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Abstract: Comprehensive geotechnical study is carried out in northern part of Iraq for a site in Duhok Governorate / Kurdistan District, where an earth fill dam supposed to be constructed on Gomel River with discharge ranging between (3 – 4) and (40 – 50)m3/s in dry and rainy seasons respectively. The preliminary design of the dam is of a small type with (15) m. height. Geomorp hologically, the area consist of high, wide and comparatively large extended mountains separated by large synclines. Stratigra phically, seven formations ranging in age from Upper Campanian - Late Ma'astrichtian to Late Miocene - Pliocene in age with Quaternary sediments are exposed in the area. The site is evaluated by studying the core of eight boreholes with total length of (193) meters, four of them along the dam axis, one in the downstream, two in the reservoir area and one in the area where infrastructures related to the dam is to be construct. Each borehole is divided into geotechnical horizons depending upon the similarity of the engineering parameter values which reflect the intensity of discontinuity planes within the rock mass, beside core state. For rock mass evaluation, three engineering parameters, namely Rock Quality Designation (RQD), Fissuration factor (C-Factor) and Number of fractures per meter (F/m) are calculated, each one of them classify the rock mass into five categories from weak to excellent with numerical values (1–5) respectively, mean value of these ranks are used by equal weight with rock rating according to Uniaxial compressive strength to get final evaluation of the rock mass for each individual geotechnical horizon. Evaluation process identifies three rock mass states which are weak, moderate and good, with mean discontinuity spacing (20, 26, 44) cm, respectively, and with mean uniaxial compressive strength (19 and 47.4) MPa for moderate and good rock states. The main ruling factor is the intensity of the discontinuity plains rather than intact rock strength, where the whole rock mass shows relatively uniform intact rock strength. INTRODUCTION Consistent and appropriate site investigation techniques were used to ensure that accurate reliable and representative data to be gained during site assessment processes where a small earth fill dam of (15) m. height is to be construct on Gomel perennial river with discharge range from (3 – 4) m3/s in dry seasons, and (40 – 50) m3/s during rainy seasons. The site is located in a mountainous area in the northern part of Iraq / Kurdistan district, Figure 1. The foundation rocks material consist mainly of claystone, marl, limestone and rare gypsum at depths, while the bulk of the abutment rock material is of limestone. The dam axis is located on Pilaspi Limestone Formation (Late Eocene), dipping about 12° towards south west. Subsurface geotechnical evaluation was conducted through studying the core of eight drilled boreholes with total length of (193) m., four of them along the dam axis, two in the upstream, one in the downstream and one in the area where the infrastructures related to the dam is to be constructed.
- Phanerozoic > Cenozoic > Paleogene > Eocene (0.75)
- Phanerozoic > Cenozoic > Neogene > Miocene (0.54)
- Geology > Geological Subdiscipline > Geomechanics (0.73)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.65)
An Overview Of The Geological, Mechanical And Physical Features Of The Lower Oligocene Limestones, Al-Ain, U.A.E.
