Environmental Engineering Reference
In-Depth Information
Depth of hole (m)
0
20
40
60
80
100
0
Probable perched
water table (15 m)
20
40
Inferred water
table (47 m)
Recovery time
after bailing
60
70 m Total depth
80
100
Depth to fluid at:
End of shift
Start of shift
Intermediate
Figure 5.28.
Method for estimating perched and main water table levels from monitoring during
drilling.
during surface mapping, recording of groundwater inflows into pits and trenches, and the
monitoring of groundwater levels during the drilling investigation. Piezometers (with
sealed tips and not just open slotted PVC “wells”) should be installed in drill-holes (see
Chapter 20) and regularly measured throughout the project. The response of groundwa-
ter levels to rainfall can provide a useful indication of mass permeability.
Measurement of seepage and flow rates, recording of rainfall, evaporation, and water
levels installed in exploration drill holes gives a broad indication of the proportion of
rainfall which infiltrates, the response of the groundwater to storms, and areas where high
groundwater flow rates may be expected. This programme provides a relatively low cost
general picture of the regional hydrogeology which can be refined by an investigation of
specific permeability values at individual sites.
Claims related to unforeseen groundwater conditions form a significant proportion of
contractual disputes. Many of these claims originate from a failure to record adequate
groundwater information during site investigations and during the period between the
completion of investigations and the start of construction.
Figure 5.28 shows how a perched and main water table can be inferred from the water
levels measured in a borehole during drilling using the DVD (depth of hole versus depth
of water) plot.
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