Soft Rock Layer Research Articles

A Study on Structural Safety of Concrete Arch Dams During Construction and Operation Phases

A methodology is proposed for static safety evaluation of concrete arch dams during their construction and operation phases. It includes foundation modeling, stage-construction process, thermal post-cooling analysis, realistic behavior of contraction and peripheral joints, reservoir filling, and operational thermal loading. The structural stability and safety is assessed using safety indices. The proposed methodology is applied to a typical concrete arch dam. The dam is three-dimensionally modeled along with its foundation using finite element method. Different foundation properties are investigated in homogeneous and inhomogeneous conditions including distinct soft rock layers. The stability and safety of the dam-foundation system is evaluated through some analysis cases, which show the importance of various features presented in the model.

Reliability and variance-based sensitivity analysis of arch dams during construction and reservoir impoundment

The static performance of arch dams during construction and reservoir impoundment is assessed taking into account the effects of uncertainties presented in the model properties as well as the loading conditions. Dez arch dam is chosen as the case study; it is modeled along with its rock foundation using the finite element method considering the stage construction. Since previous studies concentrated on simplified models and approaches, comprehensive study of the arch dam model along with efficient and state-of-the-art uncertainty methods are incorporated in this investigation. The reliability method is performed to assess the safety level and the sensitivity analyses for identifying critical input factors and their interaction effects on the response of the dam. Global sensitivity analysis based on improved Latin hypercube sampling is employed in this study to indicate the influence of each random variable and their interaction on variance of the responses. Four levels of model advancement are considered for the dam-foundation system: 1) Monolithic dam without any joint founded on the homogeneous rock foundation, 2) monolithic dam founded on the inhomogeneous foundation including soft rock layers, 3) jointed dam including the peripheral and contraction joints founded on the homogeneous foundation, and 4) jointed dam founded on the inhomogeneous foundation. For each model, proper performance indices are defined through limit-state functions. In this manner, the effects of input parameters in each performance level of the dam are investigated. The outcome of this study is defining the importance of input factors in each stage and model based on the variance of the dam response. Moreover, the results of sampling are computed in order to assess the safety level of the dam in miscellaneous loading and modeling conditions.

塔什店矿区马头门稳定性数值模拟分析

塔什店矿区马头门稳定性数值模拟分析

Optimization study on roof break direction of gob-side entry retaining by roof break and filling in thick-layer soft rock layer

This paper proposes gob-side entry retaining by roof break and filling in thick-layer soft rock conditions based on the thick-layer soft rock roof strata migration law and the demand for non-pillar gob-side entry retaining projects. The functional expressions of main roof subsidence are derived for three break roof direction conditions: lateral deflection toward the roadway, lateral deflection toward the gob and vertically to the roof. These are derived according to the load-bearing boundary conditions of the main roadway roof stratum. It is concluded that the break roof angle is an important factor influencing the stability of gob-side entry retaining surrounding rock. This paper studies the stress distribution characteristics and plastic damage scope of gob-side entry retaining integrated coal seams, as well as the roof strata migration law and the supporting stability of caving structure filled on the break roof layer at the break roof angles of -5°, 0°, 5°, 10° and 15° are studied. The simulation results of numerical analysis indicate that, the stress concentration and plastic damage scope to the sides of gob-side entry retaining integrated coal at the break roof angle of 5° are reduced and shearing stress concentration of the caving filling body has been eliminated. The disturbance of coal mining to the roadway roof and loss of carrying capacity are mitigated. Field tests have been carried out on air-return roadway 5203 with the break roof angle of 5°. The monitoring indicates that the break roof filling section and compaction section are located at 0-45 m and 45-75 m behind the working face, respectively. The section from 75-100 m tends to be stable.

