[0001] The invention concerns an improved anchoring element particularly suitable for making
slope consolidation structures, for stabilizing landslips, for making anchoring structures
in various geotechnical applications.
[0002] Anchoring elements are known and widely used, which are constituted by tie rods made
with prestressed steel strands or by solid bars in structural steel that are constrained
in corresponding holes made in the ground through a cement mix.
[0003] Anchoring elements are also known that comprise self-perforating hollow bars generally
consisting of a plurality of tubular elements that are threaded externally and axially
connected one after the other through joining sleeves as they are driven into the
ground.
[0004] The end of the first tubular element is provided with a perforating head and the
tubular element is inserted in the ground through a rotary movement or a rotary percussive
movement to which it is subjected by special power means.
[0005] While the ground is being perforated, a draining fluid is pumped into the bar, wherein
said fluid can be air, water or more commonly a mixture of cement and water, called
cement grout.
[0006] Once the desired perforation depth has been reached, a fluid consisting of water
and cement is pumped in together with additives, if any, and the hardening of the
injected fluid guarantees that the bar is anchored into the ground.
[0007] Anchoring elements of the type described, in particular those using self-perforating
bars, are widely and effectively used in many types of application. With regard, in
particular, to self-perforating bars, each one of them, when set in operation, is
capable of developing a counteracting force that increases as its cross section increases.
[0008] Obviously, even the cost of each bar increases as its cross section increases.
[0009] It is thus absolutely evident that the cost of a containment work depends on the
number and size of the anchoring elements that must be installed, which in turn depend
on the total thrust they have to be able to oppose once they have been laid.
[0010] In particular, the cost of the work increases with substantially linear proportionality
as the cross section and the number of the bars used increase.
[0011] The anchoring element is known which is described in the patent document
WO 2005/090690, and which comprises a percussion device that makes a hole in the ground and during
perforation introduces in the hole a tubular element that is placed on the bottom
thereof, as well as tensioning elements constituted by bars or strands that occupy
the entire length of the tubular element and of the hole.
[0012] A consolidation cement mix is introduced in the hole and the tensioning elements
are set rotating through the rotation of a flange to which the ends of the tensioning
elements projecting from the hole are constrained.
[0013] The limitation of this embodiment lies in that the tubular element does not project
from the hole, since it occupies just one section of its length, and thus the flange
acts exclusively on the tensioning elements to which it is connected. Consequently,
the action of constraint to the ground developed by the anchoring element does not
exploit the synergy produced by the resistance offered by the tensioning elements
and by the tubular element, as the flange acts only on the latter.
[0014] In other words, the presence of the tubular element does not create a synergy with
the tensioning elements to develop the constraining action of the anchoring element.
[0015] The present invention intends to substantially modify the limitations described above.
[0016] It is a first object of the invention to provide an anchoring element that is more
resistant than known anchoring elements having the same cross section.
[0017] It is another object of the invention to provide an anchoring element whose cost
per ton of anchoring element produced is lower compared to equivalent anchoring elements
of the known type.
[0018] It is another object of the invention to provide an anchoring element that is more
ductile than the known anchoring elements.
[0019] It is another yet not the least object of the invention to provide an anchoring element
whose ductility can be modified upon installation.
[0020] The objects described above are achieved by an anchoring element in accordance with
the main claim, to which the reader should refer for the sake of brevity.
[0021] Other details of the invention are described in the dependent claims. Advantageously,
the anchoring element of the invention represents the implementation of the well-known
anchoring technique with self-perforating bars and it can be installed with no need
for the operators to substantially modify the way they work when they install self-perforating
bars of the known type.
[0022] Still advantageously, the higher ductility of the anchoring element or, in other
words, the larger plastic deformation field allowed by it makes it possible to obtain
structures that are safer, especially in case of seismic events.
[0023] To further advantage, said higher ductility or larger plastic deformation field can
be modified during installation.