Arman, Hasan (United Arab Emirates University) | Abdelghany, Osman (United Arab Emirates University) | El Tokhi, Mohamed (United Arab Emirates University) | Hashem, Waheed (United Arab Emirates University) | El Saiy, Ayman (United Arab Emirates University)
Abstract: The city of Al-Ain is located in the eastern region of the Emirate of Abu Dhabi, United Arab Emirates. The foundational bedrock of this city is mainly the Lower Oligocene limestone. It is essential therefore to understand the geological, mechanical and physical features of this rock unit in order to minimize stability problems. This paper aims to overview the geological, mechanical and physical features of the Lower Oligocene limestone. Hence, different type of tests were carried out on the selected rock block samples. The study revealed that the unconfined compressive strength of the Lower Oligocene limestone ranges from medium to low strength. The Brazilian strength values are more or less same to the point load strength index values. The sonic velocity values are in high range. The slake durability index tests for a single cycle indicate extremely high to high class. For two cycles, the test shows high to medium high class. The rock hammer hardness test exhibites quite scattering. The bulk specific gravity, the apparent specific gravity and the bulk saturated specific gravity has very limited range. The unit weigth and the water absorbsion ratio exhibite high range. As a result, the findings may be used to overcome foundation instabilities at the city. 1. INTRODUCTION Al-Ain city is located in the eastern part of Abu Dhabi Emirates, United Arab Emirates, Arabian Peninsula near the international border of Oman and represents one of the most urbanized city in the U.A.E (Fig. 1). Most of the foundation bedrock in this city, particularly its southern part, is made mainly of limestone beds with different interbeds of marls, all are of the Lower Oligocene age. About fifty rock block samples were described and collected from three sections on the northern part of Jabal Hafit to represent the different types of the limestone beds in this rock unit. The mechanical and physical characteristics of rocks are important information for engineering applications such as design of structures either upon or inside rock, slope instability and others. They also may controlled by their petrographic characteristics, textures, composition and environmental conditions. Number of interesting research papers were already pointed out and discuss the overall effects with respect to various rock types [13]- [33]. For this study, a great number of rock samples named the Lower Oligocene limestone were collected from different locations of Jabal Hafit, Al-Ain where the Lower Oligocene limestone is well exposed (see Fig. 1). The rock block samples were in approximately 0.50x0.50x0.50 m in size. All collected rock block samples were moved to laboratory for sample preparation. Experimental studies with rock samples were carried out under laboratory conditions. Unconfined compressive strength (UCS), Brazilian strength (BRS), point load strength index (PLSI), rock hammer hardness (RHH), slake durability index (SDI), sonic velocity (SV), unit weight (UW), bulk specific gravity (BSG), apparent specific gravity (ASG), bulk saturated specific gravity (BSSG) and water absorption ratio (WAR) tests were performed to evaluate the mechanical and physical characteristics of the Lower Oligocene limestone.
Abstract: The purpose of this study is to clarify the relationship between axial point load strength and uniaxial compressive strength in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan. 1,747 rock specimens were collected for the axial point load strength test along with 326 rock specimens for the uniaxial compressive strength test. These came primarily from the earth's surface in ancient hydrothermal fields. Rock specimens in the form of cores underwent axial point load strength and uniaxial compressive strength tests using a laboratory testing machine with specimens in forced-dry and forced-wet states. An axial point load strength has a strong correlation with a uniaxial compressive strength. The estimated relationship between axial point load strength (Is) and uniaxial compressive strength (qu) is qu = 12.9 Is in soft rocks with axial point load strengths below 1.5 MPa. This combines the relationship between axial point load strength and uniaxial compressive strength in the forced-dry and forced-wet states and might be applied to onsite tests of rock with natural moisture content. Using this relationship, we can calculate the uniaxial compressive strength from only an axial point load strength test for rocks with axial point load strengths below 1.5 MPa. 1. INTRODUCTION The strength of fresh rocks and altered rocks, including hydrothermally altered or weathered rock, is generally evaluated based on uniaxial compressive strength (UCS). However, rock core pieces for the UCS test cannot be always obtained from outcrops of faulted, jointed or cracked rock masses. In these cases, the point load strength (PLS) test is a very convenient and effective alternative to the UCS test because it can be done promptly using on-site testing equipment for small rock specimens having various shapes taken from outcrops or floats. Provided that we can calculate a UCS estimate from an axial PLS value, the PLS test can lead to cost reduction and convenience. Many researchers have already studied the relationship between the PLS and UCS of hard rocks. For example, frequently cited correlations between PLS (Is) and UCS (qu) are qu % 20–25 Is [1] and qu % 24 Is [2]. The purpose of this study is to clarify the relationship between axial PLS and UCS in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan (Fig. 1). 2. ROCK SAMPLES The geology of the sampling sites consists primarily of the Upper Miocene Oteshikaushinai, Hanakushibe, and Shikerepe Formations, and the Pliocene Shikerepeyama Lava in the Okushunbetsu area of Teshikaga Town; the Upper Miocene Ikutawara Formation in the Ikutahara area of Engaru Town; and the Upper Miocene Komatsuzawa Formation in the Asahi-Nishi area of Rubeshibe Town, Kitami City, northeastern Hokkaido, Japan (Fig. 1). Rock samples, which were collected primarily from the earth's surface in ancient hydrothermal fields, are hydrothermally altered volcaniclastic rocks, including fine tuff, medium tuff, pumice tuff, lapilli tuff, and welded tuff, dacite, tuffaceous mudstone, tuffaceous sandstone, and tuffaceous conglomerate.
