1、Section 6 GROUND ANCHORS 6.1 DESCRIPTION submittal. No work on ground anchors shall begin until working drawings have been approved in writing by the Engineer. Such approval shall not relieve the Contractor of any responsibility under the contract for the successful completion of the work. This work
2、 shall consist of designing, furnishing, in- stalling, testing, and stressing permanent cement-grouted ground anchors in accordance with the plans, these spec- ifications, and the special provisions. 6.2 WORKING DRAWINGS At least 4 weeks before work is to begin, the Contrac- tor shall submit to the
3、Engineer for review and approval complete working drawings and design calculations de- scribing the ground anchor system or systems intended for use. The submittal shall include the following: (1) A ground anchor schedule giving: (a) Ground anchor number, (b) Ground anchor design load, (c) Type and
4、size of tendon, (d) Minimum total anchor length, (e) Minimum bond length, (f) Minimum tendon bond length, and (g) Minimum unbonded length. (2) A drawing of the ground anchor tendon and the corrosion protection system, including details for the following: (a) Spacers separating elements of tendon and
5、 their location, (b) Centralizers and their location, (c) Unbonded length corrosion protection system, (d) Bond length corrosion protection system, (e) Anchorage and trumpet, (f) Anchorage corrosion protection system, (g) Drilled or formed hole size, (h) Level of each stage of grouting, and (i) Any
6、revisions to structure details necessary to accommodate the ground anchor system intended for use. (3) The grout mix design and procedures for placing the grout. The Engineer will approve or reject the Contractors working drawings within 4 weeks of receipt of a complete 6.3 MATERIALS 6.3.1 Prestress
7、ing Steel Ground anchor tendons shall consist of single or mul- tiple elements of prestressing steel, anchorage devices and, if required, couplers conforming to the requirements described in Section 10, “Prestressing.” The following materials are acceptable for use as ground anchor tendons: AASHTO D
8、esignation: M 203 (ASTM Designation ASTM Designation: A 8861A 886M (Indented, 7-wire ASTM Designation: A 882lA 882M (Epoxy coated, A 416 - Uncoated, 7-wire strand) strand) 7-wire strand) 6.3.2 Grout Cement shall be Type I, II, or III Portland Cement con- forming to AASHTO M 85. Cement used for grout
9、ing shall be fresh and shall not contain any lumps or other in- dications of hydration or “pack set.” Aggregate shall conform to the requirements for fine aggregate described in Section 8, “Concrete Structures.” Admixtures may be used in the grout subject to the ap- proval of the Engineer. Expansive
10、 admixtures may only be added to the grout used for filling sealed encapsula- tions, trumpets, and anchorage covers. Accelerators shall not be used. Water for mixing grout shall be potable, clean and free of injurious quantities of substances known to be harmful to Portland cement or prestressing st
11、eel. 507 508 HIGHWAY BRIDGES 6.3.3 6.3.3 Steel Elements Bearing plates shall be fabricated from steel con- forming to AASHTO M 270 (ASTM A 709) Grade 36 minimum, or be a ductile iron casting conforming to ASTM A 536. Trumpets used to provide a transition from the anchor- age to the unbonded length c
12、orrosion protection shall be fabricated from a steel pipe or tube conforming to the re- quirements of ASTM A 53 for pipe or ASTM A 500 for tubing. Minimum wall thickness shall be 0.20 inches. Anchorage covers used to enclose exposed anchorages shall be fabricated from steel, steel pipe, steel tube,
13、or ductile cast iron conforming to the requirements of AASHTO M 270 (ASTM A 709) Grade 36 for steel, ASTM A 53 for pipe, ASTM A 500 for tubing, and ASTM A 536 for ductile cast iron. Minimum thickness shall be 0.10 inches. 6.3.4 Corrosion Protection Elements Corrosion inhibiting grease shall conform
14、to the re- quirements of the Post Tensioning Institutes “Specifica- tions for Unbonded Single Strand Tendons,” Section 3.2.5. Sheath for the unbonded length of a tendon shall con- sist of one of the following: (1) Seamless polyethylene (PE) tube having a mini- mum wall thickness of 60 mils plus or m
15、inus 10 mils. The polyethylene shall be cell classification 334413 by ASTM D 3350. (2) Seamless polypropylene tube having a minimum wall thickness of 60 mils plus or minus 10 mils. The polypropylene shall be cell classification PP21OB55542-11 by ASTM D 4101. (3) Heat shrinkable tube consisting of a
16、radiation crosslinked polyolefin tube internally coated with an adhesive sealant. The minimum tube wall thickness be- fore shrinking shall be 24 mils. The minimum adhesive sealant thickness shall be 20 mils. (4) Corrugated polyvinyl chloride (PVC) tube having a minimum wall thickness of 30 mils. Enc
17、apsulation for the tendon bond length shall consist of one of the following: (1) Corrugated high density polyethylene (HDPE) tube having a minimum wall thickness of 30 mils and conforming to AASHTO M 252 requirements. (2) Deformed steel tube or pipe having a minimum wall thickness of 25 mils. (3) Co
