Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account of the Site
and Subsidence Events
Project Ref: 10 846/100
November 2002
Client: Reading Borough Council
Civic Centre
Reading RG1 7TD
Peter Brett Associates
Consulting Engineers
16 Westcote Road
Reading, Berkshire, RG30 2DE
Funding Agency: English Partnerships
T: 0118 950 8777
F: 0118 950 8198
E: [email protected]
Prepared by:
D Emanuel
Checked by:
Eur Ing J C S Talbot
Reviewed by:
Dr C N Edmonds
On behalf of Peter Brett Associates Consulting Engineers
Peter Brett Associates disclaims any responsibility to the Client and others in respect of any matters
outside the scope of this report. This report has been prepared with reasonable skill, care and
diligence within the terms of the Contract with the Client and generally in accordance with ACE
Conditions of Engagement (Agreement D) and taking account of the manpower, resources,
investigations and testing devoted to it by agreement with the Client. This report is confidential to
the Client and Peter Brett Associates accepts no responsibility of whatsoever nature to third parties
to whom this report, or any part thereof is made known. Any such party relies upon the report at
their own risk.
© Peter Brett Associates 2002
Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Brief Overview of the Project
A large ground subsidence occurred on 4 January 2000 outside the front of Nrs 6 and 8 Field Road
Reading, (see Figures 2.1 and 2.2 for the Site Location and Site Plan, respectively) forming a crater
8m diameter and 4m maximum depth. This loss of ground support caused part of the adjacent
highway and the front and party walls of these properties to collapse and underground utilities
services to be severed. A number of residents were evacuated for health and safety reasons. Peter
Brett Associates was appointed on 5 January 2000, to advise Reading Borough Council on the
cause of the subsidence and the design of possible remedies.
Plate 2.1 Initial Collapse on 04 January 2000
The immediate task was to make safe the crater and the partially collapsed houses. This
was achieved by filling the crater with crushed concrete and erecting a temporary jackable
scaffold to support the remaining front elevation of Nrs 6 and 8 Field Road. This work was
completed by late on the day following the collapse. Investigations were commenced
immediately thereafter, to establish the nature of the ground conditions surrounding the
subsidence, and to determine the cause of the problem and its extent. These investigations
were extended in a series of stages and involved a range of techniques, including dynamic
probing, boreholes, and cctv surveys. In addition, geophysical surveys allowed a preliminary
assessment of a larger area of land around the collapse to be completed non-intrusively
using microgravity techniques. This approach permitted better targeting of follow-on
intrusive work.
The investigations found extensive historical chalk mines in the Field Road area, the full
extent of which have not yet been established. The tunnel roofs of the mine workings vary in
depth from about 7.5m to 11m below ground level. The tunnel floors are typically around
13m to 14m below ground level, while their width is predominantly 3m to 4m. The condition
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
of the mine workings was found to be poor, with many areas of the roof having collapsed.
The latter process appeared to be continuing, at a gradually increasing pace. Indeed, during
the course of the Project, before the ground had been stabilised, a further four subsidence
events occurred near the initial event, three of which caused structural damage to the rear
of Nrs 8, 10 and 18 Field Road.
Reading Borough Council successfully applied for and received funding under the
Government’s Land Stabilisation Programme, which is administered by English Partnerships
as an agency, at the time of application, of DETR (the Department of Environment,
Transport and the Regions). This Central Government body subsequently became, for a
short time DTLR (Department of, Transport and the Local Regions) prior to coming under
the aegis of the Office of the Deputy Prime Minister (ODPM).
After a review of options for treatment, it was concluded that grouting techniques
represented the most suitable and economical method for stabilising the chalk mine
subsidence problems. Grouting is a process whereby a cement-based liquid is injected into
the ground under pressure through hollow metal tubing that has been drilled into the ground.
The grout spreads through the ground infilling voids and strengthening weak ground. After
completion of the process the grout sets hard within 24 hours, gaining further strength with
time.
Plate 2.2 Stabilisation Works
The grouting works commenced at the junction of Castle Hill with Field Road and proceeded
southwards to the south end of the evacuated area, which had been fenced off. The mine
workings under the houses were partially stabilised from the road and from their rear
gardens, with additional grouting being carried out inside the even-numbered houses, using
a portable drill rig.
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Garden restoration works and repairs to the damaged houses started as soon as safe
access was available. Work on the gardens and buildings continued after the completion of
the grouting contract, and repairs were made by the various utilities to their buried services
that had been damaged by the mine roof collapses. After repairs and renewals had been
made to the buried services, and near the completion of the restoration works to the
gardens and buildings, repair of the highway took place.
Following intensive teamwork by all parties involved in the Project, the evacuated residents
were able to return to their homes, precisely on programme and before Christmas 2001,
exactly 23 months after their initial evacuation.
