US2734343A

US2734343A - asketh

Info

Publication number: US2734343A
Authority: US
Publication date: 1956-02-14
Anticipated expiration: 1973-02-14

Description

2 Sheets-Sheet l INVENTOR.

W S. M BY J. S. ASKETH UNDERGROUND SHAFTS Feb, 14, 1956 Filed June 24, 1949 Feb. 14, 1956 J. s. ASKETH UNDERGROUND SHAFTS 2 Sheets-Sheet 2 Filed June 24, 1949 ...I1...l... lr

INVENTOR. w s O/awu BY United Statesy Patent O l zas/:,343 UNDERGROUND SHAFTS Jordan Socrates Asketh, Jackson Heights, N. Y. Application June 24, 1949, Serial No. 101,034

' 3 Claims. (Cl. 61-40) This invention relates to underground shafts sunk into earth and method of construction of same, and more particularly to shafts for abstracting fluids from aquifers. The` type of shaft treated herein is the one constructed sectionally on the ground and sunk underground through excavation of unconsolidated material from within. The shaft is preferably built of reinforced concrete but it may also be constructed of any other suitable material.

In shaft-sinking technique, wherein excavation is carried out under atmospheric pressure conditions, the general practice nowadays is to cast one shaft section on the ground, sink said section by excavating from within, lift the forms and cast a second section in situ, sink the shaft again, and generally continue the process until a desired depth is reached. This method entails ineiiicieut manpower distribution, since during excavation, usually carried out by a mechanical excavator, the concrete crew remains idle, while during placing of steel for the new section the excavator crew remains idle.

Moreover, the presence of a plurality of construction joints across the shaft prohibits the use of the tensional resistance of concrete, which, if properly exploited, can furnish a stronger and more coordinated structure, capable of resisting better the stresses developed during sinking.

Again, as the shaft goes down, skin friction becomes progressively greater and retards sinking considerably. One common way of causing then the shaft to sink, is by prolonged excavation from within, below the shafts lower extremity, inducing material right around the shaft to slide down and into the hollow formed, thus relieving lateral pressure. This practice is very undesirable because earth subsidence is never uniform and may force the shaft substantially out of alignment, placing same in a laterally unsymmetrical loading condition.

lt is the object of the invention to overcome the aforesaid diiiiculties and to provide a shaft which can be sunk in earth economically, expeditiously, and effectively, through the method and means herein employed.

A further object of the inventionI is the abstraction of relatively large capacities of fluids from shallow aquifers.

A still further object of the invention is the development of naturally graduated coarse and highly permeable material right around the shaft, through which water may flow at a greatly reduced velocity, Without entrainment of line grains.

A still further object of the invention is to provide method and means for removing all line particles from the space around the shafts outer surface, thereby creating an annular layer of coarse material and reducing lateral pressure through greater arch action and less shearing resistance in this material, as explained hereinbelow.

In order that the scope of the invention be fully understood or appreciated, a description of the actions that accompany shaft sinking is given here:

When an open shaft is sunk into earth by the process of excavation from within, the force of gravity tends to cause the shaft to sink, while the bearing reaction at the bottom and the frictional resistance laterally oppose this movement. During the early stages of sinking, the frictional resistance is relatively small, since, due to the presence of some clay or silt in the surrounding earth mass (even when this is composed of sand-gravel), the force of cohesion with the assistance of shearing resistance and arch action, holds the mass together and prevents it from moving radially toward the shaft. And as is well known from Earth Pressure Theory, no lateral pressure can develop in an old undisturbed earth mass, unless actual movement is manifested, and if such movement is realized, said mass breaking away and sliding down on a surface called the rupture surface. In shaft sinking, such a rupture surface may develop under certain peculiar conditions, and for round shafts this surface is theoretically represented by an inverted frustum of a paraboloid, which, for the sake of simplicity, may be replaced by an inverted frustum of a cone, whose small base coincides with the shaft mouth at the bottom (ABBA, Fig. l).

As the shaft sinks deeper into the ground, sand par-.

ticles move closer and closer to the outer shaft-surface and some of them, particularly small particles, lodge tightly into the minutefirregularities of concrete and through shearing resistance from grain to grain support the shaft and transmit its load to the enveloping mass, thus hampering sinking progress materially. To overcome this resistance, excavation is extended far beyond the shaft's lower extremity, and material right around the shaft is caused to slide down into the hollow formed, thus relieving lateral resistance temporarily. The shaft then tends to sink a little easier, but the superimposed earth mass losing support at the bottom and sustaining the entire weight of shaft, cracks and slides down on an inclined rupture surface, as mentioned before, carrying the shaft down with it, which is thus squeezed tighter by the radial component of the earth movement. From this stage on, the shaft is virtually carried do-wn by successive earthmass ruptures and this practice is highly undesirable because the earth movement may force the shaft substantially out of alignment and place same in dangerously unsymmetrical loading conditions.

