88 Elgin Crescent
London
W11 2JL
Structural Construction Method Statement
In Support Of Planning Application
February 2014
2013153/AH
Rev A: 03/04/14
A: 6 The Courtyard, Lynton Road, London. N8 8SL
T: 020 830 4041
E: info@symmetrys.com
W: www.symmetrys.com
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
Contents
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Introduction
Existing Condition
Ground Investigation, Hydrology and Flood Risk.
Design Proposals
Construction Method Statements and Sequence of Works
Temporary Works
Stability of Neighbouring Structures
Party Wall Matters
Drainage
Summary
Appendices
A:
B:
C:
D:
E:
Proposed Drawings and Sequence of Works
Structural Calculations
Hydrogeology Study and Ground Movement – CGL
Geotechnical and Environmental Report - Chelmers
Construction Management Plan – Paul Mew Associates
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
1.
INTRODUCTION
1.1
Symmetrys Limited has been engaged by the client to carry out a basement impact assessment
for the proposed single storey basement at 88 Elgin Crescent, London. The basement is
proposed below the existing footprint of the main house as well as to parts of the rear garden. The
new gardens will be re-laid over the proposed structure once complete. It is also proposed to carry
out some refurbishment works to the main structure. This report only addresses the proposed
basement works.
Figure 1: Map showing local transport links and tunnel locations
Photo 1: Birds eye view front elevation
Photo 2: Birds eye view Rear Elevation
1.2
Our drawings and this report will be included within our client’s planning application. Our
documents are not intended for and should not be relied upon by any third party for any other
purpose.
1.3
A CAD survey of the existing property has been carried out and passed to us from Stiff and
Trevillion Architects.
1.4
This Statement has been prepared in direct response to Section 6 of the Council’s Subterranean
Development SPD from May 2009, including by reference to each of the requirements set out
under paragraph 6.1.3.
2.
EXISTING CONDITION
2.1.
The existing dwelling is located in the Royal Borough of Kensington & Chelsea, London.
2.2
The existing structure is four storeys high and the perimeter walls are built from masonry with
timber floor joists spanning front to back. The property exhibits no signs of excessive deformation
or cracking other than that would be expected of a property of this type and age.
Figure 2: Map showing local bore positions – British Geological Maps
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88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
3.0
3.1
3.1.1
3.1.2
3.1.3
GROUND INVESTIGATION, HYDROLOGY AND FLOOD RISK
3.3.1
A site specific ground investigation and interpretative report has been undertaken by Chelmer
Site Investigations Ltd and their findings and recommendations are described in their report dated
February 2013 (REF GEO/4176) appended here to in Appendix D.
3.3.2
The ground conditions are summarised as follows:
G.L to 0.9m
Made ground
0.4 m to 2.8m
Silty Clay
2.8 to 10.4m
Very Stiff fissured clay
10.4m to 12.0m
Very stiff dark brown/ grey clay.
3.3.3
The report confirms that the proposed basement development, below the existing footprint of the
building can be founded on the very stiff London clay which would allow a safe bearing pressure
of 175-200kN/m2.
Investigation / Opening-Up Works undertaken:
A report has been undertaken to encompass the following and can be found in appendices B.
1 No. borehole has been cut to determine safe bearing loads and cohesion values of the soil. As
well as the extent of any ground contamination and measured ground water and gas levels.
A series of trial pits have been cut to confirm the depth and profile of the existing foundations to
the perimeter walls once and if planning permission is granted.
Further trial pits will be undertaken, should planning be granted.
3.2
Ground Conditions Geology
3.2.1
Using the local geographical survey maps accessible via the British Geological Society website
indicated that the underlying soil strata, much like the rest of London, is London Clay. Having
reviewed boreholes cut in the area with particular respect to a borehole cut in Blenhiem Crescent
in 1987 see figure 2, firm stiff clay was confirmed down to 30m.
