US3274782A - Dewatering clayey and silty soil - Google Patents

Description

Sepf- 27, 1966 R. E. LANDAU 3,274,782

DEWATERING CLAYEY AND SILTY SOIL Filed July 5, 1965 2 Sheets-Sheet 1 RICHARD E. LANDAU BY ma@ e 1 f 1,1/2

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Sept 27 1965 R. E. LANDAU 3,274,782

DEWATERING CLAYEY AND SILTY SOIL Filed July 5, 1963 2 Sheets-Sheet 2 INVENTOR. RICHARD E. LANDAU BY 474 w Y f AT1-op ys.

United States Patent O 3,274,782 DEWATERING CLAYEY AND SILTY SOIL Richard E. Landau, Middle Village, N.Y. (717 Cornwell Ave., West Hempstead, N.Y. 11552) Filed July 5, 1963, Ser. No. 293,163 6 Claims. (Cl. 61-11) This invention relates to a method for removing water or other liquid from clayey or silty soils which will not compress to any substantial degree by such removal, and is a contnuation-in-part of application No. 746,099 led July 2, 1958, now Patent No. 3,096,622.

In the said Patent 3,096,622, a method for consolidating compressible soils by removing water and other liquids is described. This application is a continuation-in-part of said application and relates to a method of draining or removing water or other liquid from la clay or silty earth formation, wherein the earth formation does not compress to any substantial degree. In the usual case, soils of the type described will consolidate in a vertical direction when a force is applied. However, in some instances the force applied does not exceed the intergranular pressure of the soil and consequently, consolidation does not occur to any appreciable degree. For example, an earth formation may have been preconsolidated such that subsequent loading will not produce a vertical force in excess of the intergranular pressure of the preconsolidated soil. Nevertheless, it is often desirable to dewater a particular volume of earth to permit construction below the natural water level without the need for employing underwater equipment. The method of this invention provides a relatively rapid and economical process for dewatering clayey or silty soils.

Soils of the type described generally contain la substantial quantity of water (pore water) dispersed within the structural matrix. Such water is most generally associated with the level of water in the soil, commonly referred toas the water table.

The rate ,at which the dewatering will occur is related to the ability of soil matrix to permit the passage of pore water. The more fine-grained the soil the lower the rate of pore wate-r movement for a given pressure differential, or in other words, the lower the permeability of the soil.

Most clayey or silty soils are alluvial deposits, which were laid down in horizontal layers in the geologic past. Such deposits, generally, have a higher permeability in a horizontal direction along the bedding plane than in the vertical direction across the bedding planes. This condition is particularly pronounced in varved deposits which consist of alternating layers of ine and course grained materials. Varved deposits were generally formed by the stagnation of the continental glaciers, and are often found in rivers and lakes.

Clayey or silty soils generally have a greater horizontal than vertical permeability. To take advantage of this phenomenon the construction industry has resorted to the use of vertical drains, such as, wellpoints, `and sand drains. Wellpoints are usually lengths of tubing driven into the soil within the area to be drained; the tubing being porous along the length at the lower end thereof. The tubing is connected to a suction source such as .a pump which draws the surrounding soil water into and through the tubing to the surface. Sand drains are columns of sand constructed in the soil and generally used to expedite flow of water and liquid under pressure, which may be induced or otherwise exist, in the soil. These types of drains, which generally are installed vertically but may in suitable cases be installed at an angle or even horizontally, are devices in the earth that serve as low pressure points to which the pore water may ow and through which the pore watermay leave the soil forma- 3,274,782 Patented Sept. 27, 1966 tion when a suitable pressure differential exists to cause ow. To create the low pressure zone, a wellpoint is disposed in the soil at the desired level thereof. The wellpoint when in operation creates a low pressure zone by operation of a suction pump connected thereto which accelerates the ilow of water from the formation into the drain. The wellpoint is so constructed that formation liquid precolating into this type of drain will flow to the wellpoint, after which it is pumped to the surface.

Often sand drains and Wellpoints are used together especially in clay and silt soils where the permeability is so low that a large contact area between the drain and the soil is des-irable to achieve better drainage whereby the sand drains complement the drainage available from the wellpoint. In such an arrangement the sand drain is generally circular in cross-section, with the wellpoint loc-ated to a suitable depth within the sand column. lf desired a vacuum may be eifected in the sand drain by plugging the upper portion of the said column to prevent the entry of air and pumping from -a wellpoint disposed to a suitable depth within said sand column.

