Preboring Works to DWall with Air-lift & RCD Core Barrer - 17.6.14.pdf

8/11/2019 Preboring Works to DWall with Air-lift & RCD Core Barrer - 17.6.14.pdf

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PRE-BORING WORK

PRIOR TO

DIAPHRAGM WALL CONSTRUCTION


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CONTENTS

OBJECTIVE

PRE-BORE METHODOLOGY ------------------------------------------------------ 3

A) AIR-LIFT METHOD -------------------------------------------------------------- 4

Guide Wall and Casing Installation ---------------------------------------------- 4

Pre-boring --------------------------------------------------------------------------- 5

Pre-coring --------------------------------------------------------------------------- 6Backfilling -------------------------------------------------------------------------- 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


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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

deemed at the operator opinion that the collection box is fully filled.

B) RCD method: Using a boring rig attach with RCD rod instead of kally-bar, mounted with core

barrel with air supply to create suction effect to clear out drilling fragments. The RCD method

tends to be a one-time process to continuously clean out the drillings from pile toe.

The above 2 methodology of pre-boring are similar with the used of casing installed by either vibro-


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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.

4. Pre-excavate by means of backhoe or excavator to depth of guide wall if necessary at site discretion

for the initial insertion of casing setting/positioning. Commence pile 1 casing installation by vibro

hammer or casing-driving adapter of the boring rig into ground. Check the verticality of casing

during installation by spirit level. Drive casing until some refusal from ground. Commence boring

at pile 1 while pile 2 may continue to start installation of casing into hard strata by mean of vibro

Figure 1 / Section A A (N.T.S)

P1 P2


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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


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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 strata fragment

clearing from pile toe

Suction channel forhard fragment clearingfrom pile toe

Figure 3 Roller Bit Core Barrel

Note: Hard strata level shall be

recorded in the Confirmation of GIIIRock Level and witness by all and in-

charge person to sign-off as in

Appendix B.


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Enlarge Details on Core Barrel Process

Suction

channel

Air supply inletfrom compressor

Collection box collection point of fragment chiptransported by air-lift within core barrel during coring

Rock fragments

sink into

collection box


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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


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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.


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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;

Re-excavate earlier

LSS & soil

Earlier hard strata /

rock head level


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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.

P2

P2

P1

P1 P3P4

P3P4


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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.

P6P5


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Diaphragm Wall Rock Coring

1. After casing removal, position cutter for excavation as shown below with Bentonite slurry;


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2. Continue excavation rock bed until desire Dwall design depth is reached. Proceed to desanding,

lower steel cages and casting.


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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;

Air Flush &

Drill chips


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APPENDIX A

PRE-BORED PILES LAYOUT


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Preboring Works to DWall with Air-lift & RCD Core Barrer - 17.6.14.pdf