This edition of the Preene Groundwater Consulting blog discusses the assessment of hydraulic conductivity values for use in dewatering design and outlines some of the different possible methods that can be used.
Previous blogs have addressed the question what is hydraulic conductivity? and have clarified the terminology. In geotechnical language hydraulic conductivity is often referred to as coefficient of permeability, most commonly shortened to permeability, but for simplicity we will use the term hydraulic conductivity throughout this blog.
CHALLENGES OF ESTIMATING HYDRAULIC CONDUCTIVITY
There are several problems associated with carrying out tests to estimate permeability, including:
The hydraulic conductivity of the ground may vary from place to place over short distances and may be anisotropic (i.e. may be different in different directions). Soil fabric and rock structure may affect permeability. Examples include thin silt or clay layers in a sand deposit that can make the vertical hydraulic conductivity much lower than the horizontal. In rock the presence of fissures or solution features can dramatically affect the hydraulic conductivity locally.
Disturbance due to drilling and sampling
The act of drilling a borehole or of taking a sample may disturb the soil/rock and affect the hydraulic conductivity derived from the test. For example, drilling muds may invade soil pores or rock fissures (reducing hydraulic conductivity). Alternatively, drilling action may scour or open up fissures in rock (increasing hydraulic conductivity).
Effect of scale of test or sample
Small-scale tests affect small volumes of soil – laboratory tests on cores or samples recovered from boreholes (e.g. laboratory permeameter tests) or in-situ tests which are representative of only a small zone local to the test borehole (e.g. rising and falling head tests). Such small-scale tests may not observe the effects of soil fabric or rock fissures – features that tend to increase hydraulic conductivity.
Large-scale tests affect a much larger volume of soil/rock and so can observe the effect of fabric and fissures. Examples of large-scale tests include well pumping tests and dewatering trials. Large-scale tests typically give higher (and more realistic) values of hydraulic conductivity, because they influence a much larger volume of aquifer and so tend to ‘average out’ any very localised variations. These large-scale values of hydraulic conductivity are representative of those needed for good dewatering design.
POSSIBLE SOURCES OF INACCURACY IN HYDRAULIC CONDUCTIVITY ESTIMATES
A fundamental problem with estimating hydraulic conductivity is that it cannot be measured directly. In practice, field or laboratory parameters (such as water levels or flow rates) are measured directly, and hydraulic conductivity is then calculated or estimated using that field data. This offers multiple opportunities for errors or inaccuracies in the calculated hydraulic conductivity, including:
METHODS TO ASSESS HYDRAULIC CONDUCTIVITY FOR DEWATERING DESIGN
There are several methods for assessing hydraulic conductivity as part of site investigation, including:
This involves assessing the soil type or grading, or rock type and degree of fracturing, and estimating an approximate range of hydraulic conductivity based on experience or published values (as might be found in hydrogeology textbooks or dewatering guidance documents such as CIRIA Report C515). Visual assessment should be carried out on borehole or trial pit logs on every project to give an initial hydraulic conductivity estimate. This initial estimate should be used to help design subsequent testing and as a ‘reality check’ to identify gross errors in hydraulic conductivity values from testing.
These type of tests provide ‘large scale’ hydraulic conductivity values, and where time and budget allow are often the best way of obtaining hydraulic conductivity values for use in dewatering design. This type of test involves controlled pumping from a borehole, while monitoring pumped flow rate and water levels in the pumped well and in monitoring wells located at various distances from the pumped well. Pumping should last several days (in order to influence a large volume of aquifer). Extended pumping can allow these tests to provide information on aquifer boundary conditions as well as on hydraulic conductivity. Due to their duration and complexity, these tests tend to be relatively expensive. The test design and method of analysis must be appropriate to the aquifer type, so it is important to seek advice from suitably experienced groundwater specialists.
These are in-situ tests carried out in boreholes during drilling or later in monitoring wells, where water is added to or removed from the borehole to create a differential head that will induce flow of water into and out of the borehole.
Test types include:
These tests typically influence only a small volume of soil or rock around the borehole, and so provide ‘small scale’ hydraulic conductivity values. A key issue is that drilling the borehole may have caused disturbance of soil or rock, and that the observed hydraulic conductivity value could be affected as a consequence.
These are tests carried out on samples (e.g. cores) recovered from boreholes or trial pits. Typically the samples are placed in some form of laboratory permeameter, and water flow induced across the sample, with hydraulic conductivity calculated using analysis methods based on the principles of Darcy’s law. Depending on the type and size of sample, the permeameter equipment may be based on triaxial cells, oedometer cells or Rowe cells. Due to the limited size of the sample these tests tend to provide ‘small scale’ hydraulic conductivity values. Due to the difficulty of obtaining usable samples, this type of test is mainly applicable to soils or rocks of low hydraulic conductivity. It is also important to realise that the act of sampling may affect the soil density and porosity and that soil fabric may be disturbed; these effects can influence the estimated hydraulic conductivity.
Particle size correlations
This approach uses empirical correlations to relate particle size distributions (PSD) in granular soils to hydraulic conductivity. This is based on the fairly obvious principle that in granular soils hydraulic conductivity is strongly influenced by the effective pore size, which can be related to particle size distribution (PSD). Many correlations exist between PSD and hydraulic conductivity, including the most well known, Hazen’s rule. The PSD correlation method is very widely used, but has several limitations and potential drawbacks, as discussed in a previous blog.