Dam Foundation Rock Mass Research Articles

Rock Mass Structure Classification of Caves Based on the 3D Rock Block Index

In large-scale water conservancy and hydropower projects, complex rock structures are considered to be the main factor controlling the stability of hydraulic structures. The classification of rock mass structure plays an important role in the safety of all kinds of large buildings, especially underground engineering buildings. As a quantitative classification index of rock mass, the rock block index is very common in the classification of borehole and dam foundation rock mass structures. However, there are few studies on the classification of underground engineering rock masses. Moreover, their classification criteria have disadvantages in spatial dimension. Therefore, this paper takes the long exploratory cave CPD1 in the water transmission and power generation system of the Qingtian pumped storage power station in Zhejiang Province as the research object and launches a study on the structural classification of the rock mass of a flat cave based on the 3D rock block index. According to the group distribution of joints, the sections are statistically homogeneous. Additionally, the Monte Carlo method is used to carry out random simulations to generate a three-dimensional joint network model. The virtual survey lines are arranged along the center of the shape of the three different orthogonal planes of the 3D joint network model to represent the boreholes, and the RBI values of the virtual survey lines on each orthogonal plane are counted to classify the rock mass structure of the flat cave in a refined manner using the rock block index of the rock mass in 3D. The above method realizes the application of the 3D rock block index in underground engineering and overcomes the limitations of traditional rock mass classification methods in terms of classification index and dimension. The results show that: (1) Three-dimensional joint network simulations built on statistical and probabilistic foundations can visualize the structure of the rock mass and more accurately reflect the structural characteristics of the actual rock mass. (2) Based on the 3D rock block index, the rock mass structure of the long-tunnel CPD1 is classified, from that of a continuous structure to a blocky structure, corresponding to the integrity of the rock mass from complete to relatively complete. The classification results are consistent with the evaluation results of horizontal tunnel seismic wave geophysical exploration. (3) Based on the 3D joint network model, it is reasonable and feasible to use the 3D rock block index as a quantitative evaluation index to determine the structure type of flat cave rock masses. The above method is helpful and significant in the classification of underground engineering rock mass structures.

Permeability structure of the horizontally-stratified dam foundation rock mass and its engineering significance

Identifying the permeability structure of the dam foundation rock mass is important for the formulation of seepage control schemes. For the horizontally-stratified red-bed rock mass of the Guxian Dam foundation, this paper adopted a continuous, high-resolution water pressure test data processing method to analyze the relationships between rock mass permeability and elevation, lithology and shear zones, clarifiy the permeability structure of the dam foundation rock mass, and propose seepage control advice. The results indicate that the Guxian Dam foundation rock mass is characterized by a structural and random permeability behavior. The structure shows that the permeability decreases with elevation. The randomness shows that the permeability randomly fluctuates due to lithology and shear zones. For the river-bed dam foundation rock mass, the bottom elevation of the weathering and unloading zone is 450 m or so, and the 1Lu bottom boundary is at the elevation of approximately 340 m. For the rock mass of the dam abutment, the 3Lu bottom boundary is located at the elevation of 560～580 m. The section of 350～360 m elevation has a certain thickness of soft rock, a poor development of shear structure, and a low permeability. It can be regarded as a confining layer and a potential target for the optimization of seepage control schemes. In view of the universal applicability, the method proposed in this paper can be used for reference in similar projects.

Rapid in situ test and determination of dam foundation weak mudstone bearing capacity

