Packer Tests Research Articles

Relationship between fault transmissivity, flow dimensions and effective hydraulic conductivity in siliceous mudstone of the Wakkanai Formation around the Horonobe Underground Research Laboratory in Japan

Effective hydraulic conductivity is typically used to evaluate groundwater flow in faulted/fractured media. Estimating effective hydraulic conductivity relies on modelling the transmissivity and hydraulic connectivity of faults/fractures using a large dataset. As an alternative to evaluating hydraulic connectivity, flow dimension is an easily available and quantitative indicator of fault/fracture hydraulic connectivity. However, the flow dimension has not been used as much quantitatively to evaluate the effective hydraulic conductivity. This study demonstrates how flow dimensions in poorly self-sealed faults directly relate to effective hydraulic conductivities. Effective hydraulic conductivities of massive siliceous mudstone of the Wakkanai Formation around the Horonobe Underground Research Laboratory (URL) in Japan were analysed by simulation with long-term (>10 years) data of hydraulic pressure and inflow during the construction and operation of the URL. The estimated effective hydraulic conductivities varied from c . 10 −8 to c . 10 −11 m s −1 with an increase in depth from c . 250 to c . 600 m. The fault local transmissivities and flow dimensions from packer tests varied from c . 10 −6 to c . 10 −10 m 2 s −1 and from c . 2.0 or more to c . 1.0 with increasing depth, respectively. When applying the Landau–Lifshitz–Matheron formula to calculate the fault effective transmissivities, these variations correlate well and the estimated effective hydraulic conductivities are sound.

The practical limits of packer testing

Packer testing allows estimation of the hydraulic conductivity of the ground through analysis of an induced flow rate into the ground under various controlled water pressures. Consideration of the physical and practical limits of injecting and monitoring such flows and pressures is used to determine the limits of hydraulic conductivity that can reasonably be estimated from packer tests. A design chart is presented based on these practical limits and consideration of the fundamental mathematics underpinning packer test interpretation. The chart is proposed as a useful reference when planning or reviewing field testing programs.

A Two-Dimensional Planar Fracture Network Model for Broken Rock Mass Based on Packer Test and Fractal Dimension

Broken rock masses with the complexity and concealment widely exist in nature such as underground mine, collapse column, and zone. It is extremely difficult to model fracture networks and to simulate water diffusion for broken rock masses. To explore a reasonable fracture network model for broken rock masses, a new method for modeling a two-dimensional planar fracture network model is proposed in this paper. It includes packer test, empirical relationship, fractal width description, and symmetric expansion modeling. Then, the fluid-solid coupling is used to simulate the diffusion properties of water in the two-dimensional planar fracture network model. It is found that the diffusion velocities vmax and vmin do not appear in the fracture widths λmax and λmin. It indicates that the fracture widths λmax and λmin in the fracture network model for broken rock mass have little impact on the diffusion velocity. Furthermore, the fracture distribution pattern in the fracture network model is an important factor affecting the diffusion velocities vmax and vmin. The simulation results of water diffusion in the currently proposed model are almost consistent with the actual process of the packer test. Also, the validity of the two-dimensional planar fracture network model is verified by comparing the simulation results with the existing research.

Existence analysis of hydraulic conductivity representative elementary volume in fractured rocks based on three-dimensional discrete fracture network method

The complexity of fractures in rocks results in heterogeneity and anisotropy of their hydraulic properties. The existence of hydraulic conductivity representative elementary volume (KREV) is a fundamental question in comprehending the hydraulic behavior of fractured rock masses. This article introduces an analytical procedure for determining the existence of KREV in fractured rock masses, which considers the distribution of underground apertures using a discrete fracture network (DFN) model. The procedure follows four basic steps: inverse modeling to generate a 3D DFN; calibration of the effective aperture by combining the results of packer tests and flow simulation using finite difference methods; calculation of the equivalent conductivities in 10 domains and 31 different flow directions; analysis of the fit error between the conductivity ellipsoid to determine the existence of KREV. The proposed approach is applied to the fractured rocks surrounding the 3013 exploration adit near the underground powerhouse of the Three Gorges Project as a case study. The results show that most fractures in the study area have apertures ranging from 0.06 to 0.25 mm and follow a lognormal distribution. The KREV size is 14–16 times the fracture spacing. Investigation of this model has significant implications for understanding flow through fracture networks in related fields.

