Deep Overburden Layer Research Articles

Lithological and Structural Factors Affecting Groundwater Occurrence in Eruwa, South West Nigeria

The subsurface Lithological and Structural framework was investigated to understand its effects on groundwater aquiferous occurrence in this study, underlined by the Precambrian Basement Complex of Nigeria, with granite, gneiss, and schist occurring as the dominant rock types. To successfully map the subsurface, fifty-two (52) Vertical Electrical Sounding points were probed to a depth of current electrode spacing of 100m depth to obtain the variation in the resistivity measurement of the underlying weathered basement and basement rock in the subsurface. Apparent resistivity values and current electrode spacing are plotted on a bi-logarithm plot and correlated to standard curves to estimate subsurface layers' depth, thickness, and resistivity values. WinResist software uses a mathematical algorithm to estimate further the geoelectric parameters to find the best-fit model that matches the observed data. Golden Surfer software was used to represent the geoelectric parameters to aid in geospatial representation. Results show 3-5 lithological layers, which include Topsoil, Clay, Laterite, Sand, Sandy Clay, and Fresh Basement with resistivity value range 82.1Ωm - 2648.2Ωm, 21.8Ωm - 98.4Ωm, 102.3Ωm -167.5Ωm, 404.6Ωm - 2468.2Ωm and 606.8Ωm - 20,535.90Ωm indicating a weathered and fresh unweathered basement rocks layers with H, KH, HA, HKH, KHKH, QH and A geophysical curve which are also typical geometry of weathered material and limited or no fractures basement rock layer. Basement rock resistivity, Overburden thickness, Reflection coefficient, longitudinal conductance, Anisotropy coefficient range (606.8Ωm-20535.9Ωm, 2.6m - 35.6m, 0.3-1.0, 0.01-0.86 and 0.7-3.3) respectively which a typical value of shallow to deep overburden layer with limited or no-detected fracture basement rock). Incorporating the geoelectric parameters value obtained and calculated reveal limited structural deformation of the basement rocks to yield fracture capable of creating secondary porosity in the area while the lithology of the weathered layers becomes the exploitable source for groundwater recovery in the study area.

A binary-medium-based constitutive model for artificially cemented gravel-silty clay mixed soils

In situ soils of deep overburden layer of dam foundation are widely distributed in Southwest China, which are mixtures of gravel and silty clay in varying proportions and have cementation feature between soil particles. Based on a series of triaxial compression tests carried out on artificially cemented mixed soils (CMS) and remolded mixed soils (RMS) at confining pressures of 50, 100, 200 and 400 kPa with fine contents (silty clay) of 13%-70% in mass, a binary-medium constitutive model for artificially cemented mixed soils is proposed within framework of breakage mechanics theory for geological materials and homogenization theory. This model was formulated by regarding the soils as a binary medium consisting of bonded blocks (idealized as bonded elements) and weakened bands (idealized as frictional elements), whose deformation properties are described by the modified Duncan-Chang model and the Lade-Duncan elastoplastic model, respectively. The non-uniform distribution of stress and strain within a representative volume element can be given by introducing the structural parameters of breakage ratio and local stress coefficient. Comparisons of computed results with test results demonstrate that the binary-medium constitutive model can reflect mechanical properties of artificially cemented mixed soils well.

Effect of slope with overburden layer on the bearing behavior of large-diameter rock-socketed piles

Pile foundation is a typical form of bridge foundation and viaduct, and large-diameter rock-socketed piles are typically adopted in bridges with long span or high piers. To investigate the effect of a mountain slope with a deep overburden layer on the bearing characteristics of large-diameter rock-socketed piles, four centrifuge model tests of single piles on different slopes (0o, 15o, 30o and 45o) were carried out to investigate the effect of slope on the bearing characteristics of piles. In addition, three pile group tests with different slope (0o, 30o and 45o) were also performed to explore the effect of slope on the bearing characteristics of the pile group. The results of the single pile tests indicate that the slope with a deep overburden layer not only accelerates the drag force of the pile with the increasing slope, but also causes the bending moment to move down owing to the increase in the unsymmetrical pressure around the pile. As the slope increases from 0o to 45o, the drag force of the pile is significantly enlarged and the axial force of the pile reduces to beyond 12%. The position of the maximum bending moment of the pile shifts downward, while the magnitude becomes larger. Meanwhile, the slope results in the reduction in the shaft resistance of the pile, and the maximum value at the front side of the pile is 3.98% less than at its rear side at a 45o slope. The load-sharing ratio of the tip resistance of the pile is increased from 5.49% to 12.02%. The results of the pile group tests show that the increase in the slope enhances the uneven distribution of the pile top reaction and yields a larger bending moment and different settlements on the pile cap, which might cause safety issues to bridge structures.

