Case Studies on Long-term Settlement of Soft Clay Ground

Two case histories on long-term settlements of Ariake clay which is counted as one of the soft clays in Japan are described. The one of them is to report the settlement which have been observed over 25 years since construction of embankment for breakwater on the coastal Ariake deposit. The another case history is concerned with the settlement of low embankment highway on Ariake clay whose shallow surface was improved by quickline-clay mixture as a countermeasure for the settlement. The current paper is featured by the fact that the predominant secondary settlement is common with two case histories. The finite element method using an elasto-plastic mo1el was adopted to analyze the settlement of the Ariake clay observed in the above-mentioned two case histories under sustained and transient loading, respectively. It is concluded from comparison of analytical results with observed settlement that the proposed model with consideration of secondary compression is advantageous for long-term settlement prediction of soft clay

Analytical model for grouted rock bolts

This paper presents an analytical model of the grouted rock bolt in soft rock. The behavior of both the rock bolt system and the single rock bolt are discussed respectively. The coupling mechanism of rock bolt and rock mass is discussed from the viewpoint of displacement. A simple method is suggested for the rock bolt design in tunneling. According to the analysis, displacement of rock mass controls the initial force in rock mass. Case simulations confirm the previous findings that a bolt in-situ has a pick-up length, an anchor length and at least one neutral point. The theoretical prediction of single rock bolt agrees with the measured data. The position of the neutral point is not only related to the length of the rock bolt and the internal radius of the tunnel, but is also strongly influenced by the properties of rock mass. Neutral point and maximum axial load in the rock bolt tend to be constant when the anchor length of the bolt is sufficiently long, which means that increasing the length of the rock bolt may result in only a slight improvement in displacement control under certain conditions

Settlement Prediction of Soft Clay Ground under Sustained and Transient Loading

Two case studies of settlement prediction of Ariake clay ground which is counted as one of the soft clays in Japan are described. The one of them is to report a long-term settlement which have been observed over 25 years since construction of embankment for breakwater on the coastal Ariake deposit. The another case study is concerned with the settlement of low embankment highway on Ariake clay whose shallow surface was improved by quicklime-clay mixture as a countermeasure for the settlement. It is featured by the fact that the predominant secondary settlement is common with two case studies. The finite element method using an elasto-plastic model was adopted to analyze the settlement of Ariake clay observed in above-mentioned two case studies under sustained and transient loading, respectively. It is concluded from comparison of analytieal results with observed settlement that the proposed model with consideration of secondary compression is advantageous for settlement prediction of soft clay ground

Proposal of Reasonable Fill Construction Management Index in Fill Loading with Vacuum Consolidation Method Based on FEM Analyses

Vacuum consolidation method (VCM) is one of the recent methods for the improvement of soft ground. This method can forcibly drain pore water, and increase ground strength by loading of the vacuum pressure. A fill loading with vacuum consolidation method (FLVCM) is considered to be able to control lateral flow and upheaval of surrounding area during the rapid fill construction. However, the behavior of this combination on the ground deformation is not fully clarified on FLVCM, fill design and site management is depending on the experienced technique. In this study, numerical simulation has been carried out by finite element method (FEM) for the quantitative evaluation of the deformation suppression effect of FLVCM on the soft grounds of ariake clay, Japan. The utility of FLVCM was confirmed from comparison with the observed and the analytical ground deformation. In addition, the numerical simulation have been carried out under the various conditions of vacuum preand post-loading pressures and fill speeds, and it is suggested that the reasonable construction management index can be easily obtained by the site measurement of deformation

A new rheological model and its application in mountain tunnelling

The time-dependent features of soft rock, named rheology generally, should be taken into account in the long-term design and maintenance of mountain tunnels. Based on the classic Burger-MC rheological model, a Burger-deterioration rheological model is proposed in this paper and is implemented in the numerical codes FLAC3D. A deterioration threshold and two deterioration ratios are introduced in this model to consider the time-dependent strength deterioration aspect of the rock mass. The proposed model is applied to an engineering instance (Ureshino Tunnel Line I, Nagasaki, Japan) to account for the delayed deformations that occurred after its completion since November 1992. The delayed crown settlement and invert upheaval computed from simulations are featured by an exponential characteristic and a stair-typed characteristic, respectively, which agree well with the in-site monitoring data qualitatively. In addition, the realistic rheological parameters involved in the proposed model can be back-analyzed from the in-site monitoring data

MECHANISM OF SLOPE FAILURE DURING HEAVY RAINFALL IN NAGASAKI JULY 1982

On July 23,1982,299 human lives were lost in Nagasaki Prefecture due to localized heavy rainfall, nearly 90% of which were due to sediment disaster. So, it is important to clarify a mechanism of slope failure during heavy rainfall for sediment disaster prevention. In this paper, we firstly carry out a model experiment of slope failure due to infiltration and observe a phenomenon in progressive failure of slope. Secondly, we apply a finite element analysis which is divided into two stages, i.e. a saturated-unsaturated seepage flow analysis and a stability analysis taking into account the seepage force and the reduction of strength due to the rise in the degree of saturation, to two actually failured slope in Nagasaki city. And we check their failure under the conditions of preceding rainfall, rainfall on the day of failure, topography, geology and so on. Finally, we have clarified a mechanism of slope failure during heavy rainfall and shown that slope failure can be predicted if a rise in the groundwater table can be detected

Deformation and Strength of Anisotropic Sand Under Three Dimensional Stress Conditions

The three-dimensional, drained stress-strain and strength behavior of a sand prepared in cubical specimens with cross-anisotropic fabric was studied using triaxial compression, plane strain, and cubical triaxial tests with independent control of the three principal stresses. All specimens were loaded under conditions of principal stress directions fixed and aligned with the directions of the material axes. For comparable test conditions, the major principal strain was smallest and the rate of dilation was highest when the major principal stress acted perpendicular to the long axes of the sand grains. The opposite extremes were obtained when the major principal stress acted parallel to the long grain axes. The effects of initial cross-anisotropic fabric were mainly observed in the prefailure stress-strain behavior, whereas sufficient changes in the fabric had occurred at large strains to produce failure conditions which resembled those observed for isotropic sands. The three-dimensional failure surface could for practical purposes be modeled by an isotropic failure criterion

Experimental study of the hydro-mechanical behavior of rock joints using a parallel-plate model containing contact areas and artificial fractures

In recent years, geological disposal of radioactive wastes is considered to be the most promising option, which requires the understanding of the coupled mechanical, hydraulic and thermal properties of the host rock masses and rock fractures. The hydromechanical behavior and properties of rock fractures are usually determined by laboratory experiments on fracture specimens that serve as the basic building block of the constitutive models of fractured rock masses. Laboratory testing of rock fractures involve a number of technical issues that may have significant impacts on the reliability and applicability of the testing results, chief among them are the quantitative estimation of the evolutions of hydraulic transmissivity fields of fractures during shear under different normal constraint conditions, and the sealing techniques when fluid flow during shear is involved. In this study, a new shear-flow testing apparatus with specially designed fluid sealing techniques for rock fractures were developed, under constant normal load (CNL) or constant normal stiffness (CNS) constraint. The topographical data of all fracture specimens were measured before testing to constitute the geometrical models for simulating the change of mechanical aperture distributions during shearing. A number of shear-flow coupling tests were carried out on three kinds of rock fracture specimens to evaluate the influence of morphological properties of rock fractures on their hydro-mechanical behaviour. Some empirical relations were proposed to evaluate the effects of contact area and surface roughness on the behavior of fluid flow through rock fractures