Permeability coefficient of waterproof curtain by borehole packer test

Borehole packer testing is a commonly used method to obtain the in-situ permeability coefficients, which are closer to the actual subsurface conditions compared to indoor tests. Current methods for determining permeability coefficients through borehole packer testing are limited to semi-infinite spaces like rock masses, failing to accurately describe the permeability coefficient of domains with different shapes, such as waterproof curtains, thus leading to evaluation errors. This study proposed a novel approach that utilizes borehole packer tests on waterproof curtains for determination of in-situ permeability coefficients. Based on the assumption that seepage within the waterproof curtain obeys Darcy's law, and following the borehole packer testing procedures, the method derives specific formulas for different cross-sectional shapes of waterproof curtain, experimental parameters and groundwater levels. Applying this method to calculate domains with actual shapes has higher permeability coefficient values compared to the previous semi-infinite domain methods, which had underestimated the permeability coefficients of waterproof curtains. The study presents a calculation model for borehole packer tests on waterproof curtains under various geological formations and conditions, providing a more accurate approach to determine the in-situ permeability coefficients.

State of the art of hydraulic packer testing in clay rocks in the Swiss national radioactive waste disposal program

Abstract. To provide input for site selection and the safety case for deep geological repositories for radioactive waste, Nagra has drilled a series of deep boreholes in northern Switzerland. As part of the hydrogeological characterization of this exploratory drilling campaign, hydraulic packer tests in the sedimentary rocks were carried out under the supervision of Nagra, in depths from 300 to 1130 m b.g.l. (below ground level). Single or straddle-packer hydraulic tests were conducted in different low-permeability rocks. The hydraulic testing equipment used was specially designed to reduce uncertainty associated with the low-permeability testing conditions and included a downhole hydraulic shut-in valve (DHSIV), a hydraulic piston, a probe carrier, and inflatable packer(s). Pressure and temperature are monitored with sensors connected to the bottom of the borehole, the test interval, the annulus above the upper packer, and the test tubing above the DHSIV. Hydraulic test sequences consisted of multiple withdrawal and shut-in phases. Test phases started with a compliance phase to allow some of the pressure and temperature transients caused by tool installation and packer inflation to dissipate, followed by a pressure static recovery phase to allow the test interval pressure to establish a recovery trend after being isolated. In general, in the low-permeability formations the active test sequence consisted of a pulse withdrawal performed using a hydraulically operated piston displacing a precisely defined volume, a slug withdrawal, and lastly a slug-withdrawal shut-in phase. The pressure data collected during the hydraulic tests were analyzed using the well-test simulator nSIGHTS®. The use of a numerical model is necessary for the analyses of the tests in low-permeability rocks as borehole history and temperature equilibration during the test can have a significant impact on test responses. Analyses started with the definition of a borehole-formation conceptual flow model based on geological information and interpretation of diagnostic plots. Input model parameters composed of non-fitting parameters and fitting parameters are defined. Preliminary automated calibrations are performed to identify the appropriate conceptual model and obtain baseline estimates of the fitting parameters for each tested interval. Perturbation analyses entailing hundreds to thousands of optimizations are performed to obtain the final best-fit parameter values and the corresponding uncertainty ranges. Our presentation will give an overview on methodologies and workflows with emphasis on the numerical design and analysis techniques applied to the hydraulic packer tests performed. The applied methodology provided high-quality results and reliable estimates of the hydraulic conductivities in the low-permeability rocks. The results from the hydraulic packer tests will be presented, and implications for further research by Nagra will be discussed.

