The behaviour of cohesive soils under triaxial test has attracted the attention of large number of investigators because of its practical importance. The behaviour of saturated clays during shear in particular has important practical implications. The study of the effect of overconsolidation is also of great value as in nature many deposits of clay have been overconsolidated in their geological history. In order to have a thorough appreciation of stability and deformation problems, it is extremely important to understand the inter relationship between the strength, pore water pressure and volume change characteristics of the material concerned. A series of tests both drained and undrained and drained/undrained conditions alternatively on the same sample on remoulded specimen of clay (KAOLIN) to study the stress strain characteristics, response to pore pressure development and volume change during shearing were conducted and results are presented herein.
The results of triaxial tests on saturated soils are of immense value so far as the practical view points is concerned. In field, saturation of foundation soil can occur from loading. Saturation by percolation of water during the performance of a structure is a common condition requiring analysis for steady state and draw down conditions. The shear-strength parameters of cohesive soils are much influenced by its stress history. The volume changes and pore-pressure variations of normally consolidated clays are similar to those of loose sends while overconsolidated clays behave like dense sands. Factors that affect the shear-strength parameters of cohesive soil include its structure, clay content, drainage conditions, strain rate and water content (Khan, 1998). A series of tests were carried out on remoulded samples of Kaolin to study its response towards pore pressure development, volume change, failure behaviour during shearing. The results have been presented through a number of figures.
This paper presents a case study of geotechnical investigation including geophysical investigation carried out at a typical power plant site, in different stages. Under each stage the subsurface has progressively been examined in greater detail than earlier stage of investigation. Due to high water table and dense over-burden, sampling in boreholes has been difficult. In-situ tests like dynamic cone penetration tests (DCPT), plate load tests (PLT) and pressuremeter tests (PMT) were also conducted to evaluate the soil parameters. With the accumulation of data and understanding of the sub-soil, the design of foundation system has been continuously improved. The data also helped in selection of most appropriate materials like cement, reinforcement and grade of concrete. The geotechnical data helped in proper planning of relatively deep excavations with stable slopes, excavation tools to reach the desired depth under high water table, measures for dewatering. All this information has resulted in an economical foundation system and thus reducing the construction time and avoiding surprises on account of soil strata variation.
One of the major requirements in planning, design and construction of power plants is proper and adequate sub-soil investigation. The object of the investigation is to provide the engineer with as much information as possible about the substrata and other geotechnical features of the area to enable him to design the foundations of the structures economically and ensure its stability. The site of the proposed coal fired power plant (2x500 MW), plot measuring about 3.0 sq.km, is located on the east coast of India. Geotechnical investigation, comprising of various in-situ soil tests, geophysical investigation and laboratory tests has been carried out in different stages. This paper describes the geotechnical investigation to characterise the sub-surface for foundation design of light and heavy structures under static loading of the Main Power House.
The paper deals with an examination of the geotechnical aspects of a plan to renovate the “Parco delle Rimembranze”, next to the Teodorico Mausoleum in Ravenna, Italy. A wide, open square is to be built at the ground level of the Mausoleum, situated at about 1.75 m below the water level of the adjacent Candiano canal. The ground on which the square is to be built, consists of medium fine sea sand (coastal and beach deposits), below the water table and subjected to an upward hydraulic flow. Problems involved in the project include controlling the level of the water both at the provisional and at the final stage. In order to decide on the best engineering solution, accurate investigations were conducted on site, including pumping tests to evaluate how to keep the water table down and besides permeability coefficients.
The project for renovating the “Parco delle Rimembranze” next to the Teodorico Mausoleum in Ravenna includes the construction of a wide, open square with the ground level at about 1.75 m below the level of the water table which is fixed by the water level of the adjacent “Candiano” harbour canal, controlled in turn by the level of the sea. The excavation work required to build the square involves complex hydraulic rising problems; these problems may be due to piping, or to the fact that the excavation floor may be pushed up by unbalanced underthrusts on impermeable soil lenses (silt and clays) inside the incoherent sandy deposit. The plan envisages linking the square on one side to the upper surrounding area by means of grassy slopes of variable gradients, while on the other side the square will be lower than the adjacent embankments and the difference in height is guaranteed by stiff retaining walls.
Most research into clay behaviour deals with reconstituted specimens that have been isotropically compressed. While there are many ways of interpreting the test results, some of the most widely used approaches are based on the ideas of elasto-plasticity incorporated, for example, in the Modified Cam Clay model which was developed specifically for isotropic material. Real clays, that were initially anisotropically compressed, exhibit various forms of elastic-plastic components but differ in detail from those of reconstituted clays. In particular, pre-yield volume changes and yield loci are different from those proposed in Modified Cam Clay. The work reported in this paper examines the estimation of insitu stresses and the pre-yield behaviour of overconsolidated natural clays. Four consolidated undrained tests and one consolidated drained test were carried out on samples of Belfast Upper Boulder Clay obtained at various depths and locations. The effective stress in the specimens p_ k was measured using a pressure plate apparatus for each sample and it was used to estimate the insitu stresses in the ground. The measured effective stress in the sample p''k in combination with specific volume vk and the ratio of the cross anisotropic parameters J/3G* enable the prediction of the pre yield stress path during drained shearing. Comparison between predicted and measured stress paths is very good.
