Geotechnical Properties of Tropical (Lateritic) Soils and Their Implications for Road Construction: A Case Study from Bahir-Dar, Ethiopia

Mohammad Yesuf, Afzal Khan, and Argaw Asha

ABSTRACT: The research demonstrates the geotechnical properties of tropical soils and their implications for road construction. A series of standardized geotechnical and geochemical laboratory tests on lateritic soils of Bahir-Dar (Ethiopia) were conducted. Soil samples at depths of 0.6m and 1.5m were collected from five sites. The silica to sesquioxide ratio indicated that soils are lateritic. The findings show invariable particle-size distribution and Natural Moisture Content range between 4.53 to 12.2%. Average Maximum Dry Density and Optimum Moisture Content were 1.563 and 20.58 respectively. The specific gravity ranges from 1.95 to 3.09. The LL, PL, and PI range from 42 to 86.3%, 28.38 to 38.4%, and 13.12 to 49.7% respectively. The unsoaked CBR and corresponding soaked CBR values range from 4.86 to 14.36% and 1.22 to 3.88% respectively at 65 blows of the modified proctor. The results evaluated suitability of soils according to the Ethiopian Roads Authority standards and implicated that partially Test Pits-C, D & E soils satisfy to be a sub-grade or embankment material.

KEYWORDS: Road construction, Laterization, Geotechnical properties, Subgrade material, and Ethiopia.

Performance of Cost-Effective Deep Tube Wells for Groundwater Development in Bangladesh

Nazrul I. Khan, Tomoharu Hori, and Mohammad A. Kader

ABSTRACT: Different water-lifting devices have ensured irrigation and public water supplies for urban and rural areas of Bangladesh. A cost-effective deep tube well (DTW) model was developed by the Rural Development Academy (RDA), Bogura, when the government subsidised groundwater withdrawal in Bangladesh. The aim of this study was to analyse the performance of borehole technology in terms of cost, design, effectiveness, and adoption in rural Bangladesh. The cost of the RDA-developed DTW was lower (0.06–0.5 million BDT) than that of the traditional DTW, and the discharge capacity varied from 20 to 200 m3/hr. The modified borehole by the RDA used its developed design, locally available materials, local human resources, and manual drilling to construct a DTW that exhibited excellent performance with minimal cost. The output of the DTW is demand-based technology, in which the discharge capacity depends on the farm size and the number of beneficiaries. Therefore, RDA-developed DTW technology is highly appreciated by non-governmental organisations and local-level entrepreneurs in Bangladesh to improve groundwater development.

KEYWORDS: Irrigation, Groundwater utilisation, Rural development, and Water resources.

Geometry and Roughness Effect at the RCC Interface-Earth Dam Causing Leakage in Mae Suai Composite Dam, Thailand 

Soralump, K. Wannasiri, and S. Prempramote

ABSTRACT: Mae Suai dam is an earth zone dam with RCC section in the center of downstream slope located at Mae Suai district, Chiang Rai, Thailand. The differential settlement at the joint between RCC spillway and the earth-filled section caused leakage near the crest of the dam. This unusual settlement is the result of a joint where materials of different stiffness are connected, the settlement pattern depends on the shape and interface shear strength of the contact area. In this paper, non-uniform settlement of Mae Suai dam was investigated by 2D & 3D FEM using MIDAS GTS for Fully-Coupled Stress-Seepage analyses. The results of finite element analyses were verified with the geodetic measurements data and observation in rehabilitation processes of Mae Suai dam. Roughness and taper effects were studied by analyzing the settlement along the dam crest using 2D FEM analysis by varying the interface stiffness between the earth dam and RCC spillway to characterize the settlement of each condition and verified with the geodetic measurements. The analysis revealed that the effect of roughness and taper resulted in non-uniform. If these types of dam structures are selected in other projects, the pattern of settlement should be considered in accordance with the guidelines for dam crest design to prevent damage and leakage caused by the non-uniform settlement of the dam.

KEYWORDS: Composite dam, Differential settlement, Finite element analysis, and Taper effect.

