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Oak Farm Infant School

General Structural Inspection Letter Report

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Project Details:

Completion Date:

March 2020

Building Type:

School

Project Type:

Structural Reports

Location:

Hillingdon, Middlesex

Structural Engineer:

John Murphy

Introduction

This structural inspection report was prepared and commissioned under the instructions of our client, John Faley. We have been requested by our client to carry out a general site inspection of the property that is located at Hillingdon, Middlesex.

A single, non-disruptive site visit inspection was conducted on the Tuesday 25th February 2020. The weather condition at the time of the inspection was dry and cold. This report is intended primarily for information and is to be read in conjunction with any other specialist reports and investigations. In accordance with our standard practice we must point out that this report is based upon our inspection of the premises and any other information made available to us, both written and oral, which we have assumed to be correct. Structural Engineering Services cannot accept any liability to any third party for the whole or part of its content.

Brief Description of The Existing Building

The nursery is a single storey building built approximately around 1990s although the exact age of the property could not be determined. The property is likely to be supported by either load bearing walls or steel frames with exterior formed with a masonry façade. Steel columns are supporting the overhanging roof at the rear elevation and the side of the building.

A single storey flat roof extension has been built against the existing flank wall of the nursery approximately around 2005. The flank wall of the extension is formed with uPVC cladding and brickwork façade on the rear elevation wall. In this location a section of the overhanging roof has been enclosed with brickwork to form the boiler room which is subject to most of the damage observed.

The local geological map has been highlighted in the figure indicated below. It is unclear what the underlying superficial deposit is, but the underlying bedrock geology is indicated to be London Clay Formation, which is clay, silt and sand mixture. Although the superficial deposits could not be determined it is likely that clay will be present at various depths.

We carried out a non-intrusive general site inspection of the rear extension property to assess its structural condition, highlighting any potential concerns and recommending any necessary structural repairs.

Conclusion

The cracking observed are consistent with the rear elevation of the store room and boiler room extension attempting to pull away from the main nursery building. The cracks are all larger at the top than at the bottom, suggestive of a rotational movement of the cladding on the flank wall of the main building and the masonry wall of the boiler room. This is evident by the cracking pattern in the storage and boiler room  where the handrail and the steps are visibly pulling away from the main
building.

Our understanding is that the store room extension has been built on shallow foundations. Assuming that this is the case, differential settlement between the main building foundations and the relatively shallow extension foundation may be occurring. Although the weight supported by the store and boiler room foundations is relatively low we cannot exclude the potential of differential settlement as a contributing factor to the observed cracking. NHBC part 4 provides guidelines for proposed foundation depths in shrinkable soil.

The British Geological Map does not indicate the superficial deposits but it is likely that clay will be found at various depths. Considering the proximity and the height of the nearby Oak trees it is likely that the tree roots are causing the soil to heave which has likely caused damage to the foundations. It is also possible that the tree roots are physically damaging the foundations.

Furthermore, there is evidence of potentially leaking drainage. This can erode the fine particles present in the soil and cause the soil under the foundations to settle.

BRE Digest classifies cracking according to crack widths, refer to attachment on the last page of this report.  In summary, the cracking observed and discussed are likely to be a result of the ground containing significant clay content, proximity of the oak trees, damaged drains and potential differential settlement between the relatively shallow foundations below the store and boiler room and the foundations of the main building. It should be noted that a building adjacent to the main nursery building was previously demolished as a result of subsidence. This would indicate that the ground is susceptible to movement and is likely the cause of the cracking observed in addition to the above mentioned causes

Recommendations

  • Plasterboards in the store room to be exposed, condition of the masonry to be inspected and potentially repointed or repaired using crack stitching.
  • The boiler room return wall to be either toothed back in the masonry where it has cracked, alternatively packed with mortar and doweled into existing main building wall.
  • Gap between the mortar and the brickwork as indicated in image 9 should be filled with mortar to prevent water ingress and maintain weather tightness.
  • Further investigations could be undertaken to verify the condition of the foundations of the store and boiler room walls below the ground level.
  • We suggest trial holes to be dug at the rear walls of the store room and the boiler room, to inspect the foundations and the condition of soil. This would indicate whether significant localised settlement is occurring or whether there is a noticeable presence of tree roots below the base of the foundation.
  • We also recommend having the drains checked by a specialist, where they would scan the drain lines by a CCTV survey to determine their condition and fixed as necessary.
  • An arboriculturally report could be undertaken by a tree specialist to determine the influence of the trees to the property. Their surveys may indicate whether subsidence has occurred and may advise, for example, on the potential risks of removing trees near buildings. They may suggest using tree root barriers made of highdensity polyethylene to protect your property, if you are unable to remove the trees or provide suitable structural solution.
  • If subsidence is found subsequently, an appropriate structural remedy would be necessary. Either cellular structures could be installed beneath foundations and ground floor slab to reduce the downward movement. Underpinning could also be necessary where the soil itself is prone to settlement to stabilise the structure.
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OakFarmInfantSchool

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