WO2013021382A1

WO2013021382A1 - A ground anchoring system and a method for installation thereof

Info

Publication number: WO2013021382A1
Application number: PCT/IL2012/050291
Authority: WO
Inventors: Itzhak LIPSKER; Boris GOLDSHLAK; Yoram LIPSKER
Original assignee: Lipsker & Co Engineering Services (1975) Ltd
Current assignee: Lipsker & Co Engineering Services (1975) Ltd
Priority date: 2011-08-10
Filing date: 2012-08-05
Publication date: 2013-02-14
Anticipated expiration: 2014-02-10

Abstract

A method for installing an anchor system with double protection against corrosion. The method comprises the steps of providing an anchor system that comprises, at least one steel tendon having a tendon bond length and a tendon free length, at least one pre-grouted double sheathing having an inner duct and an outer duct, and an anchor head, inserting at least the one double sheathing into a borehole and introducing the tendon into an interior portion of the inner duct of the double sheathing. Then grout is injected at least into the interior portion of the inner duct, encapsulating the tendon bond length of at least the one tendon therein, injecting grout at least into a space between an exterior portion of the outer duct and the borehole, thereby forming a fixed anchor length, tensioning the tendon free length of the tendon and bringing it to a tensioned state and fixing the tendon in the tensioned state by the anchor head.

Description

A GROUND ANCHORING SYSTEM AND A METHOD FOR INSTALLATION

THEREOF

FIELD OF THE DISCLOSED SUBJECT MATTER

The present disclosed subject matter is concerned with a ground anchor system, and more specifically with corrosion protection systems for anchor systems. BACKGROUND OF THE DISCLOSED SUBJECT MATTER

Ground anchor systems consisting of tendons (e.g., cables or rods) connected to a bearing plate are often used for the stabilization of steep slopes or slopes consisting of softer soils, as well as the enhancement of embankment or foundation soil capacity, or to prevent excessive erosion and landslides. The use of steel anchors is often constrained by overall durability in placement (due to weight), and the difficulty in maintaining tension levels in the anchor. Anchor systems can hold the walls and posts of outdoor structures to the ground without a foundation or concrete-filled post holes. The strength of the ground anchor's grip is largely determined by the consistency of the site's soil.

When an anchor is installed in aggressive permeable soils, and/or according to various engineering requirements (e.g., standards), the steel tendon has to be protected against corrosion with two barriers. For example, the tendons of a permanent anchor shall be provided with two protective barriers to corrosion such that if one barrier is damaged during installation or anchor loading, the second barrier remains intact. The object of the protection against corrosion is to ensure that during the economic life of the anchorage the probability of unacceptable corrosion occurring is small.

Plastic sheaths and ducts can be used as protective barriers because they are continuous, impermeable to water, resistant to age brittleness and resistant to ultraviolet radiation damage during storage, transportation and installation. The details of the plastic duct that forms one element of protection is important as the duct has not only to provide corrosion protection, but also to transmit stresses from filler to external grout without displacement or distress. The unsure effective load transfer between duct and grout, ducts are usually corrugated. SUMMARY OF THE DISCLOSED SUBJECT MATTER

The presently disclosed subject matter provides a sequence of steps according to which encapsulation of a tendon within a pre-grouted double sheathing should be performed.

In accordance with a first aspect of the presently disclosed subject matter there is provided a method for installing an anchor system with double protection against corrosion. The method comprises the following steps:

a. providing an anchor system, comprising: a tendon, having a tendon bond length and a tendon free length; at least one pre-grouted double sheathing having an inner duct and an outer duct; and an anchor head;

b. inserting the pre-grouted double sheathing into a borehole with said at least one tendon being introduced into an interior portion of the inner duct;

c. injecting grout at least into said interior portion of the inner duct, thereby encapsulating the tendon bond length of said at least one tendon therein;

d. injecting grout at least into a space between an exterior portion of the outer duct and the ground of the borehole, thereby forming said fixed anchor length;

e. tensioning said tendon free length of the tendon and bringing it to a tensioned state; and

f.fixing said tendon in said tensioned state by said anchor head.

The term 'anchor' or 'anchor system' refers hereinafter to an installation capable of transmitting an applied tensile load to a load bearing stratum. The installation consists basically of an anchor head, free anchor length and fixed anchor length. The anchor can be installed, for example in the ground or within a stone. The anchor can be oriented in different orientation angles. According to one example, the anchor can be oriented in an inclined position, i.e., its fixed anchor length is positioned lower than its free anchor length. According to another example, the anchor can be oriented in horizontal position, i.e., its fixed anchor length is positioned substantially at same height as its free anchor length.