In a hydropower station project in the middle reaches of the Minjiang River, the dam foundation rock mass is the upper cretaceous Guankou formation (K2g) brick red weak mudstone. In previous investigations, the single-pipe core drilling technology was employed, which greatly disturbed the cores. Most of the cores were broken, and the mudstone cores quickly lost water, cracked, and disintegrated after being taken out, making it difficult to obtain several columnar cores. Indoor uniaxial compressive strength tests were conducted using only a few short columnar mudstone cores, but the obtained mudstone uniaxial compressive strength was extremely low, and the converted bearing capacity of the dam foundation mudstone was only 0.25 Mpa, which does not meet the dam foundation bearing capacity standard. Large-area pile foundation treatment is required for the dam foundation mudstone at a high cost, and the project construction period is delayed. To obtain the accurate bearing capacity of the dam foundation mudstone, we used a self-developed “self-anchored dam foundation rock mass bearing capacity rapid test device” and excavated the dam foundation ship lock section to the elevation of the foundation surface. In situ tests were conducted on 12 dam foundation mudstone rock masses in 15 days, and the longitudinal wave velocities of the rock masses were tested. Reliable bearing capacity parameters of the weak mudstone of the dam foundation were obtained, and the relationship between the bearing capacity of the dam foundation mudstone and the longitudinal wave velocity was established. In situ tests showed the minimum and maximum bearing capacities of 0.986 and 1.972 Mpa, respectively, for the dam foundation weak mudstone. After excavating the dam foundation mudstone, it showed high bearing capacity without obvious softening and relaxation. The in-situ-measured minimum bearing capacity (0.986 Mpa) is consistent with the recommended bearing capacity of dam foundation mudstone, which is much greater than the maximum load of the dam foundation (0.5 Mpa). The value meets the bearing capacity requirements for dam foundation rock masses without engineering treatment. This study obtained the real bearing capacity of mudstone for dam foundations, thereby overcoming the problem of the low bearing capacity of weak mudstone obtained through laboratory experiments. The method not only saves cost but also lays the foundation for scheduled pouring of dams.

RQD calculation method based on 3D rock discontinuity network simulation in the dam foundation of Baihetan hydropower station

The original rock quality designation (RQD) index has several limitations, such as one-dimensional (1D) direction and a single threshold. For these reasons, a novel RQD calculation method is proposed by using 3D discontinuity network simulation. In this method, a 3D discontinuity network model (3D-DNM) of the rock mass is generated by Monte–Carlo analogy procedure, in accordance with the optimal probability models of the rock discontinuity’s geometrical elements. Then, the RQD values at different azimuth on the three orthogonal planes of the 3D-DNM can be analyzed statistically. Based on the analysis result, the novel index RQD t is put forward. In this index, the threshold corresponding to the maximum standard deviation of the RQD t is defined as the optimal threshold. The RQD t with an optimal threshold has the greatest capacity to reflect the heterogeneity and anisotropy of rock mass. To illustrate the effectiveness of the proposed method, we apply it on the foundation rock mass of Baihetan hydropower station on Jinsha River. Finally, the optimal threshold is determined as 0.6 m, and the quality and anisotropy of the dam foundation rock mass are fully demonstrated by the generalized RQD 0.6.

Evaluation of Lithology Variations in Layered Red Beds with Depth: An Example of the Yellow River Guxian Dam, NW China

Abstract Lithology variations, which are recognized as rock type differences, significantly affect the physical properties of rock masses in red beds. In this paper, we introduce a statistical method for quantitatively evaluating the degree of lithology variations in layered red beds with depth. The core of this method is to use borehole logs as random variables for statistical analysis to calculate the percentage of a specified lithology at a given elevation, which involves seven steps of attitude calculation, reference point selection, distance calculation, elevation modification, data discretization, data statistics, and curve plotting. The Yellow River Guxian Dam is chosen as a field case study. We classify rock types of feldspar sandstone, fine sandstone, and conglomerate into hard rocks and that rock types of calcareous siltstone, argillaceous siltstone, and mudstone into soft rocks. Borehole logs recorded during the geological investigation are used to plot the percentage curve of hard rocks. We find that the degree of lithology variations for each lithology group differs greatly, the general behaviors of lithology variations on two sides of the Guxian Dam riverbed are quite similar but still with some differences, and that some thick lithology groups can be finely divided into several subgroups. On the basis of the hard rock percentage curve, we introduce a lithology variation index to quantitatively characterize the degree of lithology variations, which can be used as an important index to supplement the traditional methods when performing rock mass classifications in red beds. We also plot the trilithology percentage curve of sandstone, calcareous siltstone, and argillaceous siltstone, which serves for the determination of physical parameters of the dam foundation rock mass, the identification of the potential shear sliding surface, and the search for an impervious grouting bottom. Moreover, we find that the crest and trough, which are local high and low points in the hard rock percentage curve, can be used to show some characteristics of shear zones. The locations of shear zones are well represented in the form of troughs and that the development of shear zones has a good linear relationship with the hard rock percentage of the corresponding crest. The method proposed in this paper can be promoted and applied in similar projects or studies.

