Robert P. Chapuis

Practical pedotransfer functions for estimating the saturated hydraulic conductivity

The saturated hydraulic conductivity k is one of the most important and widely used geotechnical parameters, commonly involved in a diversity of applications. The value of k depends on many factors, which can be divided into three classes: properties of the fluid, pore size distribution, and characteristics of the solid surfaces. Because the latter two are not necessarily constant within a given deposit, the hydraulic conductivity may vary significantly in space. Engineers and scientists need indications about how changing factors may affect the actual k value. In this paper, the authors propose some simple expressions, based on pedologic properties, to estimate the value of k. Using experimental results of their own and taken from the literature, it is shown that the proposed pedotransfer functions can be used for quickly estimating the k value for granular and plastic/cohesive soils. Such expressions can be employed, with a useful chart format, for the preliminary design phase of a project, and also for estimating the range of k values to be anticipated within a given deposit.

Estimation of hydraulic conductivity of an unconfined aquifer using cokriging of GPR and hydrostratigraphic data

Abstract Densely sampled geophysical data can supplement hydrogeological data for estimating the spatial distribution of porosity and hydraulic conductivity over an aquifer. A 3D Ground Penetrating Radar (GPR) survey was performed over a shallow unconfined aquifer consisting of a coarse to medium sand sequence overlying an impermeable clay layer. The site is instrumented with piezometers and water levels are frequently monitored. Vertical determination of moisture and granulometry at a resolution of 10 cm were made at a few locations. The GPR reflection times were correlated with piezometric and stratigraphic information; cokriging of both data yields the spatial distribution of the radar velocities within the layers. Porosity and hydraulic conductivities are estimated using the Complex Refractive Index Method (CRIM) and Kozeny–Carman formulations, respectively. A pumping test and a tracer test, both done using a well in the center of the survey zone, provide a measure of the average hydraulic conductivity and its anisotropy. The results from cokriging in the saturated zone show that the estimated parameters agree very well with the measured hydrogeological data. The geometric mean of the porosity is close to the laboratory measurements. The geometric mean of the GPR-derived hydraulic conductivities fits the values obtained from the pumping and tracer tests. The range of estimated hydraulic conductivities is quite large and indicates that flow could be faster or slower than the one predicted from the pumping test in some places. Radar attenuation is also found to be a good indicator of porosity distribution. From the observed (high) GPR attenuations and electrical conductivities of water sampled in the piezometers, porosity is determined using Archies formula. In the vadose zone, moisture content estimated from the GPR velocities using either CRIM or Topp formulations agree well with the ones from the laboratory measurements. Cokriging of the radar reflection times and of the hydrogeological/stratigraphic data leads to an accurate estimate of the radar velocities with a precision and a spatial resolution much higher than the CDP technique. Within the limits of the interpretative models, porosity, saturation and hydraulic conductivities can accurately be estimated with a high spatial resolution over the survey zone.

Analyses of water diversion along inclined covers with capillary barrier effects.

Various types of cover systems can be used to control water infiltration into waste disposal sites. One promising option is to combine different types of soil to create a layered cover with capillary barrier effects (CCBE). A CCBE basically involves the placement of a relatively fine-grained soil, which acts as a water-retention layer, over a coarser capillary break material. On slopes, a CCBE promotes lateral water diversion. Inclined CCBEs, however, are relatively complex, as their behaviour is influenced by numerous factors. In this paper, the authors present the key results obtained from a numerical investigation into the response of steeply inclined CCBEs. The study evaluates the behaviour of covers under dry and humid climatic conditions. After a review of the physical processes and background studies, the paper presents simulation results that demonstrate the effect of key factors on the diversion length of covers, including layer thicknesses, material properties, and recharge rates. The results sh...

