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Column strip cofferdam construction method
The Japanese construction companies, Ohbaya- shi-Gumi Ltd. and Nitto Kogyo Co. Ltd., have developed a noise- and vibration-free column strip cofferdam construction method for buildings with one or two basement floors. Called the ONS con- struction method, it prevents earth slumping and damage to underground pipes, and takes only a short construction period.
The ONS method, already successfully employ- ed in Osaka and Taipei, aligns reinforced precast concrete piles of shape cross-section to form a
column strip cofferdam. The number and diameter of steel reinforcement members depend on the soil condition and depth of hole, but the use of six members having a diameter of 19 mm is standard.
Alignment of the ONS piles is shown in Fig. 2. The spacing of about 20 mm between piles is filled with soil cement. A mortar or concave pile is used for starting, ending and corner piles to ensure watertightness.
In the order of work, timbers are first laid on the ground to indicate the line along which drilling is carried out. A drilling auger with a rotary casing, called a 'doughnut auger', is suitably placed to start drilling. The drilling equipment is composed of a rotary casing pipe with a diameter of 420 mm, incorporating an earth-auger (380 mm in dia- meter), which counter rotates during drilling. Soil cement is continuously forced down through the hollow auger shaft during drilling.
When the required depth is reached both the casing and auger are drawn up smoothly while soil cement is continuously ejected from the auger shaft. Unsolidified soil cement fills up the hole when the casing and auger are removed. This soil cement contains cement, fly ash and Bentonite
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Fig. 1. Shape of ONS pile. 1. Main member 3-19 Ø; 2. hoop 9 Ø 250; 3. mould frame separator; 4. hoisting hook; 5. handling hook; 6. standard length 12.000m.
Fig. 3. Construction method (one cycle): 1. Reduction gear for casing; 2. reduction gear for auger; 3. soil cement injection hose; 4. casing 420; 5. guide timber; 6. earth auger; 7. ONS pile; 8. filled with soil cement; 9. ONS sheathing.
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Fig. 2. Alignment of ONS piles. 1. PC pile; 2. ONS pile; 3, execution order; 4. mortar pile; 5. execution method at corner.
Far East BUILDER, June 1971
Fig. 4. Water preventive effect. 1. Surface soil; 2. water; 3. sand layer; 4. shore; 5. ONS sheathing; 6. excavation ground; 7. soil cement column sheathing; 8. clay layer.
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