Statistical Simplification and Discrete Element Analysis of Seepage Model for Complex Fracture Network of Dam Foundation Rock Mass

In this paper, a model of seepage in a fracture network is analyzed by taking the rock mass of the dam foundation of Datengxia Hydropower Station, the No. 1 Project of the Ministry of Water Resources, as an example. Complex discontinuity systems are present in the rock mass of the Datengxia Hydropower Station dam foundation, and faults, soft interlayers, and structural fracture systems intersect with one another. A multi scalar structural analytical scheme that combines the deterministic analysis of large scale geological structures (i.e., faults and weak interlayers) with the statistical simulation of random discontinuities (i.e., structural fractures) is proposed in this paper to reduce the difficulty and uncertainty of the analysis of rock masses with complex structures. In particular, a chain analytical method from dominant group definition to statistical parameters (i.e., spacing and occurrence) is formed for the structural fracture system. In this way, the analysis of rock masses with complex structures is simplified to a regular and feasible scientific problem that could be solved by rock mass simulation. We then establish a seepage model of the rock mass in dam section 29# in UDEC by adopting the above structural simplification scheme. The quantitative analysis of seepage in complex structured rock masses under normal water storage and dynamic conditions is then realized. Results show that the statistical simplification of the rock mass structure and discrete element simulation can efficiently solve the seepage analysis of complex rock masses. The negative influence of pore pressure caused by seepage on the rock mass, especially under dynamic conditions, should be emphasized for safety assurance in the stability analysis of dam foundations.

Case Study: In Situ Experimental Investigation on Overburden Consolidation Grouting for Columnar Jointed Basalt Dam Foundation

The dam foundation rock mass, at the Baihetan hydropower station on the Jinsha River, is mainly columnar jointed basalt, with faults and fissures developed. Considering adverse factors such as the unloading relaxation or the opening of the fissures due to excavation blasting, consolidation grouting is needed to improve the integrity of the dam foundation rock mass. According to the physical and mechanical properties of columnar jointed basalt and the continuity of construction, the effectiveness of overburden consolidation grouting is experimentally studied. The results show that this grouting technology can obviously improve the integrity and uniformity of a dam foundation rock mass and reduce the permeability of the rock mass. After grouting, the average increase in the wave velocity of the rock mass is 7.3%. The average improvement in the deformation modulus after grouting is 13.5%. After grouting, the permeability of 99% of the inspection holes in the Lugeon test section had Lugeon values of no more than 3 LU. This improvement is considerable and provides a case to engineering application.

In situ creep tests on the long‐term shear behaviors of a fault located at the dam foundation of Dagangshan hydropower station, China

AbstractFaults in the rock mass of dam foundation are crucial in controlling dam deformation and stability. In particular, long‐term stability of dam foundation is an important factor for the safety of dam operations. The rock in the foundation of Dagangshan hydropower station is mostly granite, and there are many diabase dikes distributed in granite. Geological survey indicates that many faults exist along the interfaces between granite and diabase. Lots of diabase were metamorphosed to chlorites, which have very low strength. In order to investigate the deformation of faults in diabase dikes under long‐term loading, in situ shear creep tests were performed to estimate the creep performance of a fault. The shear creep displacement and the influence of displacement history of the specimen during multi‐level loading tests were measured and analyzed. The isochronous stress‐strain method was used to determine the long‐term strength of the fault in shear creep tests. Considering the instantaneous damage and strain softening, a new viscoelastic‐plastic softening creep model was proposed to describe the shear creep behavior of the fault. Comparisons confirmed that this creep model is capable of describing the shear creep behavior of the fault and the creep parameters were estimated accordingly using this model.

