This study aims to investigate the possibility of using biopolymer (environmental friendly material) to enhance the mechanical behaviors of collapsible soil. Two types of biopolymers were (xanthan gum and guar gum) used in this study due to their stable behaviors under severe conditions and their availability with reasonable prices. The experimental program focused on three major soil properties, i.e. compaction characterizations, collapsible potential and shear parameters. These three properties are essential in process of soil improvement. Different biopolymer concentrations were used in this study and the experimental program was performed at two curing periods (soon after mixing the soil with the biopolymer and after one week curing time). Shear parameters were measured for the treated specimens under both soaked and unsoaked conditions, while a collapsible potential test was performed under different mixing conditions (wet mix and dry mix). A numerical model was built to predict the behavior of the treated collapsible soil after and before water immersing. The results indicated that the ability of both xanthan gum and guar gum can be used as improvement materials for collapsible soil treatment. The collapsible potential has been reduced from 9% to 1% after mixing the soil with 2% biopolymer concentration in the wet case. After one week curing, the cohesion has been increased from 8.5 kPa to 105 kPa by increasing the xanthan gum concentration from zero to 2%, leading to an overall improvement in soil shear strength. It also proves that the guar gum is superior to the xanthan gum. The shear strength of soil can be increased by about 30% when using the guar gum in comparison with the xanthan gum at the same conditions; however, the collapsible potential of soil material will be reduced by about 20%.

Enhancing mechanical behaviors of collapsible soil using two biopolymers

The dragload on pile groups in consolidating ground was investigated based on a numerical analysis. The case of a single pile and subsequently the response of groups were analyzed by 2D and 3D finite element studies. Conventional continuum elements and special slip elements were used in the analyses for comparison. Based on a limited parametric study, it is shown that dragload for a single pile and group effect are normally overestimated by continuum analyses, compared with the predictions by the slip analyses. The group effect was examined from the slip analysis by considering various factors such as pile configurations, surface loading, interface friction coefficient, and axial loading on piles. An examplary analysis and one previous experimental observation of dragload and group effects were back-analysed. The case histories demonstrated that the slip analysis might predict a better estimate of dragload and group effect compared to the no-slip continuum analysis.

Slip Effect at the Pile-soil Interface on Dragload

Consolidation of unsaturated seabed around an inserted pile foundation and its effects on the wave-induced momentary liquefaction

Experimental study of barrier effect on moisture movement and mechanical behaviors of loess soil

Loess as a subcategory of collapsible soils is a well-known aeolian deposit generally characterized as a highly-porous medium with relatively low natural density and water content and a high percentage of fine-grained particles. Such collapsible soil sustains large stresses under a dry condition with natural water content. However, it can experience high and relatively sudden decreases in its volume once it reaches a certain water content under a certain load and therefore, the natural condition of the soil might not be suitable for construction if the possibility of the exposure of the soil to excessive water exists during the lifetime of the project. This research presents the utilization of an innovative method for stabilization and improvement of Gorgan loessial soil. This method uses electrokinetics and nanomaterials to instigate additives to move through soil pores, as an in situ remedial measure. To assess the acceptability of this measure, the deformability and strength characteristics of the improved collapsible soil are measured and compared with those of the unimproved soil, implementing several unsaturated oedometer tests under constant vertical stress and varying matric suction. The result emphasizes the importance of the matric suction on the behavior of both improved and unimproved soils. The test results indicate that the resistance of the soil was highly dependent on the water content and matric suction of the soil. The oedometer tests on samples improved by 3% lime and 5% nanomaterials show considerable improvement of the collapse potential. Results also reveal that stabilized samples experience notably lower volume decrease under the same applied stresses.

Feasibility of using electrokinetics and nanomaterials to stabilize and improve collapsible soils

Piles located in consolidating soil are subjected to indirect loading resulted from the settlement of adjacent soil. Both pile and soil undergo downward movements caused by the axial pile load and surcharge loading on surrounding area. Both dragload and downdrag imposed on the pile are time dependent. The purpose of this research was to analyse and quantify the dragload and down drag imposed on single pile. Effect of pile head load was considered. Shielded and unshielded piles are analysed. Three dimensional nonlinear analyses using ABAQUS 6.12 have been utilised. The research investigates the size of the surcharge loaded area, location of the pile tip and pile head load on the dragload and down drag imposed on the pile. Based on numerical results the study revealed that sacrificing piles, which are unloaded piles, “hang up” the soil between the piles in the group and, thus, the vertical effective stress around the shielded pile is reduced. Numerical results proved that an average value of the shear stress mobilisation factor β mobilised along unloaded shielded central pile varies between zero at neutral plane (NP) and 0.27. The long term dragload of the shielded floating pile is about 67% of the long term dragload of unshielded floating single pile.

Behaviour of single pile in consolidating soil

Soil collapse occurs when increased moisture causes chemical or physical bonds between the soil particles to weaken, which allows the structure of the soil to collapse. Collapsible soils are generally low-density, fine-grained combinations of clay and sand left by mudflows that have dried, leaving tiny air pockets. When the soil is dry, the cemented materials are strong enough to bond the sand particles together. When natural soil becomes wet, moisture alters the cementation structure and the soil’s strength is compromised, causing collapse or subsidence. Based on soil type and density, the potential for encountering collapsible soils throughout most of the project alignment is low. Conditions in arid and semi-arid climates like Borg El Arab, near Alexandria Egypt favor the formation of the most problematic collapsible soils. The behavior and performance of compacted sand replacement over treated collapsible soil by pre-wetting and compaction are investigated in the current study. Field investigation was performed in the form of plate loading tests conducted on compacted sand replacement over improved collapsible soil. Field plate load tests program was developed to explore the effect of compacted sand replacement thickness on collapsibility potential. Treated collapsible soil was replaced with imported cohesionless soil with variable thickness up to footing width. Results proved that the improvement of collapsible soils by sand/crushed stone replacement is possible to control/mitigate their risk potentials against sudden settlement when exposed to water. Replacement soil increases the rate and reduces the amount of footing settlement. For compacted collapsible soils, partial replacement by compacted sand/crushed stone layers decreases collapsibility potential risk. Results also, introduce the development of practical, economical and environmentally safe geochemical methods for collapsible soil stabilization and collapsible risk mitigation.

https://cyberleninka.org/article/n/282459.pdf

Naema A. Ali

Papers

Behaviour of single pile in consolidating soil

Performance of partially replaced collapsible soil - Field study