[0024] The objects and advantages described above will be highlighted in greater detail
in the description of preferred embodiments of the invention that are supplied as
indicative, non-limiting examples with reference to the enclosed drawings, wherein:
- Figure 1 shows the longitudinal cross section of the anchoring element of the invention
when installed;
- Figure 2 shows the longitudinal cross section of a variant embodiment of the anchoring
element of the invention when installed;
- Figure 3 shows a detail of Figures 1 and 2;
- Figure 4 shows a construction variant of the detail shown in Figure 3;
- Figures from 5 to 13 show different steps of the installation of the anchoring element
of the invention;
- Figures from 14 to 17 show the forces acting on the fixing unit of the anchoring element
of the invention when the latter is installed;
- Figure 18 shows, in the same load-elongation diagram, the curves regarding a self-perforating
bar of the known type and the anchoring element of the invention according to an embodiment
of the same;
- Figure 19 shows, in the same load-elongation diagram, the curves regarding a self-perforating
bar of the known type and the anchoring element of the invention according to a variant
embodiment of the same;
- Figure 20 shows, in the same load-elongation diagram, all the curves shown in Figures
18 and 19.
[0025] The anchoring element of the invention is shown in longitudinal cross section and
in the installed configuration in Figure 1, where it is indicated as a whole by
1.
[0026] According to the invention it comprises:
- a tubular bar 2 with mainly longitudinal development driven in the ground S where it is constrained by means of a cement mix C;
- one or more cables 5 or prestressed steel strands arranged inside the bar 2 and constrained by means of the cement mix C present inside said bar 2;
- a fixing unit 6 connected to the ends 5a of the cables 5 and 2a of the bar 2 projecting from the ground S to constrain the anchoring element.
[0027] It should be noted that the word "ground" indicates any means or element into which
the bar
2 is driven.
[0028] Furthermore, it should also be noted that the term "prestressed steel" indicates
a high resistance steel whose characteristics are described in the European standard
EN 10138.
[0029] The tubular bar
2 is provided with an external thread
2b and this makes it possible to make it in the desired length by connecting, through
threaded connection sleeves
15, several tubular elements
7 that are coaxial with each other and arranged one after the other.
[0030] This constructive solution is adopted especially when the tubular bar
2 is very long, since this would make the bar
2 difficult to install and to transport if it were made in single piece.
[0031] It should also be observed that the tubular bar
2 is of the known self-perforating type and is provided with a drill bit
8 arranged at its end that comes in contact with the ground
S into which it is driven.
[0032] Regarding the cables
5 in prestressed steel, they are of the known type, too, and are available on the market
in various versions with a variable number of strands.
[0033] Furthermore, the tubular bar
2 is made of structural steel, the cables
5 are made of prestressed steel and the cement mix
C that constrains the tubular bar
2 into the ground
S and the cables
5 inside the tubular bar
2 is constituted by cement grout which is a mixture of cement and water in variable
percentages according to the project.
[0034] It should be noted that the term "structural steel" indicates a steel for building
applications that is used for metal structures in the field of civil engineering and
has the characteristics described in the European standards EN 10210 and EN 10219.
[0035] In the anchoring element of the invention
1, made according to the embodiment shown in Figure 1, the cement grout
C fills the bar
2 completely, so that the cable or cables
5 is/are completely buried in the cement grout
C and thus once the cement grout has set it/they is/are constrained over its/their
entire length.
[0036] The anchoring element of the invention can be made even according to the embodiment
indicated by
30 in Figure 2, in which the cement grout
C fills only the terminal part of the bar
2 and thus once the cement grout has set the cable or the cables
5 are constrained only for the length
Cb corresponding to the cement grout
C present in said bar.
[0037] It is important to point out that in the two embodiments
1 and
30 the anchoring elements are equal to each other from a constructive point of view,
since they comprise the same bar
2, the same cables
5 and the same fixing unit
6 and differ from each other only for the different constrainment condition of the
cables inside the bar
2 that, as explained above, are completely or partially constrained in the cement grout
C.
[0038] This differentiation is obtained during the installation of the anchoring element
and, as can be better understood from the description of the installation procedure
provided here below, it makes it possible to obtain, at the end of the work, different
conditions of resistance of the anchoring element of the invention, better than those
obtainable with the known self-perforating bars. The fixing unit
6 used can be clearly seen in the detail of Figure 3, where it can be observed that
it comprises a load distribution plate
10 provided with a through hole
10a in which the end
2a of the self-perforating bar
2 is inserted, and a locking nut
11 that is screwed externally at the end
2a of the bar
2 projecting from the load distribution plate
10 and tightened against it.
[0039] There is also a cable or strand locking plate
12 positioned against the locking nut
11 and provided with one or more through holes
12a, in each one of which a cable or strand locking wedge
13 is inserted that locks the end of each strand or cable
5 to the plate
12.