Analysis of seismic damage process of high concrete dam-foundation system

In the anti-seismic analysis of concrete high dams, only the damage cracking of the dam concrete is usually considered, and the nonlinear characteristics of the dam foundation rock during the earthquake are not considered. Taking the Koyna gravity dam in India and Shapai arch dam in China as examples, this paper adopts the nonlinear time-history analysis method and takes into account the comprehensive influence of the radiation damping of infinite foundation, the hydrodynamic interaction between dam and reservoir, the nonlinear dynamic contact between various joints in the dam body and foundation, and the damage of dam concrete and foundation rock mass in the damage evolution process, analyzes damage evolution process of dam body-foundation system under earthquake action and the influence of foundation rock mass damage on seismic failure mode of concrete dam-foundation system, and compares the numerical simulation results with the actual earthquake situation. The calculation results show that after considering the damage of the dam foundation rock mass, the rock mass of the dam heel firstly appears to be damaged under the action of the earthquake and gradually expands into the interior of the foundation. The dam heel and the foundation surface are not damaged. The numerical simulation results are more in line with the actual situation of earthquake.

Study on the influence of the sediment at reservoir bottom on dynamic damage of gravity dam

Traditional linear elastic model or elastic-plastic DP model can hardly reflect the damage and failure law of the concrete and rock foundation when the tensile and compressive strengths exceed the limitation. Therefore, the concrete damage model is used to simulate the dynamic damage of gravity dam body. According to the similarity of concrete and rock mass materials, the damage model of concrete is extended to rock mass materials. The sediment at the bottom of reservoir is simulated by viscous and high density compressible fluid. Based on this, a multi-coupling simulation model is established, which can reasonably reflect the dynamic damage evolution process of the dam concrete and dam foundation rock mass. Based on the Koyna gravity dam project, taking the thickness of sediment at the bottom of the reservoir as a variable, FEM transient analysis is carried out in this paper. By comparing the damage areas of the model, the displacement of the key points of the dam body along the river and the energy dissipation index, the influence of different thicknesses of the sediment is quantified on the damage of the multi-coupling system of the gravity dam. The calculation results obtained are close to the actual earthquake damage, which verifies the feasibility and rationality of the calculation method. The results of the analysis also show that: the seismic damage of the gravity dam can be consistently reduced with the increase of the thickness of the reservoir bottom sediment. Compared with the dam body, the change of thickness of sediment has less influence on dam foundation damage.

A new and unified approach to improved scalability and volumetric fracture intensity quantification for GSI and rockmass strength and deformability estimation

Several investigators have attempted to quantify the Geological Strength Index (GSI) chart, with the latest modification of the chart (2013) utilizing RQD as a measure of blockiness. This approach has limitations where discontinuity spacing is wide (typically greater than 0.3 m) and RQD alone cannot adequately characterize the degree of blockiness, since it remains static at 100%. This paper introduces a new approach to quantifying widely spaced jointed rockmasses that is not dependent on RQD alone. At the core of the approach is a bias free volumetric fracture count (VFC) parameter (fractures/m3), that is integrated into the newly defined GSI chart as an aid to alleviate scalability and bias concerns related to the use of RQD in the quantification process. While the new GSI chart builds on the work of many, it is unique in the sense that not only is it fully quantifiable for a full range of block sizes, including block sizes much larger that possible with RQD alone, but it provides a unique approach linking the VFC parameter with P32, a parameter frequently used in DFN modelling. The correlation of VFC with P32 in particular, is possible because the VFC parameter has no constraints of a limited number and/or assumed orthogonality of discontinuity sets or rectilinear block shapes or the need for block shape correction factors. The new chart also includes correlated scales on both the vertical and horizontal axis using both the RMR and the Q-systems, providing a unified approach that is both scalable and easily quantifiable and allows for the use of all three major rock mass classification systems along with P32 within one chart, something not attempted before. Data from a dam foundation rockmass in the Lesotho Highlands are introduced and are used to validate the quantification process for the overall GSI ratings for the foundation rock mass. These ratings have been used to estimate strength and deformability parameters for the foundation rock mass using the Hoek-Brown empirical failure criteria equations and then they were compared to the large-scale in-situ test results to validate the use of the V-GSI chart and system as a new tool for use in rock engineering.

