Header pipe

(1) Hydraulic head losses caused by flow through the header pipe, reducers, tees, fittings, and valves should be computed and kept to a minimum (1 to 3 feet) by using large enough pipe. These losses can be computed from equivalent pipe lengths for various fit­tings and curves.

(2)Wellpoint header pipes should be installed as close as practical to the prevailing groundwater eleva­tion and in accessible locations. Wellpoint pumps should be centrally located so that head losses to the ends of the system are balanced and as low as possible.

If suction lift is critical, the pump should be placed low enough so that the pump suction is level with the header, thereby achieving a maximum vacuum in the header and the wellpoints. If construction is to be per­formed in stages, sufficient valves should be provided in the header to permit addition or removal of portions of the system without interrupting operation of the re­mainder of the system. Valves should also be located to permit isolation of a portion of the system in case construction operations should break a swing connec­tion or rupture a header.

(3) Discharge lines should be sized so that the head losses do not create excessive back pressure on the pump. Ditches may be used to carry the water from the construction site, but they should be located well back of the excavation and should be reasonably watertight.

4-8. Factors of safety.

a. General. The stability of soil in areas of seepage emergence is critical in the control of seepage. The exit gradient at the toe of a slope or in the bottom of an ex­cavation must not exceed that which will cause surface raveling or sloughing of the slope, piping, or heave of the bottom of the excavation.

b. Uplift. Before attempting to control seepage, an analysis should be made to ensure that the seepage or uplift gradient is equal to or less than that computed from the following equations:

i<L ————- ————– (4-5)

— y,(FS= 1.25 to 1.5) V ;

 

or

 

_______________ (/m)T___________________

yw (FS= 1.25 to 1.5)

 

Ah

 

(4-6)

 

Header pipe

seepage gradient Ah/L submerged unit weight of soil unit weight of water

artesian head above bottom of slope or exca­vation

thickness of less pervious strata overlying a more pervious stratum distance through which Ah acts

 

T = L =

 

In stratified subsurface soils, such as a course-grained pervious stratum overlain by a finer grained stratum of relatively low permeability, most of the head loss through the entire section would probably occur through the finer grained material. Consequently, a factor of safety based on the head loss through the top stratum would probably indicate a more critical condi­tion than if the factor of safety was computed from the total head loss through the entire section. Also, when gradients in anisotropic soils are determined from flow nets, the distance over which the head is lost must be obtained from the true section rather than the transformed section.

c. Piping. Piping cannot be analyzed by any rational method. In a study of piping beneath hydraulic struc­tures founded on granular soils, it was recommended that the (weighted) creep ratio Cw should equal or ex­ceed the values shown in table 4-3 for various types of granular soils,

 

I vertical seepage paths
+1/3 I horizontal seepage paths

H – he

 

Cw =

 

(4-7)

 

Table 4-3. Minimum Creep Ratios for Various Granular Soils

 

Creep Ratio

 

Soil

 

8.5

7

6

5

4

3.5

3

2.5

 

Very fine sand or silt

Fine sand

Medium sand

Coarse sand

Fine gravel

Medium gravel

Coarse gravel including cobbles Boulders with some cobbles and gravel

 

From “Securityfrom Under-Seepage Masonry Dams, "by E, W. Lane, pp. 1235-1272.

Transactions, American Society of Civil Engineers, 1935.

 

where H – he represents vertical distance from the groundwater table to the bottom of the excavation. These criteria for piping are probably only applicable to dewatering of sheeted, cellular, or earth-dike coffer­dams founded on granular soils. Once piping develops, erosion of the soil may accelerate rapidly. As the length of seepage flow is reduced, the hydraulic gradi­ent and seepage velocity increase, with a resultant ac­celeration in piping and erosion. Piping can be con­trolled by lowering the groundwater table in the exca­vated slopes or bottom of an excavation, or in either less critical situations or emergencies by placement of filters over the seepage exit surface to prevent erosion of the soil but still permit free flow of the seepage. The gradation of the filter material should be such that the permeability is high compared with the aquifer, yet fine enough that aquifer materials will not migrate into or through the filter. The filter should be designed on the basis of criteria given in paragraph 4-6c. More than one layer of filter material may be required to stabilize a seeping slope or bottom of an excavation in order to meet these criteria.

d. Dewatering systems. As in the design of any works, the design of a dewatering system should in­clude a factor of safety to cover the variations in char­acteristics of the subsurface soils, stratification, and groundwater table; the incompleteness of the data and accuracy of the formulations on which the design is based; the reduction in the efficiency of the dewater­ing system with time; the frailties of machines and op­erating personnel; and the criticality of failure of the system with regard to safety, economics, and damage to the project. All of these factors should be considered in selecting the factor of safety. The less information on which the design is based and the more critical the dewatering is to the success of the project, the higher the required factor of safety. Suggested factors of safety and design procedures are as follows:

(1) Select or determine the design parameters as accurately as possible from existing information.

