Work Method Statement for Cavity and Slumpzone Probing & Treatment

WORKS METHOD STATEMENT

CAVITY AND SLUMPZONE PROBING & TREATMENT

TABLE OF CONTENTS

1.0 WORK ORGANISATION
2.0 DESIGN CONSTRAINTS AND COORDINATION
3.0 CONTRACTING METHODS
4.0 PROCUREMENT METHODS
5.0 RESOURCE PROVISIONS
6.0 LOGISTICS PROVISIONS
7.0 MATERIAL MANAGEMENT SYSTEMS
8.0 CONSTRUCTION METHODS AND TECHNOLOGY
9.0 CONSTRUCTION PHASING AND WORKFLOW
10.0 TRAFFIC MANAGEMENT AND MONITORING
11.0 SAFETY PROGRAMME
12.0 QUALITY ASSURANCE AND CONTROL PROGRAMMES
13.0 INDUSTRIAL RELATIONS POLICIES
14.0 ENVIRONMENTAL MONITORING PROGRAMMES
15.0 APPENDICES


1.0 INTRODUCTION

This submission explains the method of construction to be adopted for the Cavity and Slumpzone Probing and treatment at the project site.

2.0 DESIGN CONSTRAINTS AND COORDINATION

Not applicable.

3.0 CONTRACTING METHOD

The conditions of Contract have been reviewed by PCSB and particularly those conditions in respect of Sub-PCSBs. The conditions are bespoke for this project and therefore be required to be reflected in any form of subcontract utilized on this project.

Please refer to PHOS-CN-010 to PHOS-CN-030 for the Contract Procedures.

4.0 PROCUREMENT METHOD

Please refer to PHOS-CN-010 to PHOS-CN-030 for the Contract Procedures.

5.0 RESOURCE PROVISIONS

5.1 Manpower / Labour

The number of manpower / labour to be used during the activities have been identified in the Manpower Record (Weekly Progress Report).

6.0 LOGISTICS PROVISIONS

The logistic control shall be in accordance with the approved Site Logistic Plan.

7.0 MATERIAL MANAGEMENT SYSTEMS

Please refer to PHOS-CP-040 for the Material Control Procedure.

8.0 CONSTRUCTION METHODS AND TECHNOLOGY

8.1 Details of the construction method are considered according to the following matters:

8.1.1 Cavity and Slump Zone Probing

Drill hole using a soil investigation machine using casing. Water shall be used as a flushing agent. Drill to rock level and proceed to core limestone bedrock to a depth of 10m if no cavity is encountered. If cavity / cavities are encountered, carry out standard penetration test at interval of 1m until next layer of rock is encountered.
The penetration speed of the cavity probing shall be recorded in every 1m penetration intervals starting from the ground level. Penetration speed versus depth with cavity/cavities shall be plotted. Then core the limestone to a depth of 10m below the last cavity encountered.

* The results of probing test shall be submitted to the R. E. for his decision on the grouting treatment required to be used.

The proposed format for verification of grouting works is included in appendix 15.5.

8.1.2 Types of Treatment

A. Drilling and Compaction Grouting for Cavity Treatment works

Drilling shall be carried out at approved pre-determined position by means of air and/or water flushing medium. Temporary casing of appropriate diameter, if necessary, shall be installed to suitable depth. Drilling sufficient depth into the bedrock as specified to determine cavity/cavities. Grouting method to be followed as:

i. Cement mortar of grade 5 shall be used where substantial cavity/cavities is/are encountered.
ii. Upon completion of drilling, install 90mm OD grout pipe and with temporary casing after flushing of hole.
iii. Connect directly from mortar pump or equivalent static pump to the grout pipe.
iv. Pump mortar and observe the working pressure reading registered at the pump set. Subject to a maximum registered pressure of 100 bar, the grout pipe shall be gradually withdrawn at about 0.5m interval within rock region and continue pumping of mortar.
v. The cycle of grout pipe withdrawal and mortar pumping shall repeat until the pipe toe reaches rock surface level. Thereafter, the drilled hole shall be mortar grouted to overflow.

Pressure release hole : Nearby bored holes before grouted shall form as a pressure release holes.

In case of multiple cavities encountered in any drilled hole, the grouting shall proceed from the lowest cavity and completed before proceeding to the next higher cavity.

The acceptance test shall be carried out 7 days after the completion of grouting process to determine the compliance to the Criteria of Acceptance as stated by the Engineer. The test shall be carried out by wash boring method, SPT shall be carried out in soil and grout cores shall be obtained for the compression test.