불규칙한 층상구조 지반에서의 발파진동 분석

측상구조를 갖는 다변성 지반에서 시행된 3개소 24회의 시험발파로부터 획득한 발파진동 데이터를 지층단면도와 연계하여 제어가 불가능한 지반 특성으로 인한 발파진동 영향을 분석하였다. 화산쇄설층인 클링커층이 불규칙하게 반복되는 층상구조에 대한 발파공학적 경험과 자료의 부족 등으로 충분한 검토가 이루어지지 않은 비효율적인 발파가 이루어지고 있다. 시험발파 지역 내 클링커층의 N값은 매우 다양한 범위로 나타나 정량화하기 매우 어렵다. 그리고 현장의 전체 발파진동 추정식(SRSD)은 감쇠지수 n의 절대값이 설계지침보다 작게 나타나 발파진동이 멀리 전파되었으며, 발파진동상수 K와 감쇠지수 n이 상대적으로 매우 작은 값을 보인 시험발파. 1의 진동특성을 지층단면도로부터 유추한 결과, 연암층 하부에 접한 클링커층 인근까지 발파천공이 진행되어 발파 시 하부에 넓게 분포하고 있는 클링커층으로 폭발 가스압이 손실되었을 것으로 판단된다. 이로 인해 암파쇄량도 절반정도로 감소하였다. 그리고 현장의 주주파수 대역은 일반적인 구조물의 고유 주파수 대역과 유사한 저주파수 대역으로 확인되었다. By comparing test blasting data experimented in three layered-structure polymorphic grounds to a geological profile, influence of blast vibration with respect to uncontrollable ground characteristics was analyzed. Inefficient blast have been performed without sufficient verifications or confirmations because insufficiencies with regard to experiments and data of blasting engineering on the layered structures to be irregularly repeated clinker layer consisted of volcanic clastic zones. It is difficult to quantify N values of clinkers within test blasting region because they have diverse ranges, or coverages. An absolute value of attenuation coefficient N in a field, estimated by blasting vibration predictive equation (SRSD), are lesser than criteria of a design instruction, meaning that vibrations caused by blast can spread far away, and the vibrational characteristics of blasting test No.1, indicating relatively small values, inferred by the geological profile, pressures of gas by the explosion may be lost into a widely distributed clinker layers by penetrating holes resulted from blast into vicinity of clinker layers located in bottom of soft rock layers at the moment of blast. As a result, amounts of spalling rocks are decreased by almost half. Also, ranges of primary frequencies in the fields are identified as similar to those of natural frequency of typical structures.

Research on water–rock (soil) interaction by dynamic tracing method for Huangtupo landslide, Three Gorges Reservoir, PR China

As groundwater activity develops in a landslide system, the water–rock (soil) interaction increasingly influences the development of the landslide hazard. In this paper, Huangtupo landsli de was chosen as the subject of our research, which is located beside the Three Gorges Reservoir and the deformation continued for a long time. First, based on a comprehensive field survey, ten types of bad geological elements which probably induce the deformation were collected from different parts of landslide. These include a soft rock layer, the sliding soil, a weak intercalation and the typical rock close to the slip zone. Then, to trace the internal relationships among these samples, the microstructure, chemical composition of minerals, the migration and evolution of clay minerals, also the particle size distribution characteristics of all the samples were analyzed separately. Comparison of the result shows, that the evolutionary paths are very obvious among these samples. With the activities of water–rock (soil) interaction, the formation and evolution of main bad geological elements can be summarized. Afterwards, the detailed mechanism of interaction is revealed by focusing on four samples which compose one of these three paths, using additional data from physical and mechanical tests, the study shows how water–rock (soil) interaction affects the microstructure and weakens the mechanical properties of rock and soil. The presented detailed research probes the water–rock (soil) interaction mechanism specifically for Huangtupo riverside landslide as a case study. Furthermore, the applied multi-sample dynamic tracing method is verified as a means to discover and illustrate the deformation mechanism of landslides.

Delineation of a paleo-channel utilizing integrated geophysical techniques at the port of duqm area, sultanate of oman