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Table of Contents for Volume 2 –
An Account of the Site and Subsidence Events
Brief Overview of the Project ................................................................................................ 1
Contents of the Final Report – Volumes other than Vol 2 ..................................................... 5
Contents of Volume 3 – Desk Studies and Research Report ................................................. 5
Contents of Volume 6 – Interpretation of Investigations and Implications .............................. 7
1
Introduction .............................................................................................................. 11
1.1
1.2
2
3
4
5
6
7
8
9
Introduction to this Volume ..................................................................................... 11
Structure of the Final Report ................................................................................... 11
Site Location ............................................................................................................ 12
Site Description ........................................................................................................ 13
Published Contemporary Geology Of The Area ......................................................... 15
Summary of Events on 04 and 05 January 2000 ....................................................... 17
Evacuations and Temporary Accommodation ............................................................ 19
The Interpreted Sequence of Initial Events................................................................ 20
Subsequent Failure Events ....................................................................................... 21
References ............................................................................................................... 26
List of Tables
Table 2.1
Summary of Geology ................................................................................................... 16
Photographs
Plate
Plate
Plate
Plate
Plate
Plate
Plate
Plate
Plate
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Initial Collapse............................................................................................................... 1
Stabilisation Works ........................................................................................................ 2
View along Field Road from the North ......................................................................... 13
Collapse Damage to Nr 6 & 8 Field Road..................................................................... 17
Scaffolding Support to Collapsed Walls of Nrs 6 & 8 .................................................... 18
View of Collapse with Adjacent Subsidence ................................................................. 20
View of Collapse Shaft at Rear of Nr 18 Field Road ..................................................... 21
“Staircase” of Mini-conveyor Belts from Garnet Hill to rear of Nr 18 ............................. 23
Rear of Nr 10 Field Road after Collapse of 08 January 2001........................................ 24
Figures
Figure
Figure
Figure
Figure
Figure
2.1
2.2
2.3
2.4
2.5
Site Location
Site Plan
The Site and its Surroundings
Extract of 1:10 000 scale Geological Map
Locations of the Recent Collapses (since 01 January 2000)
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Contents of the Final Report – Volumes other than Vol 2
Please note that relevant Figures, together with a selection of appropriate
Photographs, are presented in each volume and included in their respective detailed
Table of Contents although, for brevity, they are not listed in the following summary
of contents for each volume of the Final Report.
Contents of Volume 1 – Summary Report
Brief Overview of the Project
Table of Contents for Volume 1
Contents of Final Report – Volumes other than Vol 1
1
Introduction
1.1
1.2
2
3
Site Location
Subsidence Events And Implications
3.1
3.2
3.3
3.4
3.5
4
5
Initial Collapse Event
Cause of the Initial Event
Extent of the Site and Phasing of the Work
Evacuation of Residents
Funding of the Works
Geology and Past Use of the Site
Ground Investigations
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
6
7
8
9
Introduction to this Volume
Structure of the Final Report
Investigation Strategy
Services Clearance
Geophysical Surveys
Dynamic Probing
Window Sample Boreholes
Light Cable Percussion Boreholes
Closed Circuit TV Surveys
Interpretation of Mine Workings
Ground Stabilisation
Restoration Works to Properties
Reading Borough Council’s Role
Future Work
Appendix 11 Glossary of Terms Used in the Reports
Contents of Volume 2 – this volume – see the preceding pages for a separate
Table of Contents
Contents of Volume 3 – Desk Studies and Research Report
Brief Overview of the Project
Table of Contents for Volume 3
Contents of Final Report – Volumes other than Vol 3
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
1
Introduction
1.1
1.2
2
3
4
5
6
EnviroCheck Report and Historical OS Maps
Other Historical Maps
Summary of Historical Studies
Historical Geological Records
Previous Ground Investigations
6.1
6.2
7
8
9
10
Summary of Investigations
Discussion of Investigation Results
RBC’s Archival Records
Press Reports
Anecdotal Accounts of Past Events
Other Sources of Information
10.1
10.2
11
Introduction to this Volume
