TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
CHAPTER 2
METHODS FOR DEWATERING, PRESSURE RELIEF,
AND SEEPAGE CUTOFF
2-1. General.
water table and soil formations in the area and the
drawdown required to dewater the excavation.
a. Temporary dewatering systems. Dewatering and
b. Source of seepage flow. The source and distance
control of groundwater during construction may be ac-
L* to the source of seepage or radius of influence R
complished by one or a combination of methods de-
must be estimated or determined prior to designing or
scribed in the following paragraphs. The applicability
evaluating a dewatering or drainage system.
of different methods to various types of excavations,
(1) The source of seepage depends on the geo-
groundwater lowering, and soil conditions is also dis-
logical features of the area, the existence of adjacent
cussed in these paragraphs. Analysis and design of de-
streams or bodies of water, the perviousness of the
watering pressure relief and groundwater control sys-
sand formation, recharge, amount of drawdown, and
tems are described in chapter 4.
duration of pumping. The source of seepage may be a
b. Permanent drainage systems. The principles and
nearby stream or lake, the aquifer being drained, or
methods of groundwater control for permanent struc-
both an adjacent body of water and storage in the
tures are similar to those to be described for construc-
aquifer.
tion projects. A method often used for permanent
(2) Where the site is not adjacent to a river or
groundwater control consists of relief wells (to be dis-
lake, the source of seepage will be from storage in the
cussed subsequently in detail) installed beneath and
formation being drained and recharged from rainfall
adjacent to the structure, with drainage blankets be-
over the area. Where this condition exists, flow to the
neath and surrounding the structure at locations below
area being dewatered can be computed on the assump-
the water table as shown previously in figure 1-2. The
tion that the source of seepage is circular and at a dis-
water entering the wells and drainage blanket is
tance R. The radius of influence R is defined as the
carried through collector pipes to sumps, pits, or man-
radius of the circle beyond which pumping of a de-
holes, from which it is pumped or drained. Permanent
watering system has no significant effect on the origi-
groundwater control may include a combination of
nal groundwater level or piezometric surface (see para
wells, cutoffs, and vertical sand drains. Additional in-
4-2a(3)).
formation on the design of permanent drainage sys-
(3) Where an excavation is located close to a river
tems for buildings may be found in TM 5-818-1/AFM
or shoreline in contact with the aquifer to be de-
88-3, Chapter 7; TM 5-818-4/AFM 88-5, Chapter 5;
watered, the distance to the effective source of seepage
and TM 5-818-6/AFM 88-32. (See app. A for ref-
L, if less than R/2, may be considered as being approxi-
erences.)
mately the near bank of the river; if the distance to the
riverbank or shoreline is equal to about R/2, or greater,
2-2. Types and source of seepage.
the source of seepage can be considered a circle with a
a. Types of seepage flow. Types of seepage flow are
radius somewhat less than R.
tabulated below:
(4) Where a line or two parallel lines of *wells are
installed in an area not close to a river, the source of
Type of flow
Flow characteristics
seepage may be considered as a line paralleling the line
Artesian
Seepage through the previous aquifer is confined
of wells.
between two or more impervious strata, and
the piezometric head within the previous
2-3. Sumps and ditches.
aquifer is above the top of the pervious aqui-
fer (fig. 1-2).
Gravity
The surface of the water table is below the top of
procedure involves installation of ditches, French
the pervious aquifer (fig. 1-2).
drains, and sumps within an excavation, from which
For some soil configurations and drawdowns, the flow
water entering the excavation can be pumped (fig.
may be artesian in some areas and gravity in other
2-1). This method of dewatering generally should not
areas, such as near wells or sumps where drawdown
occurs. The type of seepage flow to a dewatering sys-
*For convenience, symbols and unusual abbreviations are listed
tem can be determined from a study of the ground-
in the Notation (app B).
2-1
Previous Page
