H. M. Shahin

STUDY ON GROWTH OF VETIVER GRASS IN TROPICAL REGION FOR SLOPE PROTECTION

River bank erosion and embankment failures happen continuously throughout Bangladesh. From a strictly economic point of view, the cost of remediating these problems is high, and the state budget for such works is never sufficient. This confines rigid structural protection measures to the most acute sections. General reasons of embankment failure are erosion due to rain splash, wave action, overtopping of storm surge. Faulty design, poor maintenance and poor construction also cause failure. The use of cement concrete blocks, stone revetments, geo- bags, and plantation etc. are commonly used for protection of embankment in traditional practices. These materials are expensive and sometimes are not effective to protect the embankments and river bank for an expected design life. On the other hand, slope stability can be augmented by using bio-engineering techniques. Vetiver grass (Vetiveria zizanioides) is being used as an efficient bio-technology for slope protection in many countries, for its special attributes like longer life, strong and long finely structured root system and high tolerance of extreme climatic condition. A few steps have only been taken recently to employ this technique for slope protection purposes in Bangladesh. This paper presents three case studies of vetiver plantation in slope protection against rain-cut and wind-induced erosion. It is found that vetiver grass grows in different soil and climatic conditions of Bangladesh and it is effective for slope protection. Prospect of vetiver plantation in protecting haor low-land is also discussed.

Interaction Effect of Retaining Wall and Existing Foundations in Braced Excavation

Two-dimensional model tests are conducted to investigate the deformation mechanism of the ground and the earth pressure of retaining wall. Numerical simulations with finite element method using FEMtij-2D are also carried out for the same scale of the model tests. Subloading tij model is used in the analyses to model the ground material. Several patterns of the model tests are performed varying the length of the retaining wall and changing the distance between the foundation and wall. It is revealed in this research that maximum surface settlement does not always occur just behind the wall, but mostly at the position of the existing building. The rotation of the foundations depends on the distance between the foundation and wall. The numerical analyses can well simulate the observed earth pressures, surface settlements and deformation mechanism of the ground.

Simple modeling of stress-strain relation for unsaturated soil.

Though stress-strain characteristics of unsaturated soils are complicated and rather different from those of saturated soils, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop’s effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten’s equation to be able to incorporate the influences of suction histories and density is proposed and applied to the proposed model. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations.

EFFECTIVENESS OF JUTE GEOTEXTILES IN FLEXIBLE PAVEMENT CONSTRUCTION

Jute, the forerunners of the manmade fibers, is used as a soil erosion control in highway side slope protection and reinforcing section of new road on poor subgrade. The effectiveness of the jute geotextiles as subgrade soil reinforcement for a flexible pavement has been investigated in this study. A systematic laboratory investigation has been carried out on the sandy soil sample collected from an Integrated Development Project in Dhaka city. Here, a comparative study is performed for the CBR values of the subgrade soil sample with jute geotextiles and without jute geotextiles. The test results show that the CBR value increases significantly with the application of jute geotextiles in subgrade soil. Further investigation has been carried out by placing jute geotextiles at different layers to find out its influence over soil keeping the same dry densities. It is found that CBR increase ratio is higher when jute geotextiles is placed at the bottom layers i.e. away from the loading point. It is also found that in case of sandy soil, the use of jute geotextiles as subgrade reinforcement is less significant when larger compaction effort has been applied.

Effect of Reinforcement on Bearing Capacity of Foundations

To increase bearing capacity of a foundation, geosynthetic is laid beneath the foundation. In previous research, the authors investigated the reinforcement mechanism by changing the length and the laying depth of the reinforcement (Nakai et al, 2009). In the present study, to get more bearing capacity, the reinforcing effect in the case that each edge of the reinforcement is fixed with the soil is investigated. For this purpose, 2D model tests and the corresponding non-linear finite element analysis are carried out. In these tests and simulation, the depth of the reinforcement is changed under constant length of reinforcement. Reinforcing effects, under concentric and eccentric load, are discussed. It is revealed that a significant increase of the bearing capacity is observed in the model tests when the reinforcement is set up in the ground with an appropriate depth under both concentric and eccentric loads. The results of numerical simulation - in which the stress-stain behavior of the soil and the frictional behavior between soil and reinforcement are properly taken into consideration - show good agreement with the observed results.