Structure of the Field Road Report
DEFRA Natural Cavity and Mining Cavity Databases
Historical Brickworks and Chalk Mining
References
Appendix 31
Appendix 32
Press Cuttings
Anecdotal Accounts Relating to Subsidence
Contents of Volume 4 – Geotechnical Investigations
Brief Overview of the Project
Table of Contents for Volume 4
Contents of Final Report – Volumes other than Vol 4
1
Introduction
1.1
1.2
2
3
4
5
6
7
8
9
10
Introduction to this Volume
Structure of the Final Report
Purpose and Scope of Investigations
Procurement of Specialist Services
Investigation Strategy
Investigation Techniques Employed
Dynamic Probing
Light Cable Percussion Boreholes
Window Sample Boreholes
Closed Circuit Television Surveys
Microgravity Geophysical Survey
Contents of Volume 5 – Ancillary Investigation Works
Brief Overview of the Project
Table of Contents for Volume 5
Contents of Final Report – Volumes other than Vol 5
1
Introduction
1.1
1.2
2
3
4
5
Introduction to this Volume
Structure of the Final Report
Utilities Services Searches
TWU Pipe Exhumation
Closed Circuit TV Surveys of Drains and Sewers
Control of the Ingress of Precipitation into the Ground
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
6
7
8
Internal Facilitating Works
Level Surveys
Groundwater Monitoring
8.1
8.2
8.3
9
Other Environmental Issues
9.1
9.2
9.3
10
Liaison with the Environment Agency
Monitoring of Groundwater Levels
Chemical Analysis of Groundwater
Grout Constituent Materials
Noise
Other
Condition Surveys
Appendix 51
Method Statement for the EA
Contents of Volume 6 – Interpretation of Investigations and Implications
Brief Overview of the Project
Table of Contents for Volume 6
Contents of Final Report – Volumes other than Vol 6
1
Introduction
1.1
1.2
2
Typical Detailed Intrusive Investigation Results
2.1
2.2
3
4
Dynamic Probes
Window Sample and Light Cable Percussion Boreholes
Summary of Intrusive Ground Investigation Results
Interpretation of Intrusive Ground Investigation Results
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
5
6
7
Introduction to this Volume
Structure of the Final Report
Interpretation of Dynamic Probe data
4.1.1
Investigation along the northern end of the road, south of Castle Hill
4.1.2
Investigation along the southern end of the Road towards Garnet Hill
4.1.3
Internal Probes in Nrs 2 to 18 Field Road
4.1.4
Ground conditions beneath Nrs 20 to 34 Field Road
4.1.5
Rear Gardens of Nrs 2 to 34 Field Road
4.1.6
Nrs 53-77 Field Road and Nrs 24-30 Garnet Hill, and the car park area
Borehole Profiles
4.2.1
Northern End of Field Road
4.2.2
Area around Initial Collapse
4.2.3
Car Park and Access Lane Area
4.2.4
Southern End of Field Road
4.2.5
Outside the Phase 1 Area
Ranges of fill depth
Top of Chalk Contours
Subsurface Features
4.5.1
Man-made Features
4.5.2
Natural Features
The Ground Model
Hydrogeology
Results of Testing for Contamination
Geophysical Interpretation Plan
Risk Assessments
References
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Contents of Volume 7 – Ground Stabilisation Works
Brief Overview of the Project
Table of Contents for Volume 7
Contents of Final Report – Volumes other than Vol 7
1
Introduction
1.1
1.2
2
Stabilisation Strategy
2.1
2.2
2.3
3
4
5
Stabilisation Method Adopted
Treatment Areas
Grouting Limitations
Risk Mitigation Measures
Tender Process
Scheme Design
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
6
7
Introduction to this Volume
Structure of the Final Report
Design Approach
Perimeter Cut-off Stage
Void Bulk-infilling Stage
Compaction Stage
Pile Base Grouting Stage
Grundomat Stage
Readymixed Grout
Monitoring of Ground Movement During Grouting
Water Table
Layout of Grout Holes
Local Variations around Collapse Areas
Evaluation of Grout Takes
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
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Background
AREA A (North End of Field Road Highway – Castle Hill to Nr 7)
7.2.1
Area A Perimeter Holes
7.2.2
Area A Void Holes
7.2.3
Area A Compaction Holes
AREA B (Mid-north Portion of Field Road Highway – Nr 9 to Nr 23)
7.3.1
Area B Compaction Holes
AREA C (Middle Portion of Field Road Highway – Nr 25 to Nr 49)
7.4.1
Area C Perimeter Holes
7.4.2
Area C Void Holes
7.4.3
Area C Compaction Holes
AREA D (Mid-south Portion of Field Road Highway – Nr 49 to Nr 67)
7.5.1
Area D Perimeter Holes
7.5.2
Area D Void Holes
7.5.3
Area D Compaction Holes
AREA E (Houses & Lower Rear Gardens of Nr 2 to Nr 18 Field Road)
7.6.1
Area E Perimeter Holes
7.6.2
Area E Void Holes
7.6.3
Area E Compaction Holes
7.6.4
Area E Grundomat Holes
7.6.5
Area E Out of Phase Works
AREA F (Upper Rear Gardens of Nr 2 to Nr 18 Field Road)
7.7.1
Area F Perimeter Holes
7.7.2
Area F Void Holes
7.7.3
Area F Compaction Holes
AREA G (Houses and Rear Gardens of Nr 53 to Nr 67 & Rear Car Park Area)
7.8.1
Area G Perimeter Holes
7.8.2
Area G Void Holes
7.8.3
Area G Compaction Holes
7.8.4
Area G Pile Base Holes
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
7.9
7.10
7.11
8
AREA H (Nr 20 to Nr 34 Field Road – Highway, Houses and
Lower Rear Gardens)
7.9.1
Area H Perimeter Holes
7.9.2
Area H Void Holes
7.9.3
Area H Compaction Holes
7.9.4
Area H Grundomat Holes
AREA I (Upper Rear Gardens of Nr 20 to Nr 34 Field Road)
7.10.1 Area I Perimeter Holes
7.10.2 Area I Void Holes
7.10.3 Area I Compaction Holes
AREA J (Houses and Gardens of Nr 69 to Nr 77 Field Road, and
Nr 24 to Nr 30 Garnet Hill)
