Quicksand and Water Wellsa
Authors: Patchick P. F. 
Published: July 2006

Quicksand is no myth, nor is it a rare phenomenon, either on the surface or in the subsurface. There is probably quicksand in every State of the U.S.A. where enough gradient is available to produce upward moving ground water. Surface quicksand is likely to occur in hilly country, especially in regions of calcareous rocks which often con‐tain solution openings and favor the occurrence of springs. It is often present near stream banks and stream beds, either along their shorelines, under water, or occasionally in the bottom of an apparently “dry” river.

Subsurface qurcksand rs found in loosely consolidated or uncemented alluvial, fluvio‐glacial, or lacustrine sediments, which are usually well sorted and clean. It has been reported to depths as great as several hundred feet, and from a few feet to as much as 200 feet thick.

In water wells, a “quick” or “heaving” condition may exist (as much as 150 feet) if a confined, permeable, loose silt, sand or gravel stratum is continuous to distant higher ground, and is subject to a greater head of water existing at the higher elevation, i. e., if artesian conditions exist.

Similar material may heave in a water‐table (unconfined) well, due to the natural tendency of granular materials to move toward a drill hole when lateral support is removed (the drill cuttings). Heaving results from a loss of shearing strength as pressure is reduced when a drill hole upsets natural equilibrium. Suction produced by suddenly withdrawing bailer or tools further contributes to disturbing natural conditions.

Quicksand is commonly sand (usually fine to medium grained) with an effective size ranging from less than 0.001 inch to 0.005 inch. It is often finer than sugar, which has an average effective size of 0.008 inch, to which it is often compared by drillers. Stoke's Law explains why fine, round‐ ed grains become “quick” much more readily than coarser sand or gravel.

If sufficiently permeable, quicksand aquifers can successfully be used for water supplies, providing proper screening devices are available. However, representative driller's samples; sieve analyses; choice of(or omission of) sand filter pack; proper slot selection; low (0.1 fps) en‐ trance velocity; and careful, patient development techniques are important prerequisites.

Permeability of quicksand varies widely, but might range from approximately 50 gpd/ft to 800 gpd/ft, and average between 100 or 175 gpd/ft2. From a 5‐feet thick quicksand bed, and at shallow depths, 1 to 10 gpm may be obtained for domestic or farm supplies using screened wells and drive points. Sufficient yields (250 gpm or more) for municipal wells are also possible.

Drilling and completion problems with cable‐tool wells can usually be overcome by: 1) watching casing alignment; 2) running bailer slowly; 3) keeping a continual high head of water inside casing; 4)using a heavy string of drill tools; 5) allowing controlled sand‐heaving; 6) using short casing drives; 7) jetting with high‐pressure water to wash‐free tools if seized by a sudden heave; 8) avoiding thin‐walled casing; 9) drilling with a mixture of bentonite or native clay and water sludge; and 10) casing above the surface if the well is flowing.

Rotary drilling and completion problems are minimal, but unless unusual pressures are encountered, quicksand aquifers may be overlooked. Caving can be prevented by providing sufficient hydrostatic differential pressure, and by using a heavy weighted drilling fluid.

Well screens can be set by bailing them down through a quicksand “plug” which may have formed, and completing using the standard pull‐back method. A self‐closing bottom (washdown fitting) can also be used to jet‐down a screen while utilizing a high‐velocity water flow.

In some cases a sand filter‐pack surrounding a continuous‐slot well screen may not be advisable in quicksand, especially if the aquifer's effective grain size is approxi‐mately 0.004 inch. A naturally developed well completed with a screen having a slot as narrow as 0.005 inch may be preferable, allowing 50 to 60 percent of the aquifer to be developed slowly and carefully through the screen. A case history comparing two well design methods is presented.

Both cable‐tool and rotary sampling methods in quick‐ sands are discussed. The techniques are basically similar, whether the material is “quick” or stable.