(1) The design and analysis of a deep-well system to dewater an excavation depends upon the configuration of the site dewatered, source of seepage, type of flow (artesian and gravity), penetration of the wells, and the submergence available for the well screens with the required drawdown at the wells. Flow and drawdown to wells can be computed or analyzed as discussed in paragraph 4-26.
(2) Methods are presented in paragraphs4-2fr and 4-3 whereby the flow and drawdown to a well system can be computed either by analysis or by a flow net assuming a continuous slot to represent the array of wells, and the drawdown at and between wells computed for the actual well spacing and location. Examples of the design of a deep-well system using these methods and formulas are presented in figures D-2 and D-3.
(3) The submerged length and size of a well screen should be checked to ensure that the design flow per well can be achieved without excessive screen entrance losses or velocities. The pump intake should be set so that adequate submergence (a minimum of 2 to 5 feet) is provided when all wells are being pumped. Where the type of seepage (artesian and gravity) is not well established during the design phase, the pump intake should be set 5 to 10 feet below the design elevation to ensure adequate submergence. Setting the pump bowl below the expecteddrawdown level will also facilitate drawdown measurements,
c. Combined systems.
(1) Well and wellpoint systems. A dewatering system composed of both deep wells and wellpoints may be appropriate where the groundwater table has to be lowered appreciably and near to the top of an impermeable stratum. A wellpoint system alone would require several stages of wellpoints to do the job, and a well system alone would not be capable of lowering the groundwater completely to the bottom of the aquifer. A combination of deep wells and a single stage owell – points may permit lowering to the desired level. The advantages of a combined system, in which wells are essentially used in place of the upper stages of wellpoints, are as follows:
(a) The excavation quantity is reduced by the elimination of berms for installation and operation of the upper stages of wellpoints.
(b) The excavation can be started without a delay to install the upper stages of wellpoints.
(c) The deep wells installed at the top of the excavation will serve not only to lower thgroundwa – ter to permit installation of the wellpoint system but also to intercept a significant amount of seepage and thus reduce the flow to the single stage of wellpoints. A design example of a combined deep-well and wellpoint system is shown in figureD-4.
(2) Sand drains with deep wells and wellpoints,
Sand drains can be used to intercept horizontal seepage from stratified deposits and conduct the water vertically downward into a pervious stratum that can be dewatered by means of wells or wellpoints. The limiting feature of dewatering by sand drains is usually the vertical permeability of the sand drains itself, which restricts this method of drainage to soils of low permeability that yield only a small flow of water. Sand drains must be designed so that thejwlll intercept the seepage flow and have adequate capacity to allow the seepage to drain downward without any back pressure. To accomplish this, the drains must be spaced, have a diameter, and be filled with filter sand so that
Qn — kpiAn — kvAn (4-11)
Qd = flow per drain
kc = vertical permeability of sand filter і = gradient produced by gravity= 1.0
Ad = area of drain
Generally, sand drains are spaced or5- to 15-foot centers and have a diameter of 10 to 18 inches. The maximum permeability kv of a filter that may be used to drain soils for which sand drains are applicable is about 1000 to 3000 x 10~4 centimetres per second or 0.20 to 0.60 feet per minute. Thus, the maximum capacity Qd of a sand drain is about 1 to 3 gallons per minute. An example of a dewatering design, including sand drains, is presented in figure D-5. The capacity of sand drains can be significantly increased by installing a small (1- or 1 Vi-inch) slotted PVC pipe in the drain to conduct seepage into the drain downward into underlying more pervious strata being dewatered.
d. Pressure relief systems.
(1) Temporary relief of artesian pressure beneath an open excavation is required during construction
where the stability of the bottom of the excavation is endangered by artesian pressures in an underlying aquifer. Complete relief of the artesian pressures to a level below the bottom of the excavation is not always required depending on the thickness, uniformity, and permeability of the materials. For uniform tight shales or clays, an upward seepage gradient i as high as 0.5 to 0.6 may be safe, but clay silts or silts generally require lowering the groundwater 5 to 10 feet below the bottom of the excavation to provide a dry, stable work area.
(2) The flow to a pressure relief system is artesian; therefore, such a system may be designed or evaluated on the basis of the methods presented in paragraphs 4-2 and 4-3 for artesian flow. The penetration of the wells or wellpoints need be no more than that required to achieve the required drawdown to keep the flow to the system a minimum. If the aquifer is stratified and anisotropic, the penetration required should be determined by computing the effective penetration into the transformed aquifer as described in appendix E. Examples of the design of a wellpoint system and a deep – well system for relieving pressure beneath an open excavation are presented in figures D-6 and D-7.
e. Cutoffs. Seepage cutoffs are used as barriers to flow in highly permeable aquifers in which the quantity of seepage would be too great to handle with deep – well or wellpoint dewatering systems alone, or when pumping costs would be large and a cutoff is more economical. The cutoff should be located far enough back of the excavation slope to ensure that the hydrostatic pressure behind the cutoff does not endanger the stability of the slope. If possible, a cutoff should penetrate several feet into an underlying impermeable stratum. However, the depth of the aquifer or other conditions may preclude full penetration of the cutoff, in which case seepage beneath the cutoff must be considered. Figure 4-36 illustrates the effectiveness of a partial cutoff for various penetrations into an aquifer. The figure also shows the soils to be homogeneous and isotropic with respect to permeability. If, however, the soils are stratified or anisotropic with respect to permeability, they must be transformed into an isotropic section and the equivalent penetration computed by the method given in appendix E before the curves shown in the figure are applicable.