HYDROGEOPHYSICAL DELINEATION OF PROLIFIC GROUNDWATER AQUIFER AROUND STUDENTS’ HOSTELS IN FUPRE CAMPUS, NIGERIA

Electrical resistivity survey was conducted around Students’ Hostels in FUPRE to delineate prolific aquifer for potable water to serve the teeming population of staff and students. Five vertical electrical soundings (VES) using Schlumberger electrode configuration with a maximum current electrode spread of 300m were employed for data acquisition. WINRESIST software was employed to execute the iteration and inversion processes of computing resistivities, depths and thicknesses of the various layers and the curve types. Results indicate that the area is characterized by 4 distinct geoelectric layers inferred differently at the VES locations. Two potential groundwater aquifer zones are delineated. The unconfined shallow aquifer zones found at VES 1, 3, 4 and 5 locations have shallow overburden depth ranging between 3.7-19.3m and coarse-grained sand columns with thicknesses ranging between 2.8-17.7m while the confined deep aquifer zone found at VES 2 location coincides with deep overburden layer at a depth of 42.6m and coarse-grained sand column with appreciable thickness of 19.1m. These results suggest that boreholes for sustainable groundwater supply around the Students’ Hostels should be sited at VES 2 location and screened at a depth ≥40.0m. However, aquifers at VES 1, 3, 4 and 5 have potentials for groundwater but are vulnerable to contamination. It is recommenced that electrical resistivity and hydrogeological surveys should be conducted at different locations in FUPRE, before any borehole(s) are drilled, to delineate the appropriate aquifer for potable groundwater supply and to avoid possible contamination.

Research on the dam foundation pit hydrogeological problems in Dadu river deep overburden layer area

Western Sichuan is located in the two and three steps of transition zone of our country on the terrain, in the area of Dadu River valley was “V” type, the valley is widely distributed deep overburden layer. The existence of deep overburden layer not only greatly increases the difficulty of hydropower development in the southwest region, but also gives birth to a series of new scientific problems. This paper analyze the basic feature and formation of deep overburden layer in the Dadu River, and then discuss the may encounter hydrogeological problems with the pit water gushing, piping and flow sand.

Analysis of overburden layer thickness influence on dynamic response of concrete face rock-fill dam

In the past, when performing dynamic response analysis of dams on deep overburden, the dam body and the overburden have often been discussed separately. In this paper, the overburden and the dam body are considered as a whole, and the dynamic response analysis is carried out by using a completely nonlinear dynamic analysis method. From the acceleration of the earth’s surface, the displacement of the dam, and the stress distribution of the panel, the dynamic response of the structure is shown to increase first and then decrease with increasing cover thickness, and the overburden layer thickness corresponding to the extreme point is called the critical thickness. The results obtained in this study can provide a design basis for a face rock-fill dam built on a deep overburden layer.

New deformation back analysis method for the creep model parameters using finite element nonlinear method

Creep deformation of rockfill and overburden affect anti-seepage systems, such as face slabs, toe slabs, and peripheral joints, which may cause structural damage, particularly in a high concrete-faced rockfill dam (CFRD) on a deep overburden layer. The deformation mechanism of rockfills is complex. It is difficult to determine the creep deformation characteristics completely with creep test. Back analysis based on measured data inversion is an effective approach to study the long-term deformation characteristics of CFRD. In this paper, a new deformation back analysis method called MPSO-BP, which integrates a modified particle swarm optimization algorithm and neural network simulator, is presented to reverse the creep model parameters of Jiudianxia CFRD based on the plate load test, pressure test, and large-scale triaxial test. The creep model parameters of rockfill and overburden are applied to analyze and forecast the long-term deformation characteristics of the CFRD by using 3D nonlinear finite element numerical analysis method. Results show that the calculated creep deformations are agreed with the monitoring data. The back analysis method and the inversion parameters are demonstrated to be reasonable. The deformation of the CFRD tends to stabilize after the dam reservoir reaches its tenth year of operation.