Assessment of Variation in Degree of Saturation Due to Rainfall for Landslide Study

Prashant Sudani and K. A. Patil

ABSTRACT: A landslide is a mass movement of the earth under gravity due to natural or man-made triggering events. Foremost of the landslides in humid countries like India are rainfall-induced landslide movements. Rainfall tends to infiltrate into the slope, leading to the recharge of the soil mass with water, which depends on the void present in the medium. This water content in the slope mass alters the slope-forming soil’s properties in many ways, i.e., increasing the weight of the slope soil, reducing the soil strength, increasing pore water pressure, and changes in the geotechnical properties of the soil mass. Numerous models are available for predicting landslides based on water present in the soil. However, the link between rainfall precipitation and corresponding real-time monitoring of water stored in the soil system is still missing. Variation in the saturation is required to be assessed corresponding to rainfall events. The present study aims to monitor the variation in the degree of saturation in the soil mass corresponding to rainfall. The leaky barrel concept is used to model the test set-up for deriving the saturation evolution in the soil mass. The output of the research interpretation successfully mapped the variation in the water saturation for the soil mass associated with rainfall events.

KEYWORDS: Landslide, Degree of saturation, and Leaky barrel.

Effectiveness of Soilcrete to Reinforce Earth Levees

Bich Thi Luong and Hoang-Hung Tran-Nguyen

ABSTRACT: Soil – cement (soilcrete) columns have high potential to create seepage cut off walls and to enhance slope stability for earth levees against annual floods in the Mekong delta. This paper evaluated the effectiveness of soilcrete walls to reinforce earth levees. The strength and hydraulic conductivity of several typical soil types taken in the Mekong delta such as soft clay, medium soft clay, and medium stiff clay mixing with cement at a content of 300 kg/m3 were determined in the laboratory. Soilcrete walls were designed basing on the tested results in the laboratory to reinforce earth levees in Dong Thap and An Giang provinces in the Mekong delta. The seepage and slope stability of the levees were analyzed using the SEEP/W and SLOPE/W softwares. The results indicate that the soilcrete walls were highly effective on cutting seepage off and increasing stability significantly in the case of rapid drawdown of floodwater. A 0.4-m single row soilcrete wall can reinforce successfully for earth levees. A 0.8-m double row soilcrete wall can reinforce earth levees sustainably.

KEYWORDS: Permeability, Soilcrete, Earth levee, Seepage, and Slope stability.

Numerical Study of Geogrid-Reinforced Retaining Wall

Marwa Feligha, Fatima Zohra Benamara , Nouaouria Mohamed Salah, Benayoun Fadila, and Souhila Rehab Bekkouche

ABSTRACT: The use of geogrid-reinforced soil retaining walls has increased considerably, due to several advantages such as their relative and rapid construction, aesthetics and good seismic performance. The behavior of these structures is complex, which requires research to better understand the effect of certain parameters on the behavior of geogrid reinforced soil structures. The aim of this work is the numerical modeling of geogrid reinforced soil retaining wall with a segmental facing geogrid-reinforced retaining wall using the finite element code (PLAXIS2D). The objective of this work is to investigate the influence of some geometrical and mechanical parameters, on the behaviour of a geogrid-reinforced soil retaining wall to analyze numerically the peak tensile strength, lateral facing displacements, critical failure surfaces and safety factor. The numerical results show that the failure plane occurred in the reinforced zone at the mid-height, this observation contradicted the triangular distribution with depth assumed in conception methodologies for reinforced soil retaining wall. The distribution of peak tensile strength with depth was bilinear at high loading increments and became trapezoidal at low loading ones.

Furthermore, it was found that the behaviour of a geogrid-reinforced soil retaining wall is independent of loading increments width beyond 0.5H, which coincides with the Ranking failure surface. It also seems that the location of the loading increments and the loading values can change the shape and the position of the peak tensile strength mobilized along the geogrid from a position closer to the facing to the soil reinforcement, which is more pronounced for the higher loading values. As far as the present study is in agreement with the AASHTO conception method according to which the failure surface, was based on the Rankine plane for a vertical facing and the Coulomb plane with an inclined facing ω ≥ 10°. It also seems that the geogrid inclination angle has a major effect on the lateral facing displacements and safety factor, these results have major implications for conception. The soil-geogrid friction ratio is not considered to have a major effect on the peak tensile strength value for friction ratio greater than 4/7φ.

KEYWORDS: Failure surface, PLAXIS2D, Retaining walls, Reinforced soil, and Tensile strength.