Integrated surface geophysical surveys were conducted along a 3 km line in the sabkha area at the Port of Duqm site in the Sultanate of Oman in order to model the thickness of the uppermost sabkha layer and to determine the existence of a paleo-channel in the area. The spatial location of the survey line was laid out by a geodetic land survey for accurate geophysical measurements. Three geophysical surveys, the shallow seismic refraction method, the 2D multichannel analysis of surface waves (MASW) and the horizontal-to-vertical spectral ratio (HVSR) survey, were conducted. Despite uncertainties that are known to be associated with each of the used methods, a good agreement was revealed between the geophysical results and data from three nearby boreholes in terms of thickness and seismic wave velocity variation. The results of the shallow seismic refraction and MASW methods show a section with three main layers; a near-surface layer with P-wave velocity varying from 300 m s−1 to 500 m s−1, S-wave velocity varying from 150 m s−1 to 350 m s−1 and thickness ranging from 2 m at the northwestern end to 10 m at the southeastern end. This low-velocity layer is interpreted as the sabkha formation. The second layer is characterized by P-wave velocity ranging from 1000 m s−1 to 1600 m s−1, S-wave velocities of 450 m s−1 to 600 m s−1 and thickness varying from 4 m to 8 m. The third layer shows P-wave velocity from 2200 m s−1 to 2700 m s−1 and a shear wave from 650 m s−1 to 850 m s−1. This could be considered as a soft rock layer. The fundamental frequencies indicated by the HVSR results at the southeastern part of the line close to the start point, where relatively thick recent alluvium deposits are present, are lower than those at the northwestern part towards the end point, where rocky outcrops of Tertiary limestone appeared. The results reflect the existence of a paleo-channel at the southeastern part of the profile in agreement with the geomorphological data.

환산SPT N값을 이용한 현장타설말뚝의 단위주면마찰지지력 산정

말뚝의 지지력은 주면마찰력과 선단지지력으로 구분할 수 있고 설계지지력을 산정 시에는 말뚝의 지지형식에 따라 두 지지력을 모두 고려하거나 둘 중의 하나만을 고려하기도 한다. 본 연구에서는 국내 현장타설말뚝의 양방향재하시험을 결과를 분석하여 각 지층별로 측정된 단위주면마찰지지력 값을 국내외의 제안식으로 산정된 값과 비교분석하였다. 토사층의 경우 SPT N 값을 이용하는 제안식 값들이 재하시험을 통하여 실측된 단위주면마찰력에 비해 작아 과소평가 되고 있음을 알 수 있었고, 암반층의 경우 국내외 제안식들이 암반의 일축압축강도를 기반으로 되어있어 코아시료 채취가 어려운 국내 풍화암이나 연암층에는 적용이 곤란하여 환산된 SPT N값을 이용한 산정식을 제안하였다. 그 결과 점성토의 단위주면마찰지지력은 <TEX>$f_s{\leq}5tf/m^2$</TEX>, 사질토의 단위주면마찰지지력은 <TEX>$f_s{\leq}15tf/m^2$</TEX>의 상한값을 제안할 수 있었다. 암반층에서는 수정 SPT N값에 따라 단위주면마찰력을 제안하였는데 풍화암층은 <TEX>$15tf/m^2$</TEX> < <TEX>$f_s{\leq}50tf/m^2$</TEX>, 연암층은 <TEX>$f_s{\geq}35tf/m^2$</TEX>으로 나타났다. Bearing capacity of a drilled shaft can be separated into side resistance and base resistance. But in domestic design procedure side resistance is usually underestimated compared with base resistance. Results of bi-directional test showed that measured side resistances in each different layers are larger than those evaluated from several suggested methods. In this study, measured side resistances in each different layer of drilled shafts installed in domestic sites are analyzed and compared with evaluated side resistances from the method using revised SPT N value. For weathered rock and soft rock layer, from which rock core can hardly be obtained, we suggested new evaluated methods using revised SPT N value instead of the method using uniaxial compressive strength of rock. Resuts showed that the ranges of side resistance of cohesive and non-cohesive layer are <TEX>$f_s{\leq}5tf/m^2$</TEX> and <TEX>$f_s{\leq}15tf/m^2$</TEX> respectively. Range of side resistance in weathered rock is <TEX>$15tf/m^2$</TEX> < <TEX>$f_s{\leq}50tf/m^2$</TEX> and that in soft rock <TEX>$f_s{\geq}35tf/m^2$</TEX>.

The Effects of Fault Stepovers on Ground Motion

Abstract Using 3D dynamic models, we investigate the effect of fault stepovers on near‐source ground motion. We use the finite‐element method to model the rupture, slip, and ground motion of two parallel strike‐slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but supershear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a damage zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homogeneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground‐motion prediction in future earthquakes on geometrically complex faults. Online Material: MPEG‐4 movies of models of dynamic rupture of fault stepovers embedded in heterogeneous material settings.

