Shaft construction method

Abstract: A method of constructing shafts and continuous walls quickly and efficiently. The construction process consists of one step in which air and a ground hardener are injected by a jetting pipe element (2) to form a column-shaped region of soil mixed with hardener (31); and another step in which a hardening inhibitor is injected into the soil-hardener mixture region (31) by the jetting element (2), the shaft then being formed by excavating the portion of the ground (32) that has not been hardened because of the presence of the inhibitor.

Description: The present invention relates to a method of constructing a shaft. When a scheduled building construction site has poor ground, the ground is generally given an improvement treatment prior to construction. Such ground treatment usually consists of hardening the soil by permeating it with a hardening agent. One method that has come into extensive use in recent years is an injection mixing method in which air and hardener are forced into the ground under high pressure to form a pile-shaped solid mass. Apparatus for implementing such an injection mixing method consists of a high-pressure pump for pumping the cement or the like that constitutes the hardener; a compressor to supply the air; and a machine to operate a double-walled boring pipe the tip of which is provided with a nozzle. The high-pressure pump is usually a plunger-type pump, which is used to pump the hardener from a mixer under very high pressure. The machine mixes the air and hardener and injects it into the ground from the nozzle of the double-walled boring pipe. For this, the boring pipe is inserted into a borehole in the ground, and for the injection process is revolved while being raised up from the bottom part of the hole. A columnar mixed region of soil and hardener is thereby formed in the ground, which the setting of the hardener forms into a pile-shaped solid mass. The injection mixing method is widely employed because it provides good operational efficiency. However, the diameter of the columnar mixed region is dependent on soil condition and pump performance, and in sandy soil or clay the standard effective diameter is limited to 2000 ±200 mm. Figure 6 illustrates a method of the prior art generally employed to construct a shaft having an inside diameter of 3500 mm, as shown in Figure 5, using an injection mixing method. The double-walled boring pipe of the operating machine is inserted into the bottom 101 of the shaft 100 in sequence from a1 to a4 to inject the hardener and form a columnar mixed region 102. The double-walled boring pipe is then inserted from b1 to b10 around the region 102 to form columnar mixed regions 103. After the hardener has set the inner portion of the mixed region 102 is excavated to thereby produce the shaft 100 having the prescribed diameter. With the conventional shaft construction method described above, the operation of removing the portion indicated in the drawing by shading to produce the requisite diameter D is rendered difficult owing to the fact that the hardener has set, which has kept operating efficiency down. In addition, the need to form numerous columnar mixed regions 102 and 103 requires much time. Another shaft construction method, which corresponds to the pre-characterising part of claim 1, is disclosed in JP-A-58-173216. According to the present invention, there is provided a shaft construction method which comprises injecting air and a ground hardener by the depthwise operation of revolving jetting means to form a columnar mixed region of soil and hardener in the ground, characterised in that the method further comprises: a step in which an inhibitor that inhibits the hardening of the hardener is injected inside the columnar mixed region, by the depthwise operation of the jetting means, the injection pressure being set to produce a portion of the columnar mixed region, hardening of which is inhibited by the inhibitor, having a diameter that is smaller than that of the columnar mixed region; and the removal of said portion of the columnar mixed region the hardening of which was inhibited by the inhibitor. Preferably, said inhibitor is injected inside the columnar mixed region before that region has set to a specified strength. Said revolving jetting means may comprise a revolving operating pipe that is operated to inject said ground hardener from a jetting element of the jetting means provided on the side of the operating pipe to form said columnar mixed region of soil and hardener in the ground, the method being such that a bottom part of said region is not injected with said inhibitor, to provide at the bottom of the shaft a hardened portion of said region of a set thickness. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a drawing showing an overall view of apparatus for performing a shaft construction method according to an example of the present invention; Figure 2 is a cross-sectional view of a high-pressure pump employed in the apparatus; Figure 3 and Figures 4a to 4f are cross-sectional views illustrating the steps of the example of a shaft construction method according to this invention; Figure 5 is a cross-sectional view of a shaft; and Figure 6 is a general view illustrating a conventional shaft construction method of the prior art. Referring to Figure 1, reference numeral 1 denotes a machine for raising/lowering and revolving a double-walled boring pipe 2; reference numeral 3 denotes a high-pressure pump for pumping hardener mixed by a mixer 4 and an agitator 5; 6 is a hydraulic unit that drives the high-pressure pump 3; and 7 is a compressor for mixing air in with the hardener pumped by the high-pressure pump 3. A pump (not shown) is used to supply the mixer 4 with water from a water tank 8. The end of the double-walled boring pipe 2 of the machine 1 is provided with a nozzle (not shown) on the side of the pipe from which hardener pumped by pump 3 is injected into the ground. The machine 1 is equipped with a drive section for general rotation of the double-walled boring pipe 2 and a drive section for rotating the double-walled boring pipe 2 within a prescribed range, and is provided with a switch-over device for selecting the drive. The drive sections rotate the double-walled boring pipe 2 by means of a rack and pinion arrangement, for example, with the pinion imparting a reciprocating action to the rack. As one example, a projecting portion is provided at each end of the rack to operate a microswitch, for example, and thereby change the direction of rotation of the boring pipe 2. The degree of rotation of the double-walled boring pipe 2 can be controlled by shifting the position of the projecting portions on the rack, thereby changing the position at which the microswitches are operated. An example of the high-pressure pump 3 will now be described with reference to Figure 2. The high-pressure pump 3 comprises a valve box 10 provided with a valve chamber 9, a plunger box 12 provided with a plunger 11, and a box 13 forming a pressure action chamber 13a disposed between the valve box 10 and the plunger box 12. The valve box 10 has an inlet passage 14 and an outlet passage 15 that communicate with the valve chamber 9 and which are provided with an inlet valve 16 and an outlet valve 17, respectively. The inlet valve 16 and the outlet valve 17 each have a seat 19 having a seat surface formed into a concave, semi-spherical shape and a multiplicity of small holes 18 that extend axially from the concave valve seat; a valve-piece 20 that has a spherical shape corresponding to the said concave valve seat; and a valve spring 21 that urges the valve-piece 20 against the seat 19. The holes 18 are for limiting the in-flow of particles that exceed a given size. In the side wall 22 of the valve box 10 is a passage 23 that connects the pressure-action chamber 13a with the interior of the valve chamber 9, and provided at the opening of the passage at the pressure-action chamber 13a end is a screening member 24 constituted of a mesh screen or the like that limits the entry of particles that exceed a given size. The end of the plunger 11, which is maintained within a cylinder 25 in the plunger box 12 via a V-packing 26 so as to project into the pressure action chamber 13a, is reciprocated at high speed by a drive means (not illustrated). A resilient membrane 27 is provided in the pressure-action chamber 13a to divide the pressure-action chamber 13a into a cylinder 25 side A and a valve chamber 9 side B. The cylinder 25 side A of the resilient membrane 27 is filled with an operating medium 28, such as oil. Construction of a shaft, such as the one shown in Figure 3, using the above apparatus will now be described. The double-walled boring pipe 2 of the machine 1 is positioned at the location where the shaft is to be constructed. The boring pipe is then used to bore a hole to the required depth, using an appropriate pipe rotation and boring rate for the conditions of the ground concerned (Figure 4a). The double-walled boring pipe 2 uses high-pressure jetting from its nozzle to bore the hole, penetrating into the ground 29 under its own weight. Alternatively, the boring pipe 2 may be inserted after the hole has been bored by another means. After the hole has been bored to the required depth, the revolving boring pipe 2 is then withdrawn up the borehole 30 (figure 4b) while the high-pressure pump 3 is operated to inject hardener from the nozzle. The rate at which the boring pipe 2 is withdrawn up the borehole and the rate at which the pipe is rotated are set in accordance with the nature and softness of the ground concerned. The hardener consists of cement such as Portland cement, for example, as the main constituent and an admixture of a water-reducing agent such as montmorillonite or calcium, suitably combined with a cement based ground improvement agent, with the proportions of the constituent components being changed to suit the ground concerned, and mixed with water. The injection of the hardener breaks up the texture of the soil ground and forms a columnar pile 31 in the ground, such as shown in Figure 4c. The double-walled boring pipe 2 is then inserted in the centre of the pile 31. This insertion of the boring pipe 2 is preferably done after the pile 31 has hardened to some extent. For example, the insertion is started after the initial hardening of the cement. The boring pipe 2 is inserted to a depth that is slightly above the depth to which it was inserted to inject the hardener. The high-pressure pump 3 is then operated to pump in an inhibitor that inhibits the setting of the hardener. The main constituent of the hardener being cement, which is alkaline, an acid inhibitor is employed. The pump delivery rate for pumping the inhibitor is set below the rate used to inject the hardener. As it revolves, the double-walled boring pipe 2 is gradually raised up from the bottom part of the pile 31 while the injection of the inhibitor from the nozzle proceeds (Figure 4d), whereby the inhibitor forms a pile 32 within the pile 31 (Figure 4e). Thus, the hardener is inhibited from setting at the inner part of the pile 31 and only sets at the outer and lower part 33. This inner part which does not harden owing to the action of the inhibitor can then be easily excavated to thereby form the required shaft (Figure 4f). The above example of the present invention enables a large-diameter shaft to be produced simply by removing soil from an inner portion, which considerably shortens the required construction time, compared with the conventional method of shaft construction by injection mixing. It is also economical, because by just changing the pump delivery rate the same nozzle can be used for the injecting of the inhibitor as well as the hardener. Also, as the setting of the hardener is inhibited in the portion injected with the inhibitor, it is easy to excavate.

