Complex Hydrogeological Conditions Research Articles

Bedding karst and multilayered groundwater flows in karstic block mountains in the northeast of Mount Xishan, Taiyuan, China

Abstract The hydrogeology of the northeastern part of Mt Xishan has historically been interpreted in terms of a simple unconfined aquifer model. The groundwater required is characterized by (1) karst terrain, (2) multilayered unconfined groundwater flow developed in block mountains involving rapid hanging discharges and (3) deep confined groundwater with a carbon 14 age of 1000–2800 years and low tritium content. The flows are partly towards the southern part of Mt Xishan, partly east towards the porous aquifers of the Nitun Basin and partly discharging through the Lancun Spring system. The spatial structure of karst aquifers in block mountains is elucidated by an understanding of stratigraphic, structural, lithological, climatic and karst geomorphological evolution. The actual complex hydrogeological conditions in the Mt Xishan area considered are demonstrated by a comprehensive model proposed by the author. Such a model may also be applied in other karstic block mountains.

Analysis of suffosion deformation of bases

For the first time, a method has been developed by the writers to analyze the degree of lixiviation of salts from soils under complex engineering-geologic and hydrogeologic conditions, which makes it possible to determine the suffosion deformations of bases.

Distributed of downward flux in unsaturated heterogeneous hydrogeology environments

We used the finite-element computer program UNSAT2 to quantify the horizontal distribution of unsaturated downward flux in porous tuff as a function of hydrogeologic heterogeneities under two different recharge rates. The distribution of downward flux is important because it influences ground-water travel time at a potential geologic repository for high-level radioactive wastes. Hydraulic properties of the Topopah Spring Member of the Paintbrush Tuff Formation at Yucca Mountain, Nevada, were selected for use in the eight simulations that were conducted. All simulations were run essentially to steady state (referred to herein as quasi-steady state). The simulations show that (1) heterogeneities in an isotropic porous rock matrix will cause downward flux to be distributed nonuniformly, (2) zones in which saturated matrix hydraulic conductivity is less than the downward flux tend to develop positive pressures that may divert flow into fractures if they exist, (3) one-dimensional analysis of vertical flow in the unsaturated zone is insufficient to ensure that fracture flow does not occur even if the true magnitude of vertical flux is known, and (4) the true spatial distribution of hydraulic conductivity above the regional water table must be determined in order to obtain the true spatial distribution of downward flux. This analysis assumes constant spatial and temporal distribution of recharge for each simulation, but the differences among simulations verify the fact that changing recharge rates also play a significant role in the distribution of downward flux owing to the influence of recharge rate on the development of positive pore pressures in the otherwise unsaturated rock. The primary significance of this research is as follows. (1) We have introduced a new method of UNSAT2 time step control that facilitates convergence even in complex, large-scale partially saturated hydrogeologic environments. Previous studies have experienced convergence problems with UNSAT2 under complex hydrogeologic conditions. (2) Nonuniform downward flow should be anticipated in heterogeneous hydrogeologic environments at a heterogeneity scale of 50 m and a model scale of hundreds of meters, even if recharge is uniform spatially. (3) The impact of heterogeneities in the unsaturated zone on downward flux can now be quantified. (4) Hydrogeologic heterogeneities can create zones of positive pressure and consequent possible fracture flow in the unsaturated zone even when the recharge rate is less than the average saturated hydraulic conductivity, but most certainly when the recharge rate exceeds saturated hydraulic conductivity. (5) This type of study provides a basis for designing hydrogeologic property testing programs and recharge distribution testing programs that are compatible with the design of models that can characterize downward flux. (6) This work shows that field documentation of the absence of positive pressures above the water table constitutes the best evidence of exclusive matrix flow (absence of fracture flow) within Yucca Mountain. If positive pressures are encountered in the field, then fracture flow probably exists within Yucca Mountain.

Experience in operating the yalta hydraulic tunnel

1. The long-term continuous operation of the tunnel shows that the main technical and economic decisions and organizational measures made during its design and construction were well-founded. The 7.3-km-long tunnel was constructed from two portal faces within 4 short years under complex geological and hydrogeological conditions with a considerable savings of the then scarce construction materials — reinforcement steel and cement. 2. Damages in the tunnel occurred and increased gradually. Cracking in the concrete lining that formed in stretches isolated from one another reached during operation 9% of the length of the entire tunnel. Noncorrespondence of the work of the lining to the calculated scheme caused by rock pressure that developed on stretches of the tunnel driven in argillites and siltstones of the Taurian series disturbed by tectonic processes should be considered the main cause of cracking. Damages that formed as a result of technological origin also occurred: poor quality of the concrete, poor placement and care of the concrete mix [1], insufficient cleaning of the rock surface before applying the shotcrete, etc. 3. The extent of the damages and labor expenditures on their elimination could have been reduced to a considerable degree by timely and systematic engineering intervention during operation of the tunnel. The absence of standards regulating the equipping of hydraulic tunnels with monitoring and measuring apparatus and procedure of observations of the condition of the structures is a significant cause of the unsatisfactory operation of the tunnel.

Performing and interpreting observations on the temperature and hydrodynamic regime for an artificially disturbed mass to evaluate water flow in mine excavations

The authors recognize the importance of the development of methods and refinement of the method of determining the vertical permeability of rock masses under field conditions for evaluating the feasibility of driving mine openings and confirming natural-protective measures taken under complex hydrogeologic conditions. Variations in the hydrostatic pressures in rock masses are observed using hydrostatic-pressure sensors. The temperature sensor is an electronic-type sensor. Investigations were performed to evaluate the excavation conditions of the stratum of coal seams located at a depth of 300-350 m. Results are presented of the measurements of groundwater pressure and temperature in the investigated interval of the mined stratum. The method shown here which is based on combined observations of the thermal and hydrodynamic regime of an artificially disturbed rock mass makes it possible to obtain reliable initial data for predicting the hydrogeologic conditions surrounding the behavior of cleaned mines beneath water-collecting structures.

Experience with chemical stabilization of saturated loesslike loams in the foundation beds of existing structures

As seen from the data presented in this article, the method of gas-silicatization with preliminary activation of the soil not only permits pumping the sodium silicate into the saturated loam without disturbing its structure, but also results in uniform distribution of the grout through the mass being stabilized and ensures a sufficiently high strength, which is important for soils in the foundation beds of buildings under complex hydrogeologic conditions.