The term 'tendon' refers hereinafter to the part of an anchor that is capable of transmitting the tensile load from the fixed anchor length to the anchor head. The tendon can be, for example, a steel bar, a fiber-glass bar, a strand, a wire/cable either singly or in groups. The term 'anchor head' refers hereinafter to the component of an anchor which transmits the tensile load from the tendon to bearing plate or structure. The anchor head may include a stressing head in which the tendon is anchored and a bearing plate by which the tendon force is transferred to the structure or excavation.

The term 'fixed anchor length' refers hereinafter to the designed length of an anchor over which the load is transmitted to the surrounding ground, through a grout body.

The term 'free anchor length' refers hereinafter to the distance between the proximal end of the fixed anchor length and the tendon anchorage at the anchor head.

The term 'tendon bond length' refers hereinafter to the length of the tendon that is bonded directly (or via additional corrosion protecting layer(s)) to the grout and capable of transmitting the applied tensile load.

The term 'tendon free length' refers hereinafter to the length of the anchor head and the proximal end of the tendon bond length.

The term 'grout' refers to a setting material which transfers load from the tendon to the ground over the fixed anchor length, and which may fill the rest of the borehole and/or contribute to corrosion protection. The grout may be, for example, a cementitious grout, a primary grout, a resinous grout, and a secondary grout.

The term 'grouting' or 'grouted' refers hereinafter to at least one of the following functions: a. to form the fixed anchor length in order that the applied load may be transferred from the tendon to the surrounding ground; b. to protect the tendon against corrosion; c. to strengthen the ground immediately adjacent to the fixed anchor in order to enhance ground anchor capacity; d. to seal the ground immediately adjacent to the fixed anchor length in order to limit the loss of grout.

The term 'duct' refers hereinafter to a tube or covering used in corrosion protection, which may be made of semi-rigid plastics and may be smooth or corrugated. The duct may be, for example, made of PVC, polyethylene, fiberglass, metal, etc.

The term 'encapsulation' refers hereinafter to a process of applying corrosion protection to a tendon bond length by grouting it within a corrosion protecting sheathing.

The term 'permanent anchor' refers hereinafter to an installation that is required to unsure the stability and satisfactory service performance of the permanent structure being supported for a period of time of more than 2 years. The term 'temporal anchor' refers hereinafter to an installation that is often used during the construction phase of a project to withstand forces for a know period of time, usually less than 2 years.

The term 'installation site' refers hereinafter to the location in which the anchor system is installed.

According to the presently disclosed method, encapsulation of the tendon within a pre-grouted double sheathing is performed only after the placement of the tendon into the borehole and its introduction into a pre-grouted sheathing. In other words, the grout between the tendon and the corrosion protecting pre-grouted double sheathing is injected therebetween only when they are already disposed within the borehole.

The pre-grouting of the pre-grouted double sheathing can be performed by injecting grout into an annulus between the inner and the outer ducts. This pre-grouting can be performed in a factory or under factory conditions. The step of pre-grouting can be also performed at the site of the installation before the step (b) of the method above.

The process of pre-grouting the pre-grouted double sheathing can comprise a step of placing spacers between the inner and the outer ducts before the injection of the grout for centralizing the inner duct with respect to the outer duct.

The pre-grouted double sheathing can be provided with at least one centralizer into which the tendon bond length of the at least one tendon is introduced therein, for centralizing the tendon bond length within the pre-grouted double sheathing.

The inner and said outer ducts can be corrugated.

The pre-grouted double sheathing can be provided as a plurality of pre-grouted double sheathings, and the method above can comprise a step of connecting the plurality of pre-grouted double sheathings to each other before the step (b) of the method.

The pre-grouted double sheathings can be connected to each other by connecting means selected from the group consisting of: screw means, fasteners, sheath connectors, couplers, and any combination thereof.

The presently disclosed method can further comprise a step of transporting the plurality of pre-grouted double sheathings when they are disconnected from each other, thereby saving space at a transportation means and simplifying the transportation.

The method can also include a step of connecting a cap to the distal end of the pre-grouted double sheathing, before the step (b) of the method. This cap can be used for a duplex anchor system. In the duplex anchor system, the step (e) of the method is performed before the step (d).

