US 3252514 A
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Description (OCR text may contain errors)
May 24, 1966 R. JOY 3,252,514
METHOD FOR PRODUCING SUBTERRANEAN WATERTIGHT SEALS Filed March 19, 1963 OHMMETER ff! 2 9"f 9 55 INVENTOR.
20.5557 JOY United States Patent 3,252,514 METHOD FDR PRODUCING SUBTERRANEAN WATERTHGHT SEALS Robert Joy, 277 E. 239th St., Bronx, Nit. Filed Mar. 19, 1963, Ser. No. 266,410 3 Claims. ((11. 166-46) The present invention relates to a method of forming a subterranean underwater seal for a piezometer of the open Well type used for the measurement of subterranean Water pressure and seal for similar purposes.
Piezometers of'this type are used in connection with highway and building projects involving the installation of an overburden on marshy soil which includes clay. Drainage is provided by means of a series of sand-filled bores in the construction area which receive horizontally flowing ground water squeezed by the weight of the overburden and permit this water to rise to the surface and drain away. Because of moisture conditions in the clay, as the overburden is loaded on top of the soil, the water pressure may build up to a point where the entire installation slides away into adjacent unfilled areas. By periodic measurement and study of the pressure of the underground water, the project may be temporarily interrupted when a dangerous reading is noted in order to permit the pressure to subside to a safe value.
More particularly, the invention pertains to an improved method for sealing the lower end of the piezometer so that accurate water pressure measurements may be obtained. The novel method involves the formation of the seal using frangible pellets of bentonite in dry pulverulent form and crushing the pellets under water in situ at the location of the seal. Y
Such seals are ordinarily formed using a series of layers of rounded pebbles about A: inch in diameter alternating with bentonite balls about /2 inch in diameter. The bentonite balls have a water content providing a putty-like consistency. The putty-like balls are rolled in talcum powder to prevent them from sticking together and stored in glass jars prior to use to prevent them from drying. When bentonite is not available, fat clay may be substituted. The moist bentonite balls and pebbles of each layer are tamped together and successive layers are added and tamped to'form the seal.
In accordance with the invention, in which compressed pellets of dry powdered bentonite are used the problem of correct moisture content is completely avoided and there is no need to prevent drying during storage.
The invention will now be described in greater detail with reference to the accompanying drawing forming a part hereof.
Referring to the drawing:
FIGURE 1 is a sectional view in elevation of an installed piezometer illustrating two seals.
FIGURE 2 is a sectional view in elevation of a tamping hammen' FIGURE 3 is a bottom view of the hammer shown in FIG. 2.
FIGURE 4 shows bentonite pellet of spherical shape.
FIGURE 5 is a sectional View taken along the line 5-5 of FIG. 4.
FIGURE 6 is a diagrammatic view of an electrical testing device for measuring the water level inside the piezometer.
Referring to FIG. 1, the piezometer comprises a casing 10 which, in the example illustrated, is formed of interconnected sections of 2" pipe. The casing 10 is driven into the ground in the usual manner by means of a wash boring equipment, successive sections being added as driving proceeds.
The casing 10 is advanced to a lowermost position indicated at 11. After cleaning and filling with clear water,
Patented May 2 lgfifi "ice thecasing is then raised about 2 ft. above the lowermost position 11 and the resulting space is filled with a first quantity of thoroughly washed sand 12 of between No.
and No. 40 mesh or preferably Ottawa Standard Sand.
A water-filled assembly consisting of a porous filter tube or point 14, connected to the lower end of a continuous length of plastic tubing 15 is then lowered to the bottom of the casing Iltl so that the bottom of the porous point 14 rests on top of the sand 12. The tubing 15 is formed of polyethylene of internal diameter and /2" external diameter. The porous point or filter tube 14 .is closed at its lower end and is formed of ceramic material from 1 ft. to 2 ft. long of 1.5 external diameter and 1.0 internal diameter. The plastic tube 15 is coupled to the porous point 14 by a frictional fit with a neoprene or rubber bushing 16. The lower end of the tubing 15 is notched at 18 to retain a plastic sleeve 19, the
A cylindrical drop hammer designated generally as 20 1 (FIGS. 2 and 3) is then lowered over the plastic tube 15. The hammer Z9 is approximately 3 ft. long and has an internal diameter of about so that it slides freely over the plastic tube 15 which has an external diameter of /2. The hammer 20 has an external diameter of 1 /8 so that it clears the internal wall of the casing 10 formed of 2" pipe leaving a space permitting it to drop freely by gravity through the water in. the casing 10 with a velocity sufficient for tamping purposes, as hereinafter described. An upwardly extending loop of cable 22 is connected to the upper end of the hammer 20. A swivel 23 connects the loop 22 to the lower end of a control cable 24 by means of which the hammer is raised and dropped using suitable means (not shown).