Effects of flow dimension in faulted or fractured rock on natural reductions of inflow during excavation: a case study of the Horonobe Underground Research Laboratory site, Japan

Major inflows of groundwater can occur during excavation in faulted or fractured rock masses, even if pre-excavation grouting is applied; postexcavation grouting may then be required to reduce these inflows. However, the diffusion equation for fluid pressure suggests that inflows may reduce naturally by 50–90% or more within days or weeks when the dimension of the flow in faults or fractures that feed the inflow (flow dimension) is close to 1, but inflow reduction is minimal when the flow dimension is close to 3. Therefore, if the flow dimension is close to 1, the natural reduction in inflow may obviate countermeasures. Nevertheless, this natural reduction being dependent on flow dimension is seldom considered explicitly when planning excavations or countermeasures. To verify the applicability of the relationship between natural changes of inflow and the flow dimension, this study measured changes in inflow at six locations at the Horonobe site, Japan, where major inflows occurred during excavations of tunnels or shafts in faulted or fractured siliceous rocks. The flow dimension at each location was assessed using pre-excavation packer tests in surface-based boreholes. The results confirm that changes in the inflows during the days and weeks immediately after their commencement depended consistently on the assessed flow dimension. Natural reductions in inflow during excavation are predictable based on the flow dimension, which can be estimated using pre-excavation borehole investigations or from the initial changes in inflow during the first several days. This approach may be helpful for improving the efficiency of excavations.

Enumerating hydraulically active fractures using multiple, high-resolution datasets to inform plume transport in a sandstone aquifer

It is widely known that flow in fractured sedimentary rock is dominated by the interconnected fractures in the rock mass, but it is difficult to determine which fractures observed in boreholes are active, meaning they transmit water under natural gradient conditions A methodology is presented that uses optical televiewer (OTV) and continuous core logs synergistically with FLUTe™ Transmissivity (T) profiling, straddle packer testing, rock core permeability testing, active line source temperature logs, and rock core VOC concentration profiles to determine the number and position of active fractures in a 196-meter borehole in sandstone. The application of this methodology indicates that 40% of the identified fractures are active in this borehole with a frequency of 0.61 active fractures per meter, which compares well with the number of fractures identified with the straddle packer Rec method used in the largest transmissive zones, while the low transmissivity zones tested are consistent with laboratory-measured rock matrix hydraulic conductivity. High-resolution, hydraulically based methods provide the necessary evidence to improve estimates of active fractures that control solute transport. This paper demonstrates how multiple, spatially resolved data sets can be used synergistically to inform fracture network parameters thereby constraining hydraulic aperture and surface area calculations that control flow and diffusion, with the aim of improving the estimation of plume migration rates and mass discharge to receptors in fractured sedimentary rocks.

3D Discrete Fracture Network Modelling from UAV Imagery Coupled with Tracer Tests to Assess Fracture Conductivity in an Unstable Rock Slope: Implications for Rockfall Phenomena

The stability of a rock slope is strongly influenced by the pattern of groundwater flow through the fracture system, which may lead to an increase in the water pressure in partly open joints and the consequent decrease in the rock wall strength. The comprehension of the fracture pattern is a challenging but vital aspect in engineering geology since the fractures’ spatial distribution, connectivity, and aperture guide both the water movement and flow quantity within the rock volume. In the literature, the most accepted methods to hydraulically characterise fractured rocks in situ are the single borehole packer test, the high-resolution flow meters for fractures, and the artificial tracer tests performed in boreholes. However, due to the high cost a borehole requires and the general absence of wells along coastal cliffs, these methods may not be appropriate in rockfall-prone areas. In this study, an unsaturated rocky cliff, strongly affected by rockfalls, was investigated by combining kinematic analysis, Discrete Fracture Network (DFN) modelling, and artificial tracer tests. The DFN model and potential rock block failure mechanisms were derived from high-resolution 3D virtual outcrop models via the Structure from Motion (SfM) photogrammetry technique. An artificial tracer was injected using a double ring infiltrometer atop the recharge zone of the slope to determine the infiltration rate and validate the DFN results. The DFN and tracer test methods are frequently used at different spatial scales and for different disciplines. However, the integration of digital photogrammetry, DFN, and tracer tests may represent a new step in rockfall and landslide studies. This approach made possible the identification of groundwater flow patterns within the fracture system and revealed about a 10-day tracer transit time from the injection area and the monitored slope, with similar conductivity values gathered from both the DFN and tracer test. Planar and wedge failures with volumes ranging from 0.1 and 1 m3 are the most probable failure mechanisms in the areas. The results were consistent with the delay between the intense rainfall and the slope failures previously documented in the study area and with their mechanisms.