(2) Use applicable design procedures and equa­tions set forth in this manual.

(3) Consider the above enumerated factors in selecting a factor of safety.

(4) Evaluate the experience of the designer.

(5) After having considered steps 1-4, the follow­ing factors of safety are considered appropriate for modifying computed design values for flow, draw­down, well spacing, and required “wetted screen length.”

Factor of Safety for Design (FS = 1.0 + (a + b + c))

Factor to be Added to 1.0

Design Data

Poor

0.25

Fan-

0.20

Good

0.10

Excellent

0.05

Experience of Designer

Little

0.25

Some

0.20

Good

0.10

Excellent

0.05

Criticality

Great

0.25

Moderate

0.20

Little

0.15

Application of Factor of Safety to Computed Values or System Design Features

Computed Value System Design Feature

Design Procedure

Remarks

Pump capacity, header, and dis­charge pipe (Q)

Increase Q to FS

Drawdown (Ah) Well spacing (a)

Decrease Ah by 10 percent 1 Decrease a by 10 percent 1

Adjust either drawdown or well spacing, but not both

Wetted screen length (hWs)

hws

Note: In initially computing drawdown, well spacing, and wetted screen, use flow and other pa­rameters unadjusted for factor of safety.

Подпись: where v = vacuum at pump intake, feet of water M = distance from base of pervious strata to pump intake, feet Hc = average head loss in header pipe from wellpoint, feet Hw = head loss in wellpoint, riser pipe, and swing connection to header pipe, feet Step 8. Set the top of the wellpoint screen at least 1 to 2 feet or more below h„—Hw to provide adequate submergence of the wellpoint so that air will not be pulled into the system. (b) An example of the design of a two-stage well- point system to dewater an excavation is illustrated in figure D-l, appendix D. (c) If an excavation extends below an aquifer into an underlying impermeable soil or rock formation, some seepage will pass between the wellpoints at the lower boundary of the aquifer. This seepage may be intercepted with ditches or drains inside the excavation and removed by sump pumps. If the underlying stratum is a clay, the wellpoints may be installed in holes drilled about 1 to 2 feet into the clay and backfilled with filter material, By this procedure, the water level at the wellpoints can be maintained near the bottom of the aquifer, and thus seepage passing between the wellpoints will be minimized. Sometimes these procedures are ineffective, and a small dike in the ex-

In addition to these factors of safety being applied to design features of the system, the system should be pump-tested to verify its adequacy for the maximum required groundwater lowering and maximum river or groundwater table likely (normally a frequency of oc­currence of once in 5 to 10 years for the period of expo­sure) to occur.

4-9. Dewatering open excavations. An ex­cavation can be dewatered or the artesian pressure re­lieved by one or a combination of methods described in chapter 2. The design of dewatering and groundwater control systems for open excavations, shafts, and tun­nels is discussed in the following paragraphs. Ex­amples of design for various types of dewatering and pressure relief systems are given in appendix D.

a. Trenching and sump pumping.

(1) The applicability of trenches and sump pump­ing for dewatering an open excavation is discussed in chapter 2. Where soil conditions and the depth of an excavation below the water table permit trenching and sump pumping of seepage (fig. 2-1), the rate of flow into the excavation can be estimated from plan and sectional flow analyses (fig, 4-27) or formulas pre­sented in paragraphs 4-2 through 4-5.

(2) Where an excavation extends into rock and there is a substantial inflow of seepage, perimeter drains can be installed at the foundation level outside of the formwork for a structure. The perimeter drain­age system should be connected to a sump sealed off from the rest of the area to be concreted, and the seep­age water pumped out. After construction, the drain­age system should be grouted. Excessive hydrostatic pressures in the rock mass endangering the stability of the excavated face can be relieved by drilling 4-inch- diameter horizontal drain holes into the rock at ap­proximately lo-foot centers. For large seepage inflow, supplementary vertical holes for deep-well pumps at 50- to 100-foot intervals may be desirable for tempo­rary lowering of the groundwater level to provide suit­able conditions for concrete placement.

b. Wellpoint system. The design of a line or ring of wellpoints pumped with either a conventional well – point pump or jet-eductors is generally based on math­ematical or flow-net analysis of flow and drawdown to a continuous slot (para 4-2 through 4-5).

(1) Conventional wellpoint system. The draw­down attainable per stage of wellpoints (about 15 feet) is limited by the vacuum that can be developed by the pump, the height of the pump above the header pipe, and hydraulic head losses in the wellpoint and collec­tor system. Where two or more stages of wellpoints are required, it is customary to design each stage so that it is capable of producing the total drawdown required by that stage with none of the upper stages function­ing. However, the upper stages are generally left in so

that they can be pumped in the event pumping of the bottom stage of wellpoints does not lower the water table below the excavation slope because of stratifica­tion, and so that they can be pumped during backfill­ing operations.