There are many ways of interpreting the behaviour of clays, some of the most effective being based on the ideas of elasto-plasticity. These suggest that concepts such as elasticity, yielding and plastic hardening can be applied to clay soils. One common elastic-plastic model is the Cam Clay Model (Roscoe and Burland 1968) which is based on evidence obtained for reconstituted materials. Natural clays differ in many ways from reconstituted clays. Natural clays are structured and reflect the conditions under which the materials were deposited.
The recent advances in ground anchor technology and the related techniques of cable bolting and rock bolting are reviewed. Collectively the technology associated with the three techniques enable design engineers to address stability problems over a range of scales and in a range of geomechanical environments. The techniques have similar aims but have developed into separate disciplines with unique attributes. The procedures for designing and creating ground anchors that meet the stringent requirements associated with modern civil infrastructure are discussed. This contrasts the very different design approaches being developed in rock bolting and cable bolting where standards are less exacting but the design problems can sometimes be more complex.
Excavations and other engineering constructions in the ground are central to many civil and mining projects. For both economic and safety reasons ground reinforcement is often a key component in the successful completion of these projects. Ground reinforcement includes, amongst other methods, the techniques of ground anchoring, cable bolting and rock bolting. Basically, all of these techniques seek to assure the stability of an artificial structure constructed within or on a soil or rock mass by the installation of structural elements within the ground. The differences between the three techniques are predominantly associated with scale and the standards of design and installation. Ground anchors tend to be longer with the highest capacity, rock bolts tend to be shorter with the lowest capacities and cable bolts have evolved to address stability problems that lie between the two. Ground anchors are usually associated with civil infrastructure projects which demand exacting standards of design and installation. This will be followed by a discussion on some of the unique attributes that characterise and differentiate the less developed disciplines of rock bolting and cable bolting.
The Savage River Iron Ore Mine, located in northwest Tasmania, was opened in 1968 to produce iron ore pellets from a magnetite orebody. Over the years four pits have been developed; South Centre Pit, Centre Pit, South Lens and North Pit. On the western walls of Centre Pit and North Pit flexural toppling failures have restricted overall slope angles to 37o. This paper discusses the geology, history and stability analyses of these toppling failures. The computer software programme UDEC has been successfully used to model excavation history, groundwater depressurisation and slope movements. The model is now being used to predict the future behaviour of the slopes to assist in mine planning.
The Savage River Iron Ore Mine, located in northwest Tasmania (refer to Figure 1), was opened in 1968 to produce iron ore pellets from the magnetite orebody. The magnetite is crushed at the mine and then sent via an 85km long slurry pipeline to Port Latta on the northwest coast for processing into hematite pellets. The original owners shut down operations in 1996 but in 1997 Australian Bulk Minerals (ABM) recommenced mining after relocating the crusher and installing a 1.3km conveyor. Over the years four pits have been developed; South Centre Pit, Centre Pit, South Lens and North Pit. The mine has had a history of flexural toppling failures in Centre Pit and North Pit that have restricted a major portion of the western walls to overall slope angles of 37o, with vertical slope heights in the order of 150m. This paper discusses the history of the toppling and the recent use of Itasca’s (2000) UDEC computer programme to model the phenomena, i.e to back analyse previous failures. Detailed geology, excavation histories and dewatering aspects have successfully been incorporated into the UDEC model.
The authors have proposed and discussed temporary storage of low temperature materials such as LNG, LPG and frozen food in openings excavated in rock mountains. In this case, as the rock mass around the openings will receive thermal hysteresis of low temperatures, it is necessary to obtain strength and deformation characteristics of rocks after receiving thermal hysteresis of low temperatures to consider the thermal behavior of rock mass around openings and the stability of openings. In this study, strength and deformation characteristics of some types of rocks were examined after receiving thermal hysteresis of low temperatures. Using the results of the experiments, temperature and stress distribution around openings were analyzed. From the results of analysis, it was found that in the case of receiving thermal hysteresis, crack length and freezing zone are about same as in the case of not receiving thermal hysteresis.
