International Journal of Civil Engineering

A study on the relationship between geotechnical properties and clay mineral composition of Hanoi soft soils in saline media

B.L. Kirov

Soft soil in Hanoi, Vietnam, is mainly originated from lacustrine 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 places in the RRD
are prepared. Mineral composition of soils determined by X-ray diffraction analysis shows 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 in soil layers, as well as their predomination 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

Simulation of collapse settlement in rockfill material due to saturation

R. Mahin Roosta

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

Study on optimized piled-raft foundations (PRF) performance with connected and non-connected piles- three case histories

A. Eslami

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 nonconnected
from the raft to reduce the settlement, which are then known to be "settlement reducer non-connected piles" to increase
the system stiffness. In this paper, two and three dimensional finite element analysis of connected and non-connected 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 is
achieved, which is considered as control parameter for optimum design. This can be considered as a criterion for project cost
efficiency. On the other hand, non-connected 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.

3D stability analysis of convex slopes in plan view using lower bound linear finite element

F. Askari

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

An investigation on effect of inclusions on heterogeneity of stress, excess pore pressure and strain distribution in composite soils

J. Jalili

A series of tests and also numerical analyses were conducted to explore the mechanical behavior of a mixture of coarse gravelsize
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 by the aid of special experiments and also numerical analyses. In addition to loading details, it is clarified through
the paper that position of inclusions relative to loading direction also affects heterogeneity of stress/strain and excess pore water
pressure distribution through the mixture. Despite the former, the latter redistributes with a rate proportional to material
hydraulic conductivity.

Strip footing behavior on reinforced sand with void subjected to repeated loading

M. Ghazavi

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.

Behavior of Rockfill Materials in Triaxial Compression Testing

A. Soroush

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. The focus of the study is
on the variations of deformation modulus of the materials (Ei and Et) with confining stress (&sigma3). 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 (&phi) in terms of particles shape, confining pressure (&sigma3), and coefficient of uniformity (Cu) are
suggested.

The effect of thermal history on thermo-mechanical behavior of bentonite-sand mixture

N. Shariatmadari

Bentonite-sand mixture is one of the most important candidates for engineering buffer element in nuclear waste repositoriesso
the analysis of its thermo-hydro-mechanical behavior is important for design purposes.An innovative 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 90◦C in
1:1bentonite-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 quantity for this buffer. For detection of this effect, XRD analysis showed that an increase in
temperature causes a decrease in basal spacing. So, the particles can come near to each other more than lower temperatures and
the amount of absorbed water in the microstructure of the clay is smaller.The effect of thermal history on behavior of bentonitesand
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.