15 AUGUST 2005
If groundwater seepage begins to exceed the capacity of the dewatering system,
conditions should not be expected to improve unless the increased flow is known to be caused
by a short-term condition such as heavy rain in the area. If seepage into the excavation
becomes excessive, excavation operations should be halted until the necessary corrective
measures are determined and affected. The design and evaluation of dewatering systems
require considerable experience that the contractor or the contracting office often do not
possess, and the assistance of specialists in this field should be obtained.
Groundwater without significant seepage flow can also be a problem since
excess hydrostatic pressures can develop below relatively impervious strata and cause uplift
and subsequent foundation or slope instability. Excess hydrostatic pressures can also occur
behind sheet pile retaining walls and shoring and bracing in shaft and tunnel excavations.
Visual observations should be made for indications of trouble, such as uncontrolled seepage
flow, piping of material from the foundation or slope, development of soft wet areas, uplift of
ground surface, or lateral movements.
Accurate daily records should be kept of the quantity of water removed by the
dewatering system and of the piezometric levels in the foundation and beneath excavation
slopes. Separate records should be kept of the flow pumped by any sump-pump system
required to augment the regular dewatering system to note any increase of flow into the
excavation. Flowmeters or other measuring devices should be installed on the discharge of
these systems for measurement purposes. These records can be invaluable in evaluating
"Changed Condition" claims submitted by the contractor. The contractor should be required to
have "standby" equipment in case the original equipment breaks down.
Surface Water. Sources
of water problems other than groundwater are surface
runoff into the excavation and snow drifting into the excavation. A peripheral, surface-drainage
system, such as a ditch and berm, should be required to collect surface water and divert it from
the excavation. In good weather there is a tendency for the contractor to become lax in
maintaining this system and for the inspection personnel to become lax in enforcing
maintenance. The result can be a sudden filling of the excavation with water during a heavy
rain and consequent delay in construction. The surface drainage system must be constantly
maintained until the backfill is complete. Drifting snow is a seasonal and regional problem,
which can best be controlled by snow fences placed at strategic locations around the
Slope Integrity. Another area of concern during excavation is the integrity of the
excavation slopes. The slopes may be either unsupported or supported by shoring and
bracing. The lines and grades indicated in the plans should be strictly adhered to. The
contractor may attempt to gain additional working room in the bottom of the excavation by
steepening the slopes; this change in the plans must not be allowed.
Where shoring and bracing are necessary to provide a stable excavation, and the
plans and specifications do not provide details of these requirements, the
contractor should be required to submit the plans in sufficient detail so that they