- Asia > Japan > Hokkaidō (1.00)
- North America > United States > Kansas > Butler County (0.24)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.55)
- Geology > Mineral > Silicate > Phyllosilicate (0.51)
Some Peculiarities And Results Of Explorations Of Deformation Processes Of The Rocks Of Adzhalykskiy Firth Valley Slopes
Freiberg, E. (Vedeneev VNIIG) | Bellendir, E. (Vedeneev VNIIG) | Golitsyn, V. (Vedeneev VNIIG) | Ablyamitov, N. (Odessa Port Plant) | Cherkez, E. (Mechnikov Odessa National University) | Tchujko, E. (Mechnikov Odessa National University) | Bich, G. (Chernomorniiproect)
Abstract: Generalization and analysis of numerous observation and research data indicate that rock massifs represent compound systems dissected by surfaces and zones of low coherence into separate structural elements (blocks) of different hierarchy [1, 2, 4]. Impact of natural and technogenic disturbances result in the fact that maximum deformation values and their derivates localize at block borders within the limits of weakness zones and in rock interlayers of relatively low strength. The job objective was to reveal basic parameters (such as distance between two homotypic planes and direction) of rocks massif composed of tectonic blocks and evaluation of its impact on development of slope rocks deformations and displacements. The territory of Yuzhniy commercial seaport (Odessa, Ukraine) is the subject of this research. Port constructions are situated on both sides of Adzhalykskiy firth - Odessa port plant is located on its right shore and an oil- terminal with onshore facilities for loading bulk product is on the left shore. During construction of the port, the firth slopes were graded and benched, dredging was undertaken and deep-water berths (18 m) were built at their toes. Meotian age Neogene rocks represented by the clay from tough to hard consistency with interlayers of tough dust sands and low-strength limestones make up geologic structure of the area up to the depth that is of interest to us. Pontic shelly limestones of rather low strength concealed under upper-Pliocene red clay and Pleistocene loess loams and sand clays lay on the eroded level of Meotian deposits. METHODS AND DISCUSSION OF THE RESEARCH RESULTS While the port is being operated, firth slopes and constructions situated on them undergo deformations which are different in magnitude and direction. This fact has brought about the necessity to perform geodetic observations, the analysis of whose results would allow to objectively define the nature of deformation processes and estimate engineering and geodynamical territory conditions. Peculiarities of changes in the deformation condition of an inclined subterranean conveyer gallery section 171 m in length have been chosen as an example in this paper. This section has been the subject of specific geodetic observations performed by the institute of Chernomorniiproect (Odessa, Ukraine) since 1989 (Fig. 1). Analysis results were performed mainly at the department of engineering geology of Odessa National University. The culvert was chosen as the object was chosen as a tool for studying structural and tectonic peculiarities of the territory as it has the following advantages in comparison with other constructions (separate buildings, berth sections, etc.) for geodetic observation. Firstly, underground section of the gallery is a long-haul linear construction that allows, in case of high density (1,5 - 3,0 m) of measurement points, to define the characteristic step of deformational inhomogeneity of the rock mass by deformation rate. Secondly, instrumentalmeasurements are made simultaneously in two parallel ranges that are at the opposite sides of the gallery on the distance of 8,7 m from each other. All these made it possible to determine direction of zones in which the deformations are most conspicuous.