Permeability tests in rigid-wall permeameters: Determining the degree of saturation, its evolution, and its influence on test results

This paper reports on a method to determine degree of saturation, S-sub-r, of a soil specimen at any time during a rigid wall permeameter test. The method indicates that the tested specimen usually is not fully saturated. It is then used to prove that the usual test termination criterion based on equality of inflow and outflow volumes may be misleading. Examples are provided where the 2 volumes were equal within 1%, whereas S-sub-r increased from 80-100% and k increased by a factor of 4. Without knowing the technique to determine the S-sub-r value at any time, the test would have been stopped prematurely and would have given some k(S-sub-r) value for an unknown S-sub-r with the risk of misinterpreting this result as k(S-sub-r=100%). New equations for gas transfer between water and gas bubbles are also established and experimentally verified for specimens permeated with either deaired water or water oversaturated with air.

Variable-Head Field Permeability Tests in Driven Flush-Joint Casings: Physical and Numerical Modeling

Two types of variable-head permeability tests in driven flush-joint casings, the end-of-casing test and the lateral injection test (Lefranc test), were performed in two sand tanks. Hydraulic conditions in the sand tanks included a constant vertical hydraulic gradient that was null, positive, or negative. The gradients were monitored by lateral piezometers, which confirmed that the large-scale hydraulic conductivity (k) value of the sand layer remained constant throughout the test program. Any interpretation method for these variable-head tests implicitly requires an assumed piezometric level (APL) for analysis. According to a common practice, the elevation of the ponded water surface above the sand deposit was taken as the APL to plot the test results as the logarithm of the difference in total head versus time. After each casing installation, permeability tests were performed for the three gradient conditions. When the vertical gradient was null in the tank, the test data provided straight lines as assumed in theory. When the gradient was positive or negative, the test data provided curved graphs that may be interpreted as giving k values that vary with time. The velocity graph method, however, provided the same constant k value for the three gradient conditions. It also detected the error in the APL and provided in all cases a local piezometric level (PL) equal to that given by lateral piezometer monitoring. Thus, the velocity graph method, a graphical representation of the conservation equation, eliminated the potential for misanalysis of variable-head tests. The experimental k values, as determined by the two types of tests, usually ranged between 0.5 and 2 times the larger-scale values determined independently by the monitored flow rates and gradients in the sand tanks. The variability in experimental k values was partly due to the local variations of void ratio and effective diameter of the sand, and partly due to the influence of casing installation. Numerical simulations of the tests confirmed the validity of the velocity graph method for providing k and the local piezometric level for a test.

Extracting Piezometric Level and Hydraulic Conductivity from Tests in Driven Flush-Joint Casings

Permeability tests in borehole casings must satisfy many conditions to give reliable results. A test can be done only in a driven flush-joint casing. When a casing is rotated, its contact against the adjacent soil does not provide a seal good enough to assess the local k-value. Only water injection can be used (either falling-head or constant-head) because water extraction creates upward forces that destabilize the soil and induce either soil heave or clogging. Other conditions are explained in the paper. Interpretation methods differ mainly in their assumptions about solid matrix deformability during the test. The paper describes an interpretation method based on the equation of mass conservation: it leads to a graph of downward water velocity in the casing. This graph provides the error made in the quick field estimate of piezometric level (PL) for a tested zone. Several examples are provided, including tests that produce hydraulic separation between soil and casing. The PL obtained with this graph was always similar to that given by a monitoring well installed at the same level after borehole completion. From analysis of many tests at the same site, information can be obtained on natural water seepage conditions using a variation of PL versus depth.

An Investigation of Factors that Influence the Water Diversion Capacity of Inclined Covers with Capillary Barrier Effects

