The invention relates to improvements in a ground, and possibly thereover, especially a weak ground such as a clay.

Various types of foundations are already known for erecting facilities, buildings, supermarkets or for obtaining an embankment for a highway, for example.

It is especially known to dispose piles in the ground above which the building, or the road, is erected.

However, it is then traditionally required to dispose a network of beams made of a « traditional reinforced concrete » for supporting the floor of the building, or the road.

If the piles are disposed in a mechanically weak ground, the strength induced by the building is typically essentially transmitted by the piles to a « harder » portion of the ground. Thus, substantially no charge of the building is transmitted to the weak ground.

It is to be noted that, in the present description, a ground is considered as « weak » if its modulus of elasticity (E) is less than about 80 to 90 bars.

A series of so-called « ballasted columns) » is sometimes used in such a situation.

Typically, such « ballasted columns » are wells digged into the ground and having a diameter between about 0.5 m and 1.20 m. The wells are filled with stones or a mixture of sand and gravels having dimensions typically comprised between 10 mm and 60 mm.

In a ground which is too weak (modulus of elasticity of the ground less than about 50 bars), such « columns » often break off, if a quite high building is erected thereon.

Embedding « rigid inclusions » within the ground is another solution.

Such « rigid inclusions » are piles having a small diameter. The distance between two piles is typically between about 1.50 m and 2.50 m.

The piles are either « micropiles » including a metallic tube, or full piles made of a traditional concrete.

But the stiffness is then typically too high.

The equivalent stiffness of a network comprising such « rigid inclusions » is very high. Then, more than 95 % of the charge is transmitted to the piles, and only 1 % is transmitted to the « weak » ground.

So, an object of the invention is to dispose in the ground « inclusions » having a modulus of elasticity (E) substantially equal to the modulus of elasticity of the so-called « ballasted columns », but which do not weak off so often.

Another object of the invention is to offer an improved concrete material having a modulus of elasticity (E) less than one of a « traditional concrete », so that a structure made of such an improved concrete does not systematically concentrate all the strengths in the ground.

In accordance with the invention, the foregoing advantages have been achieved through an improved concrete consisting of an aggregate which comprises, in dry weight and for obtaining 1 m3 of aggregate: -between substantially 100 kg and 400 kg of cement and/or lime, -between substantially 10 kg and 50 kg of an elastic substance, -between substantially 500 kg and 1,800 kg of flying ashes and/or sand.

With such a dry powder, it should be possible to obtain a concrete having a long term modulus of elasticity (E) comprised between about 500 bars and 2 000 bars, for a pressure comprised between substantially 20 bars and 120 bars.

Traditionally, the modulus of elasticity (E) and the strength o, of a concrete are defined as follows: E = 7000 oj (oj is the strength of the concrete j days after having made it).

Preferably, the abovementioned « elastical substance » is obtained from a powder of rubber, and the granulometry of said substance is preferably comprised between about 0.08 mm and 2.5 mm, while preferably including between about 30 % to 70 % of grains (or granulars) comprised between 0.5 mm and 1.8 mm, and about 70 % to 30 % of grains comprised between 0.15 mm and 0.5 mm.

Of course, a binding agent (typically water) will be used for binding the mixture and obtaining a ready-to be used concrete.

Another object of the invention is an improved pile incorporating the abovementioned concrete having a low modulus of elasticity and adapted to be disposed especially in a weak ground.

Advantageously, the pile will be a hollow tube incorporating said concrete of low modulus of elasticity.

Another object of the invention is an assembly for supporting a structure or a construction to be built on such a weak ground.

According to the invention, the assembly comprises : -a series of improved piles, as abovementioned, -the piles being « disposed » in at least a portion of the ground, below the structure, so that a distance of about two to ten times (preferably three to five times) their outer diameter separates two adjacent piles.

The method the invention refers to, teaches to dispose each pile of the above series in the ground, as follows: -a hollow drill is forced (typically screwed) in the ground for lowering it in said ground and digging therein a shape corresponding to the shape of an individual pile, while pushing the ground material forced (or pressed back) by the drill, into the surrounding ground,

-then, the drill is brought up to the ground surface again (typically by unscrewing it), and the space in the ground, released by (unscrewing) the drill and forcing the ground material, is provided with the concrete of the invention, through the drill.

According to a further feature of the invention, the drill is preferably annular for obtaining a concrete annular pile.

A detailed description follows, with reference to accompanying drawings in which: Figure 1 diagrammatically shows a section of ground including piles for erecting a structure on the ground, Figure 2 is a view from the above of the ground illustrated in Figure 1, Figure 3 is a diagrammatic view of a first embodiment of a drill adapted to be used according to the invention, Figure 4 is an external view of the second embodiment of a drill, and Figure 5 is a longitudinal section of the drill illustrated in Figure 4.