7.11.1 Area J Perimeter Holes
7.11.2 Area J Void Holes
7.11.3 Area J Compaction Holes
7.11.4 Area J Pile Base Holes
Validation Works
8.1
8.2
8.3
Validation Strategy
Interpretation
8.2.1
Referencing System for Validation Works
8.2.2
Area A (North End of Field Road Highway – Castle Hill to Nr 7)
8.2.3
Area B (Mid-north Portion of Field Road Highway – Nr 9 to Nr 23)
8.2.4
Area C (Middle Portion of Field Road Highway – Nr 25 to Nr 49)
8.2.5
Area D (Mid-south Portion of Field Road Highway – Nr 49 to Nr 67)
8.2.6
Area E (Houses & Lower Rear Gardens of Nr 2 to Nr 18 Field Road)
8.2.7
Area F (Upper Rear Gardens of Nr 2 to Nr 18 Field Road)
8.2.8
Area G (Houses and Rear Gardens of Nr 53 to Nr 67 &
Rear Car Park Area)
8.2.9
Area H (Nr 20 to Nr 34 Field Road – Highway, Houses and
Lower Rear Gardens)
8.2.10 Area I (Upper Rear Gardens of Nr 20 to Nr 34 Field Road)
8.2.11 Area J (Houses and Rear Gardens of Nr 69 to Nr 77 Field Road,
and Nr 24 to Nr 30 Garnet Hill)
Discussion and Conclusions
Contents of Volume 8 – Site Restoration and House Rehabilitation
Works
Brief Overview of the Project
Table of Contents for Volume 8
Contents of Final Report – Volumes other than Vol 8
1
Introduction
1.1
1.2
2
3
4
5
6
7
Introduction to this Volume
Structure of Field Road Report
Condition Surveys
Procurement of Contractors
Restoration of the Gardens
Rehabilitation of Houses
Restoration of Utilities
Reconstruction of the Field Road Highway
Appendix 81
Appendix 82
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Typical Tender Documents for Repairs to a House
Typical Tender Documents for the Clearance and Restoration of a Garden
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
ADDENDA REPORTS
A1
[Detailed Results and Supplementary Volumes]
EnviroCheck Report & Historical OS Maps
A2
Historian’s Account & Interpretation
A3
Previous Ground Investigations
A4
Dynamic Probing Logs (seven volumes to accommodate all probes)
A5
Light Cable Percussion Borehole Records
A6
Window Sample Boreholes Records
A7
Reports on the Geophysical Surveys
A8
Stabilisation Drilling & Grouting Records
A9
Validation Probing and Grouting Records
APPENDIX REPORTS
Completion Reports for Individual Properties
Nrs 2 to 34 Field Road
Nrs 53 to 77 Field Road
Nrs 24 to 30 Garnet Hill
- (34 properties in total)
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
VOLUME 2 AN ACCOUNT OF THE SITE AND SUBSIDENCE EVENTS
1
Introduction
1.1
Introduction to this Volume
This document, entitled An Account of the Site and Subsidence Events, forms Volume 2 of
the suite of volumes comprising the Final Report for Phase1 of the Field Road (Castle Kiln
Mines) Stabilisation Project.
1.2
Structure of the Final Report
Volume 1 provides a concise standalone summary of the whole Project, and has been
written as an executive Summary Report. For those readers who require more detailed and,
or, technical information, Volumes 2 to 8 inclusive, present comprehensive, detailed
accounts of the various engineering aspects concerning the Project. Thus they should be
treated as a whole and read in conjunction with each other. These volumes are
supplemented by the Addenda Reports in nine volumes and referenced A1 to A9, inclusive,
which contain detailed results of the investigations and stabilisation works, and they also
include reports by other parties.
Individual property completion reports have been prepared and issued separately to the
respective owners.
Reference should be made to the overall list of contents which, for convenience, is repeated
at the beginning of every main volume of the Final Report (but not the Addenda Volumes).
The overall list of contents provides a detailed guide to the location of information recorded
about the Project.
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
2
Site Location
The site is situated in Field Road, which is accessed from the westbound lane of the A4155,
Castle Hill, just to the south west of Reading town centre, as indicated in Figure 2.1. The
initial collapse occurred at Ordnance Survey National Grid reference SU 7101 7282,
beneath Nrs 6 and 8 Field Road, which are located towards the southern end of the road,
some 60m north of its junction with Garnet Hill, in a predominantly residential area of
Reading called Coley. Figure 2.2 shows the precise position of the collapse in relation to its
surroundings and all of the Field Road properties.
Coley Place lies to the east and runs approximately parallel to Field Road, while to the west
and uphill, Coley Hill also lies sub-parallel to Field Road. A short distance beyond its
junction with Castle Crescent, the southern end of Coley Hill becomes Garnet Hill, which
continues southwards then dips sharply downhill, simultaneously curving eastwards to cross
the southern end of Field Road.
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
3
Site Description
Field Road is part of the residential area of southwest Reading known as Coley and lies on
the south side of the A4155, Castle Hill, and some 200m west of the Inner Distributor Road
(A329). It is aligned in a north-south direction and is sensibly level, at an elevation of
approximately 50m above OD. Field Road is situated on the southeast shoulder of an
elevated plateau, with slopes down to the east and south, formed by the river Kennet and a
former tributary, with the Holy Brook perhaps having made a minor and uncertain
contribution to the present-day landform.