18、rrugated polyvinyl chloride (PVC) tube having a minimum wall thickness of 30 mils. (4) Fusion-bonded epoxy conforming to the require- ments of AASHTO M 284, except that it shall have a film thickness of 15 mils. 6.3.5 Miscellaneous Elements Bondbreaker for a tendon shall consist of smooth plas- tic
19、tube or pipe that is resistant to aging by ultra-violet light and that is capable of withstanding abrasion, impact and bending during handling and installation. Spacers for separation of elements of a multi-element tendon shall permit the free flow of grout. They shall be fabricated from plastic, st
20、eel, or material which is not detri- mental to the prestressing steel. Wood shall not be used. Centralizers shall be fabricated from plastic, steel, or material which is not detrimental to either the prestressing steel or any element of the tendon corrosion protection. Wood shall not be used. The ce
21、ntralizer shall be able to maintain the position of the tendon so that a minimum of 0.5 inches of grout cover is obtained on the tendons, or over the encapsulation. 6.4 FABRICATION Tendons for ground anchors may be either shop or field fabricated from materials conforming to the requirements ofArtic
22、le 6.3. Tendons shall be fabricated as shown on the approved working drawings. The tendon shall be sized so that the maximum test load does not exceed 80% of the minimum guaranteed ultimate strength of the tendon. 6.4.1 Bond Length and Tendon Bond Length The Contractor shall determine the bond lengt
23、h neces- sary to satisfy the load test requirements. The minimum bond length shall be 10 feet in rock, 15 feet in soil or the minimum length shown on the plans. The minimum ten- don bond length shall be 10 feet. 6.4.1.1 Grout Protected Ground Anchor Tendon Spacers shall be placed along the tendon bo
24、nd length of multi-element tendons so that the prestressing steel will bond to the grout. They shall be located at 10-foot maxi- mum centers with the upper one located a maximum of 5 feet from the top of the tendon bond length and the lower one located a maximum of 5 feet from the bottom of the tend
25、on bond length. Centralizers shall be placed along the bond length. They shall be located at 10-foot maximum centers with the upper one located a maximum of 5 feet from the top of the bond length and the lower one located 1 foot from the bot- I 6.4.1.1 DIVISION II-CONSTRUCTION 509 tom of the bond le
26、ngth. Centralizers are not required on tendons installed utilizing a hollow-stem auger if it is grouted through the auger and the drill hole is maintained full of a stiff grout (9-inch slump or less) durhg extraction of the auger. A combination centralizer-spacer may be used. Centralizers are not re
27、quired on tendons installed uti- lizing a pressure injection system in coarse-grained soils using grouting pressures greater than 150 psi. 6.4.1.2 Encapsulation Protected Ground Anchor Tendon The tendon bond length shall be encapsulated by a grout-filled corrugated plastic or deformed steel tube, or
28、 by a fusion-bonded epoxy coating. The tendon can be grouted inside the encapsulation prior to inserting the ten- don in the drill hole or after the tendon has been placed in the drill hole. Punching holes in the encapsulation and al- lowing the grout to flow from the encapsulation to the drill hole
29、, or vice versa, will not be permitted. The tendon shall be centralized within the encapsulation and the tube sized to provide an average of 0.20 inches of grout cover for the prestressing steel. Spacers and centralizers shall be used to satisfy the same requirements specified in Article 6.4.1.1 for
30、 grout protected ground anchor tendons. The anchorage device of tendons protected with fusion-bonded epoxy shall be electrically isolated from the structure. 6.4.2 Unbonded Length The unbonded length of the tendon shall be a minimum of 15 feet or as indicated on the plans or approved work- ing drawi
31、ngs. Corrosion protection shall be provided by a sheath completely filled with corrosion inhibiting grease or grout, or a heat shrinkable tube. If grease is used to fill the sheath, provisions shall be made to prevent it from escap- ing at the ends. The grease shall completely coat the ten- don and
32、fill the interstices between the wires of seven- wire strands. Continuity of corrosion protection shall be provided at the transition from the bonded length to un- bonded length of the tendon. If the sheath provided is not a smooth tube, then a sep- arate bondbreaker must be provided to prevent the
33、tendon from bonding to the anchor grout surrounding the un- bonded length. 6.4.3 Anchorage and Thunpet Nonrestressable anchorages may be used unless re- stressable anchorages are designated on the pians or spec- ified in the special provisions. Bearing plates shall be sized so that the bending stres