춘천지역의 천부 횡파속도를 구하기 위한 레일리파 분산곡선 역산

천부 횡파속도(<TEX>${\nu}_s$</TEX>)를 구하기 위한 효율적 방법을 모색하기 위하여, 수동적 및 능동적 방법으로 발생된 레일리파의 분산곡선 특성을 분석하였다. 춘천지역에서 반경 5 ~ 40 m인 4개 삼각형 배열을 이용하여 5분간 감지한 상시미동을 확장된 공간자기상관법으로 분석하였다. 동일한 지역에서 해머로 발생시키고 4.5 Hz 지오폰 24개로 2초간 기록한 인공적 레일리파는 다중채널 표면파 분석법으로 처리하였다. 7 ~ 19 Hz와 11 ~ 50 Hz 구간에서 상대적으로 높은 신호/잡음비를 보이는 상시미동과 인공적 레일리파의 분산곡선을 병합하고 역산한 결과를 시추공 주상도와 비교하였다. 토사층 및 연암층의 <TEX>${\nu}_s$</TEX>는 각각 221 m/s와 846 m/s 정도로 비교적 일정하나, 사력-혼전층 및 풍화암층 구간에서는 깊이에 따라 선형으로 증가하는 양상을 보인다. 횡파속도에 의한 지반분류를 적용할 경우, 풍화암/연암의 경계는 시추주상도에 표시된 깊이보다 5 m 깊은 것으로 분석된다. To evaluate methods of determining near-surface shear-wave velocities (<TEX>${\nu}_s$</TEX>), we derived dispersion curves of Rayleigh waves generated by both passive and active sources in Chuncheon, Korea. Microtremors were recorded for 5 minutes in each of four triangular arrays with radii of 5 ~ 40 m. Those data were analyzed using the Spatial Autocorrelation method. Rayleigh waves were also generated by a hammer source and recorded in the same area for 2 s using 24 4.5-Hz geophones. Multichannel Analysis of Surface Waves was applied to those data. Velocity spectra were derived with relatively high signal-to-noise ratios in the frequency ranges of 7 ~ 19 and 11 ~ 50 Hz for the microtremors and synthetically generated Rayleigh waves, respectively. The resultant dispersion curves were combined as one and then input to inversion to derive shear wave velocities that were compared with a lithology log from a nearby well. Shearwave velocities in the top soil and soft-rock layers are almost constant with values of 221 and 846 m/s, respectively; while the inverse-modeled <TEX>${\nu}_s$</TEX> increases linearly in the gravelly sand, cobbles, and weathered-rock layers. If rock type is classified based on shear-wave velocity, the inversion-derived boundary between weathered-rock and soft rock may be about 5 m deeper than in the well log.

Several Geotechnical Design and Construction Issues with Akashi Strait Bridge

The Akashi Kaikyo (Strait) Bridge, one of the major bridges of the Honshu-Shikoku bridge network connecting the main island and the Shikoku island, is the world's longest suspension bridge with a center span of about 2,000 m. The bridge was constructed over the Akashi Strait and opened to traffic in 1998 after a construction period of about 10 years. The granite stratum in the Akashi Strait was very deep, while the granite stratum provides the foundations for the other long span bridges which have been constructed in Japan. Three of the four foundations had to be constructed on either a weakly cemented gravelly soil deposit or a sedimentary soft rock layer. The geological structure was carefully investigated and the geotechnical properties, in particular the strength and deformation characteristics of these geomaterial types, were carefully evaluated. To this end, a comprehensive series of advanced triaxial compression (TC) tests were performed on rotary core tube samples of sedimentary soft rock after it was found that the due design was not possible based on unconfined compression tests. Both CD and CU TC tests and cyclic undrained triaxial tests were performed on rotary core tube samples (30 cm in dia.) of the gravelly soil. Based on the data, the long-term and seismic stabilities of the foundations were evaluated. The importance of strain- and pressure-dependent nonlinear stress-strain behaviour at small strains, as well as accurate strain measurements in the laboratory stress-strain tests, was confirmed by analyzing the full-scale behaviour of the foundation during and after construction and comparing the back-calculated stiffness values with those obtained from laboratory stress-strain tests, pressure-meter tests, plate loading tests and field shear wave velocity measurements.