The Project area, which is depicted in Figure 2.2, consisted of the entire length of Field
Road and its contiguous footpaths (collectively referred to as the highway), from Castle Hill
at the northern end, to Garnet Hill at the southern extent of the road, together with the
properties near the latter extremity (see figure 2.3).
Plate 2.3 Field Road from the North
Specifically, the properties noted above that lay within the Project area were Nrs 2 to 34 on
the west side of Field Road, and Nrs 53 to 77 on the east side, together with Nrs 24 to 30
Garnet Hill, which also abut the SE corner of Field Road. The houses on the west side of
the road are two storied in a single terrace of late Victorian age. The 17 properties on the
opposite side of the road (the east side) are all modern, having been constructed in the mid
1980s. The modern dwellings possess relatively level rear and front garden areas, and a car
park area lies behind nrs 53-67 Field Road. In contrast, the rear gardens of the Victorian
terraced properties rise steeply to the west to Garnet Hill. Some of the rear gardens are
densely vegetated.
For the first year of the Project’s duration, a security fence was erected and maintained
across Field Road, between the northern boundaries of Nrs 53 on the east side and Nr 2 on
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
the west side, and between the southern boundaries of Nrs 65/67 on the east side and Nr
18 on the west side of the highway. This area was commonly referred to, initially as the
‘fenced area’, then latterly as the ‘original fenced area’ during the Project (also known
synonymously as ‘the evacuated area’).
Within Field Road to the north of the fenced area, there are two businesses and a number
of houses. There is a single terrace of 25 brick-built, two storey, Victorian houses (Nrs 3 to
51) occupying almost the entire length of this part of the road. They are situated on the east
side of the highway, with the side of the house and garden of 111 Castle Hill (now converted
to flats on 4 floors) being located at the NE end of Field Road near its junction with Castle
Hill. On the opposite corner of this junction, (the NW end of Field Road) the boundary of Nr
113 Castle Hill abuts the Field Road highway. This property is both a business and a
residence. To the south of it are the two businesses in Field Road, Marine Ventures Ltd and
Workmates, which occupy Marven House and Hadwyn House, respectively. The only other
buildings fronting onto this side of Field Road north of the fenced area are a terrace of four,
two storey, houses, brick-built in the 1930s or 1940s, together with Field Road Garage. A
number of rear gardens to houses in Coley Hill, the next westerly road off Castle Hill and
running sub-parallel to Field Road, extend to the edge of the highway of the latter road,
between and the fenced area.
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
4
Published Contemporary Geology Of The Area
The geology of Field Road is depicted on solid and drift geological map, sheet 268, at a
scale of 1:50 000 BGS (2000), and on the 1:10 000 scale BGS unpublished sheet SU
77SW, Whitley, also as solid and drift, in draft form (see Figure 2.4). These maps indicate
that the geological sequence occurring beneath the site is essentially straightforward and
consists of part of the White Chalk Subgroup, with a mantle of undifferentiated Lambeth
Group resting unconformably on the chalk. Lynch Hill Gravel is shown to form a superficial
capping a short distance uphill from Field Road.
Since the publication of the 1:50 000 scale map (BGS, 2000) and the accompanying
explanatory booklet (Mathers and Smith, 2000), the lithostratigraphical nomenclature of the
Chalk Group has been revised, with a view to rationalising the proliferation of names in the
last 20 years. The former Upper and Middle Chalk Formations are now termed the White
Chalk Subgroup, which is part of the Upper Cretaceous Series of the late Mesozoic Era.
The upper part of the White Chalk Subgroup, corresponding to the previously named Upper
Chalk Formation that occurs in the Reading district, has been divided into the following
members, the youngest being at the top:
Newhaven Chalk Member
Seaford Chalk Member
Lewes Nodular Chalk Member
The underlying former Middle Chalk Formation is now divided similarly into:
New Pit Chalk Member
Holywell Nodular Chalk. Member
Mathers and Smith (2000) suggest that the uppermost chalk occurring either at surface or
directly beneath the Lambeth Group in Reading is equivalent to the Seaford Chalk Member,
ie the Newhaven Chalk is absent at Field Road. Sumbler (1996) describes the upper White
Chalk Subgroup as comprising a microporous, white, very fine-grained pure limestone,
which may contain over 95% calcium carbonate. It is typically soft and uncemented,
although some horizons are lithified into a more durable rock and is characterised by the
inclusion of flint, a form of translucent black or grey silica. The thickness of the Upper Chalk
Formation is reduced to a maximum of 100m in the Reading district, primarily because the
youngest beds were, during the period of the unconformity, subject to post-Cretaceous
erosion before the deposition of the strata comprising the Lambeth Group.
The Lambeth Group comprises the Reading Formation, corresponding to strata formerly
known as the Woolwich and Reading Beds. The latter incorporated, in the lowermost part,
the aptly named Bottom Beds, which are now known as the Upnor Formation and consist
generally of less than 2m thickness (but can, on occasions be up to 6m thick) of highly
glauconitic green, blue and grey clays and medium-grained sands. In the Reading district,
this basal stratum includes glauconite-coated nodular flints and, locally, abundant oyster
shells, and rests on a locally irregular and extensively bored chalk surface.
