Apiniti Jotisankasa

The Development of a Suction Control System for a Triaxial Apparatus

A new method of controlling suction in the triaxial apparatus is presented that consists of two subsystems: one each for drying and wetting that is used in combination with an independent measurement of suction. The drying system is based on a development by Cunningham et al _2003_, with improvements in the measurement of the current water content. The wetting system involves the staged circulation of air and water. This new technique enables continuous monitoring of all state variables during tests, including water content, degree of saturation, net stresses, suction, and the different components of strain. Some typical results from tests on a compacted silty clay are presented in order to demonstrate the capabilities of the system. The main advantage of the apparatus is that there is no need to raise the ambient air pressure as is required with the axis translation technique and tests may therefore be conducted at negative pore water pressure. The system may also be easily incorporated into other standard apparatus.

Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas

The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0–7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly ...

Direct Shear Testing of Clayey Sand Reinforced With Live Stake

Large direct shear tests were conducted on compacted clayey sand, reinforced with Jatropha live stakes of various ages, under saturated and unsaturated conditions. The relationship between root cohesion and the side root area ratio appeared linear. For samples with stake stabilization but no fibrous roots, a stake installed in a larger drilled hole appeared to contribute lower strength increase and greater shear displacement than a stake in a smaller hole. All the unsaturated samples at a suction of approximately 23 kPa exhibited strain-hardening behavior. The rate of strength increase due to the root was affected by the soils suction. The additional strength due to live stake and fibrous root could be reduced by a factor of 3.5 if the conditions change from unsaturated (at about 23 kPa suction) to fully saturated. A new model was proposed to account for the influence of suction on root cohesion considering the suction effects on bond stress, root tensile strength, root tensile modulus, and shear zone thickness.

Stability Analysis of Vegetated Residual Soil Slope under Rainfall Conditions

Effects of climate change including intense rainfalls have resulted in many slope failures. The bioengineering method using vegetation is an environmentally-friendly alternative for slope stabilisation. This paper presents the effect of root reinforcement on stability of vegetated slope under rainfall condition. A transient seepage and slope stability analysis was conducted based on the finite element and limit equilibrium methods. A field monitoring on a residual soil slope in Thailand was numerically modelled. In transient seepage analysis, the soil water characteristic curve and the saturated permeability measured from laboratory and in-situ tests were used and the results are comparable with the monitored values from the site. For slope stability analysis, the effective shear strength parameters from direct shear tests were used to model the well-compacted and non-compacted soil slopes. The additional shear strength of soil from the root cohesion was modelled for vegetated soil slope. Results exhibit ...

Stability analysis of vegetated residual soil slope in Thailand under rainfall conditions

Effects of climate change including intense rainfalls have resulted in many slope failures. The bioengineering method using vegetation is an environmentally-friendly alternative for slope stabilisation. This paper presents the effect of root reinforcement on stability of vegetated slope under rainfall condition. A transient seepage and slope stability analysis was conducted based on the finite element and limit equilibrium methods. A field monitoring on a residual soil slope in Thailand was numerically modelled. In transient seepage analysis, the soil water characteristic curve and the saturated permeability measured from laboratory and in-situ tests were used and the results are comparable with the monitored values from the site. For slope stability analysis, the effective shear strength parameters from direct shear tests were used to model the well-compacted and non-compacted soil slopes. The additional shear strength of soil from the root cohesion was modelled for vegetated soil slope. Results exhibit ...

Influence of the spatial variability of the root cohesion on a slope-scale stability model: a case study of residual soil slope in Thailand

The bioengineering method using vegetation is an ecological approach for slope stabilisation. However, due to a large variability of vegetation root patterns, a precise quantification of root reinforcement is relatively difficult, leading to a reluctance to use such a technique in practice. This paper presents a probabilistic framework for slope stability analysis considering the spatial variability of root reinforcement. A residual soil slope under a heavy rainfall event was used to model the seepage and stability analysis. The effect of root reinforcement was considered through an additional soil shear strength or root cohesion. Typical characteristics of the root reinforcement of vetiver grass (Chrysopogon zizanioides) in Thailand were assumed in the analysis. A probabilistic analysis was performed considering both stationary and non-stationary random fields of root cohesion. The results indicated that the failure of the vegetated slope could occur when the variance coefficient of the root cohesion was more than a critical value (a critical cov = 0.45 for the uniformly distributed root cohesion case and a critical cov = 0.32 for the case of linear decrease of root cohesion in this particular slope). In practice, the efficiency of the bioengineering method can be improved by controlling the variation of root cohesion within such limits.

Classification of Shrinkage and Swelling Potential of a Subgrade Soil in Central Thailand

This paper presents the classification of shrinkage and swelling potential of soils based on total suction measurement. Undisturbed subgrade samples were collected from highway route no. 357 located in the central region of Thailand, which encountered severe longitudinal cracking along asphalt pavement shoulder prior to trafficking. Results suggested that the proposed method based on Atterberg limits and total suction measurement provided a more reliable means of classifying shrinkage and swelling potential than that based on the swelling oedometer test.

Influence of Kaolin Mixture on Unsaturated Shear Strength of Decomposed Granitic Silty Sand

The paper reports on the influence of kaolin content on shear strength of a decomposed granitic silty sand. The granitic soil was mixed with kaolin, with the purpose of erosion protection and slope stabilization. Shear strength and water retention behaviour of the decomposed granitic silty sand, mixed with different proportions of kaolin (from zero up to 20% by weight) were tested. A tensiometer was also incorporated in a direct shear box to investigate unsaturated properties. The silty sand mixed with 10% kaolin possesses the highest saturated shear strength. The effective cohesion increases with increasing kaolin content, while the effective friction angle reaches the maximum value at the 10% kaolin. In the unsaturated condition, the angle of shear resistance with respect to suction increases with kaolin content. Based on the test results and field observation, the silty sand mixed with 10% kaolin appears to be most erosion-resistant.

Errata for “Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas” by Apiniti Jotisankasa and Warakorn Mairaing

December 2009, Vol. 135, No. 12, pp. 1932–1940. DOI: 10.1061/ ASCE GT.1943-5606.0000157 W. M. Yan, M.ASCE Assistant Professor, Dept. of Civil Engineering, Univ. of Hong Kong, Pokfulam Road, Hong Kong, China corresponding author . E-mail: ryanyan@hku.hk Ka-Veng Yuen Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Macau, Macau, China. Gil Lim Yoon, A.M.ASCE Principal Researcher, Dept. of Coastal Development and Energy, Korea Ocean Research and Development Institute, Ansan, South Korea 425-600.