- Phanerozoic > Cenozoic > Neogene > Miocene > Upper Miocene > Meotian (0.69)
- Phanerozoic > Cenozoic > Neogene > Pliocene (0.54)
SYNOPSIS: The paper presents the investigation conducted in two limestone opencast mines for the design of ultimate pit slopes. These mines cater to a cement plant and are situated in the state of Tamil Nadu in India. The main issues, which were taken into account were the presence of highway, village and other structures near mine boundaries. The investigation mainly included rock mass classification using Rock Mass Rating and Slope Mass Rating approaches, structural mapping for identifying the likely mode(s) of failure and the stability analysis for slope designs. The rock masses of these mines were classified as poor to good, with RMR varying from 38 to 62. The stability analysis was performed for plane and circular failure conditions using limit equilibrium approach. The overall slope angles in one mine were suggested at 51 - 530, 50 - 510 and 58 - 600 for ultimate slope heights of 37 m in north side (towards highway), 45 m in east side and 37 m in other sides respectively. In another mine, the overall slope angles for an ultimate slope height of 55 m were suggested at 50 - 510 for slopes facing the highway and village road and 55 - 560 for other slopes. Further, it was suggested that a minimum distance of 25 m be maintained between the ultimate slope crest and the highway and/or village road in these mines. This study has demonstrated that scientifically designed ultimate pit slopes help in the optimum recovery of mineral, which is otherwise locked up in the barriers. 1.0 INTRODUCTION The stability and design of slopes have become vitally important from the view points of safety and economics of opencast mines with the growing significance of opencast mining in mineral production. The increasing pit depth and production requirements from opencast mines subject the design engineers and planners to the pressure of working under the constraints of two conflicting requirements of safety and economics. This scenario poses a big question as to how to achieve an optimum design - a compromise between a slope that is flat enough to be safe and steep enough to be economically acceptable. The practical approach to slope stability is guided by the basic geological data, geo-technical information, ground water details and a good measure of engineering judgment (Jhanwar & Chakraborty, 2009). The factors, which mainly influence the stability of an opencast slope, are the shear strength parameters of slope mass, the presence of structural features, their characteristics and orientation vis-à-vis the slope and ground water conditions. Structural features in the form of joint, faults, etc. play an important role in defining the failure characteristics and the stability of slopes. The other extraneous factors that influence the slope design are presence of surface structures near the ultimate pit limits, influence of blasting etc. A study was conducted in two opencast limestone mines each having two separate pits and working under the constraints of important structures in the form of highway, village road, etc..
- Materials > Metals & Mining (0.85)
- Energy > Oil & Gas > Upstream (0.34)
Abstract: A large number of shotcrete slopes that were constructed in Japan during the 1970's are now more than 30 years old, and there has been a significant degree of deterioration. In Japan, many of the slopes covered with shotcrete have aged considerably. Therefore, there is a risk that slope failures may occur due to the effects of factors such as seasonal weather patterns, natural disasters, climate change, heavy rainfall, and earth quakes. Therefore, it is important to develop a method for monitoring the stability and the durability of these slopes. In this paper, we propose a technique that converts seismic velocity and electric resistivity data to porosity and saturation, which is then used to monitor weathering and groundwater fluctuation behind the slope. The evaluation results of this methodology confirm that the distribution of porosity and saturation of rock mass around the evaluated slope arrived at by this conversion system agree with those of the actual rock mass conditions evaluated using boring samples. In addition, it was possible to monitor the signs of seasonal variation and weathering in the ground by performing monitoring using this methodology over a period of multiple years. 