3.3.4
During the drilling of Borehole BH1 no groundwater was encountered. A standpipe was installed
and subsequent monitoring visits were undertaken on the 16th January and 24th January 2014,
ground water was recorded at 0.8m and 0.45m below existing ground level.
3.4
Hydrology
3.4.1
Referring to the “The Lost Rivers of London” by Nicholas Barton the closest known watercourse
is described to be on the west of the site approximately 500m away known as Counters Creek
arises near Kensal Green Cemetery and heads south to south east. See below extract from LDA
floor risk map which shows the approximate positions of the lost rivers. The man-made Regents
canal runs approximately 1000m north of the site. This is a significant distance away and is
unlikely to have any impact on the local hydrology.
3.4.3
Referring to the British Geological website the hydrology is described as HAMES GROUP Rocks
with essentially no groundwater apart from perched water, predominantly clayey sequence up to
140 m thick confining underlying aquifers. Occasional springs at base have very hard water.
3.4.4
For further details refer to the Hydrogeology study and Ground Movement report by CGL found in
appendix C
Figure 3: Geology Cross section from Alan Baxters RBKC Dec 2012
3.3
Ground Investigation
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6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
3.4.5
Flood Risk Assessment
4.0
DESIGN PROPOSALS
Referring to the RBKC strategic flood risk assessment, the proposed basement lies in Flood Risk
zone 1 and therefore having a less than 1 in 1000 annual probability of river or sea flooding any
year. As stated in the subterranean development SPD a flood risk is only required for zones 2
and 3 and therefore not provided for this planning application.
4.1
The proposal is to form a basement below the current foot print of the existing property which
extends into the rear garden. The method used to achieve this is by forming reinforced concrete
retaining walls below the existing walls in an underpin sequence. This is a well-known and most
frequently used technique to form basement structures. Temporary propping to the top of the
retaining walls will ensure that the basement doesn’t cause any excessive ground movements
whilst the construction is taking place.
4.2
Below Existing Building
To form the basement below the existing footprint of the building, the party wall as well as the
front wall and the rear wall will require being underpinned using reinforced concrete sequential Lshaped pins. The basement slab will be ground bearing and it will be tied into the toes of the
underpin structures. This will ensure that the basement slab resists any potential soil pressure
associated with heave or from hydrostatic loads from localised standing water or leaking pipes
etc. No ground water would be expected and therefore no dewatering to this area should be
required.
The expected heave forces due to overburden removal will be expected to be less than the load
of the building and therefore no significant heave is expected to this area. The basement slab will
be required to be sufficiently stiff to withstand the local heave pressures and to transfer the forces
to the perimeter underpins. These uplift forces will be resisted by the significant dead load of the
existing building, see structural calculations in appendix B. Our structural calculations also
demonstrate that the existing structure can be safely supported on the proposed underpins within
allowable parameters provided by Chelmers for ground bearing capacity.
The newly reinstated lower ground floor would be formed using a series of steel beams that span
from party wall to party wall. Between the beams a stiff floor plate will be formed using a Ribdeck
concrete slab. The new lower ground floor will act as a permanent prop to the heads of the new
basement walls. To ensure continuity between the RC retaining walls and the Masonry walls,
dowels will be drilled into the underside of the masonry walls and cast in with the RC walls.
Figure 4: LDF Flood Risk Map
4.3
To The Rear Garden
Where the basement steps down below the existing garden, traditional underpinning methods
would be used as described above. The garden podium deck would be formed in Reinforced
concrete to form a stiff box which would prop the retaining walls at ground level as well as
transfer the heavy earth loads that would be placed on the structure. The depth of the basement
would be founded at deeper level, however, due to the clay soil having similar firmness, no
differential settlement is expected.
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
The heave pressures here would be counteracted by the weight of reinforced concrete box as
well as the 1m of earth placed on top of the podium slab to form the garden above.
