SPRING/HIGH WATER TABLE

D.  SPRING/HIGH WATER TABLE LIKELY  ?  :
An aquifer is a body of water beneath the surface of the land.  Typically, an aquifer may extend over many miles in one direction.  In some cases, an aquifer can extend thousands of square miles.   An aquifer usually exists in a gravel or sand layer underlain by a relatively impermeable clay layer.  Aquifers fluctuate in depth with the seasonal rains.  With prolonged rains, surface seepage recharges the aquifer and causes the water table to rise.  With prolonged drought, the aquifer decreases, lowering the water table.  The upper reach of an aquifer is called the water table.

Springs have their origin in aquifers beneath the earth’s surface and may surface at long distances away from the main body.  A crevice that opens into an aquifer during a high water table, will not carry water when the table is low as during times of drought.  This accounts for “seasonal” springs.  The symptom of spring water in a basement is that it tends to be quite persistent.  Just as the rainfall takes a long time to swell an aquifer, it takes a long time for an aquifer to drain into a lowered water table.  If seepage occurs it tends to stay until the water table lowers.

Basements affected by seasonal springs will sometimes require water to be pumped from them for months on end.  Springs can change their course underground. This occasionally happens when nearby construction interferes with the exiting course of the spring water.  Sometimes a spring will rise quickly beneath a basement slab and force it upward.  The force is sometimes strong enough to crack the slab or even explode it upward.  Springs can undermine footings and cause foundation settlements.  A basement that is affected by a spring may tend to be damp even when the spring is not active. 

An aquifer may exist directly beneath a home.  In fact, almost all houses have aquifers beneath them.  However, most of these aquifers and the water table in general are so deep that no noticeable effects exist. Occasionally an aquifer may exist beneath a house that swells to the point where the water table rises above the basement floor.  Unless there is an extensive dewatering system operating, the water will generally flood the basement to the same height as the water table.

WHAT TO:  HOW TO:
When springs or water tables affect a basement, the only practical thing to do may be to pump the water out faster than it comes in.  A pump system may also lower the entire water table for a considerable distance around the pump.  The secret is control.  It is unwise to encourage water to move under houses and footings because soil can be displaced and settling may occur.

It is more desirable to intercept and remove the water at a point that is away from the house.  The intent is to minimize the impact of water on the house.  Removal of the water may be achieved by employing pit and pipe combinations that can drain by gravity from lots located on a slope, or employing exterior trenches and pumps where no gravity discharge is available.

Shallow pumps exert a limited zone of influence away from the pump while deeper ones exert a larger zone of influence.  The zone of influence is strongly influenced by the permeability of the earth materials.  An enormous zone of influence can be established in gravel beds while only a small zone of influence can be established in packed clay soils.

Springs and rising ground water tables will usually require some means of dewatering. If the system is electrically dependent, another backup power source should be provided for the times when power from the electric utility is not available.  It is strongly recommended that a house affected by a spring or rising water table have an inspection by a competent building analyst or soil scientist prior to implementing any dewatering system.  Be especially sure the analyst is familiar with local basement seepage and underground water conditions, etc.  The potential to do harm with an inappropriate system is great.

WARNING: BEWARE OF THE BUILT-IN BIAS of contractors who do their own inspections, and then offer to install their OWN recommendations.  Professional engineers who do not have specific experience in below grade water seepage conditions are unlikely to be of much assistance.