QUANTITATIVE ASSESSMENT OF PERMEABILITY - EX SITU TEST METHODS
An obvious and logical approach to assessing permeability is to obtain a sample of soil or rock from a borehole or trial pit and then subject it to tests in the laboratory. In relation to assessing permeability for the design of groundwater control schemes, these methods have the following limitations:
i. The act of sampling will inevitably cause some disturbance and stress relief of the soil or rock to be tested. This will affect the measured permeability and can result in very significant errors in permeability.
ii. Samples that can be tested in the laboratory are necessarily of modest dimensions (up to a few hundred millimetres in length and width). Therefore, the values of permeability obtained will be very small-scale (see Figure 12.1) and are unlikely to fully represent any permeable fabric in soil samples or fracture network in rock samples. The permeability values may not be directly relevant to groundwater control designs.
Despite these limitations, permeability data from these methods may be available; details and limitations are discussed in the following.
A specific limitation on laboratory tests is the effect of temperature. Laboratory tests are typically carried out at 20 °C, rather higher than shallow groundwater temperatures in the United Kingdom, which are typically 10-15 °C. The viscosity of water at 20 °C is lower than at 10 °C, and in this case, permeability recorded in the laboratory would be around 30 per cent higher than under comparable conditions in the field. However, in most cases, the effect of temperature is small compared with the other limitations of the tests, and temperature corrections are rarely applied.
Laboratory Testing of Soil Samples
There are a number of techniques for the direct determination of the permeability in the laboratory by inducing a flow of water through a soil sample - this approach is known as ‘permeameter’ testing. According to Head and Epps (2011a), there are two main types of permeameter testing:
- 1. Constant head test (Figure 12.6a). A flow is induced through the sample at a constant head. By measuring the flow rate, cross-sectional area of flow and induced head, the permeability can be calculated using Darcy’s law. This method is only suitable for relatively permeable soils such as sands or gravels (k > 1 x 10-4 m/s); at lower permeabilities, the flow rate is difficult to measure accurately.
- 2. Falling head test (Figure 12.6b). An excess head of water is applied to the sample, and the rate at which the head dissipates into the sample is monitored. Permeability is determined from the test results in a similar way to a falling head test in a borehole or monitoring well. These tests are suitable for soils of lower permeability (k < 1 x 10-4 m/s), when the rate of fall in head is easily measurable.
Tests may be carried out in special permeameters, oedometer consolidation cells, triaxial cells and Rowe consolidation cells. Samples are typically cylindrical in shape, with diameters of oedometer and triaxial tests typically between 38 and 100 mm. Rowe consolidation cells are typically 250 mm in diameter. Methods of testing are described in Head and Epps (2011a).
While these tests are theoretically valid, in practice they are rarely used because of the difficulty of obtaining representative ‘undisturbed’ samples of soils of high or intermediate permeability (silt, sand or gravel) that are of interest in dewatering design. Even if the sample is representative of the PSD of the soil, without specialized equipment, it is very difficult to measure the density, and hence the porosity, of granular soils in situ, especially below the water table. This means that the in situ condition of the soil cannot be reproduced reliably. Similarly, any soil fabric or layering in the sample will have a profound effect on the in situ permeability but cannot be replicated in the laboratory. There is also the question of direction of flow: many laboratory permeameter tests impose vertical flow through a sample and will therefore derive vertical permeability, while in practice it is often horizontal permeability that is of interest to dewatering designers.
The only time such tests should be considered on groundwater control projects is when the permeability of very low-permeability soils (which are typically dominated by clay-sized particles) needs to be determined during investigations of potential consolidation settlements. Even then, results must be interpreted with care, as in clays with permeable fabric the size of the test sample may result in scale effects distorting the measured permeability (see Rowe, 1972). Large (250 mm diameter) samples may give more representative results than the 76 mm diameter samples routinely tested, but such large samples are rarely available.
Laboratory Testing of Rock Samples
Rock cores can be tested in specialist laboratory permeameters in a similar way to soil samples, where flow of water is induced through core samples recovered from boreholes. The approach has similar limitations to laboratory testing in soils, in that the rock core may be disturbed or disrupted by sampling and stress relief, and the permeability values are very small-scale and are not especially relevant to the design of groundwater control systems.
Figure 12.6 Examples of laboratory permeameter test, (a) Constant head permeameter test: downward flow, (b) Falling head permeameter test. (From Head, К H and Epps, RJ, Manual of Soil Laboratory Testing. Volume II: Permeability, Shear Strength and Compressibility Tests, 3rd edition, Whittles Publishing, Caithness, UK, 2011. With permission.)
A further complication is that the permeability of rock samples is often dominated by flow through fractures, which tend to be highly directional. Depending on the orientation of the core axis relative to the significant fracture directions, permeability values from testing of the core may not represent the hydraulically significant fractures that are important to the design of groundwater control systems.
QUANTITATIVE ASSESSMENT OF PERMEABILITY - IN SITU TEST METHODS
In situ tests are commonly used to assess values of permeability. Common forms of in situ permeability tests in boreholes include:
- (i) Rising, falling and constant head tests in boreholes (Section 12.8.1)
- (ii) In situ tests in monitoring wells (Section 12.8.2)
- (iii) In situ tests in boreholes in rock (Section 12.8.3)
These tests are carried out either in boreholes (during pauses in the drilling process) or in monitoring wells installed in boreholes following completion of drilling and are based on a common principle - if the water level in a borehole is raised or lowered in a controlled manner, this will create an ‘excess head’ relative to background groundwater level. Depending on whether water levels are raised or lowered, this will induce flow out of or flow into (respectively) the borehole. Provided that the geometry of the ‘test section’ is known - this controls how water flows from the borehole - then the water level observations can be analysed to estimate permeability. These tests can only influence the soil or rock locally around the borehole. Therefore, these tests can, at best, produce ‘small-scale’ values of permeability representative of conditions around the borehole (see Figure 12.1). Such tests may be unduly influenced by any effects of soil/rock disturbance caused by drilling of the borehole or by local variations in geology close to the borehole.
Pumping tests, where water is pumped from a well for an extended period (typically several days), will typically influence a much larger volume of soil or rock and give more representative ‘large-scale’ permeability values but are more time consuming and expensive to carry out (Section 12.8.5). Occasionally, multiple wells are pumped in the form of a groundwater control trial to glean information on large-scale permeability (Section 12.8.6).
Specialist in situ tests are occasionally carried out as part of certain types of probing techniques, such as dissipation tests carried out in piezocone testing (Section 12.8.4). Borehole geophysical methods can also be used to provide permeability data (Section 12.8.7).
The test methods described in the following sections are those that are applicable to conventional civil engineering and groundwater control projects. More sophisticated methods of permeability testing have been developed in related fields, including shaft sinking for deep mining (Daw, 1984), investigations for deep geological disposal of nuclear waste (Sutton, 1996), and carbon sequestration and storage (Wiese et al., 2010). These methods are not addressed here but use the same principles as the tests described here, carried out at greater depths, at higher background pore water pressures and in lower-permeability rocks than is common for conventional tests.