The Reading Formation (which, together with the Upnor Formation, was deposited between
some 65 and 59 Ma BP) is commonly about 20m thick and consists predominantly of multicoloured mottled clays in shades of red, brown, purple, grey and orange, which resulted
from weathering and soil formation (pedogenic processes) during its depositional phase.
Lenticular cross-bedded seams and beds of fine- to medium-grained sands, commonly up
to 2m thickness, occur at all levels, and at random spatial distribution, within the formation,
although they are most common towards the base (Mathers and Smith, 2000).
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
The Lynch Hill Gravel was deposited ca 300 000 years BP, which places it in the middle
Pleistocene Series, of the Quaternary System. In common with the other River Terrace
deposits of the Thames, it occurs on the valley sides defining the former or existing course
of the River Thames. These River Terrace deposits are primarily comprised of yellow/brown
sands and gravels of variable, but dominantly flint origin, and are usually no more than a
few metres in thickness (Sumbler, 1996).
The full geological sequence encountered by this phase of the Project investigations is
summarised in the table immediately following:
Table2.1
Summary of the Geology at the Site
Made Ground
soils artificially placed and, or disturbed by human agency
Reading Formation
variably interbedded layers of clay and sand
Upnor Formation
glauconitic clayey sands with rounded flint pebbles
Upper Chalk Formation
soft white porous limestone
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
5
Summary of Events on 04 and 05 January 2000
On 4 January 2000, celebrations for the New Millennium came to an abrupt halt for many
residents of Field Road, Reading following a major ground subsidence and associated
major structural damage to two homes (see Plate 2.4, below). A concise description of
events is given below.
Plate 2.4
Collapse Damage to Nrs 6 & 8 Field Road
It is understood that the local milkman saw water issuing as small fountains around the
edges of a rectangular patch within the bituminous surfacing of Field Road, and from
between the joints between road kerbstones near Nr 55, at approximately 0500h on
Tuesday 04 January 2000.
It is further understood that one or more residents telephoned Thames Water Utilities (TWU)
to report the leak early that morning. A representative of TWU was reported to have visited
the site at 08:00h, or shortly thereafter, to inspect the situation, and then departed from the
site. TWU contractors arrived on site later in the afternoon to turn off mains stopcocks in the
road to isolate the pipe fracture zone. Yet later that day, the mains water supply was
eventually resumed to the majority of the road by a temporary over-ground connection to
bypass the pipe fracture.
Sometime near midday of the 04 January, residents noted that a large circular depression
had formed in the road outside Nrs 6 and 8 and gradually deepened. The depression
continued to increase in depth such that, by approximately 15:00h the hole had achieved a
depth of over 2 m. The continuing increase in the ground depression caused the mains
electricity supply to fail, the gas main to burst and the lower front elevation of Nrs 6 and 8 to
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
collapse, as well as the surface water sewer line to be fractured. Cable TV, mains water,
gas and electricity were severed to these properties.
The severance of the mains services, in particular the gas main, resulted in about 200
people initially being evacuated from their homes. Following emergency repairs many
residents were able to return to their homes by 22:30h that night, but 16 families from
homes surrounding the ground collapse were re-housed in temporary accommodation,
because of loss of services and the inherent danger associated with the collapse. The
emergency evacuation involved the presence at the incident of both the fire and police
services, in addition to Reading Borough Council’s Emergency Planning and Dangerous
Structures Teams.
On Wednesday 05 January, representatives from Reading Borough Council’s Building
Control and a representative of the geotechnical group of TWU called at the offices of Peter
Brett Associates’ (pba) Structures Group, who in turn requested the presence of the pba
Geotechnical Group to visit site, to inspect the damage and recommend a way forward. At
the beginning of this inspection, at approximately 10:30h, the collapse crater was measured
at 8m diameter or thereabouts, and approximately 4m depth. It was noted to be over half full
of water at that time, but had drained empty by 13:00h.
Having inspected the site and considered the problem, the immediate priority was to make
Nrs 6 & 8 structurally safe. Peter Brett Associates, acting on behalf of Reading Borough
Council, designed and supervised the necessary work: firstly by having the depression
crater infilled with recycled crushed concrete (estimated at the time as about 140 tonnes or
90 cubic metres in volume) and secondly by having a temporary jackable scaffold system
erected to support the remaining parts of the front walls (see Plate 2.5). This was achieved
by Wednesday evening. Thereafter, immediate steps were taken to initiate urgent ground
investigations to ascertain the extent and nature and cause of the ground collapse. This
work is described in volumes 3 to 6.
Plate 2.5
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Scaffolding Support to Collapsed Walls of Nrs 6 & 8 Field Road
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
6
Evacuations and Temporary Accommodation
The fracture of the gas main initiated the evacuation of all occupants of Field Road in the
afternoon of 04 January 2000. Residents were taken or directed to the nearby Salvation
Army premises, where they were looked after, with first aid support from the Red Cross and
welfare support from the WRVS. As noted in the previous chapter, the majority of the
residents were able to return to their homes by 22:30h that night after emergency repairs
had been effected. However, 16 families from the homes immediately surrounding the
ground collapse were re-housed in temporary accommodation, because of loss of services
(gas, water & electricity) and the inherent danger associated with the unsupported collapse,
coupled with the possibility of further, ongoing, subsidence.