The method can also comprise: repeating said steps (a) to (d) for inserting a plurality of tendons with their respective pre-grouted double sheathings into the borehole, and further performing said steps (e) and (f) to all the tendons together.

The method can further comprise a step of introducing the tendon bond length of the at least one tendon into the interior portion of the inner duct of the pre-grouted double sheathing.

The tendon can be a plurality of wires or a bar.

According to the method above, step (d) can be performed before or after step (c).

In accordance with a second aspect of the presently disclosed subject matter there is provided a pre-grouted double sheathing for use in the method above.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the disclosed subject matter and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

Fig. 1 illustrates an anchor system without protection against corrosion;

Fig. 2a illustrates a longitudinal cross-sectional view of a pre-grouted double sheathing;

Fig. 2b illustrates a transverse cross-sectional view of a pre-grouted double sheathing;

Fig. 3a illustrates a longitudinal cross-sectional view of an anchor system with a pre-grouted double sheathing;

Fig. 3b illustrates a transverse cross-sectional view of a fixed anchor length of an anchor system with a pre-grouted double sheathing; and

Fig. 4 illustrates the steps of the method according to which the anchor system with a corrosion protection is installed within a borehole;

DETAILED DESCRIPTION OF EMBODIMENTS

Attention is first directed to Fig. 1 of the drawing which schematically illustrates an anchor system 100 which is not protected against corrosion, but can be protected against corrosion by a pre-grouted double sheathing 10 (not shown in Fig. 1, but shown in Figs. 2a-b, 3a-b) according to the presently disclosed method. The anchor system 100 of Fig. 1 is shown for purposes of illustration without a corrosion protection sheathing in order to present the system's main elements.

The anchor system 100 of Fig. 1 comprises a plurality of tendons 20 (e.g., steel wires) each having a tendon free end 22 and a tendon bond length 24, and an anchor head 28 disposed at its distal end. The anchor system 100 is installed within a borehole 30, such that the tendon free end 22 is disposed within a free anchor length 32 and the tendon fixed length 24 is disposed within a fixed anchor length 34. The tendon bond length 24 is being grouted within the borehole 30 by a grout (e.g., a cement grout), such that when the tendon free length 22 is tensioned during installation, the tendon bond length 24 is fixed to borehole 30. By being fixed to the borehole, the tendon bond length 24 transmits load from the anchor head 28 which holds the tensioned tendon.

In order to provide a double protection against corrosion, the tendon bond length 24 of the tendon 20 is usually encapsulated within a corrosion protecting barrier, such as a corrosion protecting sheathing. According to the presently disclosed subject matter, the tendon bond length 24 is encapsulated within a corrosion protecting pre-grouted double sheathing 10 (shown in Figs. 2a and 2b and 3a and 2b), and the tendon free length 22 can be protected by various types of protecting ducts such as a duct 23 shown in Fig. 1.

Attention is now directed to Figs. 2a and 2b of the drawings, which illustrate a pre-grouted double sheathing 10 which is configured to provide two barrier (double) protection against corrosion to a tendon bond length 24 of the tendon 20 of Fig. 1.

According to the example of Figs. 2a and 2b, the pre-grouted double sheathing 10 is constructed of two concentric corrugated plastic ducts: an inner duct 4 and an outer duct 8, having an annulus 6 therebetween. The inner duct 4 is having a hollow interior portion 5 and the outer duct 8 is having an exterior portion 9. The pre-grouted double sheathing 10 has a grout 7 which is injected into the annulus 6 before the installation of the anchor system 100. The pre-grouting of the double sheathing 10 can be performed in a factory and/or under factory controlled conditions. Moreover, the pre- grouting of the double sheathing 10 can be performed before the transportation of the double sheathing 10 to an installation site of the anchor system 100. The pre-grouting of the double sheathing has to be performed under supervised condition. Therefore, it is pre-grouted outside the borehole. Reference is now made to Figs. 3a, 3b and 4 which schematically illustrate a section of an anchor system 200 in which an anchor system, which may be similar to the system of Fig. 1 , is provided with a pre-grouted double sheathing mounted thereon, and a respective method generally designated 300 in Fig. 4, according to which the anchor 5 system 200 is installed.

According to the method 300, the encapsulation of the tendon within a pre- grouted double sheathing for providing protection against corrosion is performed only after the placement of the tendon into a borehole and its introduction into a pre-grouted sheathing. In other words, the grout between the tendon and the inner duct of the pre- 10 grouted double sheathing is injected therebetween only when they are already disposed within the borehole.