In foundation shafts, walls may be made very heavy, to overcome frictional resistance, and since the entire space inside the shaft is eventually lilled up with concrete, the structure cannot be considered as uneconomical. However, in shafts where the inside space has a functional scope, walls must be suiciently strong but not excessively heavy, for reasons of economy.

ln putting the invention into effect, the shaft is provided at its lower extremity with a metallic perforated wall, forming an integral part thereof and extending all aroundv the shaft, ush with the outside face thereof. This wall serves the dual purpose of screen and cutting shoe, and is properly reinforced to act as a cutting edge and to withstand all vertical and lateral loads during or after sinking. Placed at the very bottom of the shaft, this perforatedrwall or screen permits the utilization of practically all the available uid head in aquifers, particularly shallow aquifers, by being caused to occupy the lowest part of the duid-bearing formation in the ground. Means are provided to facilitate shaft sinking through removal of fine grains of unconsolidated material from the shafts path and through said perforated wall, thus developing in situ a structure of coarse and highly permeable material right around the shaft.

The shaft proper may be of reinforced concrete either castl in place or precast in the form of annular sections so interconnected as to form a monolithic cylindrical structure, possessing the properties of a flexural member. ln another application, the shaft body is made up of two concentrically arranged sets of precast annular sections, so interconnected as to form a similar monolithic continuous structure. A coherent and integrated structure is thus obtained which may resist more effectively the forces introduced during sinking and which can be erected economically and expeditiously.

ln the accompanying drawings, wherein certain embodiments of the invention are illustrated:

Figure 1 is a vertical` section of the shaft through its axis of symmetry.

Figure 2 is a part vertical section of the shaft wall at the bottom, showing one type of perforated wall.

Figure 3 is a part vertical section of the shaft Wall at the bottom, showing another type of perforated wall.

Figure 4 is a part sectional plan of perforated wall, on line 4 4, Figure 2.

Figure 5 is a part sectional plan of perforated wall, on line 5 5, Figure 3.

Figure 6 is a part sectional plan of the lower part of the shaft on line 6 6, Figure 3, illustrating anchorage details of the perforated wall.

Figure 7 is a part vertical section of the shaft wall, illustrating one type of precast shaft construction.

Figure 8 is a part vertical section of the shaft wall, illustrating another type of precast construction.

Figure 9 is a part sectional plan of the shaft wall on line 9 9, Figure 7.

Figure 10 is a part sectional plan of the shaft wall on line 10 10, Figure 8.

In Figures 2 and 4, the perforated wall 2 is shown in its simplest form, being made of heavy metal plate rolled to the outside diameter of shaft 1, as shown, and anchored to the concrete by means of a plurality of anchors 3, arranged uniformly all around the periphery of the shaft and embedded into the fresh concrete. These anchors are welded solidly or otherwise secured to the metallic wall 2 and have perforations 4 sufficiently large to admit the passage of rods 5, which reinforce the shafts lower part and which are connected together and to the upper circular rods by means of rods 5a. The shape, size, and number of perforations in wall 2 may be such that these will readily permit the how of tine material therethrough and any arrangement which effectively performs the desired function may be employed. Perforations are either drilled or otherwise cut-in by means of any conventional method.

ln another construction, Figures 3, 5, and 6, the perforated wall 6 is made of lighter metal plate, extending all around the shaft, and is properly reinforced to resist the vertical and lateral loads during sinking. The plate is punched at and rolled to the outside curvature of the shaft diameter, as shown. According to one arrangement, the wall is reinforced vertically by means of a plurality of structural members 7, uniformly disposed all around the shafts periphery and welded solidly, or otherwise secured to the inside face of the perforated wall. These members extend well into the concrete of the shafts lower part and are anchored therein solidly. They are provided with perforations 4 sufciently large to admit the passage of rods 5 which, as in Figure 2, reinforce the shafts lower part and which are connected together and to the upper circular rods by means of rods 5a. The plate may be reinforced at its edge by means of curved channel 8 and curved angle-iron 9, for instance, solidly welded, or otherwise secured to the plates inside face circumferentially. Members 7 are preferably shaped at the bottom, as shown, and are solidly welded to channel 8.

One advantage of this construction is that bearing resistance at the shafts bottom is minimized, since the bearing edge is slender and can cut in and penetrate into the unconsolidated mass much easier than a conventional heavy shoe.