3-D fracture network dynamic simulation based on error analysis in rock mass of dam foundation

Accurate 3-D fracture network model for rock mass in dam foundation is of vital importance for stability, grouting and seepage analysis of dam foundation. With the aim of reducing deviation between fracture network model and measured data, a 3-D fracture network dynamic modeling method based on error analysis was proposed. Firstly, errors of four fracture volume density estimation methods (proposed by ODA, KULATILAKE, MAULDON, and SONG) and that of four fracture size estimation methods (proposed by EINSTEIN, SONG and TONON) were respectively compared, and the optimal methods were determined. Additionally, error index representing the deviation between fracture network model and measured data was established with integrated use of fractal dimension and relative absolute error (RAE). On this basis, the downhill simplex method was used to build the dynamic modeling method, which takes the minimum of error index as objective function and dynamically adjusts the fracture density and size parameters to correct the error index. Finally, the 3-D fracture network model could be obtained which meets the requirements. The proposed method was applied for 3-D fractures simulation in Miao Wei hydropower project in China for feasibility verification and the error index reduced from 2.618 to 0.337.

A Vibration‐Isolating Blast Technique with Shock‐Reflection Device for Dam Foundation Excavation in Complicated Geological Conditions

Under complicated geological conditions, the vibration in the dam foundation caused by blasting can lead to further deterioration of the foundation rock mass and adversely affect the safety of foundation. In order to effectively control the vibration in dam foundation rock mass, a new bench blasting technique with shock‐reflection device is proposed. It introduces a shock‐reflection device consisting of high wave impedance block and cushion material, which is placed at the bottom of vertical borehole. This shock‐reflection device can effectively reflect the explosion shock wave from vertical direction to horizontal direction after detonation, which can make blasting energy concentrated on the rock mass above dam foundation, so the vibration in the foundation can be controlled. Field blasting experiment was carried out to contrast the blasting induced vibration in foundation rock by bench blasting with shock‐reflection device and conventional bench blasting. The results indicate that the vibration in the foundation rock can be reduced by 30%~57%. In addition, the vibration at the bottom of the borehole is also demonstrated by numerical simulation, with results similar to the field experiments. The production experiment results show that the new blasting technique can replace the conventional excavation method of dam foundation in complicated geological conditions, and the new blasting technique has been successfully applied to the Baihetan dam foundation excavation.

Utilization of weathered rock mass as the China Three Gorges Project Dam foundation

The utilization of the weathered rock mass in the Three Gorges Dam Area is one of the key geotechnical issues for the China Three Gorges project (CTGP) construction. Engineering geological investigations and tests had been carried out to study the physical, chemical, mineral and mechanical properties and distribution characteristics in detail for more than 30 years. The classification of the weathered zone has been further divided. The utilization of the lower part of the weakly weathered zone as the Three Gorges dam foundation rock mass had been researched by consolidation grouting test. The research results show that: (1) The weathered rock mass in Three Gorges dam area has a great thickness of 40–70 m, with inhomogeneous behavior, physical and mechanical properties in different weathering zones. (2) It is feasible that the lower part of the weakly weathered zone could be used as the dam foundation after consolidation grouting treatment. This approach can save a large amount of construction investment. (3) The weathered rock mass can be used as different civil engineering foundation, and different weathered zone can satisfy different foundation bearing layer requirements. (4) The weathered rock mass can be utilized as construction material. The completely-weathered rock mass can be used as cofferdam filling sand material. The weakly and slightly weathered granite can be used as filling rock materials of cofferdams.

Relaxation Analysis of the Arch Dam Foundation Rock Masses of Jinping I Hydropower Project

The arch dam of Jinping I hydropower station is on obvious high ground area, and the foundation rock masses may relaxed while excavating. Limit tensile strain is put forward to be relaxation criterion. By three-dimensional elastic-plastic finite element method analyzing, and imitating the dam excavating steps, the maximum principle strain of the dam foundation rocks is obtained. The research indicates that the maximum principal strain of upstream side is greater than that of downstream side commonly, and some area would relaxed, with 41m maximal relaxed depth. By engineer analogical research with Xiaowan arch dam foundation rock mass, which have relaxed while excavating, the calculate method and criterion of this paper are reliable, and they can provide reference for engineer design.

Numerical Research on the Gravity Dam Deep Anti-Sliding with Engineering Mechanics Based on Damage Theory

Currently, researches on the gravity dam deep and shallow anti-sliding stability mainly focus on the analysis method and instability criterion, while the studies on specifically test the breakage of gravity dams due to weakening foundation rock mass and structural planes under loading are rare. Based on damage mechanics theory, this paper established a numerical model that analyzed the damage failure process of dam foundation rock mass. Taking two typical gravity dam models as the study objects, the damage processes of the dam foundations were simulated dynamically. Additionally, a comparison with other two traditional methods further validated the correctness and feasibility of the numerical model. In sum, the study findings point out that the numerical model is not only applicable to the study of the breakage mechanism of dam foundation rock mass, but also can be used as a new method to analyze problems related to deep anti-sliding stability of gravity dams.

