Characteristics of Rainfall-Induced Slope Instability in Cisokan Region, Indonesia

A 25.5 km long access road has been constructed in a hilly area in Cisokan region. Several slope instabilities occurred during the rainy season, particularly at the end of heavy rainfall. A comprehensive study was performed to understand the characteristics of rainfall-induced slope instability. The study consisted of field observation, analyses of field and laboratory test data, and numerical analyses. The study revealed that in general there were two categories of slopes with instability characteristics: (i) slopes with a significant groundwater level increase during rainfall; (ii) slopes with an insignificant groundwater level increase during rainfall. In the first category, the slope instability was caused by a loss of matric suction and eventually the pore-water pressure, uw became positive as indicated by an increase of the groundwater level. In the second category, the slope instability was caused by a loss of matric suction without a rise in pore-water pressure, uw, to a positive magnitude. Two empirical curves of slope stability were developed as a preliminary guidance to assess slope stability during rainfall in the region

Advancement in the Analysis of Seepage through Cracked Soils

Seepage is an important problem analyzed in geotechnical engineering. Conventionally, the analysis is performed in conditions where the soil is intact. The presence of desiccated cracks requires a seepage analysis that considers not only the soil matrix part of the cracked soil but also the crack network. Currently, there are three approaches in the analysis of seepage through cracked soils: (i) analysis by modeling the cracked soil as an intact material with cracks being represented as macropores; (ii) analysis by modeling the cracked soil as a material with a bimodal pore-size distribution; and (iii) analysis by modeling two components of the cracked soil separately: the soil matrix and the crack network. Each approach is reviewed and discussed in this paper. It was found that each approach is suitable for specific cases: (i) the first method is suitable for seepage analysis of cracked soil deep below the ground surface; (ii) the second method is suitable for seepage analysis of cracked soil at the ground surface under a drying process; (iii) the third method is suitable for seepage analysis of cracked soil at the ground surface experiencing rainwater infiltration. Choosing the appropriate method is essential in modeling the appropriate seepage mechanism

Advancement in the Analysis of Seepage Through Cracked Soils

Seepage is an important problem analyzed in geotechnical engineering. Conventionally, the analysis is performed in conditions where the soil is intact. The presence of desiccated cracks requires a seepage analysis that considers not only the soil matrix part of the cracked soil but also the crack network. Currently, there are three approaches in the analysis of seepage through cracked soils: (i) analysis by modeling the cracked soil as an intact material with cracks being represented as macropores; (ii) analysis by modeling the cracked soil as a material with a bimodal pore-size distribution; and (iii) analysis by modeling two components of the cracked soil separately: the soil matrix and the crack network. Each approach is reviewed and discussed in this paper. It was found that each approach is suitable for specific cases: (i) the first method is suitable for seepage analysis of cracked soil deep below the ground surface; (ii) the second method is suitable for seepage analysis of cracked soil at the ground surface under a drying process; (iii) the third method is suitable for seepage analysis of cracked soil at the ground surface experiencing rainwater infiltration. Choosing the appropriate method is essential in modeling the appropriate seepage mechanism

Pengujian Skala Penuh Dan Analisis Perkuatan Cerucuk Matras Bambu Untuk Timbunan Badan Jalan Di Atas Tanah Lunak Di Lokasi Tambak Oso, Surabaya

Bamboo pile-mattress was proposed as an alternative ground reinforcement to support road embankment on soft soils for the Surabaya Eastern Ring Road Project. A full scale test program was performed to investigate reliability of the reinforcement system since standard calculation for this type of construction was not available. The full scale program was consisted of ground settlement monitoring by settlement plates and pore water pressure monitoring by piezometers. Readings were taken for each stage of embankment construction. Analytical and numerical analyses were also performed to verify the full scale test results. Analytical calculation for immediate settlement was performed based on the elasticity theory, for consolidation settlement was performed by the 1-D Terzaghi\u27s consolidation theory. Numerical calculation was performed using the finite element method utilizing a commercial software package called PLAXIS. In the finite element analyses, subsurface soil and embankment were modeled as elastic-plastic solid materials using the Mohr-Coulomb failure criteria. Bamboo piles and mattress were modeled as elastic-plastic springs and elastic-plastics beams, respectively. Monitoring records and analysis results show that bamboo pile-mattress reinforcement has effectively increased stability. Consolidation settlement was found to be relatively uniform. Results also show that monitoring records and analysis results are relatively similar

PENGUJIAN SKALA PENUH DAN ANALISIS PERKUATAN CERUCUK MATRAS BAMBU UNTUK TIMBUNAN BADAN JALAN DI ATAS TANAH LUNAK DI LOKASI TAMBAK OSO, SURABAYA

