Method Statement for Construction

of Basement

Holly Cottage

Ketton Road

Upper Hambleton

Oakham

LE15 8TH

PCC Document Management

Job Number: 9500

Date of Issue: Nov 2016

Author: Peter Cole

This report has been prepared by PCC Consultants Ltd with all the reasonable skill, care and diligence within

the terms of the Contract with the client, incorporating our General Terms and Conditions

Contents

Contents ............................................................................................................. 1

1INTRODUCTION .............................................................................................. 1

1.1Instructions ......................................................................................................................... 1

1.2Basis of Report ................................................................................................................... 1

2DESIGN PARAMETERS .................................................................................... 2

2.1Geology and Ground Conditions ............................................................................... 2

2.2Layout of Basement ......................................................................................................... 2

2.3Construction Methods .................................................................................................... 3

2.4Conclusion ........................................................................................................................... 4

3APPENDICES .................................................................................................... 5

3.1APPENDIX 1 – Key Plans & Typical Section ............................................................ 5

3.2APPENDIX 2 – Trial Hole Logs – Mary’s Cottage Gable Wall ........................... 9

1 INTRODUCTION

1.1 Instructions

PCC Consultants have been instructed by Grange Developments Ltd. In

connection with the planning application for the construction of a new

dwelling known as Holly Cottage, Ketton Road, Upper Hambleton,

Oakham, LE15 8TH.

This report provides a method statement for the safe construction of a

basement adjacent to a listed building known as Mary’s Cottage located

to the West of Holly cottage, and Gwithian located to the East.

1.2 Basis of Report

This report has been prepared from architectural drawings supplied by

Origin Design Studios and a trial hole investigation undertaken by

subcontractors employed by Grange Developments Ltd.

A proposed sequential process for the construction of the basement is

included in the appendix of this report. We have considered within this

report options for the safe construction of the basement and our proposal

is based upon the considerations of safety, practicality and cost.

2 DESIGN PARAMETERS

2.1 Geology and Ground Conditions

The site of Holly Cottage is located at grid reference SK 90287 07561. The

geology of the location obtained from the British Geological Society maps

indicate that the site is underlain by solid geology comprising the

Northampton sand formation comprising sandstone, siltstone and

mudstone. Trial hole excavations on the site have confirmed a layer of

fragmented ironstone overlying stiff blue clay.

The geology of the site dictates the form that any temporary works and

construction method should take.

2.2 Layout of Basement

Architectural drawings indicate that the basement is set back from the

front elevation of the proposed dwelling. This layout ensures that the

basement will not be constructed immediately adjacent to the sensitive

listed building on the West boundary and sufficiently far from the

property on the eastern boundary (3.5 m) such that no special provisions

are required for the eastern side of the basement (the Part Wall (etc) Act

1996, 6 metre rule does not apply as the basement excavation and

retaining structure will not extend below a line drawn at 45 degrees from

the near face of the existing adjacent foundation). Therefore to the north,

south and east, no specific precautions need to be taken for the

construction of the basement retaining structures except as required to

comply with normal site safety standards such as setting an appropriate

angle for excavation, provision of adequate working space, and

appropriate guarding at the top of the excavation.

The south west corner of the basement is located at a distance of 2.25

metres from the rear left hand corner (north east corner) of Mary’s

Cottage and accordingly it will be necessary to take appropriate steps to

ensure that the construction of this corner of the basement (and the

whole of the west wall of the basement as this runs parallel to the newly

constructed boundary wall) is carried out in a safe manner.

1.1 Construction Methods

There are three methods of construction that have been considered for

the formation of the west retaining structure. These are;

1. The use of sheet piling. Sheet piles can be installed using low

vibration driving methods. The use of sheet piling can produce a

quick and effective temporary or permanent retaining structure. In

this instance the presence of siltstone/ironstone will not allow the

installation of the piling without extensive excavation and

backfilling first. Any rock obstructions will cause deflection of the

piles and may lead to substantial vibration or disturbance. Sheet

piling is not considered viable.

2. Contiguous bored piling. The comments made above regarding

the presence of siltstone/ironstone apply similarly to bored piles.

3. Progressive excavation and mass concrete filling. With

foundations excavated below the level of the base slab and set to

one side of the retaining structure, excavation of 2.5 to 3m long

foundations will allow for the safe installation of a temporary

retaining structure. Each section will be formed with a “Toggle”

joint to ensure adequate key between adjacent sections to ensure

stability and continuity of the permanent works. The use of a long

reach digger will allow for the progressive installation of a

“diaphragm” wall. This would include returns at the front and rear

of the basement, which would be installed first as noted upon the

attached sketches in Appendix 3.0 of this method statement.

Excavation behind the diaphragm wall will be undertaken only

when a waling beam is in place, and use can be made of a capping

beam in the zone of the foundations to provide this waling beam.

During the process of excavation and particularly when

constructing those legs adjacent to Mary’s Cottage, use should be

made of trench boxes to ensure that the sides of excavations are

properly restrained.

The basement floor will be constructed sequentially in co-

ordination with the excavation of the basement void. The

basement slab will be part of the foundation construction and

provides the support to the load bearing inner skin and internal

walls.

1.2 Site Drainage

Trial hole logs indicate the presence of ground water particularly at the

interface between the ironstone and the clay. This has been highlighted

by Stephen Johnson, the consulting engineer who has been appointed by

the owner of Mary’s Cottage. The basement excavation will necessitate

the process of dewatering, and taking into account the need to maintain

stability of excavations, well point de-watering would be prudent. Well

point dewatering should be installed as soon as possible to a depth of

approximately 4.0m below ground level and for a period that will ensure

that water is not a factor during the excavation of the diaphragm

construction.

2.3 Conclusion

Sectional construction of a mass concrete diaphragm construction will

provide a safe temporary retaining structure permitting the formation of

the basement. The construction will ensure that there is no disturbance to

the adjacent listed building.

The proposed mass concrete reinforced diaphragm wall will provide a

permanent stability and support to the adjacent foundations of Mary’s

Cottage.

P J Cole BSc CEng FIStructE MICE

PCC Consultants Ltd., Chartered Structural Engineers

Signed:

28 November 2016

Amended 11 January 2017

3 APPENDICES

3.1 APPENDIX 1 – Key Plans & Typical Section

Key Plan

Proposed Permanent Mass Concrete Sectional Diaphram Wall Plan

Proposed Section to Permanent Mass Concrete Sectional Diaphram Wall Plan

3.2 APPENDIX 2 – Trial Hole Logs – Mary’s Cottage Gable Wall