[0040] It should be noted that the words cable, cable locking plate and cable locking wedge
will be used from this point onwards.
[0041] It can be observed in particular that the length
11a of the locking nut
11 exceeds the length
2b of the end
2a of the bar
2 to which it is screwed and which is consequently spaced from the overlying cable
locking plate
12, as can be seen in Figures 1 and 2.
[0042] A variant embodiment of the fixing unit indicated as a whole by
26 is shown in Figure 4 and differs from the embodiment just described and shown in
Figure 3 only in that it is provided with a locking nut
21 whose length
21a is shorter than the length of the end
2a of the bar
2 to which it is tightened and owing to the presence of a spacer ring
23 interposed between the nut
21 and the cable locking plate
12.
[0043] It can be observed that the inner diameter of the spacer ring
23 is larger than the external diameter of the bar
2 and thus can be coupled with it with a radial play.
[0044] Furthermore, its length
23a exceeds the length
2c of the end
2a of the bar
2 that projects from the locking nut
21, which thus remains spaced from the cable locking plate
12, as shown in Figure 4.
[0045] The two embodiments
6 and
26 of the fixing unit are interchangeable, so that it is possible to obtain the anchoring
element of the invention, indicated as a whole by
1 and
30.
[0046] In practice, the installation of the anchoring element
1 is performed according to the sequence represented in Figures from 5 to 13.
[0047] The operations start, as shown in Figure 5, with the arrangement of the first tubular
element
7' that constitutes the self-perforating bar
2 in the vertical insertion position.
[0048] The drill bit
8 is arranged in contact with the ground
S and the opposite end of the tubular element
7' is connected to a mechanical unit, not represented herein, which sets it rotating
or performing a rotary percussive movement as indicated by the arrows, in order to
drive it in the ground
S according to the known art.
[0049] During the perforation of the ground, cement grout
C is injected in the bar
2, so that, according to the known art, said cement grout favours the penetration of
the bar by bringing up to the surface all the material that has been excavated during
perforation.
[0050] After the insertion of the first tubular element
7', as shown in Figure 6, a second tubular element
7" is coupled at its end through a connection sleeve
15 and then the perforation is started again and continued, as shown in Figure 7, until
the bar
2 in the desired length has been completely driven into the ground
S and completely buried in the cement grout
C present both inside and outside the bar.
[0051] Before the cement grout
C starts setting, one or more cables in prestressed steel
5 are buried inside it, as shown in Figure 8, the number and size of said cables depending
on the resistance that the anchoring element must guarantee.
[0052] In particular, the cement grout
C fills the bar
2 completely, in which case the cables
5 are buried therein for their entire length.
[0053] Once the cement grout has set, the cables will be locked in the bar
2 and will be tensioned using the known hydraulic or mechanic tensioning means, as
will be explained in greater detail below.
[0054] At the end of the operations the bar
2 will be anchored to the ground with the cables
5 tensioned and buried in the cement grout
C for their entire length, as shown in Figure 9.
[0055] Alternatively, if once the bar
2 has been driven into the ground some of the cement grout
C contained therein is extracted, the remaining cement grout fills only the terminal
part of the bar
2, up to the height
Cb shown in Figure 7a.
[0056] In this case, the cable or cables
5 that are inserted inside it will be immersed only with their terminal part, for the
length corresponding to the height
Cb of the cement grout, as shown in Figure 8a.
[0057] At the end of the operations the bar
2 will be anchored into the ground and the cables
5 will in turn be anchored inside the bar
2 only for their terminal part, as shown in Figure 9a.
[0058] Independently of the final conditions shown in Figure 9 or 9a, the ends
2a and
5a, respectively of the bar
2 and of the cables
5, must be constrained to the ground
S through one of the anchoring units
6 or
26 previously described. Taking as a reference the bar
2 driven in the ground in the configuration shown in Figure 9a and using the anchoring
unit in the embodiment indicated as a whole by
6, the load distribution plate
10 of the anchoring unit
6 is positioned in contact with the ground
S and then the locking nut
11 is tightened externally to the end
2a of the bar
2 in order to force it against the plate
10, as shown in Figure 10.
[0059] Then the cable locking plate
12 is positioned axially against the locking nut
11, as shown in Figure 11, taking care to ensure that the ends
5a of the cables
5 are arranged so that they pass through the holes
12a present in the plate
12. The cables
5 are tensioned until reaching the desired tensioning values by means of a hydraulic
jack
M that, as shown in Figure 12, is placed in contact with the cable locking plate
12.