Bamboo pile-mattress was proposed as an alternative ground reinforcement to support road embankment on soft soils for the Surabaya Eastern Ring Road Project. A full scale test program was performed to investigate reliability of the reinforcement system since standard calculation for this type of construction was not available. The full scale program was consisted of ground settlement monitoring by settlement plates and pore water pressure monitoring by piezometers. Readings were taken for each stage of embankment construction. Analytical and numerical analyses were also performed to verify the full scale test results. Analytical calculation for immediate settlement was performed based on the elasticity theory, for consolidation settlement was performed by the 1-D Terzaghi’s consolidation theory. Numerical calculation was performed using the finite element method utilizing a commercial software package called PLAXIS. In the finite element analyses, subsurface soil and embankment were modeled as elastic-plastic solid materials using the Mohr-Coulomb failure criteria. Bamboo piles and mattress were modeled as elastic-plastic springs and elastic-plastics beams, respectively. Monitoring records and analysis results show that bamboo pile-mattress reinforcement has effectively increased stability. Consolidation settlement was found to be relatively uniform. Results also show that monitoring records and analysis results are relatively similar

Compaction Control Using Degree of Saturation and Plasticity Index on Tropical Soil

Soil compaction in the field is conventionally controlled using maximum dry density, (ρd)max, and optimum moisture content, (w)opt, as the target properties. However, achieving accurate control of these target properties can be difficult due to variation of compaction energy level (CEL) and soil type. Recently, a novel soil compaction control approach using optimum degree of saturation, (Sr)opt, as the target properties has been proposed. It was argued that (Sr)opt can be a better compaction control property as the value is less sensitive to the variation of CEL and soil type. This paper presents an investigation of the compaction characteristics of tropical soils from several locations in Indonesia based on both primary and secondary data. This study was performed by exploring the relationships between (i) dry density (ρd) and Sr, (ii) (ρd) and plasticity index (PI), (iii) (ρd) and CBR, as well as (iv) (ρd) and permeability. This study showed that the (Sr)opt of the soils was 91.2%, with variation between 81.2% and 96.5%. This study also showed that (ρd)max can be related to PI at a given CEL. It is expected that the proposed relationships can be better references for field compaction control practices in Indonesia

Compaction Control Using Degree of Saturation and Plasticity Index on Tropical Soil

Soil compaction in the field is conventionally controlled using maximum dry density, (ρd)max, and optimum moisture content, (w)opt, as the target properties. However, achieving accurate control of these target properties can be difficult due to variation of compaction energy level (CEL) and soil type. Recently, a novel soil compaction control approach using optimum degree of saturation, (Sr)opt, as the target properties has been proposed. It was argued that (Sr)opt can be a better compaction control property as the value is less sensitive to the variation of CEL and soil type. This paper presents an investigation of the compaction characteristics of tropical soils from several locations in Indonesia based on both primary and secondary data. This study was performed by exploring the relationships between (i) dry density (ρd) and Sr, (ii) (ρd) and plasticity index (PI), (iii) (ρd) and CBR, as well as (iv) (ρd) and permeability. This study showed that the (Sr)opt of the soils was 91.2%, with variation between 81.2% and 96.5%. This study also showed that (ρd)max can be related to PI at a given CEL. It is expected that the proposed relationships can be better references for field compaction control practices in Indonesia

Variability of N-SPT-Correlated Undrained Shear Strength of Alluvial Deposit in Doplang Region, Central Java, Indonesia

The need to better quantify the variability of soil shear strength and its relations with the factor of safety is increasing in Indonesia. However, this aspect has not yet been studied thoroughly. This paper presents an attempt to quantify the variability of undrained shear strength in relation with the factor of safety of an alluvial deposit in the Doplang region, Central Java, Indonesia. A relationship between the undrained shear strength, su, and N-SPT for the deposit was found as su = 3.4 N-SPT. The variability of the undrained shear strength was quantified utilizing the coefficient of variance, s/m (the sample standard deviation, s over the mean, m) of the N-SPT correlated undrained shear strength. The variability of the undrained shear strength was investigated for the soil near ground surface. The deposit had a value of s/m ranging from 0.15 to 0.25 near ground surface. The variation of s/m tended to follow normal and lognormal distributions. Relationships among the coefficient of variance, the probability of failure, and the factor of safety in terms of soil strength for normal and lognormal distributions were developed. For the value of s/m near ground surface, the relationship between the probability of failure and factor of safety was obtained

Theoretical Equations for the Ratio of Undrained Shear Strength to Vertical Effective Stress for Normally Consolidated Saturated Cohesive Soils: Theoretical Equations for the Ratio of Undrained Shear Strength to Vertical Effective Stress for Normally Consolidated Saturated Cohesive Soils