In the case of sand drains, the hole is bacldilled with a material of relatively high permeability, such as coarse sand; other materials and other means are sometimes used to maintain the void and permit the free passage of fluids.

Heretofore, it has been the practice to construct drain holes for inserting Wellpoints or defining sand drains by ramming or driving a hollow casing into the formation, and where applicable, lling the casing with porous or other desired devices Aand material which in turn ills the void formed by the displaced soil. Because of the nature of the soils involved, ramming substantially decreases the horizontal permeability of the soil and -results in a slower dewateriing rate than would otherwise be possible. This decrease in permeability is due to the combined effect of smearing and remolding of the face of the void contiguous to the casing, as well as due to the disturbance of the soil in the vicinity of the hole by displacement yand energy input to the soil by the driving of the casing.

I have discovered a method for constructing drain holes in 'an earth formation which avoids the disadvantages of the prior art. Generally stated, my invention comprises helically cutting into the earth formation to the desired depth, removing the section of soil traversed by cutting, positioning a wellpoint at the desired level in the cavity formed by removing the earth, and where the wellpoint itself does not support the hole suflciently, fill-ing the cavity with a porous medium to -a suitable depth to permit the desired drainage. Where ya wellpoint drain is inserted into a sand drain column, a substantially airtight cover can be placed over or within the cavity above the porous wellpoint thus permitting the formation of a low pressure vacuum zone upon actuating the wellpoint by pumping.

By cutting into the earth, rather than ramming or pounding the earth forming the wall of the cavity, smearing and remolding is minimized, the in situ soil strength charatceristic is in no way diminished, and the soil retains substantially the degree of permeability present before cutting.

While it is not critical that the cutting unit employed be constructed so that withdrawing it from the earth also removes the section of earth cut thereby, it is convenient and preferable to use such a unit. For example, an augertype drill having continual flights has proven satisfactory; the flights on the auger defining the circumference of the cavity.

Additionally, it is desirable that the cutting unit have a hollow shaft through which the tubing of the wellpoint may be passed. In this way, tubing may be passed 3 through the shaft, while at the same time withdrawing the Vcutting unit, thereby providing continuous support for the walls of the cavity. It is noted that various field test devices such as piezometers may be installed through the hollow shaft prior to backflling.

If a hollow shaft cutting unit is employed, the bottom of the shaft may be equipped with a plug or plate so as to prevent the soil penetrated by the cutting unit from travelling up the hollow shaft. Such plug or plate, however, should be so constructed as to allow admission of the porous material into the cavity.

An `apparatus suitable for forming the cavity and backfilling same is described in U.S. application Serial No. 246,411, filed December 21, 1962. In clay or clay-like deposits, there are frequently alternate zones of permeable and impermeable strata, called varves When this is the case, the formation water often finds its way to the sand drain through the permeable varves which are often sandy or silty. The less permeable varves usually consist of clay or clay-like material, through which the water or other formation liquid cannot easily penetrate. If the earth is removed by pulling the cutting unit, there is likelihood that substantial smearing of the permeable strata with the less permeable material will occur. It is therefore desirable to effect at least one complete revolution of the cutting unit while holding the unit at substantially the same vertical position, to insure against smear. For exfample, when a continual fiight auger is used as the cutting unit, the auger is held in position so that it does not penetrate any deeper into the formation; then the auger may be rotated several times. This gently cuts the soil away from the wall; the auger flight acting as a trimming blade. This results in the cutting away of a core of earth, forming a hole in the soil formation, wherein none of the outer wall surface of the core is bonded with the remaining formation, so that, when the auger is removed, smearing and remolding will be minimized.

A convenient method for holding the cutting unit at substantially the same vertical position during the operation is to provide a gear mechanism at the top of the auger, which mechanism may be so marked that the operator will know when he has turned the auger a complete revolution or fraction thereof.

To guard against distrubance to the subsoil, it is desirable that the auger flights advance in a controlled manner until such time as the desired depth is reached. This may be accomplished by applying a tooth at the lower portion of the mechanism supporting the auger. As the auger rotates, the flights will screw themselves down on the tooth so that the auger will advance downward uniformly. The tooth may be designed to swivel back prior to the cutting operation so that the soil core may be trimmed and the entire auger removed without interference. In putting down a subsequent drain hole, the tooth may be swiveled back into position thereby removing any soil sticking in the auger iiights as the ights screw down on the tooth. Thus, the mechanism would be self cleaning.