Claims: A shaft construction method which comprises injecting air and a ground hardener by the depthwise operation of revolving jetting means (2) to form a columnar mixed region (31) of soil and hardener in the ground, characterised in that the method further comprises: a step in which an inhibitor that inhibits the hardening of the hardener is injected inside the columnar mixed region (31), by the depthwise operation of the jetting means (2), the injection pressure being set to produce a portion (32) of the columnar mixed region (31), hardening of which is inhibited by the inhibitor, having a diameter that is smaller than that of the columnar mixed region; and the removal of said portion (32) of the columnar mixed region (31) the hardening of which was inhibited by the inhibitor. A method according to claim 1, characterised in that said inhibitor is injected inside the columnar mixed region (31) before that region has hardened to a specified strength. A shaft construction method according to claim 1 or 2, characterised in that said revolving jetting means (2) comprises a revolving operating pipe that is operated to inject said ground hardener from a jetting element of the jetting means provided on the side of the operating pipe to form said columnar mixed region (31) of soil and hardener in the ground, the method being such that a bottom part of said region (31) is not injected with said inhibitor, to provide at the bottom of the shaft a hardened portion of said region (31) of a set thickness.

Inventors: KOIWA YOSHINOBU, KOIWA, YOSHINOBU
Applicants: FUJIMORI SHUICHI, KELBIN CO LTD, KOIWA YOSHINOBU, LITTLE ROCK LTD, FUJIMORI, SHUICHI, KABUSHIKI KAISHA LITTLE ROCK, KELBIN CO., LTD., KOIWA, YOSHINOBU
Application year: 1989
Main classification: E02D3
IPC codes: E02D5, E02D3