Hydrogeological Characterization of Crystalline Bedrock Using Borehole Magnetic Resonance.

We aimed to test borehole magnetic resonance (BMR) method for determining hydraulic parameters (porosity, permeability, and hydraulic conductivity) required for hydrogeological modeling in two distinct crystalline rock environments. These sites comprise Proterozoic basement rocks of different compositions: mafic rocks at the Sakatti mining development site in northern Finland and felsic rocks at the Olkiluoto Island nuclear repository site in southwest Finland. Although BMR is widely used for determining storage and hydraulic properties in sedimentary environments, there have been few studies in crystalline bedrocks. The results indicate that BMR is a suitable tool for studying lithologically and hydrogeologically heterogeneous fractured crystalline bedrocks. It can produce continuous data from hydraulic properties of bedrock in addition to more time-consuming methods such as flowmeter and packer tests and can provide guidance on where to focus additional flow measurements. The intervals display fracture and reduced matrix porosity characteristics, both of which can be enhanced or reduced locally by chemical alteration and by tectonic processes. Flow parameters vary significantly throughout the studied intervals: independently from the lithological composition, these intervals locally display relatively high porosities, and may be correlated to the more intensely fractured and/or brecciated zones. However, due to the heterogeneity in mineralogy, grain/pore arrangement, and the variability of fracture flow-driven transport in each borehole, the challenge remains in finding a unique set of permeability constants for these crystalline rock types. The permeability models could be calibrated by laboratory measurements of the core, and possibly a new permeability model suitable for crystalline bedrock could be created.

Estimating the Hydraulic Conductivity of jointed rock mass using Genetic Programming

Ground engineering through injection of cementitious grouts or polymer resins is an integral part of fractured rock mass stabilisation by improving its maximum load carrying capacity, stiffness, cohesiveness and reduction in permeability. To optimise product delivery and achieve a high penetrability of product into the rock mass, it is imperative to understand the permeability characteristics or hydraulic conductivity of the rock mass. The hydraulic conductivity of the rock mass also dictates the type of polyurethane (PU) or cement-based products to be injected. For this purpose, a Packer test has to be conducted to measure the hydraulic conductivity of a rock formation which is an expensive and time-consuming process. Alternatively, a number of empirical methods that use rock mass classifications and rock joint properties Rock Quality Designation (RQD), Q-system (Q), Geological Strength Index (GSI), Joint Spacing (JS), Joint Aperture (a) proposed by a number of researchers that are capable of predicting hydraulic conductivity (HC). In this research, a predictive model between rock mass properties and HC is proposed using new approach - genetic programming (GP). For this purpose, a database of rock mass parameters including RQD, Q, GSI, JS, Joint Aperture (a), Second Permeability Index (SPI) and Packer test results available in the literature is established. The database is split into randomly selected training and testing sets. To assess the fitting quality, the sum of the absolute difference is used, while maximum depth on trees is set to control the bloat of the model. The performance is assessed with four statistical criteria and three GP models using different input combinations are proposed. These models have been converted into simple mathematical equations to calculate HC based on collected input data. In summary, two out of three models have successfully predicted HC with high correlation to the actual HC (R2 of testing sets ≈ 0.92). Therefore, this study has shown the feasibility of applying GP models into future prediction of HC for the initial phase of rock grouting design.

