48 Geohydraulic testing
COMMENTARY ON CLAUSE 48
It is necessary to understand the groundwater regime within which a project is to be constructed for a number of reasons. These include the correct evaluation of the soil parameters for the design of the structure and its foundations, the design and implementation of temporary works to enable construction of the works and the protection of the environment, in particular groundwater as a valuable and sustainable resource during the construction and life time of the works.
Intrinsic permeability is usually measured with respect to air and is independent of the fluid.
Permeability, as used in this British Standard, is strictly the hydraulic conductivity, a measure of the rate of water flow.
The guidance given in BS EN 1997-2:2007, 2.1.4 and BS EN ISO 22282-1 should be followed. In order to meet these requirements, tests should be carried out in the field to determine the properties of the ground, in particular the permeability of the soil and the transmissivity and storage coefficients for aquifers.
In-situ permeability testing should be carried out in accordance with the various parts of BS EN ISO 22282. It is important that the correct test is carried out as the tests are often limited to a particular situation and in particular to a limited range of permeabilites.
NOTE 1 Before carrying out any tests, it is important to identify the aquifer and understand whether it is confined or unconfined. The determination of in-situ permeability by tests in boreholes involves the application of an hydraulic pressure in the borehole different from that in the ground, and the measurement of the rate of flow due to this difference. The pressure in the borehole may be increased by introducing water into it, which is commonly called a "falling head" or "inflow test", or it may be decreased by pumping water out of it in a "rising head" or "outflow test". The pressure may be held constant during a test (constant head test) or it may be allowed to vary (a variable head test). The technique is applicable only to the measurement of permeability of soils below groundwater level. The measurement of the permeability of unsaturated or partially saturated zones is extremely difficult. A variety of tests is available, ranging from the simple, which can nevertheless be used to investigate complex situations, to the sophisticated; the interpretation of the data is crucial. It is important to establish the normal fluctuations within the aquifer.
NOTE 2 For most types of ground, field permeability tests yield more reliable data than those carried out in the laboratory, because a larger (although still modest) volume of material is tested, and because the ground is tested in situ, thereby avoiding the disturbance associated with sampling.
To achieve representative test results, care should be taken in the choice of appropriate drilling methods and careful drilling techniques used to avoid disturbing the ground to be tested. In coarse soils, the ground can be loosened below the bottom of the borehole; in laminated ground, a skin of remoulded mixed material can be formed on the walls of the borehole, thus blocking the more permeable laminations; in fractured rock, the fractures might be blocked by the drilling debris.
The selection of the test method should also take account of the likely permeability to be measured. With soils of high permeability, i.e. greater than about 10–3 m/s, flow rates are likely to be large and head losses at entry or exit and in the borehole might be high. In this case, the use of field pumping tests, where the pressure distribution can be measured by piezometers on radial lines away from the borehole, should be evaluated. In lower permeability soils and in rock down to about 10–7 m/s, the test should be carried out using a standpipe or piezometer which is sealed within the test length using grout (see Hvorslev, 1951 ). In ground of low permeability, the flow rate might be very small, and measurement can be subject to error owing to changes in temperature of the measuring apparatus. The permeability in soils is influenced by the effective stress and the stress history. This should be taken into account in the test regime and in the analysis of results.
NOTE 3 Constant head tests might give more accurate results than variable head tests; but on the other hand variable head tests are simpler to perform, allow a number of different methods of analysis and can indicate other effects.
The water pressure used in testing should be less than that which disrupts the ground by hydraulic fracturing. It has been shown that serious errors can be introduced if excessive water pressures are used.
NOTE 4 When the test is carried out within a borehole using the drill casing, the lower limit of permeability that can be measured reliably is determined by the watertightness of the casing joints and by the success achieved in sealing the casing into the ground.
NOTE 5 There might be significant differences between the results of inflow tests, in which effective stress is reduced, and the results of outflow tests, in which it is increased. The permeability of soil around the borehole can also be influenced by changes in its stress history owing to installation of the borehole and any previous permeability tests performed on it.
Permeability tests should be carried out by suitably experienced persons; execution of the borehole tests requires expertise, and small faults in technique can lead to errors of up to one hundred times the actual value. Even with considerable care, an individual test result is often accurate to one significant Figure only. Accuracy can be improved by analyzing the results of a series of tests. In many types of ground, however, particularly stratified soil or fractured rock, there might be very wide variations in permeability, and the important measure of the permeability of the mass of ground may be determined by a relatively thin stratum of high permeability or a major fractures.
