US 3106068 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
Oct 8, 1963 F. BECKENBAUER ETAL v METHOD QF ExcAvATING :Fiied April 20, 1959 In venors United States Patent O 3,106,068 METHD F EXCAVATRNG Franz Beckenbauer and Ferdinand ulehla, Sulzbach- Rosenberg Hutte, Germany, assignors to Eisenwerk- Gesellschaft Maximiiianshutte A.G., Sulzbach-Rosenberg Hutte, Germany Filed Apr. 20, 1959, Ser. No. 807,421 Claims. (Cl. 61-4l) When excavating foundations or sinking shafts under compressed air physiological considerations limit the pressures that can be employed to 3 atmospheres gauge, which in exceptional cases may be raisedto 3.5 atmospheres gauge.
Consequently such work cannot be performed beyond a depth of 30 and in special circumstances beyond a maximum of 35 metres below open or ground water level.
When using compressed air in permeable ground in which the ground water level is situated at comparatively great depth the elevated pressure in the working chamber raises the original undisturbed ground water level in the surroundings, that is to say there is a build up in hydrostatic pressure in the neighbourhood of the caisson or shaft.
In an illustrative example which occurred in the St. Anna pit Iat Sulzbach-Rosenberg, this pressure build-up, ywhich is the greater the higher the pressure inside the working chamber, amounted to as much as 1.3 atmospheres gauge (the ground water level being thereby raised from a depth of 43 metres to 30 metres under the surface).
In the cited case it was therefore impossible without special approval to continue work below normal ground water level beyond a depth of 17 metres which represented the available pressure rise of 1.7 ats. gauge which remained after deducting the above 1.3 ats. gauge from the maximum permissible 3 ats. gauge. The methods which were hitherto available of penetrating beyond 35 metres below ground water level under compressed yair consisted in lowering the ground water level by sinking spring Wells in the wider environment of the caisson or shaft. However, lowering the ground water level in this Way is an extremely expensive procedure and its success depends largely upon the grain structure and homogeneity or inhomogeneity of the ground or rock that is thus to be drained.
However, in the cited case at the St. Anna pit, the invention which will be hereafter described permitted penetration to a depth of 60 metres beyond ground water level yunder moderate pressures between 2.2 and 2.5 ats. gauge. Moreover, working conditions remained entirely unchanged whilst sinking the final 16 metres of shaft, work being carried out at the same unvarying pressure o-f 2.2 to 2.5 ats. gauge without greater diiculties being experienced `at increasing depth by break-in of water or in irruption of sand and mud. After having penetrated to a depth of about 60i metres below ground water level, grown rock was won, so that the application of compressed air in the further continuation of work ceased to be necessary. Nevertheless, the invention which will be described would have readily permitted work under the same unchanged conditions to be continued without any trouble. In the worst possible ground conditions the proposed method has in practice proved fully successful.
A homogeneous permeable rock is even better suitable for applying the method.
The method consists in reducing the hydrostatic pressure in the rock surrounding the caisson or shaft by withdrawing water from the rock through the working chamber and discharging it into the free outside atmosphere.
FIGURE 1 is a diagrammatic section of the invention in use;
FIGURES 2 and 3 illustrate on the left the condition prevailing without use of the invention, and on the right the effect of the use of the invention; and
'FIGURE 4 shows a drain pipe suitable for use in the invention.
In practice the method can be performed by flushing drainage pipes into the door as well as possibly into the surrounding rock through the joints or lining of the working chamber and connecting the same inside the pressurised chamber by means of flexible'rein-forced tubing with a closed pipe system which communicates with the free outside atmosphere. This simple system of piping directly discharges the water which the pressurised air forces from the ground into the drainage pipes into the outside atmosphere. lf for reasons connected with the working procedure the height from the bottom of the shaft to beyond the roof of the caisson (i.e. to the free outer atmosphere) should exceed 6` metres, then yan air valve may conveniently be incorporated in the riser pipe of the system to act in the manner of an ejector which draws the water out of the drainage pipes and discharges it above the roof of the caisson.
During the sinking of `a shaft fat Auerbach it was observed that although water entered the shaft at various points up to metres above the door of the shaft, indicating that the water level reached this height (cavities behind the wall of the shaft ywould have interconnected any theoretically existing water tables), the pressure of the water penetrating the floor of the shaft did not exceed 0.6 ats. gauge. This fact was frequently noticed when rocks collapsed or water broke in through the door.
In the application of the herein described method to the work carried out at they St. Anna pit the `following observation was repeatedly made.
During each sinking operation in the last 16 metrespenetration was at the rate of 1.4 metres each time-flowing water brought up material from the centre of soft zones in the door. Drainage pipes were then flushed down into these soft zones and connected by flexible hose to a system of pipes leading to the free outside atmosphere. Within a -very short time the iioor was found to consolidate and the water ceased to well up. Not more than four such drainage pipes were thus hushed into an area (about 16 sq. metres) in the shaft door. Whereas welling up of the shaft oor and irruptions of mud had repeatedly occurred at much lesser depths when the described method had not been employed, work proceeded according to plan and without difficulties from the moment the novel procedure had been put into operation. The drainage pipes had :a diameter of 11/2l with a filter end 1.60 rn. in length provided with 'a woven mesh No. 12 surmounted by a 11/2 solid pipe section which was likewise 1.60 metres long. When such a drainage pipe which thus had an overall length of 3.20 metres had been flushed down into the ground it still remained effective `when the shaft floor had been sunk a further 11.40 metres because the filtering surface was then still buried `another 20 cms. below the level of the freshly sun-k floor. Before a further sinking operation began fresh drainage pipes were washed into the ground. As soon as these were in position the old ones were disconnected and drawn.
