Ground Anchor Inspector Manual

GROUND ANCHOR INSPECTOR’S MANUAL

GEOTECHNICAL ENGINEERING MANUAL GEM-17 Revision #4

AUGUST 2015

GEOTECHNICAL ENGINEERING MANUAL: GROUND ANCHOR INSPECTOR’S MANUAL GEM-17 Revision #4

STATE OF NEW YORK DEPARTMENT OF TRANSPORTATION GEOTECHNICAL ENGINEERING BUREAU

AUGUST 2015

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TABLE OF CONTENTS

I. INTRODUCTION......................................................................................................................3

II. GLOSSARY OF TERMS ..........................................................................................................4

III. CHECKLISTS ...........................................................................................................................5 A. Preconstruction ..............................................................................................................5 B. Materials ........................................................................................................................5 C. Construction Monitoring................................................................................................6 1. Drilling .....................................................................................................................6 2. Tendon Installation ..................................................................................................6 3. Grouting ...................................................................................................................7 D. Stressing and Testing .....................................................................................................8 E. Documentation ...............................................................................................................9 IV. LOAD CELL TROUBLE SHOOTING ...................................................................................10 V. REFERENCES ........................................................................................................................10 APPENDICIES ..............................................................................................................................11 A. Blank Forms (US Customary Units) ......................................................................... A-1 B. Blank Forms (International System of Units) ............................................................B-1

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I. INTRODUCTION The purpose of this manual is to provide the inspector and Engineer-In-Charge with a quick and easy-to-use set of inspection guidelines for the various aspects of tieback construction, including pre-installation inspection, installation, testing and trouble shooting load cell problems. This guide does not cover every possible situation the inspector may encounter. The enclosed checklists are intended to serve as reminders for inspectors already familiar with tieback installation. Before using this manual, the reader should refer to the chapter entitled “Ground Anchor Inspector’s Manual”, in the Task Force 27 report entitled “In Situ Soil Improvement Techniques”, prepared by the Subcommittee on New Highway Materials of the AASHTO-AGCARTBA Joint Committee. Copies of this chapter are available upon request from the Geotechnical Engineering Bureau.

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II. GLOSSARY OF TERMS 1. Anchorage – The portion of the tieback, including bearing plates, nuts, and wedges, that is used to transfer load from the structure to a tendon. 2. Bond Length – The portion of the tieback which is bonded to the soil or rock and transfers the tensile force from the tendon to the soil or rock. 3. Centralizer – A device used to center the bond length of the tieback in the hole to ensure minimum required grout cover over the tieback. 4. Creep Rate – The magnitude of total movement measured per log cycle of time during a load hold. 5. Encapsulation – The portion of the tieback which encases or encapsulates the entire length of the tieback, including the sheath in the free length, to provide an additional barrier to corrosion. 6. GUTS – The guaranteed ultimate tensile strength of the tendon. 7. Residual Movement – The permanent set of the tieback measured after stressing and releasing the tieback. 8. Sheath – The portion of the tieback which encases the tendon in the stressing length only. 9. Spacer – A device used in the bond length of strand tendons to separate each strand and permit the grout to bond with each strand. 10. Stressing Length – The portion of the tendon which is not bonded to grout. 11. Tendon – The steel used to transfer load from the anchorage to soil or rock. 12. Tendon Bond Length – The length of the tendon which is bonded to the grout. This is usually, but not necessarily, the same as the Bond Length. 13. Tieback – A system used to transfer tensile loads from a structure to soil or rock. A tieback includes all prestressing steel (tendon), the anchorage, grout, coating, sheathing, couplers, and encapsulation, if used. 14. Total Movement – The total elongation of the tieback under load measured at the anchor head. 15. Trumpet – A steel pipe or tube, integrally attached to the bearing plate, which surrounds the tendon in the vicinity of the structure.

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III. CHECKLISTS A.

Preconstruction Review the geotechnical report for the project. Review and become familiar with the plans and specification, including the construction sequence. Review the approved Grouted Tieback System (GTS) submittal and become familiar with the proposed design and method of construction. Contact the Regional Geotechnical Engineer to discuss the expected ground conditions at the site.

B.

Materials Check all Mill Test Certificates for compliance with the specifications. Ensure tiebacks are assembled according to the Contractor’s approved procedure. Specifically check: ● The spacer and centralizer spacing; ● That centralizers will ensure a minimum of ½ inch (12 mm) of grout cover all around; ● That the number of strands (or size of bar) is correct; ● That tiebacks are encapsulated, if required by the contract documents. Tendons should be covered and stored off the ground, out of the way of construction equipment. Tendons should be inspected for rust just prior to installation. Loose, powdery rust should be rubbed off, but not sanded. Tendons with nicks or pits should be rejected. Confirm the materials supplied for the grout mix match the mix design and are in compliance with the specification.