1. INTRODUCTION Slope stabilization structures made with shotcrete were constructed in large numbers in Japan in the 1970's. As more than 30 years have passed since their original construction, a variety of problems have come to light, such as cavitations behind the concrete lining, and slope failures caused by weathering of the natural ground. Currently slope management is conducted primarily by road patrols and road facility inspections to prepare disaster prevention records regarding aged slopes. If further investigation of a particular slope is found to be required, a geophysical exploration is conducted to review countermeasures to be taken against defects. For these reasons, Kusumi and Nakamura [1] devised a technique that can convert in-situ seismic velocity and resistivity to porosity and water saturation distributions (hereafter referred to as "conversion analysis"). In this study, using this conversion analysis methodology we tried to monitor the thickness of weathered rock and the degree of moisture build up in a more accurate manner, with a focus on seismic and electrical explorations that are considered to be effective in assessing the weathering of the natural ground behind the slope and variations in ground water levels. 2. GEOLOGICAL CONDITIONS OF THE SLOPE The geophysical data for the analysis in this study was acquired at a shotcrete slope (A-district) and a non-supported slope (B-district) of a national highway as shown in Figure 1. In geological terms, the overall slope consists of a sandstone layer, alternating sandstone-shale strata, and a basalt lava layer, which are located in the Tanba strata formed during the Triassic and Jurassic periods of the Mesozoic era. Figures 2 and 3 show the locations of the investigation, which are near the south side of the national road. The overall slope is comparatively large-scale, with dimensions of about 200m in length and about 50m in height.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.56)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.46)
Abstract: Although application of concrete faced rock fill dams (CFRD) have a rather long history, these have begun to be widespread lately in Turkey. These dam types are preferred generally due to being secure, comply with land conditions, their construction being practical and economic. The geological situation of the dam location is a significant factor for rock fill dams. Concrete faced rock fill dams are constructed up to now successfully at great heights. Since the whole of the concrete faced rock fill dams are dry, the earthquake do not form cavity water pressure in the rock fill cavities. Because of this property, concrete faced rock fill dams is resistant to earthquake. In this study the properties of concrete faced rock fill dams are briefly explained and Dim Dam (Antalya) is given as an example. Dim Dam is one of the most important concrete faced rock fill dams which construction continues. The dam is located on the Dim creek 13 km northeast of Antalya province Alanya town. The dam is being constructed for the purpose of energy, irrigation and fresh water and its height from the foundation is 134.50 m and crest length is 365 m. Bahçeli formation formed by Upper Permian aged schist with limestone blocks and Quaternary aged alluvium are outcropped in the dam location. Upstream face slope of the dam is 1.40 horizontal / 1 vertical, downstream face slope is 1.50 horizontal / 1 vertical. The impermeability of the dam is provided by the concrete coating on the upstream face. 1. ADVANTAGES OF CONCRETE FACED ROCK FILL DAM Dams with concrete faced have been started to be constructed intensively in the entire world in recent years. The main reasons for preferring them are summarized in articles hereunder.–Since the filling slopes are hillsideer than clay – core fillings, the body filling volume is less and it is possible to shorten the length of their discharge installations (spillway and derivation structures). –Since zones numbered 1 (clay) and 2 (semi-permanent) do not exist, it is possible to continue work during rainy days and no damage is caused on agricultural lands – In case any problems that may increase leakage occurs on the front face, its repair is easy. –The reservoir water load on the front face is safer since it is transferred to the base rock in the downstream of the dam axis. –Under the reservoir water load + earthquake loads, high safety numbers are obtained since the material parameters do not change. Furthermore, since there is no water movement within the dam, no space water pressure increases occur under earthquake vibrations and no decrease tendency is observed at sliding strength.–Injection works can be performed in parallel with the filling construction. Base improvement requirement and cost cover the region only under the heel plate, therefore it is less than the other dam types. –It is possible to construct ramps in any direction within the dam body. This minimizes the access ways in the dam.