GEOTECHNICAL HAZARD ANALYSIS OF RIVER EMBANKMENT OF BANGLADESH AND ITS PROTECTABILITY

Bangladesh has a long artificial river embankment, developed mainly for the protection of its inhabitants and resources from disastrous flash floods, tidal water, cyclone surges, river currents and others. This process increases erosion of embankments and water turbidity which bring the concerned sediments from alluvial land to the inland sides and river bed that further increase vertical soil accommodation and decrease the water depth. In this research geotechnical hazard analysis has been conducted based on the statistical riverine data and flood occurrence information, obtained from various recognized institutions and secondary literatures and scientific investigation of soil structural failures from field survey. The soil characteristics and existing conditions of the embankments of the three big rivers of Bangladesh - the Ganges, the Meghna, and the Brahmaputra-Jamuna, have been investigated. The research also includes the reasons behind the failure of different embankments that were happened in last decades. A design methodology has been proposed to make these embankments more durable and to improve the strength using a pre-designed methodological case study.

Simple Modeling of Time-Dependent Behavior for Structured Soils

A simple model to describe time-dependent behavior of various soils in 1D stress conditions is presented in this paper. The model is formulated not using the usual viscoplastic theories such as over-stress type and non-stationary flow surface type but utilizing the subloading surface concept by Hashiguchi (1980), and paying attention to the experimental results that the normally consolidation line (NCL) on the e–ln σ plane shift depending on the strain rate. The present model can describe various time-dependent behaviors not only of normally consolidated soil but also of over consolidated and naturally deposited soils in the same manner without violating the objectivities. The 1D model can easily be extended to the 3D using the t ij concept (Nakai and Mihara 1984).

EFFECTIVENESS OF REINFORCEMENT IN EMBANKMENT GROUND SUBJECTED TO REPEATED SHEAR DEFORMATION

In this research, a comparative study is done on the effectiveness of reinforcement in embankment ground. The restraint effect of ground displacement by reinforcement of sheet pile and the sheet pile combined with the nailing method are investigated by model tests and corresponding finite element analyses. An elastoplastic model and subloading tij model are used in the analyses. Two loading conditions were applied namely, the simple vertical loading to the footing and cyclic loading associated with repeated shear deformation in the ground for both series of the model tests and the finite element analyses. Soil-water coupling analysis applying an inertial force to real ground embankment is also performed. It is revealed that when the bearing capacity of the ground increases, the lateral and vertical displacements are restrained. The reinforcement by the sheet pile combined with the nailing method is effective compared with that of the sheet pile alone.

Mechanism of Support in Anchor Type Retaining Wall - Model Tests and Numerical Simulations -

To establish rational design and construction process two-dimensional model tests are carried out varying the length and inclination of ground anchors. Numerical simulations with finite element method using FEMtij-2D are also carried out for the same scale of the model tests. Subloading tij model (Nakai and Hinokio, 2004) is used in the analyses to model the ground material. It has been revealed that installation a longer anchor at the bottom part of the excavation area is very effective in reinforcing effect of earth retaining wall. Inclination of ground anchor has also significant effect on the displacement of the backfill ground and deformation of the retaining wall. The finite element analyses show very good agreement with the results of the model tests.

Simple and Unified Modelong of Time-Dependent Behavior for Various Geomaterials

A simple model to describe time-dependent behavior for various kinds of soils is presented. The present model can describe various time-dependent behaviors not only of normally consolidated soil but also of over consolidated and naturally deposited soils in the same manner without violating the objectivities. To consider the influence of over consolidation, the subloading surface concept defines a state variable ρ which represents the influence of density. To describe the behavior of naturally deposited soil, another state variableω, which represents the bonding effect, is introduced. The validity of the present models is verified using various kinds of simulations of time-dependent behavior in one-dimensional and three-dimensional conditions.