Study of Seepage Properties of Fractured Rock Mass Based on Improved K-Means Clustering Algorithm

Seepage properties of fractured rock mass are of prime importance for hydraulic engineering and accurate description of rock fracture geometry parameters is an important and basic task in rock hydraulics. In this paper, an improved K-means clustering algorithm for structural plane of fractured rock mass was first brought forward and the corresponding Matlab program for discontinuity orientations partitioning was compiled and then used in the fitting analysis of dominant orientations of certain dam foundation rock mass. On this basis, combining calculation formulas of multi-group fractures, the permeability tensor and principle value was calculated for the actual dam foundation. The results provide a theoretical and computational reference for other similar projects.

Numerical Analysis of Damage to Retained Rock Mass of Dam Foundation Caused by Blasting Excavation

On basis of the existed TCK model, the establishment process of a new rock blasting damage model is discussed by considering the relationship of crack density Cd, which indicates the shock damage extent of rock, and sound wave attenuation coefficient ap. Then, this model is inserted into the user-defined material model library of ANSYS/LS-DYNA3D software to construct the new damage model. Numerical simulation of damage to retained rock mass of dam foundation caused by rock blasting excavation in dam region with this model. Calculation results show that: as far as the blasting parameters and geological conditions in this paper, the damage to retained rock mass below caused by rock blasting excavation above is basically limited to the scope of 1.5m below the bottom of blasthole, and damage degree gradually reduces from the bottom of blasthole to faraway. Meanwhile, blasting will not cause rock mass to open along soft layer inside, whose maximum calculation displacement is only 3mm. The calculation result can be referred by the practical engineering.

The Arch Dam Foundation Rock Mass Unloading and Relaxation Deformation Characteristics and their Monitoring Result Analysis

After foundation excavation for a dam of a hydropower project, the rock mass would be unloaded and relaxed and its mechanical parameters would be lowered, thus influencing the dam stability. This paper makes an analysis of the deformation development law of the unloading rock mass of the dam foundation by analyzing the advance deformation of the dam foundation before excavation and the unloading rebound deformation of the dam foundation after excavation, and by analyzing the monitoring results of the compressive deformation after concreting of the dam and of the whole process deformation after impounding of the reservoir taking into account the results of both deformation test and acoustic test, which provides scientific basis for quality assessment of the arch dam foundation rock mass and for readjustment of the dam foundation rock mass mechanics design parameters, thus giving a reference to projects of the kind.

Stability Analyses of the Three Gorges Dam Foundation against Sliding by Strength Reduction of Finite Elements

This paper studies the dam foundation stability of the representative no. 3 powerhouse-dam section of the Three Gorges Dam (TGD), China, using elastic-plastic finite element shear strength reduction technique. In the no. 3 powerhouse-dam section area, the low-angle discontinuities, such as joints and faults, are mostly developed in the dam foundation rock mass, and constitute deep foundation stability problems. These weak structure planes affecting the dam foundation stability are modeled as continuum bodies in the finite element modeling according to their respective spatial geometrical characteristics including inclination and dip angle. Three-dimensional elastic-plastic finite element analyses are performed to obtain the stress and deformation distributions of TGD foundation and concrete structures. The factor of safety (FOS) of TGD foundation stability against sliding and its failure mechanics are comprehensively evaluated with the following criteria: (a) The maximum of the trial factors of shear strength reduction can be reached by which the dam foundation is brought to the critical equilibrium state; (b) The inflection points of the curves of the displacements of dam heel, dam toe and dam top, vs. the trial factors of shear strength reduction are located; and (c) Plastic zones develop along the potential sliding paths consisting of the dam heel, long and large joints, and other weak structure planes as the trial factors increase. The results obtained show that the dam foundation of the no. 3 powerhouse-dam section is stable under the designed normal load combinations. The evaluations of the FOS of dam foundation stability presented may provide a significant reference for the safety assessment of TGD in the Three Gorges Project, China.