International Journal of Civil Engineering
Volume:10 Issue: 2, Jun 2012

Soft soil in Hanoi, Vietnam, is mainly originated from lacustrine, marsh, river and shallow sea sediment. This is the youngest formation with around 3000 years of age. To serve the research purpose, clayey soil samples at ten areas in Hanoi and some other places in the Red River delta were prepared. Mineral composition of soils determined by X-ray diffraction analysis showed that clay minerals are predominated by Illite, Kaolinite, Chlorite, and Montmorillonite respectively. Many previous researches indicated that in saline-saturated condition, types of cation in saline water and types of clay mineral and its predomination in soil layer as well decide the changing process of geotechnical properties in other manner. In this paper the initial relationship between geotechnical properties and clay mineral composition of Hanoi soft soils in saline-saturated media is established.

In the first impounding of rockfill dams, additional settlements occur in upstream side in saturated rockfills due to collapse phenomenon; even high rainy seasons can cause additional deformation in the dumped rockfills. Unfortunately these displacements are not taken into account in the conventional numerical models which are currently used to predict embankment dam behavior during impounding. In this paper to estimate these displacements, strain hardening-strain softening model in Flac is modified based on the laboratory tests, in which same impounding process in such dams is considered. Main feature of the model is reproduction of nonlinear behavior of rockfill material via mobilized shear strength parameters and using collapse coefficient to display induced settlement due to inundation. This mobilization of shear strength parameters associated with some functions for dilatancy behavior of rockfill are used in a finite difference code for both dry and wet condition of material. Collapse coefficient is defined as a stress dependent function to show stress release in the material owing to saturation. To demonstrate how the model works, simulation of some large scale triaxial tests of rockfill material in Gotvand embankment dam is presented and results are compared with those from laboratory tests, which are in good agreement. The technique could be used with any suitable constitutive law in other coarse-grained material to identify collapse settlements due to saturation.

It has been realized that the raft (mat) foundations are capable of bearing very large loads when they are assisted with a pile group. The contribution of both raft and piles to carry the surcharge loads is taken into account, considering the stiffness and strength of involved elements in the system, i.e. piles, raft and surrounding soil. The piles are usually required not to ensure the overall stability of the foundation but to act as settlement reducers. There is an alternative design in which, the piles are disconnected from the raft to reduce the settlement, which are then known to be “settlement reducer disconnected piles” to increase the system stiffness. In this paper, two and three dimensional finite element analysis of connected and disconnected pile-raft systems are performed on three case studies including a 12-storey residential building in Iran, a 39-storey twin towers in Indonesia, and the Messeturm tower, 256m high, in Frankfurt, Germany. The analyses include the investigation of the effect of different parameters, e.g. piles spacing, embedment length, piling configuration and raft thickness to optimize the design. The role of each parameter is also investigated. The parametric study results and comparison to a few field measurements indicate that by concentrating the piles in the central area of the raft foundation the optimum design with the minimum total length of piles, which is considered as control parameter for optimum design, is achieved. This can be considered as a criterion for project cost efficiency. On the other hand, disconnected piled-raft systems can significantly reduce the settlements and raft internal bending moments by increasing the subsoil stratum stiffness. Finally, the comparison indicates that simple and faster 2D analysis has almost similar results to the time consuming and complicated 3D analysis.

Presented is a method of three-dimensional stability analysis of convex slopes in plan view based on the Lower-bound theorem of the limit analysis approach. The method’s aim is to determine the factor of safety of such slopes using numerical linear finite element and lower bound limit analysis method to produce some stability charts for three dimensional (3D) homogeneous convex slopes. Although the conventional two and three dimension limit equilibrium method (LEM) is used more often in practice for evaluating slope stability, the accuracy of the method is often questioned due to the underlying assumptions that it makes. The rigorous limit analysis results in this paper together with results of other researchers were found to bracket the slope stability number to within ±10% or better and therefore can be used to benchmark for solutions from other methods. It was found that using a two dimensional (2D) analysis to analyze a 3D problem will leads to a significant difference in the factors of safety depending on the slope geometries. Numerical 3D results of proposed algorithm are presented in the form of some dimensionless graphs which can be a convenient tool to be used by practicing engineers to esti‌mate the initial stability for excavated or man-made slopes.