Referring now to Figure l, wherein the top of the earth mass is shown by line A A and the ground water table by line 10-10, and wherein the shaft 1 is shown in sinking position, rupture of the earth mass is prevented by maintaining excavation at about the level B B, near the shafts bottom. The light agitation produced by the excavators bucket is normally sufficient to cause fine material from the outside to run through the apertures of the metallic wall 6 and to be deposited within the shaft. Material larger than the aperture size is stopped at the wall and as the shaft sinks down, it is caused to roll over it or, if gripped tightly in position, to offer fric- 4 tional resistance but practically no shearing resistance. As material is excavated from the bottom and bearing is reduced, the force of gravity causes the shaft to sink further down, while agitation of the water mass at the shafts bottom causes fine material from around the shaft to run into it.

In special conditions where the material is more packed than usual, the bucket may be worked up and down several times in lowered position to increase agitation. Or, air or water may be forced vertically through the shaft wall and down to the perforated wall 6. In especially packed material, the entire water mass may be made to move up and down within the shaft and in and out through the perforated wall and bottom, by forcing the water level inside the shaft to fluctuate up and down, until the formation is loosened and the fines are `washed into the shaft. One method of accomplishing this result is through the provision of an airtight removable cover 11, bolted or otherwise securely mounted on the shafts top, and by forcing air under pressure through perforation 12 driving the water out into the aquifer through perforated wall 6 and the bottom, until the level in the shaft assumes a new preassigned position 13 13. Air pressure is then released by opening valve 14, for instance, and letting air discharge in the open through opening 15. This sudden air pressure reduction causes the water to rise suddenly to its original level 10-10, being acted upon by the static difference 10-10 to 13 13, and to oscillate up and down until equilibrium. The surge thus produced causes a violent reversal of flow, from the aquifer to the shafts interior, partly through the perforated wall and partly through the bottom, entraining with it ne material, and loosening the formation both at the metallic wall and the bottom. Loosening of the formation at the bottom is essential because it facilitates excavation and relieves bearing resistance, which would have been appreciable if the material re mained packed. The process is repeated a number of times until all fines are collected into the shaft. The cover is then removed and excavation resumed. ln cases where application of air under pressure is required constantly, the cover may be provided with an opening and a removable airtight gate (not shown) which may be large enough to permit the passage of the excavator bucket through it and which can be opened and closed with no delay. Or, a smaller opening and removable airtight lid may be provided through which an airlift or similar pump may be lowered to the shaft bottom to remove the collected fine material.

The above described process may also bc applied to develop a coarse and highly permeable aquifer structure around the perforated wall. In this case, the process is generally applied when the shaft reaches final position, but it may be started during sinking and maintained after the shaft reaches filial position in the formation.

Functionally, the above construction serves a very useful purpose in that it provides apparatus which can be used for abstracting relatively large capacities of water from shallow aquifers. Ordinary shafts with no screen at the bottom have been used but due to lack of suf ticient waterway area between the shafts bottom edge and the impervious rock formation and consequent high infiltration velocities, they soon clog and become useless. On the other hand, ordinary wells are highly inefficient because drawdowns are usually limited to the screen top, it being considered unsafe to draw the water level below this point. In the apparatus covered by the invention, the perforated wall or screen occupies the lowest possible position in the formation and being provided with an amply large but low screen, it may yield large capacities while maintaining low and safe Water infiltration velocities. Moreover, the formation of a coarse and highly permeable aquifer structure around the screen and shaft which is naturally graduated, prevents clogging or sanding and facilitates Vgreatly the flow of water into the shaft.

To make a better use of manpower in the field, the shaft superstructure can` be made of precast annular members, so that when the shaft is being sunk, the concrete crew can be engaged inpreparing new precast sections. Generally, the perforated wall 6 is anchored into a castin-place shaft section 16, Figures 7 and 8, which is heavily reinforced. A plurality of vertical rods 17, uniformly disposed around the shaft periphery and anchored into section 16, are provided which extend above the top of section 16 and as building continues, they are further extended upward either by splicing or otherwise.

In Figures 7 and 9, precast annular sections 18 are properly reinforced and are similar in design, shape, and size, with wall thickness same as that of section 16 and weight manageable by available equipment. They are laid one on top of the other with cemented joints and they are provided with a plurality of holes 19, so spaced and sufficiently large as to permit vertical rods 17 to pass through, and to be grouted therein under pressure. As the shaft sinks, new sections are added and rods 17 are extended upward and grouted in position in holes 19 Figure 8 illustrates another type of precast shaft which is composed of cast-in-place section 16 and precast

annular sections

20, 21, and 22. The outside face of the outer precast sections 20 is made ush with the outside face of section 16, while the inside face of the inner

precast sections

21 and 22 is preferably made flush with the inside face of section 16, but not necessarily so. The annular space 23 is made large enough to permit the passage of rods 17 through and upward and to receive grout or mortar. The inside face of sections 20 and the outside face of

