Environmental Engineering Reference
In-Depth Information
Perched water table can also be a measured GWL, and represents a saturated zone
overlying an impervious stratum below which the ground is not saturated.
Artesian conditions result from groundwater under a head in a confined stratum
which is greater than the static water-table head and can result in free flow at the
surface when the confined stratum is penetrated.
Variations in conditions occur with time, and are affected by seasonal conditions,
tidal fluctuations, flooding, and pumping. Variations also occur with physical
conditions in terms of soil type and density, ground contours, surface drainage,
rock-mass discontinuities, etc.
Determining Conditions
Terrain analysis (Section 2.2.4, Interpretation of Topographic Maps) provides general
information on watertable location.
Geophysical methods (seismic refraction and electrical resistivity) provide indirect meas-
ures of the approximate depth to groundwater.
Reconnaissance methods employing excavations locate the GWL in a positive manner but
within a short time interval. Probable variations with time must always be considered dur-
ing site analysis. Test pits are the best method for measuring short-term water levels and
seepage rates. Auger borings may also clearly reveal the water table, especially in slightly
cohesive soils. In cohesionless soils, the hole will tend to collapse within a few centimeters
of the GWL but direct measurement is not usually possible.
Test Borings
The moisture condition of drive samples (SPT) may provide an indication of GWL.
Samples will range from dry to moist to wet (saturated) as the GWL is approached. The
condition should be noted on boring logs.
In cased borings, the water level is generally determined by pumping or bailing
water from the hole and permitting stabilization for 24 h. This method is reliable in uni-
form sand strata or other pervious materials, but differentiating perched from static
conditions is difficult. Site stratigraphy provides some clues for judgment. The method
is unreliable when the casing terminates in an impervious stratum which blocks
the entrance of water. The casing should be raised until it terminates in a permeable
stratum.
If the casing ends in an aquifer with an artesian head, water may flow from the casing
when pumping ceases during drilling operations, or it may rise to a point above the esti-
mated GWL.
Casing water levels should be noted periodically during boring operations and for a
period thereafter, and water loss as well as artesian and static conditions should be
noted.
Boring with a mud slurry will not provide reliable water level readings unless the hole
is flushed with clean water or unless biodegradable mud is used for boring. After flush-
ing, most holes in permeable soils collapse near or slightly above the perched or static
water level.
Borehole remote-sensing probes , such as the electric well logger and the neutron probe, pro-
vide good indications of perched or static conditions.
Piezometers ( Section 4.4.2) provide the most accurate method of measuring groundwater
conditions, and are especially useful for recording changes with time. Measurements
made during pumping tests performed at various levels are useful in differentiating
perched from static conditions.
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