Validation of a Semi-empirical Procedure for Estimating Steady-State, Groundwater Inflows in Shallow Rock Tunnels Through Case Study Analyses

The construction of tunnels in rock terrain has increased in importance around the world as the need for new transportation routes and water and wastewater conveyances has grown. Large cities and nearby suburbs have limited space for extensive, above-ground thoroughfares; therefore, underground systems may be the only viable means for developing new infrastructure. Excessive groundwater inflow into rock tunnels under construction can injury personnel and terminate the construction of the tunnel project. Therefore, it is important to accurately estimate groundwater inflow into rock tunnels. A semi-empirical procedure for estimating steady-state, groundwater inflow in shallow rock tunnels is presented and discussed in this paper. In addition, this paper presents two case study analyses which include the Elizabethtown tunnel in New Jersey and the Toledo tunnel in Ohio. Packer test (i.e., pressure test) data was analyzed for both case studies utilizing this semi-empirical procedure. This paper reviews the basic theory behind the procedure and validates the procedure through case study analyses. It also describes previous proposed modifications and clarifies the need for any such modifications. In general, good groundwater inflow estimates were derived for shallow rock tunnels utilizing this semi-empirical procedure. This research will benefit large and small municipalities (e.g., owners of major infrastructure tunnels projects) who need to install underground conveyances for water supply lines, sewer lines, railway, and roadways. By knowing the amount of water that might flow into the rock tunnel during its construction, the owners will save money, but more importantly lives if groundwater inflow are estimated accurately.

Initial Assessment on Hydraulic Conductivity of Rock Mass and its Size Effect Using Drilling Data

Predicting hydraulic conductivity and characterizing its size effect in jointed rock masses is fundamental to various geoscience and engineering disciplines. A new empirical method, using drilling data, is established to predict the hydraulic conductivity of a rock mass in the scanline direction. The effect of the scanline unit length on the distribution characteristics of hydraulic conductivity for boreholes in jointed rock masses is studied. In addition, the scanline length of boreholes was used to study the size effect of hydraulic conductivity of jointed rock masses, and a new model was proposed to determine the size effect of hydraulic conductivity. A mathematical method for representative volume element (RVE) determination was proposed based on the proposed size effect model of hydraulic conductivity. Some packer tests were conducted to validate the developed method for predicting hydraulic conductivity. The results show that (a) the predicted hydraulic conductivities are close to those determined from the packer tests; (b) the scanline unit lengths influence the distribution characteristics of hydraulic conductivity; (c) a large but fluctuating increase in hydraulic conductivity with increasing scanline length, and the fluctuation decreases with fewer fractures and joints; (d) the mathematical method for predicting RVE and KRVE is effective to make an initial evaluation on jointed rock masses with high fracture frequency.

A Numerical Evaluation of Coal Seam Permeability Derived from Borehole Gas Flow Rate

Coal seam permeability is a critical factor in coal seam gas extraction and gas outburst control. In Australian coal mines, coal seam permeability is normally estimated using a packer test or drill stem test. In contrast, Chinese coal mines generally estimate a parameter called the “gas conductivity coefficient” by measuring natural gas flow rates from an underground borehole drilled through a coal seam. With this method, it has been frequently reported that the permeability of many Chinese coal seams is between 0.0001 mD and 0.01 mD, which is extremely low compared to that of Australian coal seams (1–100 mD). It is therefore natural to wonder how closely the Chinese method measures permeability. Resolving this question will allow knowledge and experience in outburst management to be shared between Australian and Chinese coal mines. This question is investigated by the numerical modelling of gas desorption and flow through a seam of known permeability and by using the model’s borehole gas flow rate to estimate the permeability using the Chinese method. A total of 126 simulations were run with various input reservoir parameters. The results suggest that the Chinese method estimates permeability at an accuracy of 85% to 100%, which is adequate for mine pre-drainage design and outburst control. For the high diffusion rate (e.g., high gas content and short desorption time) and low Darcy flow rates (e.g., low permeability), these errors are reduced.