After being lowered to the top of the sand 12, the porous point 14 is tapped firmly into position by means of the hammer 20.
The casing 10 is then raised to the position shown in FIG. 1 so that its lower end is spaced about 1 ft. above the top of the porous point 14. Simultaneously, watersaturated sand is poured into the top of the casing 10 to fill the space around the porous point 14 and above the porous point 14 with a second quantity of sand 26. The second quantity of sand 26 extends above the lower end of the casing 19 to a depth of about 2% ft.
The first seal 27 isthen formed. The seal 27 consists of six 1 layers of pebbles alternating with five 3" layers of bentonite pellets such as balls 30 shown in FIGS. 4 and 5. Each pellet consists of compressed dry bentonite which is packed s-ufiiciently hard to permit handling and which is sufficiently frangible to break up into small fragments under the action of the drop hammer 20. Shapes other than spherical may be used, if desired.
The first layer of pebbles 31 is dropped into the upper end of casing 10. This is followed by the first layer of bentonite pellets 32. The second layer of pebbles 33 is then dropped into the upper end of the easing 10. The hammer 20 is then used to break up the layer 32 of pellets 30 between the layers of pebbles 31 and 33 using a drop of about 6" and about twenty blows. The frangible pellets 3i) drop rapidly through the water in casing 10. There is no tendency to stick to the plastic tube or to the internal surface of the casing 10. Additional layers are added to complete the seal 27. In each instance, the pellets 30 are covered by an overlying layer of pebbles such as layer 33 during the application of the hammer blows. This prevents the fragments of the broken pellets from adhering to the hammer 20.
A third quantity of sand is then added above the first seal 27 to a depth of about 2 ft. The second seal 36 is then installed above the sand 35 in the same manner as the first seal 27. A fourth quantity of sand 37 is then placed on top of the second seal 36 to a depth of about 12 ft. above the second seal 36. The portion of the casing above the sand 37 may be left filled with Water or may be filled with earth. However, the space above the fourth quantity of sand 37 may be left open, if desired.
The plastic tubing 15 is passed through a bearing plate or a loosely mounted fitting 39 which rests on the upper end of the casing 10. The fitting 39 has an axially extending hole 40 formed therein. The diameter of the hole 40 is slightly larger than the external diameter of the plastic tubing 15 so that the tubing 15 may be pulled taut. A rubber bushing 41 is fitted over the upper free end portion of the tubing 15 and is forced downwardly against the upper surface of the fitting 39 with the tubing 15 held taut. The tubing 15 and bushing 41 are then secured together by tightening a hose clamp 43 around the bushing 41.
A threaded sleeve 44 is then fitted on the upper end of the casing 10 for lateral enclosure of the upper end of the tubing 15. A plug 45 is then threaded into the upper end of the sleeve 44 for complete enclosure of the upper end of the tubing 15 so that exposure to the elements is avoided. The entry of dirt into the tubing 15 is also prevented. The sleeve 44 is provided with a small breathing aperture 47 to maintain atmospheric pressure at the upper end of the tubing 15.
After the seals 27 and 36 have hardened, clear water filters through the sand 26 and through the porous point 14 and enters the tubing 15. The water rises to a level which is determined by the underground water pressure in the vicinity of the porous point 14. If the seals 27 and 36 should be defective, the slow seepage of water will enter the casing 10 instead of being confined to the smaller diameter tubing 15. Accordingly, a much greater volumetric displacement of Water will be required for an accurate reading of water pressure at the porous point 14. For purposes of accuracy, it is essential that the seals 27 and 36 be able to withstand the water pressures involved without appreciable leakage.
