12th ISRM Congress

ABSTRACT:A large number of casing collapses are reported in one of the largest carbonate oil fields in south of Iran. Most of the casing collapses have been occurred in Gachsaran overburden formation. Compaction of reservoir is one of the prime suspects of this phenomenon. To quantify the compaction, the lithology across the failure depth (3200m for well M-42, as the first failed well) was reviewed and an interface between anhydrite (as a stiff rock) and marl (as a soft rock) was detected. To obtain static elastic parameters of the host rock, uniaxial compressive test on the anhydrite and marl cores were carried out. The samples were taken from 12m to 195m depth of Gachsaran formation. To assess the thermal effect on these rocks, uniaxial and triaxial compressive tests were performed on the anhydrite and marl at different temperatures (range 23°C to 100°C). The relations between E, UCS and failure angle with temperature were obtained in anhydrite and marl. To analyze whether the anhydrite/marl interface in well 42 was prone to failure because of depletion and ensuing reservoir compaction, numerical modeling reproducing triaxial laboratory testing was carried out. The stress and interface condition were defined the same as reality in collapse depth of well 42. The shear stress across interface due to reservoir depletion doesn’t exceed the shear strength of marl. As a result, reservoir compaction is not the only cause of casing collapse in this oil field and participation of other factors are also necessary which needs further researches.

ABSTRACT: To reduce the emission of CO₂ into the atmosphere, it is proposed to store CO₂ into deep un-mineable coalbeds. Many studies have been reported investigating the effect of CO₂ injection on the flow properties to provide a means of estimating the recoverable CH₄ and storable CO₂. However, assessment of the long-term integrity of stored CO₂ and the potentially damaging effects of CO₂ on the mechanical response of coal have been largely neglected. Understanding the geomechanical response of coalbeds to CO₂-injection can be crucial in site selection, in designing and planning CO₂-ECBM and coalbed geo-sequestration operations. To investigate the effects of CO₂ adsorption on the mechanical strength and stiffness of coal, 45 core specimens of Australian black coal from the Sydney Basin were tested. The behaviour of dry, water-saturated and CO₂-saturated specimens were investigated at confining pressures up to 1.0MPa at room temperature. Results showed that triaxial compressive strength of CO₂-saturated coal is 20% less than that of water-saturated coal at confining stress of 1.0MPa. Hoek & Brown failure criterion was used to explain the behaviour of coal and it was found that the rock constant m_i of CO₂-saturated coal is significantly lower. It can be concluded that CO₂ changes the mechanical behaviour of coal by changing the way cracks and fractures propagate and by changing the surface properties of fractures.

ABSTRACT: The research significance of the present paper concerns the need of knowing the mechanical properties of stone existing in the ancient constructions, when a suitable methodology for their rehabilitation or strengthening is followed. The rehabilitation of ancient constructions, mainly the ones pertaining to the architectural heritage is a demand of modern societies. The deep knowledge of mechanical and fracture properties of the stone is also an important advantage, when structural analysis is needed for the evaluation of the safety conditions of ancient constructions. Thus, this paper aims at providing relevant experimental data on the engineering properties of granites of existing ancient masonry buildings, discussion of the main features of the compressive and tensile behavior of granites, assessment of the influence of the planar anisotropy, weathering state and physical properties on the mechanical behavior of granites. Besides, a comparison between compressive and tensile behavior is performed and statistical correlations between compressive and tensile engineering properties are provided.