[0060] The ends
5a of the cables
5 are then locked in the respective holes
12a through the cable locking wedges
13 that, having a conical profile that matches the conical profile of the respective
holes
12a, lock the cables
5 through radial compression.
[0061] It is evident, as already explained, that the description provided above can be completely
referred also to the use of the fixing unit
26, in which case the spacer ring
23 is superimposed to the locking nut
21 and the cable locking plate
12 is placed against the spacer ring
23 in the configuration shown in Figure 4.
[0062] At the end of installation the anchoring element
30 shown in Figure 2 will thus be obtained.
[0063] At this point, the space inside the bar
2 where there are the cables
5 is filled with cement grout, not shown in the figures, in order to obtain a single
block in which the structural steel of the bar
2 and the prestressed steel of the cables
5 work in synergy.
[0064] Furthermore, the filling with cement grout preserves the metal from corrosion.
[0065] It should be noticed that the term structural steel means a steel of the known type
suited to be used for geotechnical applications and having a low carbon percentage,
while the term prestressed steel means a highly elastic silicon steel of the known
type, used in the field of prefabricated building structures.
[0066] It should also be underlined that both the bar
2 and the cables
5, instead of being made of steel, can be made with other metallic or non metallic materials.
The description provided above can be completely referred to the case where the bar
2 driven in the ground in the configuration of Figure 9 is used, in which case, once
the operations have been completed, the anchoring element
1 shown in Figure 1 will be obtained.
[0067] Advantageously, the anchoring element of the invention bears loads that are considerably
higher than those born by equivalent anchoring elements of the known type, as could
be verified by means of tests that will be illustrated further on in the description.
[0068] For this purpose, it is appropriate to describe the static behaviour of the anchoring
element
1 when installed, and in particular of the fixing unit
6 that constrains its end to the ground
S as shown in Figures from 14 to 16.
[0069] When the locking nut
11 is tightened against the load distribution plate
10, as shown in Figure 14, this receives the thrusting action
T from the underlying ground
S and discharges it onto the nut
11.
[0070] When the cables
5 are tensioned by the tensioning jack
M as shown in Figure 15, they are thrust upwards in the direction indicated by the
arrow
F and the reaction of the tensioning jack
M is discharged onto the nut
11, generating a compression force
F' contrary to the tensioning force
F.
[0071] When, at the end of the tensioning operation, the cables
5 are locked by the cable locking wedges
13 and the tensioning jack
M is removed, as shown in Figure 16, the cables
5 force downwards the cable locking plate
12 to which they are connected and discharge the compression force
F' against the nut
11. Therefore, when the anchoring element
1 is in operation, the nut
11 is loaded on its opposing faces by the forces
T and
F' that are contrary to each other, as shown in Figure 16.
[0072] The same considerations apply even if the fixing unit
26 provided with the spacer ring
21 is used, in which case the compression force
F' will act on the nut
21, not directly but through the interposition of the spacer ring
21, as shown in Figure 17.
[0073] The force generated by the cables
5 is discharged onto the load distribution plate
10 and at the end of installation only the load due to the tightening force exerted
on the bar
2 will be born by the nut
11 or
21.
[0074] Advantageously, the anchoring element of the invention described herein, in both
its embodiments
1 or
30, makes it possible to obtain structures that resist to higher loads compared to the
structures that would be obtained using self-perforating bars of the known art.
[0075] Laboratory resistance tests have made it possible to verify that the anchoring elements
1 and
30 of the invention guarantee much higher resistance than, for example, self-perforating
bars of the known type having the same diameter. Advantageously, this higher resistance
involves also a considerable economic advantage.
[0076] In fact, the construction cost of a containment structure using the anchoring elements
of the invention and being able to guarantee the same total resistance, expressed
in tons, is much lower than the construction cost of structures made with anchoring
elements of the known type.
[0077] In other words, the use of the anchoring elements of the invention reduces the cost
per constrained ton, as the addition of the metal cables
5 to the self-perforating bar
2 involves a very low percentage cost increase vis-à-vis the percentage increase in
the achievable resistance, which instead is very high. Therefore, using the anchoring
elements
1 of the invention it is possible to make containment structures that develop greater
counteracting forces even though they use a smaller number of anchoring elements compared
to analogous structures made with self-perforating bars or other known anchoring elements.