Oftentimes when an area has been dewatered, it is desirable to limit the radius of influence of a wellpoint system, for example, where adjacent soils will compress when water is removed. To avoid compression and consequent settling damage to structures in the area, the soil is recharged to elevate the water table to its original level beyond the area where dewatering is desired. To accomplish recharging a wellpoint may be installed according to the method described herein and water pumped from the surface through the tubing of the wellpoint and into the formation. This technique of installing the wellpoint has the advantage of reducing soil disurbance without adversely affecting the permeability of the adjacent soil to any substantial degree. When the water level in the cavity is higher than the surrounding water table, there is a pressure differential which forces additional water into the soil to accomplish rewatering.

In a broad sense, the technique for helically cutting into the soil may be used to advantage where ever percolation into the clay or silt formation is desired. For example, the effluent from a septic tank may be conducted into one or more cavities containing a porous material. By helically cutting the clay or silt to form the cavities, the natural permeability of the soil is not adversely affected and maximum percolation will occur. Other uses embracing the idea of creating a plurality of cavities in a clay or slit formation and allowing the formation water to drain into the cavity or passing into the formationfrom said cavity will be apparent to those skilled in the art.

Particular aspects of my Iinvention maybe seen with reference to FIGURE 1 which shows a sectional view of different kinds of cavities, and FIGURE 2 which shows sand and -a wellpoint being injected into a cavity through a hollow-shafted auger.

Referring to FIGURE 1, A, B, C, D, and E refer to various congurations involving formation of the soil cavity and the disposition of permeable media and equipment within the cavity. It is noted that the invention is not limited to the conditions therein presented to illustrate the method referred to in the invention.

The subsoil 1 may be stratified heterogeneously or homogeneously, or need not be stratified. The permeability of the subsoil in its natural state is greater than its permeability when remolded or otherwise disturbed as is generally the case in silty and clayey soils.

The cavity is formed by Ithe insertion of an auger into the soil in a controlled manner so as to minimize remolding and rel-ated disturbance to the periphery 4 of the cavity, FIG. 1A. The core of soil contained within the flights 3 on shaft 2 of the auger may be fully or partially removed from the subsoil depending upon the specific configuration desired and the equipment employed. If it is desired to place medium 5 of specific permeability or other characteristic into the lower or any specific portion of the cavity, as in FIG. 1B and 1C, this may be accomplished and the remainder of the cavity may be back-filled with any desired material 6. Although instances occur where the cavity may be found to be self supporting, therefore requiring no backfill or cavity support medium, such is not generally the case. As collapse of the cavity will result in undesirable soil movements, resulting in a loss in soil permeability, cavity support may not always be indispensable but should be provided.

The usual means for dewatering or recharging is apipe -7 to which is connected a well-point 8 such as described in U.S. Patent 1,570,697, FIGS. 1B and E. However, depending upon the rate of water flow desired and the character of the support mediums into which it is inserted, a simple pipe 7 may suflice and function efficiently, FIGS. 1C and 1D. The best means for given conditions will be evident to those familiar in the art.

The natural water table is shown as X-X, and for convenience is shown to be above the bottom of the cavity. Where the objective is dewatering requiring the lowering of the water table X-X, the cavity would necessarily penetrate the level of the water table; whereas for recharging purposes the cavity need not necessarily be carried through the level of the water table X-X, although it may, at times, be desirable to do so. Where medium 6 is not permeable, the well-point 8 or the low point of the pipe 7 must be below medium 6 and situated within the range of the permeable medium 5. For dewatering purposes, a suction is effected by means of a suitable pump attached to pipe 7 to cause the water or other fluid in the soil to flow from the soil 1, through the cavity periphery 4, through the porous medium 5 (if used), into the pipe 7 and well-point 8 (if used) and out of the system.

By lowering the pressure sufficiently to effect a lowering of the water table, the rate at which the water is extracted from the soil must exceed the rate at which the water table will be replenished. The suction causes a lowering of the water table in the cavity to Z-Z, and because the water or fluid in the soil will flow into the cavity there will be tapering of the water table in the surrounding soil for a distance beyond the cavity. The higher the permeability of the cavity periphery, the less the suction that will be required to produce a given level beyond the cavity; or for a given suction the greater the dewatering effect. By putting the fluid in the pipe 7 under pressure the reverse action occurs, raising the effective pressure in the cavity to Y-Y with a flow into the soil occurring and tapering for a distance beyond the cavity. Where the desired level of Y-Y is to be higher than the existing ground surface, then an impermeable plug may be employed in the place of medium 6 in FIGS. 1B and C able to withstand the fluid pressure applied through the pipe 7 without being forced out of the ground. Other plugs, caps, and seals may be employed as will be evident to those fami-liar in the art. As in the case of dewatering, the higher the cavity periphery permeability, the more efllcient the system. It is noted that w represents the fluid taken from the soil or placed into the soil.

