- Areas of Expertise
- Construction Methods
- Machinery & Equipment
TEMPORARY PRESTRESSED ANCHOR CONSTRUCTION METHOD
1.0-) SCOPE OF WORK.
Anchors are horizontal elements constructed in deep excavations for the prevention of vertical displacements of and rotation within structures.
2.0-) MACHINERY AND EQUIPMENT
- Drilling Rig designed for rotary plus percussive operation
- Rods + Down The Hole Hammer + Consistent Bit
- Continous Flight Auger + Consistent Bit
- Casings and bentonite mixing and pumping set
- Maxbit / Odex
- Air compressor
- Cement Injection Unit (pump + mixer + agitator)
- Prestressing Unit
- Water Pump
- Generator (If needed)
3.0-) DETAILS OF METHOD OF CONSTRUCTION
The machinery tools and drilling system chosen according to soil conditions initiate.
Rotary, Drive, Overburden drilling systems are used in anchors.
3.1.1-) Rotary Drilling
Drilling is realized with the trust and the rotational torque by the transmission of the rods/augers to the ground with suitable bits. Temporary casings or drilling fluids are precautions taken whenever there are collapsing soil conditions.
3.1.2-) Percussive Drilling Method
Used in rough soil structures. Two different applications exist: Top Hammering and Down the Hole Hammering. In Top Hammering by the hammer located at the top of the rotary head, in Down the Hole Hammering by the hammer located at the bottom of the hole, percussions are created to break and grain the soil particles. In percussive methods, generally compressed air is used to flush and cleanse the hole from excavated earth and debris.
3.1.3-) Drilling Method by using Top Hammer and Temporary Casings
This method is applied whenever the soil layers are not stable-tends to collapse- and do not form a clean hole for the purposes of constructing a good load bearing anchor. Temporary casings are driven and extracted by using Top Hammer.
As an alternative, the use of bayonet adaptors are used for cased anchor drillings.
Additionally, when soil condition necessitates, special drilling methods, namely Odex and Maxbit, are used.
3.2-) Procedure for Preparation of Prestressed Anchor Body
Primarily, with the project in mind, steel tendons are cut to the required length. Fixed length of the anchors are prepared by spacers and centered to the sheath by centralizers. The free length is insulated by HDPE pipes. The transition region in between the fixed and free regions is insulated again by pitch thus preventing the transmission of the injection to the free length. From one end to the other, the anchor strands are covered with one or two HDPE pipes.
3.3-) Cement Injection
The adhesion force between grout and tendon depends on mixture ratio of ingredient of the injection, type of preparation and method of injection. For hard and rocky soils, prestress strand is placed to the hole by manpower.Using the first HDPE pipe, the grout is injected into the hole and after two hours procedure is repeated by using the second pipe, which is a procedure fulfilled for safety. For clayey or fill soils, pre-grouting is accomplished by placing a pilot pipe before placing anchor strands. Portland cement is used. The water in the mix should have the same quality as the concrete. The definitive property of the cement slurry is w/c ratio, the optimum being in between 0.42 and 0.45. Expander, retarder, accelerator, fluidifier etc.should at this stage be added to the mixture. Briefly, the prepared mixture at desired ratios are injected to the hole at high pressure. During injection, it is most important that the present water within the hole migrates.
- Tendon materials
- Bearing Plate
- Anchor Head
- Wedge Plates
Diameter and number of tendon materials, dimensions of bearing plates, diameter of anchor head and number of holes, number of wedge plates are all calculated depending on anchor forces in the project.
3.4-) Pre-stress Test
Bearing plate, wedge plates and the anchor head are placed to the anchor, minimum 7 days after its' completion. The anchor strand should extend outwards 90-100 cm from the front face of the soldier beam. By the means of using the hydraulic jack with the power unit, readings are taken at the 25, 50, 75, 100 and 125% of the design load. Deformations are read likewise with a measuring scale and recorded. The load will eventually be decreased back to design load, the anchors are locked at the prespecified design load.
For each anchor, a record card is filled. It includes date of drilling and injection, length and quantity. Filled sheets are sent to the employer for approval.
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