ABSTRACT: Bakhtiary Dam & Hydropower Project is located on Bakhtiary River (the main branch of Dez River), about 70 km North-East of the city of Andimeshk in South-West of Iran. The main dam is a double-curvature concrete arch type with maximum height of 325m and crest length of 509m at El. 840 masl. It will be the highest arch dam in the world now at the design stage. The hydropower plant consists of an underground powerhouse complex with a total installed capacity of 1,500MW which is designed for annual energy output of 3,000 GWH.
Bakhtiary dam site and its reservoir are located in the Folded Zagross tectono–sedimentary province. The dam site is completely placed on siliceous limestones of Sarvak formation (Cretaceous period), divided to 7 units from Sv1 to Sv7. Bakhtiary dam and hydropower plant project is unique for its outstanding characteristics. One of them is the extensive engineering geological and rock mechanics investigations carried out during its design studies. Subsurface exploration at Bakhtiary dam site consisted of the drilling of more than 178 boreholes with the total length of about 12,500mand excavation of 11 galleries with the total length of 2,850m for performing engineering geological investigations and in-situ rock mechanics tests. Geotechnical investigations performed in all the boreholes included RQD measurements, lithological and joint survey and analysis of permeability tests’ results.
A large number of laboratory rock mechanics tests were performed on rock samples selected from the drill cores of the exploratory boreholes including index tests, ultrasonic wave velocity measurements, unconfined compression tests, triaxial tests and direct shear tests. In order to make a realistic estimate on the geomechanical parameters of the existing rock masses at the dam site, an extensive in-situ rock mechanics test program was performed in the exploratory galleries excavated in the dam foundation and underground powerhouse areas. This test program consisted of deformability tests including Plate Load, Large Flat Jack and Dilatometer tests, in-situ stress measurement including Borehole Slotter and Hydraulic Fracturing tests and Direct Shear test for determining the shear strength parameters of the bedding planes as the most frequent and dominant discontinuity at the dam site. Since the Plate Load Tests provided the most reliable results for estimating the deformability characteristics of the rock masses therefore; a correlation was defined between the overall modulus of deformation of the rock masses and their ratings based on GSI classification system. Results of Hydraulic Fracturing tests at Bakhtiary dam site revealed that the average orientation of the major horizontal principal stress at the dam site (N57° ± 36° ENE) fitted well with the general stress regime by which the Zagross mountain ranges had been created, in the vicinity of the borderline of Arabian and Eurasian tectonic plates. Engineering classification of the existing rock masses at Bakhtiary dam site were performed based on the detail information gathered from engineering geological mapping and discontinuity survey data on the surface and in the exploratory galleries using RMR₈₉, GSI and Q systems. Then the modulus of deformation of the rock masses were estimated based on the correlation between Plate Load Tests results and GSI values of the tested rock masses using Hoek and Diederichs formula (2006) and their ultimate bearing capacity were calculated using the Serrano et. al. method (2000). The estimated geomechanical parameters for the “Group A” rock masses confirm that a 325m high arch dam can be constructed at Bakhtiary dam site.

ABSTRACT: Many problems result when coal mine shafts are built in complicated geological conditions and serious shaft lining ruptures have often occurred in the eastern part of China since 1987. Several types of treatment methods were used for shaft lining reinforcement such as grouting method, sets of wall reinforcement method and stress-relief slot method; based on an analysis of these methods and the mechanism of the shaft lining rupture, a new treatment method was proposed for shaft lining treatment with underground continuous impervious curtain. In this research, the various conditions of the UCIC built around the shaft lining were analyzed, and the effect of the (UCIC) range of improvement was discussed, then the optimum condition was determined. The results show that the effect of the UCIC for treatment depends on the range and width and also the material’s property, the safety factor of the shaft lining can be the evaluation criterion and had great changes in the boundary of the different layers.

ABSTRACT: Rock cutting is an important operation in some rock engineering works such as oil well construction and machine driven excavation of rock tunnels. The selection of drill bits and drilling parameters is crucial for the economics of the operation. It can be very important to evaluate both rate of penetration and drill bit life during the planning the construction of a well. Single cutter laboratory experiments are used to generate data to help evaluating the drill bit interaction mechanisms. The paper describes one two-dimensional and one three-dimensional continuum finite element model of rock cutting, allowing a close simulation of the cutting process during the single cutter test. The numerical simulation presented takes into account confining pressure, large deformation and high strain rate.

ABSTRACT: The two twin derivation tunnels of a large hydroelectric project in Brazil were 20m apart one from the other, separated by a rock septum. The failure of the gate at the inlet of one of the tunnels promoted the sudden draw down of the reservoir. The maximum velocity of the water was calculated as being of some 50 m/s, causing heavy erosion of the tunnel in its first 50 m, reducing the rock septum thickness to only 12m.
A concrete plug was to be built at the middle portion of the damaged tunnel and then the reservoir would be filled again, but maintaining the neighbor tunnel closed and empty for some time, before it also receives a concrete plug. The problem consisted in the assessment of the strength of the rock septum and its stability when subjected to a hydrostatic pressure of 18MPa, which could cause failure of the rock wall, leading to a disaster with the emptying of the reservoir again.
The paper describes the studies performed, comprising geological and geomechanical characterization of the rock septum, finite element analysis under diverse conditions and the conclusion derived as well as the protection measures taken. The concrete plugs were built, the reservoir was filled successfully and the project is in operation for some years now without any problem.

ABSTRACT: The structural design of permanent reinforced concrete linings of tunnels involves significant uncertainties, principally related to the simplistic representation of the concrete structure interaction with the surrounding geological medium and the inherent limitations pertaining to direct application of standard limit state design concepts. Notably, the characteristic of nonlinear structural behaviour of concrete is rarely considered in a rigorous manner. Geo-materials are often represented simplistically, including discontinuous rock masses which may still be approached as pseudo-continua. This paper reports on an integrated modelling approach allowing for the variable stiffness behaviour of reinforced concrete at different stages of flexural cracking and the effects of creep in interaction with discontinuous rock surround. The concrete behaviour model was implemented in the Universal Distinct Element Code (UDEC) and demonstrative results from modelling cases will be presented.