In step 210, the anchor system 200 is provided with the following components: tendons 120, having a tendon bond length 134 and a tendon free length 132, a pre- grouted double sheathing 101 having two corrugated ducts: an inner duct 104 and an 15 outer duct 108; and an anchor head (not shown).

In step 220, the pre-grouted double sheathing 101 is inserted into the borehole 130 with the tendons 120 being introduced into an interior portion 105 of the inner duct 104.

In step 230, a grout 140 is injected into the interior portion 105 of the inner duct 20 104, resulting in encapsulation of the tendon bond length 134 of the tendons 120 therein.

In step 240, a grout 142 is injected into the space 143 between the exterior portion 109 of the outer duct 108 and the ground 150 of the borehole 130, resulting in formation of the fixed anchor length 124. The step 240 can be performed before the step 25 230.

In step 250, the tendon free length 132 is tensioned and brought to its tensioned state. According to different examples, the sequence of step 230, 240 and 250 can change according to different needs and requirements.

And finally, in step 260, the proximal end (not shown) of the tendon free length 30 (not shown) is fixed in the tensioned state by the anchor head (not shown).

Before the step 210, the method 300 can further comprise an additional step in which the tendon bond length 134 of the tendons 120 is introduced into the interior portion 105 of the inner duct 104 of the double sheathing 101. This can be performed when the double sheathing 101 is located outside the borehole 130.

When forming vertical or inclined grouted duct encapsulations, the grout is introduced from the bottom of the vertically restrained sheath to unsure complete expulsion of air and to provide good grout contact with the contained tendon.

Referring now back to Figs. 3a and 3b in which the final result of the method 300 is shown. Fig. 3a illustrates a longitudinal cross-sectional view of the fixed anchor length 124 of the anchor system 200 with the pre-grouted double sheathing 101 mounted thereon, and Fig. 3b illustrates a transverse cross-sectional view of a fixed anchor length 124 of the anchor system 200 with the pre-grouted double sheathing 101 mounted thereon.

Figs. 3a and 3b show the anchor system 200 with the pre-grouted double sheathing 101 mounted on the tendon bond length 134 of the tendons 120 for providing protection against corrosion. The pre-grouted double sheathing 101 has a grout 107 which was injected into the annulus 106 before the installation of the anchor system 200. The pre-grouting of the double sheathing 101 could be performed in a factory and/or under factory controlled conditions. Moreover, the pre-grouting of the double sheathing 101 could be performed before the transportation of the double sheathing 101 to an installation site of the anchor system 100.

As shown in Fig. 3a, the tendon free length 132 can be protected by ducts 136 and 137 which provide double protection against corrosion to the tendon free length 132.

The pre-grouting of the double sheathing can include a step of placing spacers between the inner and the outer ducts before the injection of the grout for centralizing the inner duct with respect to the outer duct. Moreover, the double sheathing can comprises centralizers into which the tendon bond length of the tendons is inserted, for centralizing said tendon bond length within said double sheathing.

The sequence of the steps and/or the location in which the steps of the method 300 are performed, provides various advantages with respect to another sequences and/or locations of performing the installation of the anchor system.

For example, if the step 230 of the encapsulation would have been performed in a factory or in factorial conditions at the installation-site but outside the borehole, this could result in a construction which already comprises the grout 140. This construction would have an additional weight which would require much complicated procedure of raising the already encapsulated tendon for inserting it into the borehole. Moreover, this extra weight may damage the construction of the borehole by harming the walls of the borehole during the insertion of the system therein. Furthermore, the lately the encapsulation of the tendon is performed during the installation, the chances of causing the encapsulating grout to crack are lower. Therefore, the encapsulation should be performed as lately as possible as in the method of the presently disclosed subject matter.

If the encapsulation of the tendon(s) is performed in a factory, there may be logistical difficulties of transporting a heavy encapsulated construction to the installation site (because of the additional weight of the encapsulating grout). This transportation of the encapsulated tendon(s) may require special transpiration means (e.g., tracks) with a special elongated length (due to a length of the tendon bond length which may exceed 12 meter).

According to some examples of the presently disclosed subject matter, the anchor system can be provided with a plurality of double sheathings which are connectable to each other before the installation of the system within the borehole. The double sheathings can be connected to each other by connecting means such as: screw means, fasteners, sheath connectors, and couplers. The connecting means can have an interior space into which grout can be injected while it is already within the borehole. This can be implemented for providing an additional support to the anchor while it is installed within the borehole.