34、ses in the plate and average bearing stress on the con- crete, if applicable, do not exceed the allowable stresses described in Article 10.3.1.4.3, “Anchorage Devices with Distribution Plates.” The size of bearing plates shall not be less than that shown on the plans or on the approved working drawi
35、ngs. The trumpet shall be welded to the bearing plate. The trumpet shall have an inside diameter at least 0.25 inches greater than the diameter of the tendon at the anchorage. The trumpet shall be long enough to accommodate move- ments of the structure during testing and stressing. For strand tendon
36、s with encapsulation over the unbonded length, the trumpet shall be long enough to enable the ten- dons to make a transition from the diameter of the tendon in the unbonded length to the diameter of the tendon at the anchorhead without damaging the encapsulation. Trum- pets filled with corrosion-inh
37、ibiting grease shall have a permanent Buna-N rubber or approved equal seal pro- vided between the trumpet and the unbonde length cor- rosion protection. Trumpets filled with grout shall have a temporary seal provided between the trumpet and the un- bonded length corrosion protection. 6.4.4 Tendon St
38、orage and Handling Tendons shall be stored and handled in such a manner as to avoid damage or corrosion. Damage to tendon pre- stressing steel as a result of abrasions, cuts, nicks, welds and weld splatter will be cause for rejection by the Engi- neer. Grounding of welding leads to the prestressing
39、steel is not permitted. A slight rusting, provided it is not suffi- cient to cause pits visible to the unaided eye, shall not be cause for rejection. Prior to inserting a tendon into the drilled hole, its corrosion protection elements shall be ex- amined for damage. Any damage found shall be repaire
40、d in a manner approved by the Engineer. 6.5 INSTALLATION The Contractor shall select the drilling method, the grouting procedure and grouting pressure to be used for the installation of the ground anchor as necessary to sat- isfy the load test requirements. 6.5.1 Drilling The drilling method used ma
41、y be core drilling, rotary drilling, percussion drilling, auger drilling or driven cas- ing. The method of drilling used shall prevent loss of ground above the drilled hole that may be detrimental to 510 HIGHWAY BRIDGES 6.5.1 the structure or existing structures. Casing for anchor holes, if used, sh
42、all be removed, unless permitted by the Engineer to be left in place. The location, inclination, and alignment of the drilled hole shall be as shown on the pians. Inclination and alignment shall be within plus or minus 3“ of the planned angle at the bearing plate, and within plus or minus 12 inches
43、of the planned location at the ground surface (point of entry). 6.5.2 Tendon Insertion 4 The tendon shall be inserted into the drilled hole to the desired depth without difficulty. When the tendon cannot be completely inserted it shall be removed and tine Uii noie cieaned or reriiied IO permit inser
44、tion. Partially inserted tendons shall not be driven or forced into the hole. 6.5.3 Grouting A neat cement grout or sand-cement grout conforming to Article 6.3.2 shall be used. Admixtures, if used, shall be mixed in quantities not to exceed the manufacturers rec- ommendations. The grouting equipment
45、 shall produce a grout free of lumps and undispersed cement. A positive displacement grout pump shall be used. The pump shall be equipped with a pressure gauge to monitor grout pressures. The pressure gauge shall be capable of measuring pressures of at least 150 psi or twice the actual grout pressur
46、es used, whichever is greater. The grouting equipment shall be sized to enable the grout to be pumped in one continuous operation. The mixer shall be capable of continuously ag- itating the grout. The grout shall be injected from the lowest point of the drill hole. The grout may be pumped through gr
47、out tubes, casing, hollow-stem augers or drill rods. The grout may be placed before or after insertion of the tendon. The quantity of the grout and the grout pressures shall be recorded. The grout pressures and grout takes shall be controlled to prevent excessive heave of the ground or fracturing of
48、 rock formations. Except where indicated below, the grout above the top of the bond length may be placed at the same time as the bond length grout, but it shall not be placed under pres- sure. The grout at the top of the drill hole shall stop 6 inches from the back of the structure or from the botto
49、m of the trumpet, whichever is lowest. If the ground anchor is installed in a fine-grained soil using a drilled hole larger than 6 inches in diameter, then the grout above the top of the bond length shall be placed after the ground anchor has been load tested. The entire drill hole may be grouted at the same time if it can be demonstrated that the ground anchor system does not de- rive a significant portion of its load resistance from the soil above the bond length portion of the ground anchor. If grout protected tendons are used for ground anchors anchored in rock, then pressure grouting