Considerable care should be exercised in interpreting the test data. Where a reliable result is required, the programme of borehole permeability tests is normally followed by a full-scale pumping test.
NOTE 6 Many effects of the geohydraulic tests are not only influenced by the ground itself, but stem from the testing procedure. Historically, the water pressure test was evaluated based on the assumption that the stationary behaviour was achieved. Recent advances in geohydraulics have shown that transient phenomena are often present. BS EN ISO 22282-3 attempts to address the limitations of certain testing procedures without restricting the required equipment too stringently. The water pressure test can be carried out in a borehole of any orientation and diameter. The test section can be located either below or above the groundwater level.
For open piezometer systems, the guidance in BS EN ISO 22282-2 should be followed.
NOTE 7 BS EN ISO 22282-2 specifies requirements for the determination of the local permeability in soils and rocks below and above groundwater level in an open hole by water permeability. Constant flow rate test method is suitable for k-value greater than 10–6 m/s; the variable head test method is suitable for k-value between 10–6 m/s and 10–9 m/s; the constant head test method is suitable for k-value between 10–4 m/s and 10–7 m/s.
Water pressures tests (WPT) in boreholes drilled into rock (also known as "Packer tests") should be carried out in accordance with BS EN ISO 22282-3.
NOTE 8 The tests are used to investigate the following:
- hydraulic properties of the rock mass, which are mainly governed by discontinuities;
- absorption capacity of the rock mass;
- tightness of the rock mass;
- effectiveness of grouting; and
- geomechanical behaviour, e.g. hydrofracturing, hydrojacking.
When undertaking pumping tests as part of geotechnical investigation, BS EN ISO 22282-4 should be used. It applies when tests are performed on aquifers whose permeability is such that pumping from a well can create a lowering of the piezometric head within hours or days depending on the ground conditions and the purpose. It covers pumping tests carried out in soils and rock. It should be used for evaluating the hydrodynamic parameters of an aquifer and well parameters, such as:
- permeability of the aquifer;
- radius of influence of pumping;
- pumping rate of a well;
- response of drawdown in an aquifer during pumping;
- skin effect;
- well storage; and
- response of recovery in an aquifer after pumping.
Pumping tests should be used to determine the transmissivity, T, and the storage coefficient, S.
NOTE 9 A pumping test consists in principle of drawing down the piezometric surface of the groundwater by pumping from a well (the test well) and measuring the pumped discharge and the water level in the test well and piezometers, before, during and after pumping, as a function of time.
When undertaking testing for the determination of the water permeability of an existing geological formation or of treated or compacted materials BS EN ISO 22282-5 should be used. The infiltrometer test is used to determine the infiltration capacity of the ground at the surface or shallow depth.
NOTE 10 The infiltrometer test is a simple test for determining the permeability coefficient. The method can be applied using either steady-state or transient conditions, in saturated or unsaturated soils. Surface infiltration devices include single and double-ring infiltrometer designs of the open or closed type. The measurement devices and measurement procedures are adapted to different ranges of permeability. Open systems are adapted to permeability ranges from 10–5 m/s to 10–8 m/s and closed systems for permeability lower than 10–8 m/s. This test is not commonly used in the UK.
When undertaking testing on for closed system tests, described as "pressure pulse tests" but are also known as "transient" or "slug tests", BS EN ISO 22282-6 should be followed. (Neither of these test methods is used commonly in UK.)
Soakaway "infiltration" tests are often undertaken in connection with drainage design and should be carried out in accordance with BRE Digest 365 [N3]. These tests are generally performed in pits or trenches and are specific to the design procedure in the BRE Digest. They are infiltration tests and should not be used to assess permeability.
The method of determination should give representative results for the proposed site of the soakaway or system. This is achieved by:
- Excavating a trial pit of sufficient size to represent a section of the design soakaway. The infiltration test requires a trial pit (or pits) located in the vicinity of the proposed soakaway. The depth should match the depth of the proposed soakaway, i.e. the total depth from the finished ground level should be equal to the depth to invert of the surface water drain (0,75 m — 1,5 m) plus the proposed operating depth of the soakaway
- Filling the pit three times in quick succession whilst monitoring the rate of seepage, to represent soil moisture conditions typical of the site when the soakaway becomes operative (only filling as high as the invert level).
- Examining site data to ensure that variations in soil conditions, areas of filled land, preferential underground seepage routes, variations in the level of groundwater, and any geotechnical and geological factors likely to affect the long-term percolation and stability of the area surrounding the soakaway have been assessed.
- Following the BRE Digest method fully.
Groundwater should not rise to the level of the base of the soakaway, during annual variations in the water table.