As abovementioned, the invention relates to an improved concrete having a low modulus of elasticity « Using an elastic, or rubbery, material to be incorporated in a traditional concrete, reduces the modulus of elasticity of such a traditional concrete, without notably reducing the strength of the resulting concrete.

Thus, incorporating such a rubbery material in a traditional concrete will induce a long term modulus of elasticity « E » comprised between 800 bars and 1 500 bars for a mechanical strength comprised between 40 bars and 80 bars.

The above-disclosed concrete is adapted for obtaining such a result.

According to a specific embodiment, said specific concrete can be composed as follows (it is to be noted that the following composition is for obtaining 1 m3 of powdered concrete, excluding water). So, the composition in dry weight is as follows: 1°) Including incorporated flying ashes: -flying ashes: 600kg to 1,200 kg -cement (and/or lime): 150 kg to 280 kg -powder of rubber: 10 kg to 50 kg 2°) Incorporating a powder of sand: -sand (having a granulation of about 0.8 mm to 4 mm): 800 kg to 1,600 kg, -cement (and/or lime): 150 kg to 300 kg -powder of synthetic rubber: 10 kg to 50 kg For obtaining the abovementioned results, the granulation of the rubber will be comprised between 0.15 mm and 1.8 mm, including 40 % to 60 % of granulates comprised between 0.5 mm and 1.8 mm and 60 % to 40 % of granulates comprised between 0.15 mm and 0.5 mm.

Any other material having an elasticity comparable with the elasticity of the rubber could be used.

Whatever it may be, such an « elastical concrete » can be especially used for erecting piles adapted for supporting a structure to be built over a ground, and especially a ground having a weak mechanical strength, such as a weak clay.

According to the invention, a ground is considered as « weak » if its modulus of elasticity is less than about 80 to 90 bars (and typically comprised between 5 and 80 bars).

In such a weak ground referenced as 1 in Figure 1, supporting piles 3 made of the so-called « low modulus concrete » are vertically disposed.

As illustrated in Figure 2, the piles 3 are regularly distributed in the ground 1 and the distance e between two piles is of about 1.5 m to 2.5 m.

The piles 3 are preferably cylindrical and have a circular section.

The external diameter dl of the piles is preferably comprised between 20 cm and 60 cm and the piles are distributed in the ground so that the distance e therebetween is substantially three to five times greater than the external diameter dl.

Above the mesh of piles 3, a structure 4 is to be erected.

Another feature of the invention relates to the drill used for disposing the piles in the ground, such a drill being adapted to be forced into said ground to be reinforced.

As illustrated in Figure 3, a « full » drill 5 can be used (viz. not annular).

The drill 5 is a cylindrical tube having an axial helical rib 7 around its outer surface 5a.

The bottom end 5b of the drill is provided with a movable trap 9.

The top end 5c of the drill is connected to a series of tubes 11 connected end to end for controlling the screwing of the drill along its longitudinal axis 13.

Operating means (not illustrated) are connected to the series of tubes 11 for moving the tubes and the drill around the axis 13.

Typically, the diameter d2 of the drill is larger than the diameter d3 of the tubes 11.

The operation of the drill is preferably as follows : The drill is vertically disposed just above a zone of weak ground to be reinforced.

The drill is screwed into the ground and the tubes 11 are successively connected end to end, while the drill 5 penetrates into the ground.

It is to be noted that the drill 5 does not induce an extraction of (viz. does not expell from the ground) the ground material moved by the

helical rib 7. On the contrary, said « swept back » ground material is forced into the surrounding ground which, thus, is compacted.

At a predetermined depth in the ground 1, below the ground surface, the drill 5 is stopped.

Then, the drill is unscrewed from the ground and the abovementioned « low modulus concrete » is injected into the drill, through the series of hollow tubes 11. The pressure of said concrete which circulates into the inner axial duct 5b of the drill, ejects (or opens) the trap 9 and progressively fills the hollow space liberated by unscrewing the drill.

The diameter of the full pile 3 so obtained is such that dl = du.

However, in many cases, obtaining a hollow pile in the ground is preferable.

Then, an"annular"drill such as illustrated in Figures 4 and 5 (large scale) is recommended.

The drill 15 illustrated in Figures 4 and 5 is a hollow, annular drill.

Such a drill comprises a central hollow tube 17 having a longitudinal axis 19 (generally vertically disposed).

The central tube 17 comprises a bottom end 170 around the outer surface 170a of which is disposed an helical rib (or blade) 23.

At the bottom, the free end of the tube 17 is closed by a plug 25 provided with sharp and hard tooth 26 for digging the ground.

