Detailed information on permeability testing of soil
S.1 Test procedures
(1) Twice as much material as needed for the number of specimens to be tested should be taken from the stratum.
(2) The specimens to be tested should be selected to represent the extremes in relevant soil properties, i.e. composition, density index, void ratio, etc.
(3) As a guideline, the hydraulic gradient in clays and silts should be less than 30 and less than 10 in sand.
(4) Depending on soil type and required accuracy of coefficient of permeability, the required degree of saturation in the permeability test should be considered.
NOTE Examples of procedures for lusting the permeability of soil are given in the documents listed in X.4.7.
S.2 Number of tests
(1) Table S.1 gives a guideline for the minimum number of tests required as function of the variability of the soil and existing comparable experience with the type of soil.
|Variability in measured coefficient of permeability (k)||Comparable experience|
|kmax/kmin > 100||5||4||3|
|10 < kmax/kmin ≤ 100||5||3||2|
|kmax/kmin ≤ 10||3||2||1a|
|a A single test and classification tests to verily compatibility with existing knowledge.|
(2) In Table S.1, a specification of only one test represents a verification of the existing knowledge. If the test results do not agree with the existing data, additional tests should be run.
S.3 Evaluation of test results
(1) There are four widely used methods to determine the coefficient of permeability (hydraulic conductivity):
- field tests, such as pumping and borehole permeability tests;
- empirical correlations with grain size distribution;
- evaluation from an oedometer test;
- permeability tests on soil specimens in the laboratory.
The evaluation of the coefficient of permeability can be optimised by a combination of these methods.
(2) Even in a homogeneous soil stratum, there can be a large variation in the coefficient of permeability due to small changes in stresses, void ratio, structure, particle size and bedding. The most reliable method to obtain a value of the coefficient of permeability is a field testing method.
(3) Even in a homogeneous soil stratum, the coefficient of permeability of a soil layer should be described by upper and lower limit values.
(4) For silts and clays, the derivation of the coefficient of permeability from incremental oedometer test results only gives an approximate estimate. Constant rate of strain oedometer tests provide a more direct measure of the permeability.
(5) In homogeneous sand, the coefficient of permeability may be assessed in a reasonably accurate manner from correlations with the grain size distribution.
(6) For clay, silt and organic soil where undisturbed samples of high quality can be obtained, laboratory tests may give reliable test results. The representativeness of the specimens tested should be carefully checked.
(7) For some types of soil, the degree of saturation may influence the coefficient of permeability up to as much as three orders of magnitude.
(8) The chemistry of the permeant may change the coefficient of permeability by several orders of magnitude.
Preparation of specimen for testing on rock material
(1) The ISRM Suggested Methods for Rock Characterisation, Testing and Monitoring do not contain a specific requirement for preparation of rock specimens. However, most of the test methods contain a section on preparation of samples, with requirements on sample volume, sample quality, preparation method, specific dimensions and tolerance checks on dimensions and shape.
(2) Examples of the common practice for preparing rock core specimens and determining dimensional and shape tolerances are given in the document listed in X.4.8. In the following paragraphs, extracts of, and comments on, these documents are given.
(3) It is not always possible to obtain or prepare rock core specimens which satisfy the desirable criteria given in the ISRM suggested methods, for example for weaker, more porous, and poorly cemented rock types and rock types containing structural features.
(4) All instruments and assemblies for determining straightness, flatness and perpendicularity of end surfaces should be controlled on a registered regular time basis having tolerances satisfying at least the requirements of the specific rock tests.
(5) Most unfractured cores taken by single tube, double tube or triple tube core barrels using rotary drilling techniques can be used with or without re-coring after a trimming of the end bearing surfaces. Blocks collected directly from a rock formation may also be used, if the orientation of the block is clearly indicated on the sample that will be used for re-coring the test specimens.
(6) The required sample volume depends on the test programme. For many purposes, samples 300 mm to 1000 mm long with a diameter greater than 50 mm should be sufficient for preparing rock specimens for a group of classification, strength and deformation tests.
(7) The required quantity of cores depends highly on the natural and induced fissuring of the rock material. The initial description of the core should include an evaluation of the degree of fissuring and homogeneity. This description should be used when selecting the core sections for testing.
(8) Selection of test specimens from zones of the core without fractures may lead to non-representativeness of the test specimens for the formation. This should be taken into account in the reporting.
(9) For weaker rocks (sedimentary rocks), the sample treatment is extremely important for deformability, strength and swelling tests. The rock samples for such tests should be packed in the field as soon as obtained from the core barrel. Even a short exposure may change the water content and the inherent properties of the rock.