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C.

Construction 1.

Drilling Record appropriate information for each anchor installation on a copy of the “Ground Anchor Installation Form” included in this manual. Verify the anchor hole location. Verify that the anchor declination angle at the time of drilling is the same as that shown in the plans. Log the soil and rock cutting brought up by the drilling operation. Check that the anchor hole is positioned so that the tendon does not have to be pulled over laterally or vertically to be attached to the structural system.

2.

Tendon Installation Inspect uncased holes with a mirror or flashlight for caving or loose debris. The hole should be open and clean. Make sure the tendon is handled carefully to prevent damage. If the bond length of an encapsulated tieback is grouted before insertion into the hole, the Contractor should provide uniform support to prevent the grout from cracking. Temperature of the tendons must be above 32° F (0° C) to prevent the grout from freezing on contact. Verify that the bond length is degreased prior to installation. Make sure the tendon is inserted to the minimum unbonded length. Do not permit the Contractor to force the tendon in with machinery. The trumpet should be long enough to extend well over the sheathed tendon, but not for the entire unbonded length. The tendon should not contact the sides of the trumpet at any time.

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3.

Grouting Contact the Materials Group to inspect and approve the grouting equipment. The grout should be pumped at the lowest point of the drill hole so that the hole is filled progressively from bottom to top. The Contractor should continue pumping grout until it flows free of impurities and lumps. The Contractor should continue to pump grout as the grout tube, auger, or casing is removed. The grouting equipment should be equipped with a working pressure gauge. Monitor and record the grout pressure used and verify agreement with the approved mix design. Clean the grout pressure gauge at least daily to prevent clogging. Measure and record the grout volume placed in the hole. Perform Quality Assurance Testing – Density of Fluid Grout: Use a Baroid Mud Balance in accordance with the American Petroleum Institute (API) Recommended Practice (RP) 13B-1: Standard Procedure for Testing Water-Based Drilling Fluids to check the density of each batch of mixed grout prior to placement. If the State elects to test compressive strength, perform the following: Grout Cubes: Cast a mold set (three (3) grout cubes) for every 50 yd3 of grout placed in accordance with the procedure contained in Materials Bureau Test Method NY701-19E Grout Cube Molding Procedure The Contractor should not reverse the auger rotation while grouting. The trumpet and anchorage should be completely filled with grease or grout, after testing has been completed.

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For grease-filled trumpets, be sure the seal is in place and the grease level does not drop. D.

Stressing and Testing Obtain the bar or strand properties (i.e. steel modulus and area), necessary to calculate elongation, from the approved GTS submittal and verify the delivered materials are the same as specified. The dial gauge should be in proper working order and should have appropriate travel length. Obtain calibration curves for each jack and pressure gauge set, and for each load cell. The jack or pump should have a bleed-off valve. The load cell and jack should be aligned concentrically with one another. The dial gauge should be aligned with the axis line of the anchor. Do not allow the jack to drop onto or lay on the anchor. The load in the anchor should not drop below the alignment load. For a bar tendon, be sure the spherical nut inside the jack chair does not come into contact with the jack. When strand tendons are used, watch the wedges to ensure that the strands are uniformly loaded. During a creep test, the load should be held constant. The load must be held within 25 psi (175 kPa) if a jack pressure gauge is used or within 200 lbs (900 N) if a load cell is used.

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Do not allow the top row of anchors to be stressed until the backfill material has been placed and compacted above the anchor behind the structure. Record all reading and other pertinent information during testing.

E.

Documentation Make copies of all appropriate forms for anchor installation and testing. Record and log each anchor installation, which includes: drilling, subsurface conditions encountered, tendon installation, and grouting. Record and plot test results for each anchor. All forms and calculations should be complete, accurate, and up to date. Log dates, times, and weather conditions on all records. Keep a photographic record along with the written documentation.

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IV. LOAD CELL TROUBLE SHOOTING Lack of agreement between a load cell and pressure gauge has three common causes: 1. Eccentric loading. This can be resolved by using deformable washers, angle washers, or spherical seats to ensure perpendicularity, and centralizers to center the tendon. 2. Improper load cell calibration. This can be avoided by performing the calibration with a dummy tendon to simulate field conditions, rather than between the platens of a testing machine. 3. Improper calibration of hydraulic jack and gauge. This can be avoided by activating the jack against a passive testing apparatus rather than activating the testing machine. The full range of ram travel should be tested as well as the effect of off-center loading.

V. REFERENCES Subcommittee on New Highway Material (1990) “Ground Anchor Inspector’s Manual”, In Situ Soil Improvement Techniques, AASHTO-AGC-ARTBA Joint Committee, pp. 200-286.

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APPENDICIES

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