Covers with capillary barrier effects (CCBEs) can be used as an alternative to more traditional covers that rely on materials with a low saturated hydraulic conductivity. A typical CCBE includes a fine-grained soil layer placed on a coarser material. Other layers can also be added to help the cover play its role(s) efficiently. In a cover built on a flat area, the capillary barrier effect at the interface between the fine and coarse materials allows the finer soil layer to store incoming water, which can later be released by evaporation. Such Store-and-Release covers can be quite convenient in arid and semi-arid conditions. In sloping areas, a CCBE also acts as a lateral water diversion system. Part of this diversion occurs along the sloping interface between the two superimposed soils. It can contribute significantly to the reduction of water percolation deeper into the underlying wastes. However, inclined CCBEs are more complex, as they are influenced by many factors that are not yet fully understood. In such layered covers, moisture is not evenly distributed along the length of the slope. Under some conditions, the moisture-retaining layer can reach a critical degree of saturation at a certain down dip location, which increases infiltration of water into the coarse material and reduces the cover efficiency. This paper presents some of the main results of an ongoing investigation of inclined CCBEs that includes testing on a physical model, field work on large scale covers, and numerical analyses of various cover scenarios. Emphasis is placed here on numerical simulations to assess some of the influence factors that affect the diversion capacity and moisture distribution. The numerical results show how the diversion capacity is affected by the saturated hydraulic conductivity of the fine-grained soil, by the precipitation rate and duration, and by the layer thickness. The findings presented here can be useful for the design of inclined CCBEs.

Detecting and Quantifying Leakage Through Defective Borehole Seals: A New Methodology and Laboratory Verification

A new method for quantifying leakage through poorly sealed boreholes is presented and verified using a laboratory scale sandbox experiment. The method applies to a leaky borehole between two aquifers separated by an aquitard. A nonreactive tracer is injected into an upper aquifer piezometer, and the lower aquifer is pumped at a fixed rate. First, the presence of the tracer in the recovered water indicates the existence of the hydraulic short-circuit and cross-contamination. The leakage rate associated with the pumping rate can then be determined by measurement of the recovered tracer concentration. By correlating the leakage rate with the pumping rate, the hydraulic properties of the defective seal can be determined and the degree of cross-contamination can be predicted for any pumping rate. The method will be useful for practitioners who need to evaluate the quality of a borehole seal. The method is successfully tested using a laboratory sandbox experiment.

NUMERICAL SIMULATIONS OF LONG TERM UNSATURATED FLOW AND ACID MINE DRAINAGE AT WASTE ROCK PILES

The authors present a numerical modeling study of unsaturated water flow and acid mine drainage in idealized (but representative) waste rock piles and using observed climatic recharge data. The simulations were used to help understand the long term hydrogeological behaviour and to help design and assess in situ groundwater monitoring methods. The flow simulations showed that when the same annual cycles of average monthly recharge are applied each year at the top of the piles, the water content profiles become periodic after a few years. The water distribution within the piles then becomes independent of the preceding hydraulic conditions for the cases considered here. Also, the results indicate that the amplitude of water content variations inside the pile between humid and relatively dry seasons is generally small (a few percent). Consequently, typical measurements of the water content variations can, in some cases, be limited for in situ monitoring because their level of precision is often of same order of magnitude as the expected changes. Long term simulations of oxygen diffusion and acid mine drainage through the waste rock piles showed that oxygen is generally not a limiting factor in the unsaturated zones of these types of systems and that preferential flow, moisture content and grain size can have a significant influence on oxidation rates and pH distribution. ______________________ 1 Paper presented at the 7 International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Ecole Polytechnique, PO Box 6079, Stat. Centre-Ville, Montreal, QC, Canada, H3C 3A7, 3 Universite du Quebec en Abitibi-Temiscamingue, 445 boul. de l’Universite, RouynNoranda, Qc, Canada, J9X 5E4, Tel. 514-340-4711, ext. 4046 ; fax. 514-340-4477 ; michel.aubertin@polymtl.ca

A numerical modelling approach to assess long-term unsaturated flow and geochemical transport in a waste rock pile

This article presents an approach to investigate unsaturated water flow and geochemical transport in sulphidic waste rock piles using numerical simulations with stochastically distributed material properties. The water retention curve and hydraulic conductivity function are used to represent spatial variability for water flow, while distributed geochemical parameters are used for reactive transport. The results illustrate how the spatial correlation of hydraulic properties affects the moisture distribution and flow within the pile, which in some cases creates local preferential flow paths. The numerical simulations also show how sulphide mineral oxidation rates can be influenced by the grain size, sulphide fraction and moisture content distribution, which may explain the large spatial variations observed in the composition of drainage water. Such simulations help to better understand the long-term response of a typical waste rock pile.