(1) Cement and chemical grout curtain. Pressure injection of grout into a soil or rock may be used to reduce the permeability of the formation in a zone and seal off the flow of water. The purpose of the injection of grout is to fill the void spaces with cement or chemicals and thus form a solid mass through which no water can flow. Portland cement, fly ash, bentonite, and sodium silicate are commonly used as grout materials. Generally, grouting pressures should not exceed about 1 pound per square inch per foot of depth of the injection.
(a) Portland cement is best adapted to filling voids and fractures in rock and has the advantage of appreciably strengthening the formation, but it is ineffective in penetrating the voids of sand with an effective grain size of 1 millimetre or less. To overcome this deficiency, chemical grouts have been developed that have nearly the viscosity of water, when mixed and injected, and later react to form a gel which seals the formation, Chemical grouts can be injected effectively into soils with an effective grain size D10 that is less than 0.1 millimetre. Cement grout normally requires a day or two to hydrate and set, whereas chemical grout can be mixed to gel in a few minutes.
(b) Cement grouts are commonly mixed at water-cement ratios of from 5:1 to 10:1 depending on the grain size of the soils. However, the use of a high water-cement ratio will result in greater shrinkage of the cement, so it is desirable to use as little water as practical. Bentonite and screened fly ash may be added to a cement grout to both improve the workability and reduce the shrinkage of the cement. The setting time of a cement grout can be accelerated by using a 1:1 mixture of gypsum-base plaster and cement or by adding not more than 3 percent calcium chloride. High-early – strength cement can be used when a short set time is required.
(c) Chemical grouts, both liquid and powder – based, are diluted with water for injection, with the proportions of the chemicals and admixtures varied to control the gel time.
(d) Injection patterns and techniques vary with grout materials, character of the formation, and geometry of the grout curtains. (Grout holes are generally spaced on 2- to 5-foot centers.) Grout curtains may be formed by successively regrouting an area at reduced spacings until the curtain becomes tight. Grouting is usually done from the top of the formation downward.
(e) The most perplexing problem connected with grouting is the uncertainty about continuity and effectiveness of the seal. Grout injected under pressure will move in the direciton of least resistance. If, for example, a sand deposit contains a layer of gravel, the gravel may take all the grout injected while the sand remains untreated. Injection until the grout take diminishes is not an entirely satisfactory measure of the success of a grouting operation. The grout may block the injection hole or penetrate the formation only a short distance, resulting in a discontinuous and ineffective grout curtain. The success of a grouting operation is difficult to evaluate before the curtain is complete and in operation, and a considerable construction delay can result if the grout curtain is not effective. A single row of grout holes is relatively ineffective for cutoff purposes compared with an effectiveness of 2 or 3 times
that of overlapping grout holes. Detailed information on grouting methods and equipment is contained in TM 5-818-6.
(2) Slurry walls. The principal features of design of a slurry cutoff wall include: viscosity of slurry used for excavation; specific gravity or density of slurry; and height of slurry in trench above the groundwater table. The specific gravity of the slurry and its level
above the groundwater table must be high enough to ensure that the hydrostatic pressure exerted by the slurry will prevent caving of the sides of the trench and yet not limit operation of the excavating equipment. Neither shall the slurry be so viscous that the backfill will not move down through the slurry mix, Typical values of specific gravity of slurries used range from about 1.1 to 1.3 (70 to 80 pounds per cubic foot)
U. S. Army Corps of Engineers
with sand or weighting material added. The viscosity of the slurry for excavating slurry wall trenches usually ranges from a Marsh funnel reading of 65 to 90 seconds, as required to hold any weighting material added and to prevent any significant loss of slurry into the walls of the trench. The slurry should create a pressure in the trench approximately equal to 1.2 times the active earth pressure of the surrounding soil. Where the soil at the surface is loose or friable, the upper part of the trench is sometimes supported with sand bags or a concrete wall. The backfill usually consists of a mixture of soil (or a graded mix of sand-gravel-clay) and bentonite slurry with a slump of 4 to 6 inches.
(3) Steel sheet piling. Seepage cutoffs may be created by driving a sheet pile wall or cells to isolate an excavation in a river or below the water table. Sheet piles have the advantage of being commonly available and readily installed, However, if the soil contains cob bles or boulders, a situation in which a cutoff wall is applicable to dewatering, the driving may be very difficult and full penetration may not be attained. Also, obstructions may cause the interlocks of the piling to split, resulting in only a partial cutoff.
(a) Seepage through the sheet pile interlocks should be expected but is difficult to estimate. As an approximation, the seepage through a steel sheet pile wall should-be assumed equal to at least 0.01 gallon per square foot of wall per foot of net head acting on the wall. The efficiency of a sheet pile cutoff is sub stantial for short paths of seepage but is small or negligible for long paths.
(b) Sheet pile cutoffs that are installed for longterm operation will usually tighten up with time as the interlocks become clogged with rust and possible incrustation by the groundwater.
(4) Freezing. Freezing the water in saturated porous soils or rock to form an ice cutoff to the flow of groundwater may be applicable to control of groundwater for shafts or tunnels where the excavation is small but deep. (See para 4-12 for information on design and operation of freezing systems.)