Water is removed to the borehole to lower water levels, inducing flow from the surrounding strata into the borehole.įigure 12.8 Slug tests in boreholes, (a) Falling head phase of slug test. Water is added to the borehole to raise water levels, inducing flow from the borehole into the surrounding strata, (b) Rising head test. At the end of theįigure 12.7 Variable head tests in boreholes, (a) Falling head test. Instead, a heavy rod (termed a ‘slug’) is quickly lowered below the water level in the borehole to displace water and hence rapidly raise water levels - analogous to a falling head test (Figure 12.8a). However, in a slug test, no water is added to or removed from the borehole. Again, this involves applying rapid changes to the water level in a borehole and then observing the rate at which the water level returns to the background or natural water level. Alternatives are to use airlift equipment (Howell, 2013) or suction pumps or submersible pumps.Īn alternate form of variable head test is the ‘slug test’. The most obvious way to do this is by using a bailer, which is adequate in soils of moderate permeability, but it can be surprisingly difficult to significantly lower water levels if soils are highly permeable. 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. It involves rapidly removing water from the borehole and observing the rate at which water rises in the borehole. If the natural groundwater level is close to ground surface, it may be necessary to extend the borehole casing above ground level to allow water to be added.Ī rising head test (Figure 12.7b) is the converse of a falling head test. It can be difficult to carry out falling head tests in very permeable soils (greater than about 5 x lO- 3 m/s), because water cannot be added quickly enough to raise the water level in the borehole. Particular attention should be given to the cleanliness of tanks, buckets, etc, so that the water does not become contaminated by those means. 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.
#PERMEABILITY TEST ON UNDISTURBED SAMPLE MANUAL#
Alternatively, if a pressure transducer linked to a stand-alone datalogger is available (Section 22.6), this can be suspended below the water level in the borehole and used to record water levels more frequently than can be achieved by manual dipmeter readings. The necessary equipment comprises a dipmeter (to measure water level in the borehole), a bucket, a stopwatch and a supply of clean water (perhaps from a tank or bowser). 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 dimensions and geometry of the ‘test section’ have a key influence on flow conditions during the test and hence on the assessed permeability.įor a falling head test (Figure 12.7a), water is rapidly added to raise the water level in the borehole.
The ‘test section’ of exposed strata is between the bottom of the casing and the base of the borehole.
Typically, the upper portion of the borehole is supported by temporary casing (which should be sealed into the upper strata to exclude groundwater from those levels). If necessary, the start of the test should be delayed until readings show that the pre-test groundwater level has stabilized. It is essential that a representative ground water level is obtained. The borehole is advanced to the proposed depth of test, and the original groundwater level is noted. These tests in boreholes are distinct from tests carried out in monitoring wells (Section 12.8.2) following completion of the borehole, where tests can be carried out only at the fixed level of the response zone.Įxecution of variable head tests is straightforward and requires only basic equipment.
0 kommentar(er)
