Investigation of the causes of movement and damage to an office building in the UK founded on Carboniferous Coal Measures mudstone. William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS, UK SPECIALISTS IN FORENSIC INVESTIGATION Robert Jessep MEng CEng MICE MIStructE FGS Associate Head of Geotechnical Engineering

Introduction William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Appointed as an independent Expert under English Law (Civil Procedures) by Solicitors acting for a developer of an office building. The brief was to establish:- The causes of movement and damage to the building; (which was about 6 years old at the time of receipt of the instructions) Whether the movement was ongoing or had ceased; An appropriate remedial scheme; and Who was responsible for the shortcomings which led to the damage.

Description of the site William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Geology – Carboniferous Coal Measures Mainly mudstone weathered to a soft to stiff clay at surface. Complex site history:- Storage of steelworks waste for 30 years Stockpiling of up to ~32m depth of opencast mining arisings Above material removed about 10 years before development Re-compacted Coal Measures mudstone fill placed in 800mm thick layers Granular fill containing slag placed as a working platform. Trial pits sunk before building No boreholes sunk in the mudstone Ground Conditions:-

Description of the building William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS 4 Storey Office building with a plan area of ~1000m 2, formed of:- Steel framed construction with precast concrete floors, built on Pile caps with perimeter ground beams, founded on Hollow steel piles of 244mm diameter driven to 4 to 6 metres depth. Masonry built of ground beams with glazing and curtain walling above. Ground level floor slab formed from ground bearing reinforced concrete slab. Piles driven to a set which indicates high dynamic resistance. Dynamic re-strike tests on piles with CAPWAP analysis. No static maintained load testing of the piles.

Description of the damage William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Crushed partitions Cracking and distortion of partitions and glazing

Description of the damage William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Cracking in blockwork walls Cracking in between blockwork walls and columns

Results of soil investigations William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Samples of ‘soil’ subject to oedometer swelling test Results:- Up to 4.4% strain in fill 1% strain in a sample of mudstone Slag chemical analysis/tests Results:- Little expansion potential in slag Evidence of past expansion Mudstone rotary cored Results:- Slightly to completely weathered Very stiff clay in places above 6m depth Very weak rock below 6m depth

Results of level surveys William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Precise level surveys were carried out for twelve months. Followed BRE Digest 386 guidance using two deep ‘stable’ datums. 2.5m grid of points on the ground slab. Selected low-level perimeter brickcourse. Underside of accessible first floor level steel beams. Ground slab Relative levels of the slab 7 years after construction (m) Floor slab showed variation in levels of 70mm.

Results of level surveys William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Ground slab movement in the 4 th to 6 th years (mm) Generally heave Up to 7mm Isolated settlement Up to 7mm Continuing heave Up to 4mm Isolated settlement Up to 1mm Some reversal from above Ground slab movement in the 7th year (mm)

Results of level surveys William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Brickcourse Survey points at pile/column locations and mid-span between those locations Level differences of 40mm Typically low points at piles and high points at mid-span points Maximum differential of nearly 30mm or a slope of 1:130 Settlement at 3 pile locations of up to about 1mm in 7th year One part of the building heaved by 3.3mm in 7th year Relative levels of a perimeter brickcourse 7 years after construction (mm)

Results of level surveys William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS Steelwork Limited surveys in 6th year showed settlement of up to 9mm at pile locations Precise levelling showed the level of the steel beams spanning from the perimeter to internal columns was lower at the internal end. Typically by 10mm Up to 22mm Precise levelling in 7th year generally showed little ongoing movement but Settlement at some internal columns of up to 1.5mm 2.5mm heave in one part of the building corresponding with the brickcourse

Analysis – movement of foundations Anticipate pile end bearing limited by weaker softer Coal Measures Ultimate end bearing capacity of the piles would be less than 280kN Normally ignore limited shaft resistance in design Pile design loads ranged from 270 to 640kN (only around 60% ever applied) Given disparity in loads and capacity – excessive settlements not surprising –Surveys suggest settlements of up to 40mm or 15% of pile diameter Dynamic pile tests predicted a factor of safety ranging from 2.3 to 3.6 It is recommended that these tests are calibrated with a static load test because: –The results can be in error, typically by a factor of 2 –Dynamic testing in Coal Measures may be unrepresentative due to relaxation No static load tests were carried out at this site Undue pile settlement caused by shortcomings in the design due to a lack of site investigation Static load tests would probably have revealed the inadequate pile load capacity William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS

Analysis – movement of ground slab William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS The working platform contained slag Largely blastfurnace slag which is less expansive than other slags Petrographic analysis showed evidence of past expansion Accelerated expansion tests showed little future expansion potential May only have contributed to a minor extent to the heave of the building Fill from mudstone normally subject to settlement In particular collapse settlement upon inundation or percolation with water Probable explanation for settlements observed in the past in areas of the slab The fill at this site came from a mine that was up to 95m deep Stress relief could be expected to give rise to long term swelling with water absorption Particularly with gravel and boulder sized lumps of mudstone Swelling tests indicate potential for up to 100mm future heave but probably 40mm Likely swelling of this fill has been the significant cause of the heave at this site Minor contribution to heave from swelling of Coal Measures caused by stockpiling surcharge and subsequent unloading Excessive heave and settlement caused by inadequacies in the design due to a lack of adequate desk study, site investigation and testing of the subsoils

Conclusions The movement and damage at this building has been largely caused by:- –Lack of adequate desk study, and –Lack of adequate site investigation This case study highlights:- –The importance of good desk study and site investigation –The need to consider stress history of sub-soils –The general risks associated with fill materials –The need to calibrate dynamic pile tests with static maintained load tests William J Marshall & Partners CONSULTING ENGINEERS AND ARCHITECTS

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