Estimating Effective Porosity in Bedrock Aquifers.

Flow in many bedrock aquifers is through fracture networks. Point to point tracer tests using applied tracers provide a direct measure of time of travel and are most useful for determining effective porosity. Calculated values from these tests are typically between 10-4 and 10-2 (0.01% to 1%), with these low values indicating preferential flow through fracture and channel networks. Tracer tests are not commonly used in site investigations, and specific yield is often used as a proxy for effective porosity. The most popular methods have used centrifuge measurements, water table fluctuations, pumping tests, and packer tests. Specific yield varies substantially with the testing method. No method is as reliable as tracer testing for providing estimates of effective porosity, but all methods provide complementary insights on aquifer structure. Temporal and spatial scaling effects suggest that bedrock aquifers have hierarchical structures, with a network of more permeable fractures and channels, which are connected to less permeable fractures and to the matrix. Consequences of the low effective porosities include groundwater velocities that often exceed 100 m/d and more frequent microbial contamination than in aquifers in unconsolidated sediments. The large uncertainty over the magnitude of effective porosity in bedrock aquifers makes it an important parameter to determine in studies where time of travel is of interest.

Effect of fault activation on the hydraulic connectivity of faults in mudstone

An injection test and repeated packer tests were conducted for a fault in siliceous mudstone in order to activate the fault and investigate changes in hydraulic connectivity of the fault before and after fault activation. The injection test successfully induced a significant dilational shear failure within the fault. Pressure changes measured by the repeated packer tests were analyzed before and after the failure, where log-log plots of the pressure derivatives changed after the failure from an upward trend indicating a limited extent of fluid flow paths to a horizontal trend suggesting well-connected fluid flow paths. After the borehole had been open for six weeks, the pressure derivatives were restored to an upward trend. This reversible change in pressure derivatives means that the hydraulic connectivity of the fault increased temporarily during and just after the injection test, but fault activation did not irreversibly affect the initially low hydraulic connectivity of the fault. This transition in the hydraulic connectivity of the fault is also consistent with the variation in fluid pressure monitored at a neighboring observation hole. We propose that analyzing the pressure derivatives obtained by repeated packer tests before and after an injection test in a single borehole is effective for assessing the sensitivity of the hydraulic disconnectivity of faults to fault activation, which is key information for risk assessment of radioactive waste disposal.

An Analytical Solution for Non-Darcian Flow on a Constant Head Packer Test in the Interlayer Staggered Zone

Groundwater flow in an aquifer has frequently been found to be non-Darcian by performing in situ tests. A novel analytic model is proposed in this study for describing the unsteady non-Darcian flow in a confined aquifer by taking advantage of the observed flow rate and injection pressure during the constant head packer test. A linearization approximation of the Izbash equation is used to approximate the nonlinear term in the governing equation. This analytic model is applied to describe the non-Darcian flow in the interlayer staggered zone at the Baihetan hydropower station, China. The test results inversed by the genetic algorithm show that non-Darcian flow happened during the test under the injection pressure 0.3 MPa with the power index n is 1.278, non-Darcian hydraulic conductivity k 1 is 1.613 × 10 − 5 cm/s and the specific storage S s is 9.757 × 10 − 5 m-1, respectively. The sensitivity analysis indicated that when the power index n or the specific storage S s is larger, and the hydraulic head will increase more slowly and needs longer to stabilize, but the non-Darcian hydraulic conductivity k 1 shows the opposite trend. Moreover, the hydraulic head is more sensitive to the power index n compared to other parameters at late times. The findings of this study reveal the non-Darcian flow during the constant head packer test and provide a simple and fast way to estimate parameters for more accurate seepage field simulation.

