Site investigation for groundwater lowering
6.6 Determination of permeability
6.6.6 In situ tests in boreholes: rising, falling and constant head tests
This group of tests includes:
i Rising and falling head tests (collectively known as variable head tests), ii Constant head tests.
These tests are carried out in the field on the soil in situ. They, therefore, avoid the problems of obtaining representative undisturbed samples that limit the usefulness of laboratory testing. Tests in boreholes are those car-ried out during interruptions in the drilling or boring process. When the test is complete drilling recommences – this allows several tests at different depths to be carried out in one borehole. These tests are distinct from tests carried out in observation wells following completion of the borehole, where tests can be carried out only at the fixed level of the response zone.
Execution of variable head tests is straightforward and requires only basic equipment. The borehole is advanced to the proposed depth of test, and the original groundwater level noted. The upper portion of the bore-hole is supported by temporary casing (which should exclude groundwater from those levels). The ‘test section’ of exposed soil is between the bottom of the casing and the base of the borehole.
For a falling head (or inflow) test (Fig. 6.8(a)) water is added to raise the water level in the borehole. Once the water has been added the water level in the borehole is recorded regularly to see how the level falls with time as water flows out of the borehole into the soil. The necessary equipment is a dipme-ter, bucket, stopwatch and a supply of clean water (perhaps from a tank or bowser). It is essential that any water added is absolutely clean, otherwise any suspended solids in the water will clog the base of the borehole test section and significantly affect results. Particular attention should be given to the cleanli-ness of tanks, buckets, etc, so that the water does not become contaminated
by those means. It can be difficult to carry out falling head tests in very per-meable soils (greater than about 1093m/s) because water cannot be added quickly enough to raise the water level in the borehole. If the natural ground-water level is close to ground surface it may be necessary to extend the bore-hole casing above ground level to allow water to be added.
A rising head (or outflow) test (Fig. 6.8(b)) is the converse of a falling head test. It involves removing water from the borehole and observing the Figure 6.8 Variable head tests in boreholes. (a) Falling head (inflow) test, (b) rising
head (outflow) test.
rate at which water rises in the borehole. The test does not need a water supply (which can be an advantage in remote locations), but does require a means of removing water rapidly from the borehole. The most obvious way to do this is using a bailer, which is adequate in soils of moderate perme-ability but it can be surprisingly difficult to significantly lower water levels if soils are highly permeable. Alternatives are to use airlift equipment or suction or submersible pumps.
For the relatively permeable soils of interest in groundwater lowering problems, variable head tests are analysed using the work of Hvorslev (1951) which is the basis of the methods given in BS5930 (1999). Hvorslev assumed that the effect of soil compressibility on the permeability of the soil was negligible during the test, and this is a tolerable assumption for most water-bearing soils. If in situ permeability tests are carried out in relatively compressible silts and clays different test procedures and analysis may be required (see Brand and Premchitt 1982).
For the Hvorslev analysis, permeability k is calculated using k:ᎏF
where A is the cross-sectional area of the borehole casing (at the water levels during the test), T is the basic time lag and F is a shape factor dependent on the geometry of the test section. T is determined graphically from a semi-logarithmic plot of H/H0versus elapsed time as shown on Fig. 6.9. H0is the excess head in the borehole at time t:0 and H is the head at time t (both H and H0are measured relative to the original groundwater level). Additional notes on the analysis of variable head tests are given in Appendix 6B.
Values of shape factor F for commonly occurring borehole test section geometries were prepared by Hvorslev (1951) and are shown in Fig. 6.10.
Shape factors for other geometries are given in BS5930 (1999). The simplest test section is when the temporary casing is flush with the base of the bore-hole, allowing water to enter or leave the borehole through the base only.
If soil will stand unsupported it may be possible to extend the borehole ahead of the casing to provide a longer test section. If the soil is not stable the bore-hole could be advanced to the test depth, and the test section be backfilled with filter sand or gravel as the casing is withdrawn to the top of the test section.
Constant head tests (Fig. 6.11) involve adding or removing water from a borehole at a known rate in order to maintain a constant head, which is recorded. Constant head tests are most often carried out as inflow tests, but outflow tests can also be carried out. The equipment required is rather more complex than for variable head tests, as some form of flow measurement (typically by the timed volumetric method) is required. In the simplest form of the test, appropriate to relatively permeable soils, the flow rate is adjusted until a suitable constant head is achieved, and the test allowed to continue
until a steady flow rate is established. A consistent supply of clean water is required for tests, and this can be a disadvantage in remote locations.
Permeability k is calculated from
where q is the constant rate of flow, Hcis the constant head (measured rel-ative to original groundwater level) and F is the shape factor (from Fig. 6.10).
Variable and constant head tests in boreholes have a number of limita-tions, and may be subject to a number of errors. When carrying out these tests (and when reviewing the results) it is essential that these factors are considered.
Figure 6.9 Analysis of variable head tests.
1 Tests in boreholes only involve a relatively small volume of soil around the test section. If the soil is heterogeneous or has significant fabric, such tests may not be representative of the mass permeability of the soil. Large scale tests (such as pumping tests) may give better results.
2 Results of inflow tests (falling head and constant head tests) can be sig-nificantly affected by clogging or silting up of the test section as water is added. It is vital that only totally clean water is added but, even then, Figure 6.10 Shape factors for permeability tests in boreholes (after Hvorslev 1951).
silt already in suspension may block flow out of the borehole. It is not uncommon for inflow tests to under-estimate permeability by several orders of magnitude.
3 In loose granular soils outflow tests (rising head and constant head tests) may cause piping or boiling of soil at the base of the borehole.
This could lead to over-estimates of permeability.
4 The drilling of the borehole may have disturbed the soil in the test section, changing the permeability. Potential effects include particle loosening, compaction, or smearing of silt and clay layers.
5 Reliable analysis of test results requires that the original groundwater level be known (this is discussed in Appendix 6B). However, if the natural groundwater level varies during the test (due to tidal or other influences) the test may be difficult to analyse. If significant groundwater level fluctuations are anticipated during a test of say one or two hours duration, tests in boreholes are unlikely to be useful.
Although these tests have a number of limitations they are inexpensive to execute, and are widely used. It is good practice to carry out both rising and falling head tests in the same borehole to allow results to be compared.
In any event, results should be treated with caution until supported by permeability estimates from other sources.
Figure 6.11 Constant head inflow test in boreholes.