[0078] Even in this case, accounting checks have shown cost reductions that can reach even
60% per ton of counteracting force achievable.
[0079] Furthermore, as already explained, the anchoring element of the invention is more
ductile than the known anchoring elements.
[0080] For this purpose, a series of loading tests have been performed on self-perforating
bars of the known type and on anchoring elements of the invention in the embodiment
indicated as a whole by
1 and also in the embodiment indicated as a whole by
30, with the cables
5 constrained in the bar
2 respectively for their entire length or partially.
[0081] The diagrams shown in Figures 18 and 19 have thus been obtained, in which:
- the letter A identifies the diagram regarding the self-perforating bar of the known type;
- the numbers 1 and 30 identify the diagrams regarding the anchoring elements of the invention in the two
variant embodiments, respectively 1 and 30.
[0082] In the diagrams, the values of the loads are given in the Y-axis and correspond to
the pressure, in bars, applied to operate the hydraulic jack that tensions the elements
being tested, while the elongation values are given in the X-axis and expressed in
millimeters.
[0083] With reference to Figure 18, it can be observed that:
- the values of the ultimate tensile strength R of the self-perforating bar 2 of the known type and of the anchoring element of the invention 1 are almost the same and equal to 340 bars;
- in the presence of the same load, the anchoring element 1 of the invention always shows higher elongation values compared to those of the self-perforating
bar A;
- in the anchoring element 1 the yield effect Rs appears in the presence of a load of approximately 200 bars and therefore much earlier
compared to the self-perforating bar A, where it appears in the presence of a load of approximately 260 bars;
- in the anchoring element 1 the plastic phase P extends for a much greater section compared to the self-perforating bar A and this makes it more ductile.
[0084] With reference to Figure 19, in addition to the curve relating to the self-perforating
bar
A of the known type, it is possible to observe also three curves relating to three
anchoring elements
30 of the invention, each one of which has the cables
5 tensioned with different loads.
[0085] In particular, the curves
30a, 30b and
30c are related to increasing tensions on the cables
5.
[0086] It can be observed that:
[0087] the values of the ultimate tensile strength
R of the self-perforating bar
A of the known type and of the anchoring element
30 of the invention are practically the same and equal to 340 bars;
- in the presence of the same load, the anchoring elements 30 always show higher elongation values compared to those of the self-perforating bar
A;
- in the anchoring elements 30 the yield effect Rs appears in the presence of a load of approximately 220 bars and therefore earlier,
also in this case, than in the self-perforating bar A, where it appears in the presence of a load of approximately 260 bars;
- as the tension of the cables 5 increases, also the stiffness of the anchoring elements 30 increases, which is shown by their lower elongation values in the presence of the
same load;
- in any case, the plastic phase P of the anchoring elements 30 of the invention extends for a much larger section compared to the self-perforating
bar A.
[0088] Observing the diagram of Figure 20 that combines the diagrams of Figures 18 and 19,
it can be understood that the anchoring elements
1 and
30 of the invention have larger plastic phases
P than the self-perforating bars
A of the known type, compared to which, therefore, they are more ductile, thus achieving
another object of the invention.
[0089] In fact, the combination of the known cables
5 in prestressed steel with the known self-perforating bars
2 in structural steel makes it possible to obtain a synergy among the different materials
used and to obtain a high ductility of the anchoring element, much higher than that
obtainable using the known self-perforating bars.
[0090] Said increase in the ductility of the anchoring elements is extremely important when
it comes to increasing the safety of the structures, above all those destined to operate
in a seismic environment, where it is important to ensure that the structure, even
if damaged, will not collapse.
[0091] It can be observed that using the anchoring elements carried out according to the
embodiment
30 and properly pre-tensioning the cables
5 means offering the designer the opportunity to modify the ductility of the anchoring
element depending on the project, and thus to obtain characteristics that cannot be
achieved with the known self-perforating bars, which constitutes a further advantage
of the invention.
[0092] It can thus be understood that the anchoring element of the invention, in both its
variant embodiments
1 and
30 described herein, achieves all the set objects and advantages.
[0093] It is obvious that the anchoring element can be made using tubular bars in any length
and with any diameter, and that any number of cables can be placed inside it according
to the loads defined for the project.
[0094] Any variant embodiments, not described herein and not illustrated in the figures,
of the anchoring element of the invention, must however be considered protected by
the present patent, provided that they fall within the scope of the claims that follow.