Compaction grouting shall only be deemed to have been successfully completed if :

i. For soil within treatment zone, the individual ‘N’ value at any depth shall not less than 20 and the average ‘N’ value shall not less than 25.
ii. No void is encountered.
iii. Unconfined compression strengths of the cores are in excess of 2 N/mm2.


B. Pressure Grouting (Tube a Manchette Grouting)

Drill hole using appropriate tools to the required level. Water shall be used as drilling fluid in soil while in the rock, water and compressed air shall be used.

Prepare 50mm diameter PVC tube with Tube a Manchette at the appropriate location where treatment is required. Lower 50mm diameter PVC tube into complete drill hole. Fill drill hole with bentonite cement grout mix as sleeve grout. Mix proportion is :

Bentonite 40kg
Cement 200kg
Water 850litres

When grout overflows, stop grouting. Withdraw casings and top up grout before all the casings are withdrawn.

One day after grouting, wash off grout in the PVC pipe with water. After about 1-2 days, proceed with pressure grouting. The mix proportion is : Water / cement ratio is 0.45.

A double pecker is used for pressure grouting and the pressure used must not be greater than overburden pressure +5psi. This is to avoid excessive uplift due to excessive pressure. Pump capacity shall be up to a maximum pressure of 60 bars.

The pressure grouting shall be terminated if either of the followings criteria is encountered :

i. The ground surface heave up as measured from continuous precise leveling.
ii. Achieving the injection pressure specified.

Pressure release hole : Nearby bored holes before grouted shall form as a pressure release holes.

In case of multiple cavities encountered in any drilled hole, the grouting shall proceed from the lowest cavity and completed before proceeding to the next higher cavity.

Flush the grout within the PVC pipe with water so that future pressure grouting can be carried out.

Monitor level of ground surface using leveling during grouting.




c. Large Empty Cavity Treatment

Install 500mm diameter temporary casing to rock head. Then bore out soil within temporary casing. Core rock to puncture roof of large empty cavity. When the empty cavity and the temporary casing is filled up with the aggregate, extract the temporary casing.

9.0 CONSTRUCTION PHASING AND WORKFLOW

Please refer appendix 15.3

10.0 TRAFFIC MANAGEMENT AND MONITORING

10.1 Maintenance of Existing Roads, Footpaths and Service Drains, Etc.

The construction team shall maintain all existing site access, roads, footpath, service drain, etc. and reinstate any damage caused by any reason whatsoever during the progress of the works.

It will be the Logistic Manager’s responsibility to ensure that ingress and egress to the site are kept free from obstruction brought about by the work on this site and in no way shall cause hindrance to traffic or ancillary works either by his own vehicles or by his work people, material, etc.

11.0 SAFETY PROGRAMME

11.1 Safety Hoarding, Temporary Works and Public Safety

The Logistic Manager shall be responsible for submission of plans and drawings to the relevant Authorities for the construction of all the necessary temporary fencing and protection hoardings, temporary drains and desilting pits, safety nettings, screens, etc. for the works.

The Logistic Manager shall provide adequate signboards at all strategic positions warning the public to keep away from the work site and erect temporary fencing and barriers where necessary around the site to prevent unauthorized trespassing during works.

11.2 Pollution and Disturbance

The HSSE Manager shall be responsible to take necessary measures to ensure that noise and air pollutions are orderly controlled to satisfy the full requirements of the relevant authorities.

He shall also ensure that disturbance due to noise and air pollutions caused by the works to the neighbourhoods and public are kept to an absolute minimum.
In particular, the HSSE Manager shall ensure that the existing driveway areas are constantly wet to prevent excessive dust / air pollution during the demolition work.


12.0 QUALITY ASSURANCE AND CONTROL PROGRAMMES

Please refer to Appendix 15.1 for Inspection Test Plan and Appendix 15.2 for the checklist.

13.0 INDUSTRIAL RELATIONS POLICIES

Not Applicable.

14.0 ENVIRONMENTAL MONITORING PROGRAMMES

Please refer to PHOS-EMP-001 Environmental Management Plan.

15.0 APPENDICES

Appendix 15.1 Inspection Test Plan
Appendix 15.2 Inspection Checklist
Appendix 15.3 Cavity Probe Layout
Appendix 15.4 Job Safety Analysis
Appendix 15.5 Proposed format for verification of grouting works