Study on Non-Darcian Flow in Interlayer Shear Weakness Zones in the Basalt by High-Pressure Packer Tests

The interlayer shear weakness zone (ISWZ) is a special structural plane with different widths and spacing in stratified rock masses, it has higher permeability compared with surrounding rocks which is a risk factor for the safety of the hydropower station project. The high-pressure packer test (HPPT) by step injection is always applied to characterize the permeability of ISWZ. However, the non-Darcian flow is easy to appear under high pressure, which makes the Darcy law model no longer applicable. In this study, two non-Darcian flow analytical methods for confined aquifer were proposed to investigate the non-Darcian flow permeability parameters. The equivalent permeability coefficients of different non-Darcian models were derived as well. The in situ tests were conducted on the ISWZs at the Baihetan hydropower station to verify the proposed methods. The results indicate that the flow is non-Darcian flow in the test section from integrity to destruction during the whole HPPT process. Izbash’s law has a better fit than Forchheimer’s law in this complicated test situation. The equivalent permeability coefficients after destruction are one or two orders of magnitude larger than those before. Meanwhile, it is necessary to pay attention to the increased difference of two expressions of the equivalent permeability coefficients under higher gradient (i) or velocity (v). In general, these methods can be used to evaluate the characteristic of ISWZ to analyze the impact on engineering stability.

Design of seepage control system around a large-scale underground powerhouse: A case study of Baihetan Hydropower Station

Located at the lower reach of Jinsha River, the Baihetan Hydropower Station is the world’s second largest hydropower station. It consists of a double-curvature arch dam with a maximum height of 289 m and two large-scale underground cavern systems on each side of the bank, which is the largest underground cavern system of hydropower project in the world. The construction site is characteristic of complex geological conditions, and multiple interlayer staggered zones are developed between different strata. Based on the results of on-site packer tests, the interlayer staggered zones generally have a higher hydraulic conductivity than the surrounding rocks, and therefore may act as the potential seepage flow path. Since the underground powerhouse is close to the upstream reservoir and these is a large difference of water level between the upstream and downstream reservoirs, the control of groundwater flow in the surrounding rocks of the underground caverns becomes one of the key technological issues. A large-scale seepage control system including grout curtains and drainage curtains was designed around the underground powerhouses. Besides, the hydraulic conductivity of the interlayer staggered zones was systematically characterized and several anti-seepage tunnels were designed and constructed specifically for the control of seepage flow through the interlayer staggered zones. On this basis, numerical simulation of seepage field was conducted, and the results show that the designed seepage control system is effective in lowering the groundwater level and reducing the amount of seepage around the powerhouse.

The Physical Reasons to Have Underdamped or Oscillating Variable-Head (Slug) Tests: A Review and a Clarification

Variable-head permeability tests are performed in monitoring wells (MWs) or open holes between packers to assess the local value of hydraulic conductivity, K, of gravel, sand, and fractured rock. This article examines physical reasons for having an oscillating water level in the riser pipe, called underdamped (UD) response. The UD solutions are incomplete because they solve one half of the problem—for the water in the pipe—and ignore the second half, seepage in the tested material. This article shows that these half-solutions violate mathematical rules for the second half and thus, are incorrect. Four reasons for oscillations are examined: (1) solid matrix elasticity (storativity), (2) trapped gas, (3) sealing defect, and (4) hydraulic fracturing. Reason (1) yields small oscillations but not large oscillations. Reasons (2–4) can yield large oscillations and explain field data for occurrence, frequency, and damping. Thus, a free space along the casing or some trapped gas is needed to observe large oscillations. Hence, a UD slug test response only proves that the MW (or a packer) is poorly installed. This MW yields wrong values for hydraulic head, gradient, and water analyses. The data of oscillating slug tests are shown to depend upon parameters that are not easy to evaluate. This is why the UD slug tests are found to give poor estimates of K. It is recommended to use suitable drilling techniques to avoid overexcavation, to take 2–4 hours to correctly install a MW, to avoid using compressed gas to start a slug test, and to apply head differences that are high enough to have good results but lower than those producing hydraulic separation. For packer tests, it is recommended to monitor the water pressure between the packers, in the hole below the lower packer and above the upper packer, using three pressure sensors.