Abstract: The Aloft dam diversion tunnel with length of 450 m and diameter of 9 m is to be excavated in left abutment of Aloft dam and the currently under study. The lithology mostly comprises of jointed Asmara and Arum limestones. The tunnel passes the AS2 zone of Asmara formation belongs to dolomitic limestones of Miocene age. The rock mass discontinuities orientations were studied by a field survey and a statistical analysis. Also the boreholes logs and outcrop surveys were used to determine the basic characteristics of the discontinuities. The rock mass surrounding the tunnel is classified using the Q, RM and GI methods. The engineering properties and temporary tunnel support systems were determined based on the methods. A series of 3D analytical (stress-strength method) and numerical continuum (FEM) stability analyses were conducted for the rock mass surrounding excavation face based on the excavation scheme proposed by the empirical method based on RM for supported and unsupported tunnel. Based on the analyses, there is no need for intense support installation. For verification and to determine the influence area of the face, the longitudinal displacement profile values were extracted, drawn and compared with the available literature. 1. Introduction The Aloft dam is to build on Aloft River located 77 km from Shah record city, Chaharmahal-Bakhtiary Province, is still under feasibility studies. The dam diversion tunnel has the length of 450 m, diameter of 9 m and inlet invert elevation of 1037 above sea level. 2. Geology of the study area The lithology mostly consists of Avrom formation limestones, dolomitic limestones and limestones of Asmara formation which belong to lower Miocene age. Structural geology studies did not show any major fault in the tunnel route area, but a few minor and small local faults are observed in river bed and right abutment which are represented by vertical movement of less than a meter along with surface typical morphological evidences. 2.1. Orientation of discontinuities To classify the discontinuities of the left abutment where the tunnel is to be excavated, 300 data pairs of discontinuities dip/dip directions were measured from the available outcrops. 2.2. Discontinuities characteristics Basic characteristics of discontinuities including aperture, weathering, in fillings, spacing, roughness, persistence(continuity), Rock quality Designation (RED) and also ground water flow state and quantity, were extracted from 3 available boreholes logs (BH1 nearly inlet portal, BH2 in middle part and BH3 nearly outlet portal) and longitudinal geological tunnel profile. The data ranges used in the frequency analysis were uniquely determined from the ranges utilized for classifying the parameters used in famous classification systems such as Q, RM and GI. 3. Geotechnical Evaluation and classification of rock mass along the tunnel route Considering longitudinal geological profile of tunnel route, structural geology of the tunnel, 3 available boreholes logs and discontinuities orientations and characteristics, the tunnel route was divided into 3 parts as inlet portal, middle part and outlet portal. The rock mass along the 3 parts were individually classified using well-known classification systems such as RM, Q, and GI.
Abstract: A hydraulic bypass tunnel of about 850m in length and 9m in diameter is planned at the La Penna dam, along the Arno river (Tuscany, Italy). The tunnel has the purpose to laminate the floods of the Arno river. In this note the preliminary study for the Geotechnical characterization of the rock masses involved in the tunnel excavation is presented. The geological and Geotechnical settings of the area were investigated by means of field surveys and the rock-mass behaviour was investigated both in terms of wedge-fall hazard and stress-strain behaviour. The study outlined the presence of two alternating homogeneous rock masses with different strength properties which affect in different ways the excavation of the tunnel. More detailed in-situ investigations and laboratory testing are required in order to minimise the uncertainty in the design and execution stages. INTRODUCTION The La Penna hydroelectric dam has also the function to laminate the flow of the Arno river upstream Florence (Italy). The dam is a concrete arc-dam realized in the 1954–57 (Fig.1). An hydraulic by-pass tunnel of about 850m in length and 9m in diameter is planned for excavation in order to improve the discharge capacity of the dam from 630m3/s to 1350m3/s In this note the geological and Geotechnical studies performed in order to verify the feasibility of the La Penna by-pass tunnel are presented. GEOLOGICAL SETTING The La Penna basin is located on the Mt. Faltering Formation (Fig.2) which is constituted by arenaceous and arenaceous -politic turbidities with inter-layers of TNT (Thin Bedded Turbidities), the Mt. Faltering Formations has a maximum thickness of about 900m and it is dated to the Late Oligocene-Early Miocene [1, 2, 3, 4, 5]. Figure 2 – Geological map of the area around the La Penna dam. Key: FALL = Mt. Faltering Formation; L = overlying lacustrine plio-pleistocene deposits; Q = quaternary deposit; a = alluvial; Gk = geostructural ego mechanics station S = prognostic drill hole; T = planned by-pass tunnel. The lower portion of the Mt. Faltering Formation is constituted by thick beds of arenaceous turbidities with Ta, Ta-e and Ta/c-e Bowman sequences with massive sandstones (S3 interval of Lowe [6]). The upper portion of the Formation is mainly a turbidity siltstone, characterized by Tc-e and TD Bowman sequences. The petrographic composition is mainly due to quartz, feldspar and micas. The general geostructural setting is a monoclinic with dipping towards ENE/ESE of about 15°–25°. A geological cross-section across the by-pass tunnel is shown in Fig. 3. GEOSTRUCTURAL SETTING The Mt. Faltering Formation is characterized by the principal bedding and by three main joint sets (Fig.4). The bedding dips towards S0: 95°/16, and the three sets of joint towards J1: 183°/85°, J2: 244°/86°, J3: 313°/83°. The joint sets J1 and J2 are referable to a hk0 ^b system, while the J3 set are referable to the ac system (sense Hancock [7]).This structural setting is coherent with the general tectonic setting of this portion of the Northern Apennines chain, and indicates the absence of local tectonic disturbances.
