First, the performance of woven water hyacinth limited life geosynthetics (LLGs) was investigated in the laboratory using index and hydraulic tests. Second, the field soil erosion control tests were conducted to confirm the performance of woven water hyacinth LLGs with and without vegetation using artificial rainfall. The average flow rate from the permittivity test on woven water hyacinth LLGs increased with higher hydraulic gradient while the transmissivity values decreased at higher normal pressures. From laboratory test results, woven water hyacinth LLGs can be utilized for erosion control in field applications because the moisture retained in woven water hyacinth was high thus it can help the growth of vegetation. In the full scale field erosion control test, the surface runoff was reduced when using woven water hyacinth cover combined with Ruzi grass which was further reduced with increasing growing periods. The soil loss was lowest when combining Ruzi grass with woven water hyacinth. Therefore, woven water hyacinth can be used effectively to control soil erosion and can be enhanced by using in combination with vegetation. In the absence of vegetation, the smaller opening size of woven water hyacinth LLGs with 8×8 mm has better erosion resistance than the larger opening size of 12×12 mm when runoffs were similar for both opening sizes. The results also indicated that using woven water hyacinth with increasing growing period of Ruzi grass the soil loss was greatly reduced. Finally, it can be concluded that for soil erosion control, water hyacinth LLGs combined with Ruzi grass can be applied in sloping ground surface.

Thermal conductivity of soft Bangkok clay

In relation to an investigation into thermal ground improvement using prefabricated vertical drains, thermal conductivity of soft Bangkok clay specimens was measured, at different porosities and temperature levels, using a simple non- destructive steady state test method. In addition, a simple model to simulate the thermal conductivity of saturated fine-grained soils behaviour at different porosities has been introduced. It is formulated in terms of thermal conductivity and volume fraction of each soil phases (solid and water), and a morphological parameter that determine the share of both extreme heat flow models (series/parallel). The proposed model has been validated against thermal conductivity results reported in literature and results obtained from the present investigation. Reasonable agreement has been obtained between the predicted and the measured thermal conductivity values.

The use of polymeric and metallic geogrid on a full‑scale MSE wall/embankment on hard foundation: a comparison of field data with simulation

A full-scale reinforced earth embankment was designed and constructed by the Department of Highways of Thailand on a hard foundation in Phitsanulok Province, Thailand. Two types of reinforcement were used in the embankment. One side was reinforced with polymeric reinforcement consisting of polyester (PET), polypropylene (PP) and high-density polyethylene (HDPE) and referred to as a reinforced steep slope (RSS), with an angle of 70° from horizontal. On the other side, the embankment was reinforced with metallic reinforcement consisting of metallic strips (MS) and steel wire grids (SWG) combined with vertical segmental concrete facing and referred to as a mechanically stabilised earth wall (MSEW). The behaviour of the reinforced soil slope and the mechanically stabilised earth wall on a hard foundation were observed and compared with predictions from the PLAXIS 3D software. The lateral displacements and settlements were very small in the case of the MSEW with inextensible reinforcement. The corresponding lateral and vertical deformations in the RSS were much larger due to its extensible reinforcing materials. The stiffnesses of the reinforcing materials decrease in the following order: MS, SWG, PP, HDPE and PET. The results obtained from three-dimensional (3D) finite element method simulations (using PLAXIS 3D) were in good agreement with the field measurements in terms of vertical and lateral deformations and strains in the reinforcement.

Recent Developments of PVD Soft Ground Improvement: Laboratory Test Results and Simulations

This chapter focuses on the recent developments of soft ground improvement using prefabricated vertical drain (PVD) combined with surcharge, vacuum, and heat preloading in shortening the consolidation time. The laboratory tests were conducted in a large-scale consolidometer with reconstituted specimens using PVD combined with surcharge (PVD only); PVD combined with surcharge and vacuum pressure (Vacuum-PVD); PVD combined with surcharge and heat up to 90 °C (Thermo-PVD); and PVD combined with surcharge, vacuum pressure, and heat up to 90 °C (Thermo-Vacuum-PVD). Analyses were carried out to determine the flow parameters by back-calculation in terms of the horizontal coefficient of consolidation (C h) and the ratio between the horizontal permeability in the undisturbed zone (K h) to the horizontal permeability in the smear zone (K s) or (K h /K s). The C h values for reconstituted specimens with PVD, Vacuum-PVD, Thermo-PVD, and Thermo-Vacuum-PVD were 1.93 m2/year, 2.23 m2/year, 4.17 m2/year, and 4.38 m2/year, respectively, with corresponding K h /K s values of 3.0, 2.7, 1.4, and 1.1, respectively. The results of FEM numerical simulations using ABAQUS software yielded good agreement with the measured settlements and excess pore water pressures. Moreover, the higher temperatures resulted in reduced viscosity of water, which resulted in the increase in the horizontal permeability. The Thermo-PVD and Thermo-Vacuum-PVD resulted in faster rates of consolidation and higher magnitudes of settlement because of the reduction of the drainage retardation effects in the smear zone surrounding the PVD, which resulted in the reduction of K h /K s and increased coefficient of horizontal consolidation, C h.