90 Brighton Road, Surbiton, Surrey Civil and Structural Engineering Basement Feasibility Report Mixed Use Scheme 108802-REP-002 March 2015
Contents 1. Executive Summary... 3 2. Site Description and Constraints... 4 2.1. Site Location, Description, and Current Use... 4 2.2. Geotechnical conditions and constraints... 5 3. Structural Design... 7 3.1. Existing Substructure... 7 3.2. Proposed Substructure... 7 3.3. Superstructure... 10 PROJECT TITLE: REPORT TITLE: Team Client Architect Building Services Engineering Consultant Civil and Structural Engineering Consultant PROJECT REFERENCE: 108802-REP 002 Issue and Approval Schedule: 90 Brighton Road, Surbiton, Surrey Indigo Scott Harper Downie Fairhurst GGA Civil and Structural Engineering Mixed Use Basement Feasibility Report Rev. 1.0 Name Signature Date Appendix A Basement Construction Methodology Appendix B Site Investigation & Environmental Impact Assessment Prepared by James Toon 10/03/2015 Appendix C Underground Drainage Report Reviewed by Nick McSpadden 10/03/2015 Approved by Nick McSpadden 10/03/2015 Revision Record: Rev. Date Status Description By Chk. App. A 16/03/15 Revised to suit comments JT NMcS NMcS B This report has been prepared in accordance with procedure OP/P02 of Fairhurst Quality Assurance System. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 2
1. Executive Summary This report covers the Civil and Structural Engineering Basement Feasibility Report for the development of a single storey basement at 90 Brighton Road, Surbiton, Surrey. The aim of this document is to convey the civil and structural issues considered for the construction of the basement. The proposed mixed use development is to construct two new blocks with the rear residential building 4 storeys and the front 3 storey building mixing commercial at ground floor and residential above. A single storey basement covering the majority of the site for underground parking and the lower level of the dwellings. The basement is accessed via a ramp to the south western corner. Investigation of local ground conditions suggest that the water table is relatively low, however some perched local groundwater may be found. The flood risk for the site is low and the localised damming effects of the basement also anticipated to be low. Green roof systems have been proposed, which will provide attenuation for the drainage of surface water from the site. This will control the flow into the existing sewer system. Foul water drainage for the residential areas of the development will be gravity discharged into the existing sewerage system above ground and pumped for the lower levels. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 3
2. Site Description and Constraints 2.1. Site Location, Description, and Current Use 2.1.1. Site Location The site is located in Surbiton, Surrey. It is bordered by Brighton Road to the south and the rear gardens of properties on Andrews Square to the North. The eastern and western boundaries are the adjacent properties on Brighton Road. 2.1.2. Site History The site has been subject to a number of changes of use since 1840 when residential properties covered the site. This stayed with minor alterations until 1974 where a petrol filling station is shown for the first time. Between 1986 and 1987 the garage was developed to the current configuration and has remained largely unchanged until present day. Figure 1. Location of the site. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 4
2.2. Geotechnical conditions and constraints The production of the basement construction methodology has been developed in accordance with the documents required by London Boroughs for the construction of basements within inner city sites. Issues raised by the requirements of these documents are as follows: - 2.2.1. Local Geology Geological records suggest the site is underlain by London Clay. An outline description of the in-situ ground conditions can be found in Table 1 below from a site specific ground investigation for the site conducted in January 2015 and a Phase 1 Geo Environmental Risk Assessment conducted by SLR Global Environmental Solutions (refer to Appendix B). The following data was based on the boreholes dug across the plan of the site. The boreholes were dug to 6m with the London Clay not encountered, however existing borehole data held by the BGS for sites within 1000m of the site show London Clay. Stratum Depth to top of stratum (mbgl) Depth to base of stratum (mbgl) General description 2.2.3. Slope Stability The proposed development poses no slope stability issues. The site is slopes gently to the north east and south west whilst the surrounding area slopes gently to the north west towards the River Thames. The basement construction method has been proposed to resist the temporary and permanent loads with very small movements and hence pose little risk to structures at ground level. 2.2.4. Impact on Drainage, Surface and Groundwater Flows and Levels The current development is predominantly covered by hard standing and the proposed development will therefore have limited impact on the drainage of surface water (refer later within this report). A report has been completed by Odyssey Markides in January 2015 for the Drainage Assessment and Strategy (refer to Appendix C). Within this report it is stated that all the surface water drainage will discharge under gravity into the existing connection to the Thames Water sewer network. Surface water within the basement will be pumped to the outfall via a rising main. Attenuation of the surface water will be achieved by a series of green roofs across the development. Compliance with the London Plan and the reduction of flow rates is considered within this report. Made ground 0.00 0.00-1.60 Langley Silt 1.10 2.40 1.10 2.40 Concrete over grey, black and brown sandy clayey gravel of flint, brick and concrete. Orange-brown clayey sand / very sandy clay. The groundwater level has been monitored as part of the ground investigation with a depth ranging between 3.841m and 5.105m below ground level. From this data it is anticipated that some groundwater will be encountered within the basement excavation that will need to be pumped out as part of the works. Hydrostatic pressures on the basement construction will be minimal with only the lower section of the basement within the water. The Odyssey Markides report notes that the encroachment of the new basement into the groundwater will be minimal and so will not affect the surrounding buildings during or after the construction. Kempton Park Gravels 2.40 2.40 6.00 Dark brown sand, becoming sandy gravel with depth. The River Thames runs 350m north west of the site with no other watercourses in close proximity. London clay 6.00 Undetermined Not encountered Table 1 Summary of site specific ground conditions. 2.2.2. Impact of Temporary and Permanent Works on Integrity of Adjacent Structures There are no underground structures in the vicinity of the proposed development. A method for construction of the basement has been proposed that will limit the surface ground movements to very small amounts and so will ensure the integrity of the adjoining pavement and roadway are maintained. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 5
