Types and source of seepage

a. Types of seepage flow. Types of seepage flow are tabulated below:

Type of flow Flow characteristics

Artesian Seepage through the previous aquifer is confined

between two or more impervious strata, and the piezometric head within the previous aquifer is above the top of the pervious aqui­fer (fig. 1-2).

Gravity The surface of the water table is below the top of

the pervious aquifer (fig. 1-2).

For some soil configurations and drawdowns, the flow may be artesian in some areas and gravity in other areas, such as near wells or sumps where drawdown occurs. The type of seepage flow to a dewatering sys – tern can be determined from a study of the ground­water table and soil formations in the area and the drawdown required to dewater the excavation.

b. Source of seepage flow. The source and distance L* to the source of seepage or radius of influence R must be estimated or determined prior to designing or evaluating a dewatering or drainage system.

(1) The source of seepage depends on the geo­logical features of the area, the existence of adjacent streams or bodies of water, the perviousness of the sand formation, recharge, amount of drawdown, and duration of pumping. The source of seepage may be a nearby stream or lake, the aquifer being drained, or both an adjacent body of water and storage in the aquifer.

(2) Where the site is not adjacent to a river or lake, the source of seepage will be from storage in the formation being drained and recharged from rainfall over the area. Where this condition exists, flow to the area being dewatered can be computed on the assump­tion that the source of seepage is circular and at a dis­tance R. The radius of influence R is defined as the radius of the circle beyond which pumping of a de­watering system has no significant effect on the origi­nal groundwater level or piezometric surface (see para

4- 2o(3)).

(3) Where an excavation is located close to a river or shoreline in contact with the aquifer to be de­watered, the distance to the effective source of seepage L, if less thanR/2, may be considered as being approxi­mately the near bank of the river; if the distance to the riverbank or shoreline is equal to about R/2, or greater, the source of seepage can be considered a circle with a radius somewhat less than R.

(4) Where a line or two parallel lines of wells are installed in an area not close to a river, the source of seepage may be considered as a line paralleling the line of wells.