(a) The design of a conventional wellpoint sys­tem to dewater an open excavation, as discussed in paragraph 4-2b, is outlined below.

Step 1. Select dimensions and groundwater co­efficients (H, L, and k) of the formation to be de­watered based on investigations outlined in chapter 3.

Step 2. Determine the drawdown required to dewater the excavation or to dewater down to the next stage of wellpoints, based on the maximum ground­water level expected during the period of operation.

Step 3. Compute the head at the assumed slot (he or h0) to produce the desired residual head ho in the excavation.

Step 4. Compute the flow per lineal foot of drainage system to the slot Qp.

Step 5. Assume a wellpoint spacing a and com­pute the flow per wellpoint, Qw = aQP.

Step 6. Calculate the required head at the well­point hw corresponding to Qw.

Step 7. Check to see if the suction lift that can be produced by the wellpoint pump V will lower the water level in the wellpoint to hw(p) as follows:

V^M-hw(p) +Hc +Hw (4-8)

cavation just inside the toe of the excavation may be required to prevent seepage from entering the work area. Sump pumping can be used to remove water from within the diked area.

(2) Jet-eductor (well or) wellpoint systems. Flow and drawdown to a jet-eductor (well or) wellpoint sys­tem can be computed or analyzed as discussed in para­graph 4-26. Jet-eductor dewatering systems can be de­signed as follows:

Step 1. Assume the line or ring of wells or well- points to be a drainage slot.

Step 2. Compute the total flow to the system for the required drawdown and penetration of the well screens.

Step 3. Assume a well or wellpoint spacing that will result in a reasonable flow for the well or well­point and jet-eductor pump.

Step 4. Compute the head at the well or well­point hw required to achieve the desired drawdown.

Step 5. Set eductor pump at M = hw—Hw with some allowance for future loss of well efficiency.

The wells or wellpoints and filters should be selected and designed in accordance with the criteria set forth under paragraph 4-6.

(a) If the soil formation being drained is strati­fied and an appreciable flow of water must be drained down through the filter around the riser pipe to the wellpoint, the spacing of the wellpoints and the perme­ability of the filter must be such that the flow from formations above the wellnoints does not exceed

Qw = kvAi (4-9)

where

Qw = flow from formation above wellpoint kv = vertical permeability of fdter A = horizontal area of filter і = gradient produced by gravity = 1.0 Substitution of small diameter well screens for well – points may be indicated for stratified formations. Where a formation is stratified or there is little avail­able submergence for the wellpoints, jet-eductor well – points and risers should be provided with a pervious filter, and the wellpoints set at least 10 feet back from the edge of a vertical excavation.

(b) Jet-eductor pumps may be powered with individual small high-pressure centrifugal pumps or with one or two large pumps pumping into a single pressure pipe furnishing water to each eductor with a single return header. With a single-pump setup, the water is usually circulated through a stilling tank with an overflow for the flow from wells or wellpoints (fig. 2-6). Design of jet eductors must consider the static lift from the wells or wellpoints to the water level in the recirculation tank; head loss in the return riser pipe; head loss in the return header; and flow from the wellpoint. The (net) capacity of a jet-eductor pump de­pends on the pressure head, input flow, and diameter

of the jet nozzle in the pump. Generally, a jet-eductor pump requires an input flow of about 2 to 2% times the flow to be pumped depending on the operating pressure and design of the nozzle. Consequently, if flow from the wells or wellpoints is large, a deep-well system will be more appropriate, The pressure header supplying a system of jet eductors must be of such size that a fairly uniform pressure is applied to all of the eductors.

(3) Vacuum wellpoint system. Vacuum wellpoint systems for dewatering fine-g-rained soils are similar to conventional wellpoint systems except the wellpoint and riser are surrounded with filter sand that is sealed at the top, and additional vacuum pump capacity is provided to ensure development of the maximum vac­uum in the wellpoint and filter regardless of air loss. In order to obtain 8 feet of vacuum in a wellpoint and filter column, with a pump capable of maintaining a 25-foot vacuum in the header, the maximum lift is 25-8 or 17 feet. Where a vacuum type of wellpoint system is required, the pump capacity is small. The ca­pacity of the vacuum pump will depend on the air per­meability of the soil, the vacuum to be maintained in the filter, the proximity of the wellpoints to the exca­vation, the effectiveness of the seal at the top of the filter, and the number of wellpoints being pumped. In very fine-g-rained soils, pumping must be continuous. The flow may be so small that water must be added to the system to cool the pump properly.