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PRE-BORING WORK PRIOR TO DIAPHRAGM WALL CONSTRUCTION
CONTENTS OBJECTIVE PRE-BORE METHODOLOGY ------------------------------------------------------ 3 A) AIR-LIFT METHOD -------------------------------------------------------------- 4 Guide Wall and Casing Installation ---------------------------------------------- 4 Pre-boring --------------------------------------------------------------------------- 5 Pre-coring --------------------------------------------------------------------------- 6 Backfilling -------------------------------------------------------------------------- 8 Diaphragm Wall Soil Excavation ----------------------------------------------- 10 Pre-boring P3 & P4 --------------------------------------------------------------- 11 Pre-boring P5 & P6 --------------------------------------------------------------- 12 Diaphragm Wall Rock Coring --------------------------------------------------- 13 B) RCD(REVERSE CIRCULATION METHOD) ------------------------------- 15 APPENDIX A - LAYOUT PLAN OF PRE-BORING - ENLARGE LAYOUT OF PRE-BORING
Objective: To remove hard strata prior to diaphragm wall construction 1. Pre-boring Methodology
Pre-boring is selected to ease the excavation for diaphragm wall at hard strata / high rock level in these area prior to diaphragm wall excavation. It entails the process since inception of guide wall construction, pre-casing installation, coring and backfill with LSS(Liquefied Soil Stabilizer) and lean concrete at above rock bed and within rock bed zone for backfilling respectively. Based on current available bore-log information, pre-boring of 1000mm diameter along Dwall alignment, please see Appendix A have been selected for such works. Thus, 2 pre-boring methodology will be used to carry out, namely; Air-lift and RCD(Reverse Circulation Drill) methods. A) Air-lift method: Using a boring rig attach with kally-bar mounted with a self-collecting box core barrel to perform pre-boring with air supply to create “suction” effect to clear out drilling fragments from pile toe. The air-lift method of clearing the drilling from pile toe are air-lifted into “collection box” situated above the core barrel, thus every interval of an hour may be required to retrieve its kally-bar to empty those loose rock fragment being collected in the “collection box” or
A) Air-lift method Casing Installation
1. Construct the guide wall for diaphragm wall alignment accordingly to pre-bore location layout, catering for the outer casing diameter of 1050mm. 2. Only applying for pre-bore location, its guide wall internal face to internal face width shall be constructed as in Figure 1 to suit the casing installation for pre-boring works. 3. Pre-arrange with marking of each diaphragm wall(D-wall) joints on guide wall top for initial casing setting out for pile 1 and 2 at every joint of D-wall.
P1
P2
Pre-boring
1. Pre-bore to clear out materials with boring bucket within casing at pile 1 until hard strata/rock top level as shown below Figure 2; similarly for P2 casing and its pre-boring works as well as for all the intended pre-boring location are apply.
P1
P2
Pre-Coring
1. Replace boring bucket with Roller Bit Core Barrel, Figure 3 next page; to continue rock head coring. The core barrel is fitted with a conceal pipe within its wall, and connected to the suction channel to supply air from a compressor unit, so as to create a ‘suction-effect’ within the suction channel to air-lift the crush strata beneath the roller bit at pile toe level into the collection box located above the roller bit.
Suction channel – for hard fragment clearing from pile toe
Suction channel – for hard strata fragment clearing from pile toe
Enlarge Details on Core Barrel Process Air supply inlet from compressor
Suction channel
Rock fragments sink into collection box
Backfilling
1. Once the desire depth of that D-wall panel is reached, retrieve core barrel and kelly-bar. Moved off boring rig to pile 2 continue pre-boring works in similar manner described earlier. 2. While pile 1 is prepared with the lowering of tremie pipe, G20 shall be used for the backfilling from pile toe to the earlier hard strata/rock level. Remaining from hard strata/rock level to guide wall toe/base shall be backfilled with LSS/Grade7 lean concrete as shown below;
P1
P2
LSS / G7
in jumping manner at the construction manager discretion so as to provide a continuous pre-boring within site constraint. 4. Pre-bore and cast in similar manner to the rest of the pile. In event of more pre-bore holes are requires to carry out at others location other-than those shown in Appendix A, upon further information are obtained from the probing soil investigation, new location to be pre-bored shall be added and revised for information.
Diaphragm wall Soil Excavtion
1. Commence grabbing or use cutter with Bentonite slurry, if available to pre-excavate out those earlier backfilled LSS/G7 lean concrete and remaining soil shown below until earlier hard strata / rock head again;
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Re-excavate earlier LSS & soil
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Earlier hard strata /
Pre-boring P3 & P4
1. Installed casing into excavated trench to setout remaining pre-bore location P3 and P4 on guide wall top. Repeat using roller bit to desintegrate the hard strata / rock bed again at P3 & P4.
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P1
P4
P3
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P2
Pre-boring P5 & P6
1. Upon hard strata / rock bed are cleared. No backfilling of G20 is required for these last pre-bore piles but to retrieve casing for the cutter for further excavation with Bentonite slurry.
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P5
P6
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Diaphragm Wall Rock Coring
1. After casing removal, position cutter for excavation as shown below with Bentonite slurry;
2. Continue excavation rock bed until desire Dwall design depth is reached. Proceed to desanding, lower steel cages and casting.
B) RCD method
In RCD methodology, the pre-boring procedures and coring of hard strata / Rock bed drilling arrangement is similar described as in air-lift method, but for RCD method, RCD drilling section of pipe are used instead of kally-bar; core barrel is connected to RCD pipe with supply of air into RCD pipe to create “suction-effect” within RCD pipe to air-lift the drilling chippings/fragments with bentonite slurry within core barrel via RCD pipe into collection tank at ground level as shown below;