The Emergency Planning Officer, in his log for the first day’s events, recorded that 16
households were displaced from their homes, with 7 families making their own
arrangements, 3 households being accommodated by RBC in the local Travel Lodge, and
the remaining six or so being accommodated elsewhere, as arranged by RBC resource
officers from the emergency housing unit. By the end of the first week after the initial event,
7 families were accommodated in the Travel Lodge, a further 7 made their own
arrangements, 1 family were accommodated in Battle Hospital, where one of the household
worked, and 1 was housed by RBC.
Soon after the various loss adjusters representing the affected households’ insurers had
been appointed, a meeting was held and it was agreed that their principals would make ex
gratia payments for temporary accommodation. Concurrently, RBC Housing and Social
Services Directorate had been making considerable progress in finding short-term rented
accommodation for those households who had not made their own arrangements.
During the course of the Project, when the time scale had been extended on a number of
occasions, the rentals were exchanged to longer periods, with attendant 2 months notice to
terminate the rental agreements.
In March 2000, the occupants of Nr 18 Field Road were evacuated, and found temporary
accommodation. Again, in mid-October 2000, additional temporary housing was required for
the occupiers of Nrs 20 to 34 and 69 to 73 Field Road, following a major ground subsidence
event at the rear of Nr 18 Field Road. Finally, short-term accommodation was obtained by
the occupiers’ insurance or by private arrangement for the households of Nrs 75 & 77 Field
Road, and 24 to 30 Garnet Hill, between June and September 2001, while grouting
stabilisation works were performed beneath their houses.
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
7
The Interpreted Sequence of Initial Events
Following intensive geotechnical investigations described in detail in Volume 4,
understanding of the cause of the initial collapse has been revised. It is thought that the
heavy rainfall in the week preceding the event caused minor ground subsidence around a
cast iron water main. This is considered to have been the final triggering event to a long
cumulative series of ground movements that had been taking place gradually over a long
period of time. The fracture of the water main and the ensuing sudden and large volume of
water washed soil into large cavities, now known to be mine gallery voids. The wetting of
the overlying materials both weakened them and increased their weight, thus contributing to
the final stages leading up to the major collapse.
Plate 2.6
View of the Initial Collapse with Adjacent Subsidence
Closed circuit tv footage obtained during investigations later in the Project proved that the
cavities throughout the site were in a very advanced state of collapse with considerable delamination of the roof and walls occurring. The de-lamination had led, in many cases, to the
complete loss of the original chalk roof. In such instances the roofs of the cavities were
spanned entirely by clays and sands, which are much more prone to destabilisation by
excessive water than is chalk.
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
8
Subsequent Failure Events
Three subsequent ground collapse events have occurred within the site at varying times
after the initial collapse of 04 January 2000 and before stabilisation works were completed.
The first such incident happened at about 13:00h, on Wednesday 18 October 2000, at Nr 18
Field Road, which had been evacuated some 6 months previously, as a result of a risk
assessment, after site crews had heard loud underground noises from the adjacent house.
On 18 October a vertically sided crater of 4m diameter and 7.8m depth opened within
minutes. The crater was located in the rear garden of Nr 18 Field Road, only 1 m or so from
the rear corner of the property. The crowns of mine galleries could be seen near the base of
the void.
Plate 2.7
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View of Collapse Shaft at Rear of Nr 18 Field Road
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Engineers from Peter Brett Associates were on site at the time of the collapse and were at
the scene within minutes of the next-door neighbour (Nr 20) seeing and hearing the crater
form. For some 18-24 hours thereafter, the crater was observed to increase slightly and
very slowly in size, this lateral movement being towards the building. Immediate action was
taken to evacuate the occupiers of Nrs 20, 22 and 24 Field Road and further risk
assessments took place the next day, Thursday 19 October 2000, when it was decided to
evacuate the rest of the terrace, comprising Nrs 26 to 34 Field Road. The evacuation and
arrangements for temporary alternative accommodation, as needed, were made for these
residents by RBC housing officers.
The decision was taken to stabilise the void by filling it with reject stone as a temporary
solution, until the contract grouting works would be able to effect a permanent stabilisation.
Consideration of the logistics of the problem revealed that the bulk infill material would have
to be brought to the site of the crater via Garnet Hill and down the seemingly steep rear
garden, because the length of the terrace precluded a practical route from Field Road,
around either end of the terrace to the void.
Initially, it was thought that the reject stone could be delivered into the crater from Garnet
Hill using nesting tapered cylindrical plastic or steel builders chutes. Work continued late
into the night of 18 October attempting to deliver the reject stone into the void by this
method, but it failed, always becoming blocked two or three metres down the slope. In the
clarity of daylight the next morning, it was patently obvious that the steepness of the garden
(its gradient) was less than the angle of internal friction of the reject stone, which meant that
the latter would not flow under gravity.
The delivery system was rapidly reviewed and the use of a staircase of mini-conveyor belts
was sought for instant hire. These units, which were about 2m length each, eventually
arrived late on the evening of 19 October. At the same time, crushed recycled concrete was
ordered in place of the reject stone, to reduce the cost. The conveyor units were erected on
their scaffold support frame (see Plate 2.8 below) and filling commenced briefly, before
being abandoned at about 02:00h in the morning of Friday 20 October, when several of the
electrically powered conveyors failed.