Toppling mechanisms of rock slopes considering stabilization from the underlying rock mass

Soft-hard interbedded structures are commonly found in anti-dip rock slopes and endow the slopes with special properties. In this work, a simple two-beam model was first used to discuss the possible failure modes of rock layers with different lithology. The exact failure sequence of rock layers depends on their tensile strengths, elastic moduli, and thicknesses. Then, based on the deformation compatibility of rock layers, a novel theoretical method called the DCM for assessing the stability of soft-hard interbedded anti-dip rock slopes was proposed. A centrifuge test and a numerical model were used to verify the feasibility of the DCM. The calculated critical g-level of 47 g for the centrifuge model and a safety factor of 0.96 for the numerical model agree well with the results of the centrifuge test (45 g) and numerical simulation (0.98). Moreover, the traditional limit equilibrium method (LEM) was also adopted to explore these two models. The calculated results of 68 g and 1.79 using the traditional LEM were found to be much larger than the validated values, with deviations of more than 50%. This finding implies that the traditional LEM is not suitable for assessing the stability of soft-hard interbedded anti-dip rock slopes. In addition, the failure evolution of toppling failure in soft-hard interbedded anti-dip rock slopes was investigated using numerical modelling. The results show that the soft and hard rock layers do not fail at the same time; the rock layers of one type of lithology are destabilized first, then the other rock layers begin to fail until the entire slope undergoes flexural toppling.

A Generalized Backward Euler algorithm for the numerical integration of an isotropic hardening elastoplastic model for mechanical and chemical degradation of bonded geomaterials

AbstractAn extended version of the classical Generalized Backward Euler (GBE) algorithm is proposed for the numerical integration of a three‐invariant isotropic‐hardening elastoplastic model for cemented soils or weak rocks undergoing mechanical and non‐mechanical degradation processes. The restriction to isotropy allows to formulate the return mapping algorithm in the space of principal elastic strains. In this way, an efficient and robust integration scheme is developed which can be applied to relatively complex yield surface and plastic potential functions. Moreover, the proposed algorithm can be linearized in closed form, thus allowing for quadratic convergence in the global Newton iteration. A series of numerical experiments are performed to illustrate the accuracy and convergence properties of the algorithm. Selected results from a finite element analysis of a circular footing on a soft rock layer undergoing chemical weathering are then presented to illustrate the algorithm performance at the boundary value problem level. Copyright © 2002 John Wiley &amp; Sons, Ltd.

A study on problems associated with finite element excavation analysis by the stress-flow coupled method

This study attempts to address some problems associated with excavation analysis by means of the stress-flow coupled method. This is done through the investigation of two examples, both of which are excavation analyses modelling a sequential excavation process. The first example is a three-dimensional elastic analysis of a tunnel constructed in soft rock layer. The second example is an axi-symmetric elasto-plastic analysis of a shaft constructed in slightly overconsolidated clay layer. In order to clarify the effect of sequential excavation process on the analysis results, a conventional two-dimensional analysis on each example is also carried out. Through this study, we will demonstrate that it is essential to consider in detail the incremental advance of a tunnel face when one investigates the change of pore water pressure and stresses surrounding a tunnel face during excavation. Additionally, on the basis of the analytical results obtained, a proposal is presented for evaluating the stability of rock mass surrounding a tunnel during construction.

Some Thoughts on the Origin, Development and Retreat of Slopes

AbstractA senior Moscow University geomorphologist suggests that the limited Russian literature on slope development interprets “slope processes” too narrowly, restricting the term largely to mass wasting. Moreover, the convex-concave slope profile is viewed as the mature type under all climatic conditions. Such an approach tends to ignore the common phenomenon of slope retreat, which is presented as a more significant factor in slope development than the processes taking place on the slope itself. Several examples of slope retreat over long distances are given, suggesting that the profiles of retreating slopes depart from the convex-concave model. The author also takes issue with the view that stepped slope profiles tend to be smoothed by mass wasting. In his view, stepped profiles may in fact be further accentuated in slope development, for example, on valley sides made up of alternating soft and hard rock layers and in the case of so-called antiplanation terraces in high-latitude mountains. The author ...