The authors have proposed and discussed temporary storage of low temperature materials such as LNG, LPG and frozen food in openings excavated in rock mountains. This storage method has the advantages of efficient utilization of land, large scale storage, preservation of the environment, reduction of heat loss etc. In this case, the storage quantity of low temperature materials changes according to their use. For this, as the rock mass around the openings will receive thermal hysteresis of low temperatures, it is necessary to obtain strength and deformation characteristics of rocks after receiving thermal hysteresis of low temperatures to consider the thermal behavior of rock mass around openings and the stability of openings. In this study, strength and deformation characteristics of some types of rocks were examined after receiving thermal hysteresis of low temperatures. The effects of the amount of thermal hysteresis, atmospheric temperature, porosity, etc. on strength and deformation characteristics of rocks were discussed.
In this paper, stability analysis of the Ertan arch dam was conducted by using three- dimensional nonlinear finite element methods and geomechanical model tests. The results were compared with data of prototype observations before, during and after completion of the dam. A three-dimensional nonlinear F.E.M., TFINE program, based on a damage-fracture mechanics model for jointed rock mass, has been developed and applied to evaluate the stability of the dam. Geomechanical model tests were also carried out to verify the conclusions drawn from numerical computation. A full- scale monitoring system was setup at the dam site, which enabled the observations during construction and operation of the dam. Construction of the dam was finished and the dam has been operated satisfactorily since impoundment of the reservoir in June 1998. The behavior from numerical computation, geomechanical model tests and dam actual observations are in good agreement.
CONFIGURATION OF ERTAN ARCH DAM
The Ertan arch dam is located in the downstream sector of the Y along River in south western China. The geological conditions at the project site are well formed for a high concrete double curvature arch dam. The rock mass at the dam site is generally composed of igneous crystalline rocks . Primary design and optimization were accomplished in 1985 and in 1987 respectively. Construction of the project began in September 1991 and after 7 years the main buildings of the dam were finished. The reservoir was impounded and flood was discharged in June 1998. The up stream face of the dam can be seen from Figure 1 and general layout of the project is shown in Figure 2. The crown cantilever section of the dam body is shown in Figure 3. Detail data about the project can be seen from Table 1 .
This paper presents the analytical displacement of the sheet pile braced excavation system by means of the FEM technique based on case studies of deep excavation in Bangkok city with full geotechnical instrumentation. The research sites consists of 4 projects where the field measurement was recorded corresponds to the construction techniques and sequences. These four case studies had different boundary conditions with the excavation depth varied between 8-11 m. while the bracing spans and construction techniques were different. The back-figured values for soil stiffness in terms of Young’s modulus are calculated as Eu/Su = 150, 250 and 1000 for soft Bangkok clay, medium clay and stiff silty clay, respectively. The results have later been confirmed with the first self-boring pressuremeter tests in Bangkok subsoils.
Deep excavations in Bangkok City of Thailand are still necessary even though the country has faced an economic crisis. There are still required the necessary civil works to serve the demands and to solve the environmental problems such as our first underground MRT subway project as well as the waste water treatment schemes. Bangkok subsoil conditions consist of a very soft dark gray clay about 12-14 m. thick, underlain by stiff clay to about 23-25 m. depth then followed by the first dense silty sand layer. Excavation in such soft dark gray clay requires efficient retaining structures such as reinforce concrete diaphragm walls for deep excavation and temporary sheet piles for shallower excavations. The sheet pile work is generally applied for excavations up to about 10-11 m. depth. Use of sheet pile systems is very popular because the construction cost is cheaper, the piles are reusable and simple for the contractor. Lateral wall movement that will not induce any damage to the adjacent areas normally controls the depth of excavations.
The influence of Poisson''s ratio on overcoring was investigated using data from 47 tests. In these tests both the CSIRO HI triaxial cell and the triaxial cell developed by Swedish State Power Board were used. Boreholes drilled parallel to the major principal stress resulted in higher Poisson''s ratio compared to boreholes drilled at an angle to major principal stress. Numerical modelling by FLAC was used to simulate overcoring and biaxial testing. When the major principal stress is acting perpendicular to the borehole, the extensional strain is larger on the inner surface of the core during overcoring than when parallel to the borehole. Simulations of a biaxial test of a virgin core show extensional strain, which may be larger than critical extensional strain of the rock type, indicating non-elastic behaviour.
Äspö Hard Rock Laboratory (HRL) in southern Sweden is a full-scale laboratory for a future deep repository of spent nuclear fuel. In 1990 the excavation of the ramp started. The ramp is about 3600 metres long and down to 450 meters below ground surface. The first part was excavated by a conventional drill and blast method, while the last 400 meters was excavated by a Tunnel Boring Machine (TBM). A comprehensive analysis of all results from rock stress measurements performed to date at Äspö HRL has been done. The magnitudes of rock stresses obtained from overcoring were higher than those predicted by hydraulic fracturing. Statistical analyses of the elastic properties from the Äspö data showed that there was a significant difference between the mean values of Poisson''s ratio obtained from biaxial tests of cores performed with the two different cells. The analysis gave a Poisson''s ratio of 0.34 for the CSIRO HI-cell and 0.23 for the SSPB-cell (Lundholm, 2000).