ANCHOR

CONSTRUCTION

 Tendon Storage and Handling

Anchor tendon consists of prestressing steel strands, polyethylene injection pipes, centralizers and spacers. Tendons shall be handled and stored in such a manner as to avoid damage or corrosion. Damage to the prestressing steel, as a result of abrasions, cuts, nicks, welds and weld splatter can be cause for partly or wholly rejection of the damaged material by the Engineer. The prestressing steel shall be protected if welding is to be performed in the vicinity. Prestreesing steel shall be protected from dirt, rust, or deleterious subtances. A light coating of rust on the steel is acceptable. If heavy corrosion or pitting is noted, the Engineer shall reject the affected tendons.

1. The Contractor shall use care in handling and storing the tendons at the site. Prior to inserting a tendon in the drill hole, the contractor and the Engineer shall examine the tendon for damage.
2. Lifting of the pre-grouted tendons shall not cause excessive bending, which can debond the prestressing steel from the surrounding grout.
3. Presressing steel shall not be exposed to excessive heat (i.e., more than 230 C).

Anchor Hole Drilling

Methods for anchor hole drilling are left to the discretion of the contractor. The selection of drilling method should account for special corcerns identified in project specifications such as noise, vibrations, hole alignment, and damage to existing structures. The inability of the contractor to establish a stable hole of adequate dimensions and within specified tolerances may be cause for modification of drilling method.
Drilling will be carried out with a fully hydraulic, crawler type drill rig by means of rotary drilling technique with augers or odex or by means of top hammer or down the hole hammer of suitable diameter.
Depending on the soil properties the anchor holes can be drilled by air pressurised rotary drilling method by using drilling machine, drilling tools called augers and drill bit. The air pressure will be used in order to assist the auger erode the soil and expose the disturbed soil outside the hole. Air pressure will be approximately 8 bars and will be supplied by using air compressor and air hose. Holes for anchors shall be drilled at the locations and to the length, inclination and diameter shown on the Contract Drawings or the approved Working Drawings. The drill bit or casing crown shall not be more than 3 mm smaller than the specified hole diameter. At the ground surface the drill hole shall be located within 300 mm of the location shown on the Contract Drawings or the approved Working Drawings. The drill hole shall be located so the longitudinal axis of the drill hole and the longitudinal axis of the tendon are parallel. In particular, the ground anchor hole shall not be drilled in a location that requires the tendon to be bent in order to enable the bearing plate to be connected to the supported structure. At the point of entry the ground anchor shall be installed within plus/minus three (3) degrees of the inclination from horizontal shown on the Contract Drawings or the approved Working Drawings. At the point of entry the horizontal angle made by the ground anchor and the structure shall be within plus/minus three (3) degrees of a line drawn perpendicular to the plane of the structure unless otherwise shown on the Contract Drawings or approved Working Drawings.
Soil and rock types and ground conditions should be recorded during drilling. Unexpected conditions should be carefully documented, any where appropriate, samples should be taken. Drill cuttings and soil exposed in the excavation should be visually classified to identify ground which may be susceptible to caving. Ground which may be susceptible to caving includes: (1) cohesionless soils below the groundwater table; (2) highly fractured or weathered rock; and (3) ground where artesian water pressures exist. Signs of caving include: (1) an inability to withdraw drill steel; (2) a large quantity of soil removed with little or no advancement of the hole; (3) abnormally large drill spoil pile in comparison to other holes; (4) settlement of ground above the drilling location and (5) an inability to easily insert the anchor tendon the full length of the drill hole. Where excessive caving occurs drilling should be halted, and alternative drilling methods should be used to stabilize the drill hole.

1. Tendon Insertion

After drilling, holes should be thoroughly cleaned to remove loose material within the design length. For uncased holes in cohesionless soils, excessive cleaning should be avoided such as would cause significant ground loss. After cleaning is complete, uncased holes should be inspected with a mirror, high light, or by probing. If the hole is to be grouted prior to insertion of the depth be measured to ensure that the tendon can be installed to the full depth. Drill holes may be considered clean if the tendon can be easily inserted to the desired depth.
The dimensions of each tendon should be checked to ensure that the minimum bond and unbonded lengths are equal to or exceed the minimum values specifield for that anchor. Anchors may have specifield maximum values if right-of-way restrictions exist.
Centralizers must be securely affixed to tendons at required intervals and properly sized such that grout may flow freely of the borehole arround the tendon. Spacers must separate individual strands too, so that an adequate thicks of grout covers each strand and to prevent the intertwining of adjacent strands.
If caving occurs during installation, the tendon should be withdrawn and the hole redrilled. The tendon must not be driven.