Seepage analysis of the fractured rock mass in the foundation of the main dam of the Xiaolangdi water control project

The foundation of the main dam in the Xiaolangdi water control project is located on the riverbed with a deep overburden layer. Here, the bed rocks have complex geological conditions and developed faults and fissures. Therefore, seepage within the fractured rock mass of the dam foundation has a significant impact on the safety operation of the dam. Using the fractured rock mass seepage theory, a three-dimensional hydrogeological model of the main dam foundation in the Xiaolangdi water control project is established in this paper which takes into account the distribution of faults and fissures in the foundation. The hydraulic conductivities of the major structures and faults are obtained by back-analyzing the data measured by osmometers embedded in the dam foundation. The model is validated by comparing measured water levels and flow rates with calculated results. On this basis, a sensitivity analysis was made of the different factors affecting seepage in the dam foundation. The following conclusions can be made: (1) Seepage is positively correlated with the reservoir’s water level, but there is hysteresis in the correlation, (2) the concrete cutoff wall in the dam’s foundation effectively controls the amount of seepage. Also, an increase in the curtain depth on both banks reduces seepage a certain amount. Furthermore, the right bank is clearly more impermeable than the left bank, (3) the deep-coverage layer of the foundation has a notable influence on seepage. The drainage curtain in the downstream of the dam effectively controls the amount of seepage in the dam foundation and improves the stability of the foundation seepage, (4) faults in the dam’s foundation have a significant influence on seepage. If grout-strengthening technology is applied to the curtain, seepage control at the intersection between the curtain and faults should be especially highlighted, (5) the sediment deposit upstream of the dam body has no impact on the quantity of seepage from the dam’s foundation, but it does significantly reduce the hydraulic head.

Long-Term Deformation Analysis of the Jiudianxia Concrete-Faced Rockfill Dam

Creep deformation of rockfills affects anti-see page systems, such as concrete face, toe slab, and peripheral joint. In particular, it may cause structure damage, such as water stop failure and concrete face cracking, especially in a high concrete-faced rockfill dam (CFRD) on a deep overburden layer. In this paper, viscoelastic constitutive models for concrete materials and bedrocks (a new creep constitutive model for rockfills) are introduced. A comprehensive study method on the stress and deformation of a high rockfill dam on a deep overburden layer is established based on field plate loading tests, laboratory triaxial creep tests, and prototype monitoring data analysis. The long-term deformation characteristics of Jiudianxia CFRD with complex topography and geological characteristics are analyzed and forecasted by using three-dimensional nonlinear finite element model analysis considering the effect of creep. The deformation of the dam shows an evident increment compared with that without considering the creep effect. Rockfill creep obviously influences the deformation and stress of the concrete-face slab. Meanwhile, comparative study of numerical calculation results and monitoring data showed good agreement between the computed settlements and measured values. Overall, the creep model and the adopted parameters are demonstrated to be reasonable, and the deformation of Jiudianxia CFRD is basically stable.

Stability and Seismic Behavior of Landslide with Deep Overburden Layer in Kala Hydropower Station, China

Landslide induced by earthquakes is one of serious seismic secondary disasters. For some large-scale landslide as discussed in the study, both rigid limit equilibrium method and dynamic finite element method are applied to comparatively analyze the stability against sliding under typical working conditions, and to reveal dynamic response characteristics. Results show that rainfall and earthquake have significant influences on the stability of landslide mass, and the safety factors under rainfall and seismic conditions do not satisfy relevant specifications. The similar results are obtained by dynamic finite element method, but the safety factors are larger than that of rigid limit equilibrium method. The landslide belongs to thrust load caused landslide, featuring upper tension damage and lower shear damage. Some measures, such as cutting slope and reducing load, shear tunnel, anchor tunnel, are recommended to reinforce the landslide mass.

Seepage Calculation and Analysis of the Upstream Earth-Rock Cofferdam of the Pinecone Hydropower Station

The upstream cofferdam of the Pinecone Hydropower Station was an upstream earth-rock cofferdam built on the deep overburden layer of upstream foundation pit, which has many characteristics such as big reservoir capacity in front of the cofferdam, deep foundation pit behind the cofferdam and long anti-seepage time. In order to provide reference and a basis for seepage control construction, the sensitivity analysis on permeability coefficients of the seepage control system was made to analyze the seepage characteristics of upstream cofferdam at design water level and to discuss the permissible value range of permeability coefficients. The calculation results show that correct and reasonable seepage control measures are able to realize an effective control of the seepage field. Besides, the engineering design parameters are obtained when the optimization analysis on permeability coefficients meets the seepage control requirements, and thus provide the basis for engineering optimization design.