The greater efficacy of this measure in more permeable, this is to say more sandy rock, is due to the fact that in rock of this type the embedded drainage pipes create a much larger drainage basin, i.e. a basin of much greater volume, within the rock, Iand because a much greater volume of water is removed from the rnore permeable strata (cf. FIGS. 3 and 4). It may be observed that the reduction in hydrostatic pressure around the caisson or shaft in rock or soils having K-values in the order of 10*5 cm./sec. (i.e. strata of relatively low permeability) is not accompanied by a lowering of the ground water level. By :allowing the water to liow out of the rock via the drainage pipes through the closed pipe system' into the free outer atmosphere the potential energy of hydrostatic pressure is converted into the kinetic energy of the flowing water. In coarse sands, gravels, and loose rock, the desired etlect is likewise yachieved by a reduction in the `ground water level.
The above described method, on the one hand, permits the working chamber to be sun'k to a much lgreater depth below .ground water level whist maintaining the usual pressure than was hitherto possible and, on the other hand, the depths of penetration that could in the past be achieved can now be reached with only low pressures in the working chamber. These lower pressures are less likely to have adverse physiological effects and permit longer working times. Moreover, in cases in `which the sinking of caissons causes difficulties due to the ibuoyancy of the air i.e. owing to the high pressures in the working chamber, and high ball-ast loads are needed in sinking, Ithis work can now be greatly facilitated by the reduction in the necessary pressure.
An object of the invention lies in the attainment of a lowering of the hydrostatic pressure at the point of operation without drainage of the soil except at the point `of operation, i.e. without lowering of the ground water table or the water level surrounding the caisson upwardly of the leading edge of the caisson.
The accompanying drawings illustrate the method proposed by the invention.
FIG. 1 shows the shaft wall at a, the ceiling of the pressurised chamber at b; the closed pipe system is indicated by c. A drainage pipe d flushed into the floor of the pressurised -chamber or pneumatic caisson comprises an upper solid pipe section e with a flexible tube f connecting it with the pipe system a. Additional drainage tubes g have been flushed into the ground behind the wall. An
,air injector h is incorporated in the riser pipe of the system which also includes shut-off valves at i.
For a clear understanding it is necessary only to consider that the most important volume to lbe considered is the interior of the caisson. This volume must ybe kept at a pressure that can be sustained by the workmen who dig out the soil. Consider applicants drainage connection and pipe d, e in FIGURES 2 and 3. It will be seen that the water in the soil in advance of caisson a will flow into well point or drainage pipe d and the water pressure in the soil surrounding the solid pipe section e will be reduced. There is, of course, a pressure in the caisson that is above the atmospheric pressure to push water above well point d downwardly, and a pressure in pipe e and f equal to not more than the static head of water from the caisson to the level `of the point at which it can be disposed of by co-nvent-ional pumps. The pressure in pipes e and f may be considerably lower, Iand preferably is lower, since the more rapid removal of -wa-ter will mean that the pressure within the caisson can be reduced.
As seen in FIGURE 1 -at g, g, screened well points n I may be extended from the sides of the shaft wall rz as required to reduce the ow of water downwardly adjacent the wall where the earth has been disturbed, these drainage tubes or well points are not for the purpose of reducing the surrounding water table generally. Points g `will not lbe necessary in all cases but only where the downflow between the wall and the surrounding earth is excessive. The free water lbetween the earth land the wall is then removed but no -attempt is made, or is necessary, to re move Water from the earth far beyond the space between the undisturbed earth and the wall. It will lbe seen, Ithen, that the hydrostatic head of the water surrounding the lower end of the caisson is reduced without particularly iniluencing the water pressure within the undisturbed earth surrounding the caisson above the level of the caisson end.
The structure of the drainage pipe shown lin FIGURE 4 may be any convention-al well point as used for the `so-called dnive wells in which such a point is mounted on the end of Va pipe and ldriven into the earth. Such well points are common articles of commerce and their structure is adequately described by the term well point.
What we claim is:
1. A method of driving shafts or the like using a pneumatic caisson including the steps of projecting at least one well point into the permeable material in the direction in which the shaft is being driven, connecting said well point to -a piping system discharging beyond said caisson at atmospheric pressure, and subjecting the well `point to a vacuum to draw water in the permeable material in the direction away from the caisson, where-by the permeable material `at the caisson is dewatered, and performing an excavating operation to lower the working surface in advance of the caisson.
2. The method of claim 1 in which well points are also projected sideways through the caisson wall into the surrounding permeable materi-al.
3. The method of claim 1, in which an ejector device is inserted in the piping system `and w-ater owing into the well points is lifted to a space beyond the caisson by said ejector.
4. The method of claim 1 in which said well point is driven into the permeable material in the direction in which the shaft is being driven a distance such that the water is removed from the permeable material at least as far beyond the working `surface as the depth of material that is to be removed in one excavating operation.
5. The method lof claim 4, in which upon completion of one excavating operation at least one additional well point is driven in advance of the first said well point and the rst said well point is removed.
References Cited in the le of this patent UNITED STATES PATENTS 962,612 Batten June 28, 1910 989,110 Billings Apr. 1l, 1911 1,010,642 Knorre Dec. 5, 1911 2,126,575 Ranney Aug. 9, 1938 FOREIGN PATENTS 11,902 Great Britain July 3, 1902 388,367 Germany June 17, 1924 264,683 Italy May 7, 1929 504,427 Germany Aug. 4, 1930 698,315 Germany Nov. 7, 1940 733,806 Germany Apr. 2, 1943