7.0
GROUND STABILITY AND STABILITY OF NEIGHBOURING STRUCTURES
7.1
Due to the robust engineering principles and construction method applied, the extent of
movement is unlikely to be significant. We can confirm that the proposed structural design and
method of construction of the basement has been developed with a view to ensuring structural
safety, and that if constructed in accordance with this document the works will be able to be
completed without any adverse impact on the structural stability of the neighbouring properties,
other adjacent structures, adjoining land and gardens or the adjoining public highway.
7.2
The reinforced concrete structure will be designed to accommodate surcharges from the
neighbouring property, public highway and ground pressures. The structure will have adequate
stiffness to ensure that the lateral deflections do not exceed the appropriate limits recommended
by British Standards Codes of Practice in order to ensure that potential ground movements be
kept to acceptable limits.
7.3
The structures will be designed to withstand any uplift due to hydrostatic pressures as well as
being designed to transfer vertical loads into the ground safely.
7.4
Figure 1, shows the positions of the Circle Line relative the subject site, these are a significant
distance away, with the closest line being 300m away. Due to this significant distance no
consultation with the London Underground Asset Protection team will be undertaken.
7.5
Ground movement assessment has been undertaken CGL, this can found in appendix C.
Based on experience from the adjacent sites; the proposed development at 88 Elgin Crescent is
similar in size and depth as well as being founded in similar ground conditions we expect the
category of movement to be a category 1 or less based on the below table.
For further information on the heave forces expected please refer the CGL report in Appendix C.
4.3
Waterproofing
BS8102 sets out guidance for the waterproofing of basement structures according to their use.
With this in mind the use of tanked, integral and/or drained methods of waterproofing will have to
be considered. These items will be considered once a tanking specialist has been employed.
4.4
Internal Alterations
In terms of load bearing elements, only the party wall spreader beam at lower ground floor will be
disturbed. See 6.0 temporary works for further details on how this will be retained.
5.0
CONSTRUCTION METHOD STATEMENTS AND SEQUENCE OF WORKS
See appendix A on SK01-SK03
6.0
TEMPORARY WORKS
6.1
The method of construction adopted minimises the need for temporary works in the form of
propping. However, propping during the underpinning sequencing will be required. Please see
appendices A for construction sequence and method statements.
The central load bearing wall would be needled and supported off a series of beams which in turn
would be supported off a section of basement slab that would be cast ahead of the needling
works. This would produce an unhindered area for the basement to be excavated and formed.
Once the central load bearing wall has been supported the remaining perimeter walls can be
underpinned as per our drawings SK03.
The appointed contractor will be responsible for all temporary supports and for the stability of the
structure during the works. To ensure that Symmetrys design intent is correctly interpreted by the
contactor, they will be required to submit all temporary works proposals and method statements
to the approved engineer for review a minimum of 7 working days prior to commencing
excavation.
The contractor should also submit a dewatering strategy in case water is encountered.
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
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88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
The Contractor shall monitor the position and movements of the elevations of the adjacent
properties around the perimeter of the proposed excavation. The monitoring shall be undertaken
by a specialist survey company. The monitoring system will have at least the following
characteristics:
1)
The existing facades of the neighbouring properties will be monitored near ground level and at
roof level, at intervals not exceeding 3m centres. Furthermore the retaining walls to the garden
structure will be monitored.
2)
Monitoring points (targets) shall be firmly attached, to allow 3D position measurement, for the
duration of the work, to a continuous and uninterrupted accuracy of -/+ 1mm. A suitable
remote reference base/datum unaffected by the works will be adopted, one located at least
50m from the site.
3)
Points / targets shall be measured for 3D positioning on, at not less than the following
intervals:
• Before any works commence (base reading)
• Weekly during the period of basement excavation/construction
• Monthly during the course of the remainder of the works.
• Six months after the completion of all construction works.
5)
All measurements shall be plotted graphically, to clearly indicate the fluctuation of movement
with time. The survey company shall submit the monitoring results to the Engineer
(Symmetrys Ltd) and to the Adjoining Owners Party Wall Surveyors/Engineer within 24 hour of
measurement, graphically and numerically.