Periodically, pressure measurements are made by measuring the water level in the tubing 15. The height of the water level measured in feet above the lower end of the tubing 15 may be converted to pounds per square inch using a conversion factor of 0.43. Thus, if the water level in the tube 15 is 100 feet above the lower end of the tube 15, the water pressure at the lower end of the tube 15 is 43 pounds per square inch. Whether or not this is a dangerously high pressure will be determined by the nature of the soil formation and other factors individual to each installation.
For measurement of the water level the apparatus shown in FIG. 6 may be employed. This apparatus comprises a length of shielded wire such as microphone cable 50 and an electrical continuity indicator such as an ohmmeter 51.
One terminal 52 of the ohmmeter 51 is connected to the upper end of the shield conductor 53 of the cable 50. The other terminal 55 of the ohmmeter 51 is connected to the inner conductor 56 of cable 50. At the lower end of the cable 50, the outer insulation is cut away to expose an end portion of the outer shield conductor 53 so that it may establish electrical contact with the water in the tubing 15. The inner insulation 59 of cable 50 extends for about 4;" beyond the exposed lower end portion of the shield conductor 53 and the inner conductor 56 protrudes about beyond the end of the inner insulation 59 for contact with the water.
For guide purposes, a sleeve 60 formed of A external diameter plastic tubing is secured near the lower end of the cable 50 just above the end of the outer insulation 57. The sleeve 60 thus slides freely within the /8" internal diameter of the taut plastic tubing 15. A series of copper sleeves 61 is secured to the lower end portion of the cable 50 at 1 foot intervals. About nine such sleeves are desirable for weighting the cable so that it moves downwardly to the water level. These sleeves are also desirably about A" in external diameter like the plastic sleeve 60.
When the cable 50 is lowered into the plastic tubing 15, the indicating needle of the ohmmeter 51 will give a pronounced kick when the lower end of the shield conductor 53 reaches the surface of the water. This may be repeated several times to assure an accurate depth reading. Advantageously reference marks may be provided, One of which is indicated at 63. Conveniently, the reference marks 63 may be formed by circumferentially extending bands of tape disposed at five foot intervals along the cable 50. The length in feet of cable 50 which is required to reach the water level, when subtracted from the known length in feet of the tubing 15, will determine the height of the water level above the lower end of the tubing 15. This may be converted to pounds per square inch, as described above.
When the water pressure is such that the water level is located above the upper end of the tubing 15, a pressure measuring device such as a Bourdon gauge or mercury manometer (not shown) is used. The pressure measuring device is connecteed to the upper end of the tubing 15 with appropriate precautions to assure the exclusion of air.
It will be appreciated that various changes and modifications may be made in the methods and apparatus herein shown and described without departing from the spirit and scope of the invention as defined in the appended claims.
What is claimed is:
1. The method of forming a watertight seal between two concentric vertical tubular members adjacent to the lower ends thereof in an annular space therebetween, the lower portion of said space being filled with sand and with water above said sand, said method comprising the steps of dropping pebbles into said space in a quantity sufficient to form a layer of pebbles above said sand, dropping frangible pellets of dry bentonite in pulverulent form into said space in a quantity sufficient to form a layer resting on said layer of pebbles, dropping further pebbles into said space in a quantity sufficient to form a further layer of pebbles resting on said layer of pellets, and applying downwardly directed mechanical force to said further layer of pebbles to break said pellets into fragments.
2. The method according to claim 1, wherein said mechanical force is percussively applied.
3. The method according to claim 1, wherein said force is applied by the downwardly directed impact pressure of a cylindrical hammer which moves freely in said annular space.
References Cited by the Examiner UNITED STATES PATENTS 715,141 12/1902 Plotts 16623 2,193,775 3/1940 Stratford 16620 2,193,808 3/ 1940 Dieterich 16620 X 2,597,554 5/1952 West 16620 2,634,098 4/1953 Armentrout 166-432 X 2,642,268 6/ 1953 Armentrout 166-29 X 2,836,555 5/1958 Armentrout 166--29 X 3,111,031 11/1963 Kuritza 73295 CHARLES E, OCONNELL, Primary Examiner.