A series of tests and also numerical analyses were conducted to explore the mechanical behavior of a mixture of coarse gravel-size particles floating in a matrix of silt, sand or clay. The research is a step forward in an ongoing investigation on behavior of composite clay, which is used as the core material of some large embankment dams all over the world. After providing the reader with an overall image about behavior of such materials through the literature, the paper focuses on a predominant feature of the composite soil behavior: increase of non-deformable solid inclusions in a mixture leads to formation of heterogeneity of stress field, excess pore water pressure and strain distribution along the specimens. This paper mainly probes formation of such heterogeneity during experiments by utilizing some miniature pressure transducers inside relatively large hollow cylinder and also cyclic triaxial specimens. The same triaxial specimen geometry is also modeled by finite element code to explore such trend of behavior, results of which are included as the last part of the article.

This paper describes a series of laboratory model tests on strip footings supported on unreinforced and geogrid-reinforced sand with an inside void. The footing is subjected to a combination of static and cyclic loading. The influence of various parameters including the embedment depth of the void, the number of reinforcement layers, and the amplitude of cyclic load were studied. The results show that the footing settlement due to repeated loading increased when the void existed in the failure zone of the footing and decreased with increasing the void vertical distance from the footing bottom and with increasing the reinforcement layers beneath the footing. For a specified amplitude of repeated load, the footing settlement is comparable for reinforced sand, thicker soil layer over the void and much improved the settlement of unreinforced sand without void. In general, the results indicate that, the reinforced soil-footing system with sufficient geogride-reinforcement and void embedment depth behaves much stiffer and thus carries greater loading with lower settlement compared with unreinforced soil in the absent of void and can eliminate the adverse effect of the void on the footing behavior. The final footing settlement under repeated cyclic loading becomes about 4 times with respect to the footing settlement under static loading at the same magnitude of load applied.

This paper studies thoroughly and deeply the results of about one hundred triaxial compression tests on thirty types of rockfill materials. The materials are categorized in accordance with their particles shape (angular / rounded) and gradation characteristics. The main tool of the study is the Hyperbolic Model developed by Duncan and Chang [1]. The focus of the study is on the variations of deformation modulus of the materials (Ei and Et) with confining stress (3). Features of the mechanical behavior of the rockfill materials, as compared with the general behavior of soils, are highlighted through the exponent parameter (n) of the Hyperbolic Model. It is shown that high confining stresses may have adverse effects on the deformation modulus of the rockfill materials and make them softer. The particle breakage phenomenon which happens during compression and shearing is found as the main factor responsible for the above effects and, in general, responsible for controlling the behavior of the materials. For the rockfill materials of this study, two correlations for estimating the initial elasticity modulus (Ei) and the internal friction angle (φ) in terms of particles shape, confining pressure (σ3), and coefficient of uniformity (Cu) are suggested. Key words: Rockfill Materials; Hyperbolic Model; Triaxial Test; Particle Breakage; Deformation Modulus; Internal Friction Angle.

Bentonite-sand mixture is one of the most important candidates for engineering buffer element in nuclear waste repositories; so the analysis of its thermo-hydro-mechanical behavior is important for design purposes. An innovational setup of classic oedometer was used for swelling and compression study at high temperatures in this research. A fully calibration program was utilized to include high temperature effects on measurements. This research shows that the elevation of temperature from 25 to 90oC in 1:1 bentonite-sand mixture in distilled water reduces free swelling potential and strain about 20 percent. The required time for equalization of swelling is less in high temperature due to increasing in permeability. Also, the high temperature causes increasing in compressibility rate and quantities for this buffer. The effect of recent thermal history on behavior of bentonite-sand mixture has been showed and tried to clarify it. At similar stress-temperature states, thermal history causes different deformation in samples. The highest temperature that bentonite has been experienced, controls its behavior in the next thermal cycles.