6)
The following trigger levels for movement are proposed for agreement. In the event of a
trigger value being reached the Contractor will immediately stop any work that might cause
further movement, assess the situation and propose alternative methods for proceeding, with
definitive further movement limits for those later steps.
7)
Trigger movement limits are proposed as follows:
A)
Facades and Party Walls Horizontal and Vertical Movement
Amber +/-8mm
All parties notified.
Red +/-10mm
Works reviewed
Figure 5: Building damage categories used by the IStructE and ICE
Figure 5: Building damage categories used by the IStructE and ICE
8.0.
PARTY WALL MATTERS
8.1
The scope of works falls within the Party Wall Act 1996. Procedures under the Act will be dealt
with by the client’s Party Wall Surveyor. The Party Wall Surveyor will prepare and serve
necessary Notices under the provision of the Acts and agree Party Wall Awards in event of
disputes. The Contractor will be required to provide the Party Wall Surveyor with the appropriate
drawings, method statements and all other relevant information covering the works notifiable
under the Act. The resolution of the matters under the Act and provision of Party Wall Awards will
protect the interests of all owners.
8.2
Monitoring
B)
The Existing Garden Walls
Amber +/-10mm
Red +/-15mm
All parties notified.
Works reviewed
It is proposed that the structural stability of the surrounding/adjacent properties is safeguarded by
a system of movement monitoring.
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
9.
DRAINAGE
10.0.
SUMMARY
9.1
The development is an extension of a single family dwelling house. As no additional utilities or
houses are being created there will be no material change in the requirements of the local
drainage infrastructure.
10.1
The main engineer employed carry out the works should review all temporary works, contractors’
method statements and calculations for these works, prior to works starting. The permanent
works will also be submitted to Building Control and the necessary Party Wall Surveyors for
approval prior to the works commencing on site.
9.2
There will be appropriate drainage installed to the landscaping (hard and soft) on the site. As the
basement to the rear is covered by at least a meter of soil there will be no effect on surface water
on the site.
10.2
The proposed basement at 88 Elgin Crescent has been designed with robust structural principles
and methods of construction that are commonly used and well known in the industry. This will
ensure the integrity of neighbouring structures and roadways are not compromised during the
works.
9.3
As RBKC stated in their SPD on subterranean development: 4.5.1 “With regard to the impact of
subterranean development on ground water flows and levels, groundwater flows will find an
alternative route if blocked by a subterranean structure, although there may be very small rises in
level (16).These changes in level are likely to be significantly less than the natural variations in
the water table associated with seasonal variations.”
9.4
The above ground drainage will be, subject to invert levels, drained by gravity to the existing
combined sewage system. The below ground drainage will be drained to a submersible package
sewage station situated below the basement slab which will then be pumped via a rising drain to
the nearest available inspection chamber on the existing gravity drainage system. This can then
flow by gravity into the existing combined sewage system.
9.5
There have been recent cases where the sewers within RBKC have not been able to cope
(notably in July 2007). To mitigate for this suitable measures such as non-return valves will be
incorporated into the drainage design.
This assumed Method Statement and Structural report has been completed by Symmetrys
Limited and checked by Christopher Atkins CEng MIStructE who is the Managing Director of
Symmetrys Limited.
Ashwin Halaria
MSc BEng
Project Engineer
Christopher Atkins
CEng MIStructE
Director
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
APPENDICES
APPENDIX A: Proposed Drawings and Sequence of Works
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APPENDIX B: Structural Calculations
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
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88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
APPENDIX C: Hydrogeology study and Ground Movement – CGL
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
88 ELGIN CRESCENT – BASEMENT IMPACT ASSESSMENT
APPENDIX D: Ground Investigation and Environmental Report – Chelmers
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Symmetrys Limited Consulting Engineers, Registered In England And Wales, Company No. 5873122
6 Linzee Road, London, N8 7RE
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APPENDIX E: Construction Management Plan
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