Abstract Two theoretical equations are developed to calculate the ratio of undrained shear strength to the vertical effective stress (the ratio of (su/sv’)) for normally consolidated saturated cohesive soils. The effective stress approach is used as the basis in the development of the theoretical equations. The theoretical equations are developed by relating the total and the effective stress paths. The development of the excess pore-water pressure is quantified using Skempton A and B pore-water pressure parameters. The theoretical equations are developed for two initial stress conditions: (i) an initially hydrostatic condition and (ii) an initially Ko (non-hydrostatic) condition. The performance of the theoretical equations of this study is compared with field and laboratory measurement data obtained from the literature. The close results between the theoretical equations and the measurements show that the theoretical equations of this study can compute the ratio of (su/sv’) well. Using the theoretical equations, the values of the ratio of (su/sv’) commonly used in engineering practice can be explained from the soil mechanics framework. Keywords: Saturated cohesive soils, c/p ratio, normally consolidated soil, undrained shear strength, effective shear strength, theoretical equation. Abstrak Dua persamaan teoritis dikembangkan untuk menghitung rasio kuat geser tak teralirkan dengan tegangan efektif vertikal (rasio (su/sv’)) untuk tanah kohesif jenuh terkonsolidasi normal. Pendekatan tegangan efektif dijadikan dasar dalam pengembangan kedua persamaan teoretis ini. Persamaan teoretis tersebut dikembangkan menghubungkan lintasan tegangan total dan lintasan tegangan efektif. Kenaikan tekanan air pori ekses dikuantifikasi menggunakan parameter tekanan air pori A dan B dari Skempton. Persamaan teoretis dikembangkan untuk dua kondisi tegangan awal: (i) tegangan awal hidrostatik dan (ii) teganan awal Ko (non hidrostatik). Kinerja kedua persamaan teoretis tersebut dibandingkan terhadap data pengukuran lapangan dan pengujian laboratorium yang diperoleh dari literatur. Persamaan teoretis dari studi ini memiliki kinerja yang baik dalam memperhitungan rasio (su/sv’) yang ditunjukkan dengan dekatnya hasil perhitungan menggunakan persamaan teoretis dan hasil pengukuran lapangan maupun pengujan laboratorium. Dengan persamaan teoretis tersebut, nilai rasio (su/sv’) yang biasa digunakan dalam rekayasa praktis bisa dijelaskan secara mekanika tanah. Kata-kata Kunci: Tanah kohesif jenuh, rasio c/p, tanah terkonsolidasi normal, kuat geser tak teralirkan, kuat geser efektif, persamaan teoretis.

Wetting hydraulic properties of cracked soils

In the field, desiccation cracks in soil occur commonly at the ground surface. The presence of cracks results in an increase in water flow rate in this zone due to lesser resistance to flow of water through crack openings. Some models have been proposed previously to analyse flow through cracked soils. However, a model to quantify lateral flow rate through a cracked soil and a model to analyze change of water content of cracked soil incorporating an actual crack network in the soil has not been fully developed. Laboratory experiments to investigate performance the model also need to be performed. In addition, variation of crack network in the soil need to be incorporated in the model. In this study, a framework to analyze lateral flow through cracked soil was developed. The framework consists two aspects: a model to calculate flow rate through the crack network and a method to analyze change in water content and matric suction of a cracked soil. In this study, the crack network in the soil is idealized as series of linear cracks. In the proposed model to calculate flow rate through the crack network, the flow through a single crack is modelled as a flow through parallel plates and the flow rate through the idealized crack network is calculated by incorporating the conservation of mass principle and the additional head losses due to the change in crack aperture. In the proposed method to analyse change in water content of cracked soils, the idealized crack network is modelled as head boundary conditions and the boundary conditions are then incorporated in a numerical analysis. In addition to that, average water content and average matric suction was proposed to represent the variation in water content and matric suction in a horizontal plane of a cracked soil. Experiments were performed to investigate performance of the proposed model. The experiments mainly consisted of small and large scale lateral flow tests. Small scale and large scale lateral flow apparatuses were developed to perform the tests. The large scale lateral apparatus was developed to capture large number of cracks in the specimens. Cracked soil specimens were obtained by desiccating the soil in room temperature. Flow rates were measured during the lateral flow tests. Water contents were measured during and at end of tests. Instrumentations were utilized to measure change of water content during one of the large scale lateral flow tests. Numerical analyses were performed to investigate performance of the proposed model to analyze change in water content and matric suction of the cracked soil. The results obtained from numerical analyses were compared with those obtained from laboratory experiments. A parametric study was performed to investigate the effect of variation of crack network on change in water content and matric suction of cracked soils. A computer code to generate random crack networks was developed. The computer code generates random crack network from the statistical parameters of the crack network. Connectivity among cracks endpoints are incorporated in the calculation to imitate the desiccation crack network in soils which is interconnected each other. Numerical analyses using the proposed method of change in water content were performed incorporating variation of crack networks. A comparison of the predicted and measured lateral water flow rates showed that the proposed model was able to predict the lateral flow rate through the crack network quite well. Proposed method to analyze change in water content and matric suction of a cracked soil was also able to predict change in water content and matric suction of cracked soils. The results were closer to the measured values than those obtained by modelling the cracked soil as a continuum. This finding indicated that in order to model change in water content and matric suction of cracked soils, the cracks should be modelled as head boundary conditions. In addition, the parametric study showed that with the same statistical parameters of crack network, different average of water content and matric suction can be obtained. Values of maximum variations in average water content and average matric suction were found in this study.Doctor of Philosophy (CEE