Comparative Study of Soil Properties for Stability Analysis of Residential and Rice-Field-Converted-To-Subdivision Land Areas in Bulacan, Northern Philippines

Date of Award

7-1-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics

First Advisor

Joel T. Maquiling, PhD

Abstract

Knowledge of soil properties is useful in any investigation that focuses on human management of land resources. The soil is not an undifferentiated medium but is rather a dynamic one that is constantly in formation and undergoing transformation. In reality, soils are not only fine-grained material, it is a combination of several sizes of grain from coarse to finer particles. Several geo-technical properties of soil affect the stability of built structures. The objective of this study is to conduct an investigation on the soil properties of local residential (LR) and rice-field-converted-to-subdivision (RFCS) land areas in the same town in Bulacan, Northern Philippines. Soil samples to a depth of about 30 cm were taken from each area and subjected to geo-technical soil analysis. These core soil samples were tested for index, basic and engineering properties of soil. Results reveal that the RFCS contained 95% aggregates finer than 75 μm while the LR contained only 18% aggregates smaller than 75 μm. Moreover, the RFCS was found to have a plastic index of about 27 while the LR was approximately 0, and is therefore considered non-plastic. Specific gravity values are 2.68 and 2.56 for the RFCS and LR respectively. In addition, the RFCS has lower Maximum Dry Density (MDD) of 1616 kg/m3 compared to 1775 kg/m3 of LR. Also, a lower permeability of 1.3090 X 10⁻4 cm/s for RFCS while 9.0812 X 10⁻² cm/s for LR. The RFCS is also a highly expansive soil with Free-Swell Index (FSI) of 123.59% compared to only 20.42% of LR. California Bearing Ratio (CBR) values at 100% MDD are 14% for LR and only 2% for RFCS. The shear strength measured by the soil’s cohesion and angle of friction for LR (c=2.63; ∅=41.82°) and RFCS (c=34.266; ∅=31.07°). Based on the geotechnical soil analysis, LR is more stable than RFCS since it has greater entropy due to varying size and distributions of particles; denser particles that provides higher resistance due to additional interlocking between its particles; and a moderate degree of expansiveness, non-plastic and greater shear strength at higher normal stress. The addition of ashfall to RFCS decreases its plasticity and free swell index while increases its percolation rate. On the contrary, no significant effect was observed in LR-AF proportions. These results show that there should be considerable geo-technical research done on the prevalent practice of converting rice fields to low-cost subdivisions in Bulacan, Northern Philippines.

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