Groundwater characteristics

a. An investigation of groundwater at a site should include a study of the source of groundwater that would flow to the dewatering or drainage system (para 2-2) and determination of the elevation of the water table and its variation with changes in river or tide stages, seasonal effects, and pumping from nearby wa­ter wells. Groundwater and artesian pressure levels at a construction site are best determined from piezom­eters installed in the stratum that may require dewa­tering. Piezometers in pervious soils may be commer­cial wellpoints, installed with or without a filter (para

1- 6c) as the gradation of foundation material requires. Piezometers in fine-grained soils with a low perme­ability, such as silt, generally consist of porous plastic or ceramic tips installed within a filter and attached to a relatively small diameter riser pipe.

b. The groundwater regime should be observed for an extended period of time to establish variations in level likely to occur during the construction or opera­tion of a project, General information regarding the groundwater table and river or tide stages in the area is often available from public agencies and may serve as a basis of establishing general water levels. Specific conditions at a site can then be predicted by correlat­ing the long-term recorded observations in the area with more detailed short-term observations at the site.

c. The chemical composition of the groundwater is of concern, because some groundwaters are highly cor­rosive to metal screens, pipes, and pumps, or may con­tain dissolved metals or carbonates that will form in­
crustations in the wells or filters and, with time, cause clogging and reduced efficiency of the dewatering or drainage system. Indicators of corrosive and incrust – ing waters are given in table 3-3. (Standard methods for determining the chemical compositions of ground­water are available from the American Public Health Association, Washington, DC

d. Changes in the temperature of the groundwater will result in minor variations of the quantity of water flowing to a dewatering system. The change in viscosi­ty associated with temperature changes will result in a change in flow of about 1.5 percent for each 1° Fahrenheit of temperature change in the water. Only large variations in temperature need be considered in design because the accuracy of determining other parameters does not warrant excessive refinement.

2- 4. Permeability of pervious strata. The

rate at which water can be pumped from a dewatering system is directly proportional to the coefficient of permeability of the formation being dewatered; thus, this parameter should be determined reasonably accu­rately prior to the design of any drainage system. Methods that can be used to estimate or determine the permeability of a pervious aquifer are presented in the following paragraphs.

a. Visual classification. The simplest approximate method forestimating the permeability of sand is by visual examination and classification, and comparison with sands of known permeability. An approximation of the permeability of clean sands can be obtained from table 3-4.

b. Empirical relation between Dl0 and k. The per­meability of a clean sand can be estimated from em-

Groundwater characteristics

1.0 10 Effective groin size (D)q) in mm

(From “Ground Wafer Hydrology" by D. K. Todd, 1959, Wiley & Sons, Inc. Used with permission of Wiley & Sons, Inc.)

 

Figure 3-3. Specific yield of water-bearing sands versusDw, South Coastal Basin, California.

Table 3-2. Specific Yield of Water-Bearing Deposits in Sacramento Valley, California

Specific

Yield

percent

 

Material

 

Подпись: 25Gravel

Sand, including sand and gravel, and gravel and sand 20

Fine sand, hard sand, tight sand, sandstone, and related

deposits 10

Clay and gravel, gravel and clay, cemented gravel, and

related deposits 5

Clay, silt, sandy clay, lava rock, and related fine­grained deposits 3

(From “Ground Water Hydrology byD. K. Todd, 1959, Wiley & Sons, Inc. Used with permission of Wiley & Sons, Inc.)

Table 3-3. Indicators of Corrosive and Incrusting Waters

Indicators of Indicators of

Corrosive Water__________ _______ Incrusting Water

Подпись:A pH greater than 7

Total iron (Fe) in excess of 2 ppm

3. Подпись: 3. Hydrogen sulfide (H^S) in excess of 1 ppm, detected by a rotten egg odor Total manganese (Mn) in excess of 1 ppm in con­junction with a high pH and the presence of oxygen

Подпись:4 Total dissolved solids in excess of 1,000 ppm indicates an ability to conduct electric current great enough to cause serious electro­lytic corrosion

5 Carbon dioxide (CC^) in excess of 5 0 ppm

6. Chlorides (Cl) in excess of 500 ppm

(Courtesy ofUOP Johnson Division)

Table 3-4. Approximate Coefficient of Permeability for Various Sands

Подпись: ft/minПодпись: 10

Type of Sand (Unified
Soil Classification System)

Coefficient of Permeability k x 10 -4 cm/sec x. „ -4

Подпись: Sandy silt Silty sand Very fine sand Fine sand Fine to medium sand Medium sand Medium to coarse sand Coarse sand and gravel

5-20 20-50 50-200 200-500 500-1,000 1 ,ooo-1,500 1,500-2,000

2,0- 5,000

10-40 40- 100 100-400 400-1,000 1,000-2,000

2.0- 3,000

3.0- 4,000

4.0- 10,000

U. S. Army Corps of Engineers

pirical relations between D10 and к (fig. 3-4), which were developed from laboratory and field pumping tests for sands in the Mississippi and Arkansas River valleys. An investigation of the permeability of filter sands revealed that the permeability of clean, rela­tively uniform, remolded sand could be estimated from the empirical relation:

k=C(D10)[1] (3-2)

where

к = coefficient of permeability, centimetres per second

C = 100 (may vary from 40 to 150)