excavation of the basement area. Through the design and the construction of the basement the impact on the adjacent structures will be minimal. The Thames Water Ring Main South crosses the site (refer to figure 4). The depth of the tunnel is currently unknown and will be subject to further investigation. Works above the tunnel and exclusion zones are to be established to Thames Water prior to the commencement of the works. Figure 2 Local river courses 2.2.5. How the Design has Assessed Geological and Hydrological Concerns The choice of walling method for the basement has been influenced by the local geology: A stable basement excavation has been designed in the temporary case; Basement walls have been designed for the permanent case; The basement water control has been designed to have minimal impact on the adjoining structures by ensuring no dewatering will occur outside the basement. Should dewatering be required then sufficient flow paths will be maintained that no significant change to the water table will occur due to the basement installation. 2.2.6. Engineering Details The principles of the structure of the basement and its construction are considered later within this document. 2.2.7. Impact on Stability of Local Buildings Below Ground The basement construction will be constructed using steel sheet piles, which have been designed to ensure that construction movements are minimised during excavation of the basement. Suitable propping in the temporary and permanent cases will be designed to ensure there is little ground movement during 2.2.8. Impact on Existing Trees Figure 3 Thames Water Ring Main South A number of established trees cover the rear of the site. The eastern corner of the basement has been recessed, which is to avoid an established tree on the boundary that currently has a Tree Preservation Order (TPO) in place. It is not anticipated that the proposed works will have a detrimental effect on the trees, which will be monitored during the construction. An Arboricultural Impact Assessment, Arboricultural Method Statement and Tree Protection Plan has been produced by Advanced Tree Services and is available from the client on request. The NHBC document for the construction of foundations near trees will be referred to for guidance, however this is more applicable for ground level domestic buildings. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 6
3. Structural Design 3.1. Existing Substructure There are believed to be a number of tanks currently on the site as part of the petrol station. Some of the tanks are understood to be still in operation with the others redundant. The condition of the tanks is unknown and will be subject to further investigation. The tanks will be removed as part of an enabling works package prior to the commencement of the main works. The existing Thames Water tunnel is described in the previous section with easements and construction still to be confirmed. A build over agreement and confirmation of the depth will be required from Thames Water as well as the agreement of any temporary works above to the form the basement. Current Thames Water guidance recommends that easements to sewers are 3m from the outer wall of the sewer and is assumed to be applicable to the construction above the tunnel. It is generally accepted that vibration through construction activities are to be avoided adjacent to sewers and so the sheet piling installation will need to be confirmed as acceptable by the local authority and Thames Water. Basements to adjacent structures are currently unknown and will be subject to further investigation. 3.2. Proposed Substructure Figure 4 Silent piling installation machinery The advantages of this form of piling is the achievement of close proximity to adjacent structures (not applicable on all the boundaries of this site), but more importantly the minimal vibration during construction. This will have little or no impact on the adjacent residential properties and adjacent Thames Water sewer through ground based vibration. Clay soils are best suited for sheet piling with the reduction in the risk of ground obstructions (as discussed previously London clay soils have been assumed through existing borehole data). Localised obstructions can be overcome by either water jetting or pre-augering. 3.2.1. Structure The current architectural proposal is for a single storey basement accessed by a ramp to the south western corner of the site. The basement contains the lower level of each dwelling as well as parking for each residence, bin stores, bicycle stores and storage areas. The perimeter to the excavation is to be constructed using steel sheet piles that are pushed in using the silent piling technique (refer to figure 4). A trench is excavated prior to the installation of the piles along the line of the wall for the construction of the reinforced concrete capping beam after the sheets have been installed. A preliminary construction method statement has been included in Appendix A. Figure 5 Installation of the steel sheet piles Following the completion of the sheet piles the reinforced concrete capping beam can be constructed (refer to figure 6), which fulfils a number of functions, i.e. ties the head of the sheet piles and the concrete liner wall (constructed later in the process). The ground floor slab will also be tied into the capping beam. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 7