1. Anchor Grouting

Grouting means filling the anchor hole with water-cement mix (grout). Grouting should be performed either before insertion of the anchor tendon or as soon as practial afterwards to minimize the potential for hole caving. Grouting will be performed using injection set.  The injection set consists of two mixers, a pump and injection hose. Holes open longer than 8 to 12 hours should be recleaned prior to insertion of the tendon or grout. Grouting equipment should allow for continuous and completion of grouting of each anchor in less than one hour. For strand tendons, the strands must be aligned to allow installation of the anchorage while the grout is still fluid (i.e., before the grout starts to harden).
Grouting of the tendon should be performed in one stage. In single stage grouting, the bond and unbonded lengths are filled with grout in one injection sequence. The grouting in the unbonded length should be placed under gravity or low pressure. The drill hole should be grouted to a level which will allow an approximately 300 to 600 mm gap behind the beginning of the hole.
The water / cement ratio according to the weight shall be maximum 0,45.
Grout should be injected at the lowest point in the drill hole to fill the hole without generating air voids.
Grouting equipment usually includes a pressure gauge at the pump. The gauge should be checked periodically and cleaned at least daily.
The grout volume placed in the hole should be measured and recorded. The grout take is defined as the volume of grout actually placed divided by the estimated hole volume. Excessively high grout takes may indicate that grout is lost though a hydraulic fracturing of the surrounding ground or flowing into voids caused by caving of the hole or preexisting voids in the ground.
Post-grouting is performated by injecting grout under high pressure after the initially placed bond zone grout has initially hardened (4- 6 hours after primary grouting). The post grouting pressure shall be in between 10-15 bars. The tendon must be equipped for post-grouting prior to insitallation of the tendon.

1.  Anchorage Installation 

Once grouting is complete, the anchorage (bearing plate and wedge plate) should be installed. The anchorage and tendon must be properly aligned.
The anchor bearing plate must be installed perpendicular to the tendon with the tendon centered in the bearing plate, without bending or kinking the prestressing steel elements. Wedge holes and wedges must be clean and free of rust to prevent slippage of strands and promote proper seating of the wedges.

ANCHORAGE STRESSING & LOAD TESTS

2.1 Introduction

The selected ground anchors are tested for the determination of the safe load carrying capacity after installation and prior to being assigned as load carrying structural elements within an achored system.
The basic principal behind the anchor testing is to determine the bond zone capacity of the soil or rock and is based on measurment of creep of the grouted body under constant load. The rate of creep of the bonded zone is directly related to the applied load.
Each ground anchor is load tested to verify its capacity. The load test are performed at the ground level and consists of tensioning and pre-stressing stell elements and measuring load and movement and performed as part of the stresssing of the anchors.
Test other than loading (proof), creep and performence test are also conducted to find out the ultimate and safe load carrying capacity of anchors in time.

2.2  Testing & Stressing Equipments

2.2.1 Hydraulic Jack and Pump

A hydraulic jack, working with hydraulicly locking system is being used for the tests. The jack is excited by an electricity powered power-pack and is capable of applying a concentric load to the tendon and transfers the load to all of the pre-stressing elements of the tendon simultaneously.  

2.2.2 Pressure Gauges and Load Cells

A 100 mm 0-700 bars gliserrine manometer is used as pressure gaues, attached to the power-pack. The reading on the manometer indicates the absulute value of the applied load.
For proof test and lift-off tests a pressure gague alone is used for mesuring load.  But in all creep test where an extended time for the load periods are requred an apropriate load cell shall be used in concert with the pressure gauges.
The pressure gauges and the load cell shall have calibration certificates not older than 6 months.  

2.3 Anchor Load Testing    

A unique aspect of ground anchors, as compare to other structural systems, is that every ground anchor that is to be part of a completed structure is load tested to verify its load carrying capacity and load deformation behavior before being put into service. The acceptance or rejection of ground anchors is determined based on the the results of proof test.
Proof tests are the most common as it is part of an anchor stressing procedure. Each ground achor undergoes one of the above stated tests. The results of these tests are used to determine whether any particular ground anchor is meeting the acceptance criteria which are based on allowable creep and elastic movement of the anchor during load testing and can be put into the service.
2.3.1 Proof Tests

The proof test shall be performed by incrementally loading the ground anchor in accordance with the schedule shown in Appendix.I.
 The load shall be raised from one increment to another immediately after recording the ground anchor movement. The load shall be monitored with the primary pressure gauge. At load increments other than the maximum test load, the load shall be held just long enough to obtain the movement reading.
The intermediate load steps in a proof test shall be held for at least five (5) minutes and the maximum load step for ten (10) minutes.

2.3.2 Procedures for Anchors Acceptence Criteria

Anchors must satisfy the minimum and maximum elongation criteria (shown in Appendix.I)  Otherwise, it shall be either rejected and replaced at no additional cost to the Owner or locked off at not more than 50 percent of the maximum acceptable load attained.

2.3.3 Anchor Lock-Off

After testing has been completed, the specified lock-off load is applied to the anchor tendon and the strands shall be locked-off by inserting the wedges into the wedge plate. The magnitude of the lock-off load shall be according to the project specifications.