When a double sheathing is configured to be formed of a plurality of connectable double sheathings, its transportation to the installation site can be simplified when the connectable double sheathings are disconnected from each other. The transportation is simplified due to a shorter length of each connectable double sheathing which results in a requirement for a shorter transportation means. Transportation of connectable double sheathings to the installation site is also much simpler due to the reduced weight thereof. For example, instead of transporting a heavy pre-grouted double sheathing, it can be transported in parts. Moreover, transportation of connectable double sheathings to the installation site before the encapsulation prevents cracking of the encapsulating grout during the transportation due to reduced weight of the double sheathings. According to another aspect of the presently disclosed subject matter, the anchor system can be provided with a cap which is connectable to the distal end of the double sheathing, before the step 220. This cap can be used for a duplex anchor system in which the step 250 is performed before the step 240.

According to different examples, the grouts 140, 142 and 107 can be made of materials which can provide an additional protection against corrosion, but a standard grout which may crack because of different reasons, usually does not constitute a part of a corrosion protection system because the grout quality and integrity cannot be assured.

Claims

A method for installing an anchor system with double protection against corrosion, comprising the following steps:

a. providing an anchor system, comprising: at least one steel tendon, each having a tendon bond length and a tendon free length; at least one pre-grouted double sheathing having an inner duct and an outer duct; and an anchor head;

b. inserting at least said one double sheathing into a borehole with at least said one tendon being introduced into an interior portion of the inner duct of the double sheathing;

c. injecting grout at least into said interior portion of the inner duct, thereby encapsulating the tendon bond length of at least said one tendon therein;

d. injecting grout at least into a space between an exterior portion of the outer duct and the borehole, thereby forming a fixed anchor length;

f. fixing said tendon in said tensioned state by said anchor head.

The method according to Claim 1 , further comprising a step of pre-grouting said pre-grouted double sheathing by injecting grout into an annulus between the inner and the outer ducts.

The method according to Claim 2, wherein said step of pre-grouting is performed in a factory or under factory conditions.

The method according to any one of Claims 2 to 3, wherein said step of pre- grouting is performed at the site of the installation before said step (b).

The method according to any one of Claims 2 to 4, wherein said step of pre- grouting further comprises a step of placing spacers between the inner and the outer ducts before the injection of the grout for centralizing said inner duct with respect to said outer duct.

The method according to any one of the preceding claims, wherein said pre- grouted double sheathing further comprises at least one centralizer into which the tendon bond length of said at least one tendon is introduced therein, for centralizing said tendon bond length within said pre-grouted double sheathing.

7. The method according to any one of the preceding claims, wherein said inner and said outer ducts are corrugated.

8. The method according to any one of the preceding claims, wherein said at least one pre-grouted double sheathing is a plurality of pre-grouted double sheathings;

5 said method further comprising step of connecting said plurality of pre-grouted double sheathings to each other before said step (b).

9. The method according to Claim 8, wherein said pre-grouted double sheathings are connected to each other by connecting means selected from the group consisting of: screw means, fasteners, sheath connectors, couplers, and any combination

10 thereof.

10. The method according to any one of the preceding claims, further comprising a step of transporting a plurality of pre-grouted double sheathings when they are disconnected from each other, thereby saving space at a transportation means and simplifying the transportation.

15 11. The method according to any one of the preceding claims, further comprising a step of connecting a cap to the distal end of the pre-grouted double sheathing, before said step (b).

12. The method according to Claim 11, wherein said cap is used for a duplex anchor system.

20 13. The method according to Claim 1 1, wherein in said duplex anchor system, said step (e) is performed before said step (d).

14. The method according to any one of the preceding claims, further comprising repeating said steps (a) to (d) for inserting a plurality of tendons with their respective pre-grouted double sheathings into the borehole, and further

25 performing said steps (e) and (f) to all the tendons together.

15. The method according to any one of the preceding claims, further comprising a step of introducing the tendon bond length of said at least one tendon into the interior portion of the inner duct of the pre-grouted double sheathing.

16. The method according to any one of the preceding claims, wherein said tendon is 30 a plurality of wires.

17. The method according to any one of the preceding claims, wherein said tendon is a bar.

18. The method according to any one of the preceding claims, wherein said step (d) is performed before said step (c).

19. A pre-grouted double sheathing for use in the method according to any one of the preceding claims.