Dio = effective grain size, centimetres Empirical relations betweenD10 and к ;irc only approx­imate. and should be used with reservation until a cor­relation based on local experience is available.

c. Field pumping tests. Field pumping tests are the most reliable procedure for determining the in situ permeability of a water-bearing formation. For large dewatering jobs, a pumping test on a well that fully penetrates the sand stratum to be dewatered is war­ranted; such tests should be made during the design phase so that results can be used for design purposes and will be available for bidders. However, for small dewatering jobs, it may be more economical to select a more conservative value of k based on empirical rela­tions than to make a field pumping test. Pumping tests are discussed in detail in appendhC.

d. Simple field tests in wells or piezometers. The permeability of a water-bearing formation can be esti­mated from constant or falling head tests made in wells or piezometers in a manner similar to laboratory permeameter tests. Figure 3-5 presents formulas for determining the permeability using various types and installations of well screens. As these tests are sensi­tive to details of the installation and execution of the test, exact dimensions of the well screen, casing, and

filter surrounding the well screen, and the rate of in­flow or fall in water level must be accurately meas­ured. Disturbance of the soil adjacent to ^borehole or filter, leakage up the borehole around the casing, clog­ging or removal of the fine-grained particles of the aquifer, or the accumulation of gas bubbles in or around the well screen can make the test completely unreliable. Data from such tests must be evaluated

Groundwater characteristics

0.05 0.1 0.2

EFFECTIVE GRAIN size

(D,

0.5 1 .o 2.0

,) OF STRATUM, m m

Figure 3-4. Dw versus in situ coefficient of horizontalpermeability – Mississippi River valley and Arkansas River valley,

NOTATION

D = DIAM, INTAKE, SAMPLE, CM d – DlAM, STANDPIPE, CM L = LENGTH, INTAKE, SAMPLE, CM H c = CONSTANT PI EZ HEAD, CM H , = PIEZ HEAD FOR t = t, , CM H2 = PIEZ HEAD FOR t = t2 , CM q= FLOW OF WATER, CM3/SEC t = TIME, SEC

k’ = VERT PERM CASING, CM/SEC

,v

ky = VERT PERM GROUND, CM/SEC

kh =HORIZ PERM GROUND, CM/SEC

к MEAN COEFF PERM, CM/SEC m

m = TRANSFORMATION RATIO

km = Vkhkv m = Vkh/kv

In =loge = 2.3 log,0

Groundwater characteristics

WELLPOINT FILTER AT IMPERVIOUS BOUNDARY

 

WELLPOINT
FILTER IN
UNIFORM
SOIL

B

 

CASE

 

CONSTANT HEAD

 

VARIABLE HEAD

 

Groundwater characteristics

d2ln(^b) h

k.= ——- ^-2-^ In— FOR —— > 4

h SL (t2 – t,) H2 D

 

Groundwater characteristicsGroundwater characteristicsGroundwater characteristicsGroundwater characteristicsGroundwater characteristics

Groundwater characteristics

ASSUMPTIONS

SOIL AT INTAKE, INFINITE DEPTH AND DIRECTIONAL ISOTROPY (ky AND Ц CONSTANT) – NO DISTURBANCE, SEGREGATION, SWELLING, OR CONSOLIDATION OF SOIL – NO SEDIMENTATION OR LEAKAGE – NO AIR OR GAS IN SOIL, WELLPOINT, OR PIPE – HYDRAULIC LOSSES IN PIPES, WELL­POINT, OR FILTER NEGLIGIBLE.

U. S. Army Corps of Engineers

Figure 3-5. Formulas for determiningpermeability from field falling head tests.


carefully before being used in the design of a majorde­watering or drainage system.

3-5. Power. The availability, reliability, and capacity of power at a site should be investigated prior to selecting or designing the pumping units for a dewa­tering system. Types of power used for dewatering sys­tems include electric, natural gas, butane, diesel, and gasoline engines. Electric motors and diesel engines are most commonly used to power dewatering equip­ment.

3-6. Surface water. Investigations for the con­trol of surface water at a site should include a study of precipitation data for the locality of the project and de­termination of runoff conditions that will exist within the excavation. Precipitation data for various localities and the frequency of occurrence are available in pub­

lications of the U. S. Weather Bureau or other refer­ence data. Maps showing amounts of rainfall that can be expected once every 2, 5, and 10 years in 10-, 30-, and 60-minute duration of rainfall are shown in figure

1- 6. The coefficient of runoff c within the excavation will depend on the character of soils present or the treatment, if any, of the slopes. Except for excavations in clean sands, the coefficient of runoff c is generally from 0.8 to 1.0. The rate of runoff can be determined as follows:

Q = ciA (3-3)

where

Q= rate of runoff, cubic feet per second C= coefficient of runoff і = intensity of rainfall, inches per hour A = drainage area, acres

Groundwater characteristics

Groundwater characteristics

10-MIN RAINFALL

 

Groundwater characteristics

30-MIN RAINFALL

 

Groundwater characteristicsGroundwater characteristicsGroundwater characteristicsGroundwater characteristicsGroundwater characteristicsGroundwater characteristics

Groundwater characteristics

1-HR RAINFALL

(U. S. Department of Agriculture Miscellaneous Publication No. 204)

Figure 3-6. Inches of rainfall during 10■ and 30-minute and 1-hour periods.