Figure 6 Construction of the reinforced concrete capping beam The excavation of the basement can then commence with a soil berm against the sheet pile retaining wall, which starts just below the capping beam at a 45 degree incline into the basement. Once the excavation reaches formation level then concrete thrust blocks are constructed within the ground as the base of the temporary raking props to the sheet pile wall (refer to figure 7). These blocks are required at regular centres running parallel with the wall. De-watering to the excavation will be undertaken as necessary and discharged into the local sewer network at a rate and to the agreement of Thames Water. Figure 8 Installation of raking props Once installed the remainder of the basement can be excavated to formation (refer to figure 9). Figure 9 Remainder of soil excavated Figure 7 Reduced level excavation and construction of thrust blocks Steel temporary props extend from the top of the thrust blocks to the capping beam running around the perimeter of the basement (refer to figure 8). The clutches to the sheet piles will then be welded to help against any water ingress whilst the ground is prepared for the new basement raft. Hardcore will be compacted in 150mm layers prior to the laying of a 50mm concrete blinding (refer to figure 10) and the construction of the reinforced concrete basement raft. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 8
3.2.2. Heave Ground heave occurs when the ground beneath a building expands upwards and is usually found within clay soils when they get wet or have trees or loads removed. The removal of trees causes the sub-soil to re-hydrate and expand and the extend will be dependent on the size, number and species of trees, i.e. the water demand and extent of the root system. The removal of load also causes heave with the unloading of the soil allowing the clay to swell. The effects of heave are anticipated to be low for the construction of the proposed basement and its effects will be overcome using the mass of the structure, i.e. the weight of the structure will be greater than the heaving force upwards. 3.2.3. Basement Waterproofing Figure 10 Basement raft and liner wall construction An external tanking system is proposed for the waterproofing below the basement raft running up the face of the steel sheet piles to the underside of the capping beam (refer to waterproofing section later in the report). Following the installation of the waterproofing and the completion of the raft the reinforced concrete liner walls are constructed to the perimeter to the underside of the concrete capping beam. The internal columns and the reinforced concrete ground floor completes the substructure facilitating the removal of the temporary props with the ground floor acting as the permanent prop for the top of the retaining wall. Adjacent to the rear of the properties a series of opening against the retaining wall allow light wells into the lower ground floor. In these locations the retaining wall is buttressed by concrete walls between the properties joining perpendicular. The intended use of the basement is habitable space. The requirement for residential living areas is a grade 3. These grades are what depicts the environment of the space and have been taken from the CIRIA Design Guide R139, Water Resisting Basements. Three different types of system are recognised for the waterproofing of underground structures: - 1. Type A - Tanked protection 2. Type B - Structurally integral protection 3. Type C - Drained protection The following table shows the various options for achieving the required standard: - Grade of Basement Basement Usage Performance Level Form of Protection Comments on grade Grade 1 Car Parking Plant Rooms (excluding electric equipment) Workshops Some seepage and damp patches tolerable Type B Reinforced to EC2. concrete Visible water may not be acceptable Design to EC2 Part 1 may result in crack widths unacceptable in permeable ground Groundwater should be checked for chemicals Building Regulations are more likely to require Grade 3 for workshops Figure 10 Ground floor slab construction and buttressing walls adjacent to lightwells Grade 2 Car Parking Workshops Plant Rooms (requiring drier environment) No water penetration but moisture vapour tolerable Type A or Type B Reinforced to BS8007 concrete Performance relies on good workmanship Groundwater should be checked for chemicals A high level of 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 9
Grade of Basement Basement Usage Performance Level Form of Protection Comments on grade Grade 3 Storage Areas supervision of all stages of construction is necessary Ventilated Residential and Working Areas Offices Restaurants Leisure Centres Dry environment Type A or Type B Reinforced concrete to BS8007 or Type C With walls and floor cavity and DPM As Grade 2 Multi element systems may be required in highly permeable ground Table 2 Basement waterproofing CIRIA Design Guide R139 Water Resisting Basements Note: Grade 4 omitted as it would not be applicable for the intended building use (i.e. generally used for book archives and mainframe computer rooms). For a grade 3 basement, a fully tanked solution is as the current proposal discussed earlier in this report. The employed systems will be very dependent on the contractor for its effectiveness as any construction defects (i.e. punctures or poor joints) would allow the penetration of water through the structure. Water could then track along the back of the wall and enter the building through any defects in the concrete. This is minimised with the concrete liner wall and drained cavity (for additional protection). Water bars and hydrophilic strips will be included at all construction joints and interfaces within the construction of the new retaining wall. Puddle flanges must be installed where the proposed services puncture the basement structure. Where sheet piles the clutches between the adjacent sheets will be welded to provide temporary protection against water ingress, however this will not be part of the permanent waterproofing strategy. One system will be constructed for the whole basement. 3.3. Superstructure The proposed superstructure for the development is currently being developed and is anticipated to be a reinforced concrete frame. 108802-REP-002 Basement Feasibility Report Mixed Use Scheme 10/03/2015 Rev A Page 10
Appendix A Basement Construction Methodology
Appendix B Site Investigation & Environmental Impact Assessment
Appendix C Drainage Report
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