Above the helical rib 23 is disposed a larger helical rib 27 which extends axially up to the top end 270 of the tube 17, therearound.

The pitch Pl of the helical screw 27 is substantially two times larger than the coaxial pitch P2 of the bottom helix 23, and the external diameter D1 of the screw 27 is substantially three times larger than the external diameter D2 of the helix 23. The central hollow tube 17 has a diameter D3 which is substantially half the diameter D2.

One pitch after the top end 270 of the tube 17, the helix 27 is equipped with a peripheral lining element 31 extending longitudinally (parallel to the axis 19) at the periphery of the helical rib 27, and even beyond.

Such a lining closes the outer surface 27a of the helix 27. So it has an helical shape.

For screwing and unscrewing the drill, the outer surface 31a of the lining is provided with an helical extension 35. The extension 35 radially extends (outwardly) the transverse wall of the helix 27 and the two helices (27,35) have consequently the same pitch Pl and are in phase. The diameter D4 of the most external helical blade 35 is about three times D2.

The peripheral helical lining 31 longitudinally extends closer to the helix 23 than the helix 27 does: the helix 27 stops one pitch above the helix 23, whereas the peripheral lining 31 stops around the first upper pitch of the helix 23.

Typically, the helix 27 has two turns, whereas the helical lining 31 and the external helical extension 35 thereof have two turns and a half.

Upwardly, the lining 31 and its helical extension 35 begin at the end of the first turn of the helix 27.

The bottom end of the helix 27 is longitudinally disposed at the middle of the external outward helix 35.

The bottom end of said outward helix 35 is disposed substantially at the end of the first upper pitch of the lower central helix 23.

Further, between the level of the one and a half turn of the helix 27 (the reference is the top of the helix) and its bottom end (end of helix 35), the lining 31 is a double-walled lining comprising an outer wall 330 and an inner wall 340 defining an annular, inner chamber 37 therebetween.

At its bottom free end 310, the helical lining 31 has an opening 39 which is in fluid communication with the annular chamber 37.

In operation, the opening 39 is closed by a trap 41 (Figure 5).

Upwardly (at its top end) the chamber 37 is further in fluid communication with a radial duct 43 extending transversally to a local portion of the helix 27 for connecting the chamber 37 and the inner space 370 of the central tube 17.

So, the chamber 37 is in fluid communication with the central hole 370, through the radial conduit 43.

At the level of the conduit 43, the bottom end of the axial inner space 370 is closed by a transversal plate 45 having a curved shape.

With such an arrangement, the drill 15 has an annular cavity 47 interposed between the inner tube 17 and the outer lining 31.

The diameter of said annular space 47 (the shape of which is an helix) is Dl-D3 (except at the level of the helix 23). The length of the annular space 47 is presently 2.5 x P1. Whereas the top of the annular space 47 is vertically closed by a portion of the helix 27, its bottom is completely opened, as illustrated in Figures 4 or 5.

At the top end 270 thereof, the tube 17 is adapted to be coaxially connected to one tube of the abovementioned tubes 11.

The operation for digging a well in the ground 1 with the drill 15 is preferably as follows.

The drill 15 is vertically screwed into the ground 1.

The larger diameter of the drill (D4) (external diameter of the helix 35) can, for example, be of about 70 cm to 150 cm, while the operative inner diameter of the drill (D1) will then be typically comprised between 55 cm and 140 cm.

The radial length E1 of the helical blade 35 is typically between 6 cm and 10 cm and the radial thickness E2 of the chamber 37 (including the thickness of the wall 330,340) is typically between 5 cm and 15 cm. So, the thickness of the annular space created into the ground 1 by screwing the drill 15 will be of about 5 cm to 15 cm.

At a predetermined depth in the ground, the drill 15 is stopped.

The removal of the drill is operated by unscrewing it While unscrewing the drill, the appropriate concrete, and especially the abovementioned « low modulus concrete » of the invention is injected through the series of hollow tubes 11 and then through the inner space 370 of the tube 17. The pressure of injection directs the concrete in the chamber 37, through the duct 43, as illustrated by the arrows in Figure 5. At the end 310 of the chamber, the concrete pushes the plug 41 and throws into the annular space created by the drill in the ground.

So, said annular space is progressively filled by the concrete from the bottom of the well to the ground level.

It is to be noted that the drill 15 forces the ground material into the ground and does not push it outside, over the ground level.

So, screwing and unscrewing the drill induces a compacting effect in the ground, around the digged well.

Of course, the concrete injected in the hole drilled in the ground according to anyone of the above described embodiments is allowed to harden in said ground and the corresponding pile is so maintained in the ground for supporting any structure built thereon.