The state of art comprises an earth consolidation method according to which only cement grout is injected to obtain reinforcing columns. This method comprises a first step of drilling with an excavating tool a preliminary hole having a diameter of about 60 to 90 mm to a depth equal to the depth of the reinforcing column to be formed, usually between approximately 10 m and 30 m. If the excavating head has to drill layers of compact ground that are difficult to drill, it is necessary to exert an impulsive stress repeatedly on the top of the excavating tool, using a percussion hammer for example, in order to break up the layers of compact ground. The impulsive stress may also be exerted on the excavating head, using a so-called "boring bottom" pneumatic percussion hammer.
If the ground has a medium resistance to drilling, a supply of cleaning water is injected into the hole while the drilling is in progress; said supply of water being interrupted before grout injecting is started. When pneumatic tools are used, the supply of cleaning water may be replaced by a supply of compressed air generated by the tools.
The method comprises a second step of injecting grout into the preliminary hole.
If a boring bottom hammer has been used to drill the preliminary hole, before starting the second step of the method, the excavating tool must be drawn from the hole and the excavating head of the tool replaced with a rotary grout distributing device. The tool is then reinserted in the hole and grout injection is started. The grout is injected via a pair of jets positioned diametrically opposed in the excavating head, the jets having a flow diameter of between 1 and 3 mm.
While the grout it injected, the distributing device is gradually drawn from the hole at a constant translation speed to form a column having a diameter of between 60 and 120 cm.
The cross-section of the column is substantially circular if the earth is homogeneous, but may be irregular, it the earth is not homogeneous.
Another method according to the prior art comprises drilling a hole into the ground using a pneumatic percussion hammer and injecting the grout while the hammer is moving upward. The grout is injected through the same conduits previously used for the compressed air supply.
The apparatus used for this method comprises hollow, rotary drills, equipped at the bottom with excavating heads provided with three-cone or three-blade bits, if a soft ground has to be drilled; however, if a ground comprising rock intrusions or layers of hard material has to be drilled, the excavating heads are equipped with pneumatic boring bottom hammers.
While drilling is in progress, the rotary drills are fed either by a water pump unit or by an air compressor driving the pneumatic hammers.
When the hole has been completed, a grout is injected at a pressure of about 500 bar and at a flow rate of 150 1/min through radially opposed nozzles with a flow diameter of approximately 1 to 3 mm, said nozzles being distributed around the periphery of a grout distributor fitted at the base of the drill; the flow of grout being supplied to the drill via a rotary joint-type manifold with a radial inlet perpendicular to the axis of the drill.
This method however is time-consuming and considerably complicated because regular cleaning and frequent replacement of obstructed nozzles are necessary.
Furthermore, slight deviations of the drilling axis may often occur due to a lack of uniformity of the earth composition, particularly when three-cone or three-blade drilling equipments are used, said deviations resulting in irregular columns with inadequate resistance that are unsuitable to constitute a formation of closely-packed or interpenetrating elements making up, for example, waterproof barriers or surroundings for wells.
With respect to the plant, it is to be noticed that the rotary joint manifold is subject to a stress substantially perpendicular to the axis of the drill which causes an uneven wear of the manifold seals. The uneven wear of the seals allows grout to leak out from the internal chamber of the manifold between the internal rotary section and the external sliding section of the joint manifold, thereby causing the seizure of the joint.
In addition, the use of percussion hammers at the top of the drill requires the use of large supporting pylons, thereby increasing the size of the plant. Furthermore, if pneumatic boring bottom percussion hammers are used, then it is necessary to remove the pneumatic percussion hammer before injecting grout, because the delicate mechanical components of the hammer would be irreparably damaged if they were to come into contact with the consolidating grout. The removal of the hammer causes a considerable waste of time.
In addition, if the earth is not stable around the drilled hole, the insertion of the grout distribution into the hole may be slowed down by earth collapsed into the hole.
Attempts to inject the grout through the same conduits used to supply compressed air to the percussion hammer have failed, mainly because residuals of grout inside the conduits damages the hammer when it is supplied with compressed air, unless the conduits are thoroughly cleaned every time they are used to inject grout, with a considerable reduction in productivity.
As far as the consolidating column is concerned, the substantially circular shape limits the working load thereof. It is true that the irregular shape of the column obtained when the earth is not homogeneous, as well as the presence of large-sized lithoid elements in the column, increases the vertical sliding friction thereof: however the presence of such elements and the dimensions thereof cannot be controlled or predicted.
The techniques previously described are open to considerable improvement in order to avoid the above mentioned problems.