sections

21 and 22 are made especially rough, or they may be provided with one, two, or more circumferential recesses (not shown), in order to increase the bond between the concrete of the precast sections and the grout or mortar. The first inner section 22 is built with one-half the height of the other sections 21 in order to cause construction joints between sections 21 to fall midway opposite sections 2t) and construction joints between sections 20 to fall midway opposite sections 21. Precast sections 20 are provided with circular reinforcing bars 24 and sections 21 and22 with similar bars 25. No vertical bars are provided in these precast sections and bars 24 and 25 may be secured in position by thin wire, being suspended from the top, for instance. The object of this construction is to insure an uncracked concrete section in the vertical sense and to fully develop the tensional resistance of concrete in the same sense. Had Vertical rods been provided, cracks would have been developed in the Vertical sense due to concrete shrinkage during setting.

This arrangement furnishes a coherent and integrated structure which possesses the properties of a non-sectional monolithic shaft. Full advantage of the tensional resist ance of concrete can be taken, and if, for instance, the shaft is hung or subjected to beam action during sinking, the wall has the tendency to fail along, or close to, a plane 26-26 or 26a-26a. Due to complete bond between grout or mortar fill and concrete, slippage of the individual sections is prevented and the full tensional load is thrown on the uncracked section along said plane 26-26 or 26a-26a, which due to substantial wall thickness, possesses great tensional resistance. If, however, the applied load is greater than the concrete resistance and the wall fails in tension, the load will automatically be thrown on reinforcing bars 17, which would thus hold the cracked structure together.

In ordinary shaft construction, tensional resistance of concrete cannot be counted on, because of horizontal joints extending right across the shaft and because the vertical reinforcement prevents the concrete to shrink uniformly, thus inducing numerous cracks, particularly around said reinforcement.

I claim:

l. A substantially vertical double wall segmental shaft, sunk into earth as a unit of uniform wall thickness, comprising two concentrically arranged and spaced sets of precast concrete tubular members, one set Within the other, cementing means lling the annular space between the concentric sets of said tubular members to integrate said sets so as to form a monolithic hollow shaft sinkable underground, the members of each set being aligned and in end contact with each adjacent member with the lines of contact between the tubular members of the outer set being in staggered relation to the lines of contact of the tubular members of the inner set, said shaft having at the bottom termination a cast-in-place reinforced concrete tubular section of the same thickness and being connected, continuous and flush With said shaft.

2. A substantially vertical double wall segmental shaft, sunk into earth as a unit of uniform wall thickness, comprising two concentrically arranged and spaced sets of precast concrete tubular members, one set within the other, cementing and reinforcing means filling the annular space between the concentric sets of said tubular members to integrate said sets so as to form a monolithic hollow shaft si'nkable underground, the members of each set being aligned and in end contact with each adjacent member with the lines of contact between the tubular members of the outer set being in staggered relation to the lines of contact of the tubular members of the inner set, said shaft having at the bottom termination a castin-place reinforced concrete tubular section of the same thickness and being connected, continuous and ush with said shaft, the said bottom termination having a cutting edge.

3. A method of underground shaft construction, consisting in casting a section of this shaft on the ground having projecting reinforcing rods therein, excavating earth from within and sinking same, adding on top and on each side of said rods two concentrically spaced precast concrete tubular members dissimilar in height to maintain uniform shaft diameter and uniform wall thickness, filling the annular space containing said rods and between the outer and inner tubular members with cement mortar, sinking the shaft through an excavation from within, adding on top another two concentrically spaced precast concrete tubular members similar in height so as to maintain said uniform shaft diameter and wall thickness and to create a staggered relationship between the joints of the outer tubular members and the joints of the inner tubular members, filling the annular space between the outer and inner tubular members with cement mortar, sinking the shaft through the excavation from within and repeating the process of shaft building and sinking through superposition of concentrically spaced precast tubular members similar in height and excavating from within until the shaft bottom reaches a predetermined depth in the ground.

References Cited in the file of this patent UNITED STATES PATENTS 255,664 Pettingill Mar. 28, 1882 460,545 Wolf Sept. 29, 1891 657,951 Mooney Sept. 18, 1900 932,261 Flynn Aug. 24, 1909 933,776 Moran et al. Sept. 14, 1909 1,038,973 Schnyder Sept. 17, 1912 1,488,662 Cater Apr. 1, 1924 1,645,719 Paulsen Oct. 18, 1927 1,682,697 Thorpe et al. Aug. 28, 1928 1,709,222 Lawlor et al Apr. 16, 1929 1,710,471 Cater Apr. 23, 1929 1,907,943 Fitzpatrick May 9, 1933 FOREIGN PATENTS 173,767 Germany 1906 744,431 France Apr. 20, 1933