Where only recharging is desired, and the required level of water to produce the desired flow is lower than the natural ground surface, the permeable medium 5 may extend to the ground surface and the fluid w may be applied to the top of the porous medium 5 at a suitable rate through a suitable pipe or orifice 9. If this type of system is desired, but the Water level required is above the ground level, a suitable container may be provided to retain the water at a higher Ilevel than the ground surface. Implementation and variations of the foregoing will be evident to those familiar in the art.

Where it is desired to measure the level of fluid in the soil, the pipe 7 may be a passive pipe such that the level of fluid within the pipe will equalize with that in the soil or otherwise reflect the iluid pressure in the soil. A suitable rod, gage or other compatible system may then be inserted into the pipe and the water level in the system ascertained. This is useful in specific instances such as when pore pressure of fluid in the soil is related to the effective stresses in the soil which may be important in the control of the safety of specific types of dams. Other uses will be evident to those familiar in the art. By effecting the highest peripheral permeability in the cavity periphery, the response time for the fluid to have the desired information is reduced and greater accuracy of the reading is obtained.

I claim:

1. A method for locally controlling the fluid level in clayey or silty soils comprising (l) forming a cavity in the soil by (a) penetrating into the soil to the desired depth by helically cutting with a flight auger into the earth formation thereby to form a core of earth,

(b) removing the core from the soil by withdrawing the auger thereby to form a cavity,

(c) disposing a well-point at the desired depth in the cavity,

(2) actuating the well-point as a conduit for fluid flow,

(3) controlling the fluid flow in the well-point to control locally the fluid level in the soil.

2. The method of claim 1, wherein after penetrating into the formation to the desired depth, a cylindrical cut is circumscribed throughout the depth of the penetration by making at least one complete revolution of the auger While maintaining the auger in the same Vertical position as that occupied at the termination of its penetration into the earth.

3. The method of claim 1, wherein the auger is hollow shafted and wherein the cavity is filled with porous material through said hollow shaft.

4. The method of claim 1 wherein the fluid level is controlled by introducing fluid into the soil and said wellpoint is a conduit extending to a desired depth.

5. The method of claim 4, wherein the eilluent from a septic tank is the fluid that is introduced into the soil.

6. The method of claim 1, wherein the fluid level is controlled by removing fluid from the soil.

References Cited by the Examiner UNITED STATES PATENTS 193,348 7/1877 Robinson 175-394 1,570,697 6/1926 Moore. 2,729,067 6/1956 Patterson 6l-53.58 2,920,455 6/1960v Ryser et al 61-63 X 3,096,622 7/1963 Landau 61-10 FOREIGN PATENTS 652,570 4/ 1951 Great Britain.

OTHER REFERENCES Construction Methods and Equipment, pub. April 1956, pp. 74, 75 and 76.

Engineering News-Record (pub), Apr. 6, 1944, pp. 81 to 85.

CHARLES E. OCOINNIELL Primary Examiner.

JACOB SHAPI'RO, EARL J. WITMER, Examiners.

Claims (1)

1. A METHOD FOR LOCALLY CONTROLLING THE FLUID LEVEL IN CLAYEY OF SILTY SOILS COMPRISING (1) FORMING A CAVITY IN THE SOIL BY (A) PENETRATING INTO THE SOIL TO THE DESIRED DEPTH BY HELICALLY CUTTING WITH A FLIGHT AUGER INTO THE EARTH FORMATION THEREBY TO FORM A CORE OF EARTH, (B) REMOVING THE CORE FROM THE SOIL BY WITHDRAWING THE AUGER THEREBY TO FORM A CAVITY, (C) DISPOSING A WELL-POINT AT THE DESIRED DEPTH IN THE CAVITY, (2) ACTUATING THE WELL-POINT AS A CONDUIT FOR FLUID FLOW, (3) CONTROLLING THE FLUID FLOW IN THE WELL-POINT TO CONTROL LOCALLY THE FLUID LEVEL IN THE SOIL.

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