After modifying the electrical supply and substituting spare conveyor panels, filling resumed
at approximately 08:30h and continued with few stoppages, until late that night, then
throughout the following day (Saturday 21 October). The hole was eventually filled using a
total of approximately 240t, or 170m3 of crushed concrete, with the operation being
completed by 11:30h on Sunday 22 October 2000. An emergency temporary closure of part
of Garnet Hill, which had been effected to facilitate the works, was lifted the following
working day.
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Peter Brett Associates
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Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
Plate 2.8
“Staircase” of mini-conveyor belts from Garnet Hill to rear of Nr 18
A further subsidence occurred almost a month later, during the afternoon of Sunday 12
November 2000, when the previously filled crater in the rear garden of Nr 18 was
reactivated. On this occasion, the subsidence caused a crater, 4m deep and over 5m
diameter, to appear. Much of the next day was taken up in organising the backfilling of this
recurrence. The grouting Contract was on the point of commencing on site, but at the north
end of Field Road and the contractor, Keller Ground Engineering (KGE) was persuaded to
start site operations early to stabilise the collapse as soon as possible, as the crater was
increasing in diameter, such that it started undercutting the foundations to the rear
extension of the adjacent Nr 16 Field Road.
KGE proposed to pump low viscosity ready-mixed grout into the void, using their usual bulk
fill grouting techniques and plant. Initial attempts to stabilise the hole, using cementitious
grout, were halted owing to the late arrival of plant and the incorrect specification of grout
being delivered late on the night of Monday 13 November. Filling resumed on Tuesday
morning, using the correct grout mix, and eventually, by that evening, the hole had been
filled to within 1.5m of the ground surface. The grout fill level was deliberately kept low,
because it was thought undesirable to permit any structural bearing between the concrete
and the foundation of the adjacent Nr 16. Further stability would be afforded to the area by
the injection of grout via inclined boreholes drilled under the properties from the front, at the
appropriate time in the works programme.
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Peter Brett Associates
Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
During the continuing investigations and about a month after the grouting stabilisation works
had started at the northern end of Field Road, a third collapse occurred, some time before
08:00h on 08 January 2001. It occurred under the southwestern corner of the single storey
rear extension of Nr 10 Field Road, resulting in the loss of this extension. The subsidence
crater was measured at ground level, as approximately 5m in diameter, and 8m depth. The
sides of the crater continued to collapse and by that afternoon, the rear two-storey section
of the building had collapsed (see Plate 2.9 below).
Plate 2.9
Rear of Nr 10 Field Road after collapse on 08 January 2001
The crater posed an immediate threat to the stability of the rest of Nr 10 and the adjoining
houses, so it was decided to backfill it as soon as possible. This was achieved by pumping
readymix concrete into the crater via a pipe running through Nr 10 from Field Road at the
front of the house. Keller Ground Engineering had already commenced the stabilisation
works in November 2000, thus the equipment used to pump the readymix concrete was
already on site. The pipe was moved into position using ropes, thus eliminating the need to
have personnel enter the dangerous structure. The crater was filled to within a few
centimetres of the base of the exposed sewer pipe that was severed during the collapse.
The backfilling took three days to complete and required 72m3 of concrete. The remainder
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
of the crater was backfilled once the sewers were reinstated, using the surplus crushed
concrete left from the first backfilling of the collapse at the rear of Nr 18 in October 2000.
Following the backfilling of the crater at the rear of Nr 10 the building was inspected
internally and externally, and scaffolding was erected to support the remaining structure.
In the afternoon of 05 February 2001 Project staff discovered a void beneath the floorboards
in the rear room of Nr 8 Field Road, during routine structural inspections. The void was
roughly circular in plan, with a diameter of approximately 3m, and was up to 3.1m deep. The
void had a thin crust of soil spanning over part of it and extended beneath the rear wall of Nr
8 and the party wall between it and Nr 6 Field Road.
The void was immediately backfilled by Keller Ground Engineering using site-batched grout.
The plant used was already at the site. The void was backfilled to within 0.2m of the
foundation of the house using 11m3 of grout. Support was added to the foundations, using
solid packing, when the grout had set.
The positions of the collapses relative to the houses at Field Road are shown in Figure 2.5.
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Field Road Chalk Mines
Stabilisation Project - Phase 1
Volume 2
An Account Of The Site
And Subsidence Events
9
References
British Geological Survey (2000) Reading. Solid and Drift Geology England and Wales Sheet 268.
1: 50 000 scale. Br. Geol. Surv., Keyworth.
Mathers, S J and Smith, N J P (2000) Geology of the Reading District: A Brief Explanation of the
Geological Map Sheet 268, Reading. Br. Geol. Surv., Keyworth.
Powell, J H (1998) A Guide to British Stratigraphical Nomenclature. Constrn. Ind. Res. Inf. Assoc.,
London.
Sumbler, J H (1996) British Regional Geology: London and the Thames Valley. (4th Edition).
HMSO, London, for Br. Geol. Surv.
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