- Phanerozoic > Cenozoic > Neogene > Miocene (0.69)
- Phanerozoic > Cenozoic > Paleogene > Oligocene (0.69)
- Phanerozoic > Cenozoic > Quaternary > Pleistocene (0.55)
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.96)
Abstract: Large time dependent convergences of a tunnel excavated across a carboniferous rock mass are analyzed. These delayed convergences were simulated using an elasto-viscoplastic constitutive model that included a rock strength degradation process. The 3D numerical simulation of the tunnel excavation was performed accounting for both stage construction sequence and rate of excavation advancement. A comparison of the numerical results with the data measured on site allowed for a good calibration of the model parameters. 1 INTRODUCTION In this paper, a case study of a tunnel which has experienced large time-dependent displacements due to rock creep and rock damage is presented. This phenomenon has been studied by several authors, for example, Body et al. (2002); San drone et al. (2006); Strep and Gilda (2009); Pellet and Rosebud (2007) and Pellet et al. (2009). Despite these previous studies, there is still a lack of input, especially when comparing numerical results to measured field data. Therefore, additional investigations are still needed to validate the numerical simulations of the observed behaviour of rock masses. In this study, special attention is paid to properly calibrate parameters of the constitutive model. 2 PRESENTATION OF THE TUNNEL UNDER STUDY 2.1 Geological context and construction process The tunnel under study was excavated in the Alps for a high speed railway project, as reported by Rettighieri et al. (2008). It was excavated in a highly fractured carboniferous rock formation where large convergences of the tunnel wall, of about 1.5 meter, were observed (Figure 1). Due to the nature of the geological formations, it was foreseen that the principal difficulties in the tunnel excavation would be due to: High overburden stress in the highly disturbed and vectorized geological formation, Rocks prone to creep with poor mechanical properties (squeezing rocks), The tunnel was excavated in the following sequence: 5-meter long sections were excavated using the "drill and blast" method, Light temporary supports made of shotcrete, rock bolts, and steel sets were installed, The next section of excavation was undertaken. The average advancement rate was 1 meter per day. Therefore, the time to complete each cycle was approximately 5 days. 2.2 Characterization of the mechanical properties of the rock mass The mechanical properties of the rock mass were assessed based on in-situ investigations (Russ, 2009) and based on rock mass classifications. Geological Strength Index (GI) and Rock Mass Rating (RM) were used to constrain the Hook and Brown failure criterion parameters at the scale of the rock mass. Based on the ISM suggested method (ISM, 2007), the following set of values for the peak strength and for the residual strength were selected. 2.3 Data from tunnel monitoring During the tunnel construction an extensive monitoring program was undertaken. Tunnel convergences of the tunnel wall were measured, and extensometer were used to measure displacements inside the rock mass (Rettighieri et al. 2008). The maximum convergence was more than 70 cm after 80 days. It has to be noted that the displacements increased with time even when the excavation was stopped.