LOCATION:   Needham Market Quarry, Suffolk

PROJECT TYPE:   Ground investigation, supplementary contamination & geotechnical assessments and validation

VALUE:   £165K

BACKGROUND

Needham Market Quarry in Suffolk was operated by Needham Chalks Ltd for more than 50 years, having previously been farmland. Hopkins Homes purchased the site for redevelopment, to provide 266 homes.

 

Harrison Group was employed at the pre-acquisition stage by Hopkins Homes, to provide factual ground investigation information for consultant Coffey Geotechnics Ltd, and continued to provide geotechnical services throughout design and construction.

SCOPE OF WORK

The objective of the main ground investigation, carried out in October 2013 while the quarry was still operational, was to provide geotechnical, groundwater and contamination information. It comprised:

27 cable percussive boreholes, to a maximum depth of 26m, with groundwater and gas monitoring standpipes installed at 15 locations

20 cone penetration tests, to a maximum depth of 15m

20 dynamic continuous sampling boreholes (for investigation next to buildings, vegetation and along the edge of the quarry lake), to a maximum depth of 4.1m

14 machine-excavated trial pits to 3.5m depth and four trial trenches, to 4m depth, to gather data on shallower ground conditions, including material stockpiles

Six soil infiltration tests to 1.8m, to provide an overview of drainage potential

Geophysical surveys: resistivity and electromagnetic tomography profiling

Monitoring ground gas and groundwater levels, plus groundwater sampling.

 

Investigations revealed most of the site was covered with between 0.1m and 13.8m of variable made ground, typically reworked from glacial sand & gravel and Diamicton. This material was excavated to expose the underlying Newhaven Chalk and used as backfill after quarrying or was stockpiled. Ground gas and low levels of soil contamination were also recorded.

 

Based on its assessment of the investigation results, Coffey Geotechnics recommended:

Ground improvement of an area of thick fill, to mitigate differential settlement

Slope remodelling

Excavation through an embankment for a new access road

Installation of ground gas protection measures

Drainage and backfilling of the quarry lake with engineered fill (a new drainage basin was installed as part of the development’s drainage strategy).

 

Hopkins Homes engaged Ingent Consulting Engineers (formerly JMS Consulting Engineers) to develop the engineering design and J Breheny Civil Engineering to undertake groundworks. We were employed to carry out further ground investigation and assessments, between 2015 and 2020, including:

Slope stability modelling and assessment, comprising further investigation, laboratory analysis; modelling carried out using Slope/W

Advising on the safe working and development of slopes, considering drainage and planned construction of roads and houses

Reviewing slope, drainage and earthworks design

Assessing ground gas, advising on protection requirements and reducing the area requiring protection

Further investigation of the chalk in one area, reducing the need for piling

Additional soil infiltration testing, to support detailed soakaway design

Site visits to sample and analyse waste generated during groundworks

Contamination investigations after earthworks exposed organic and potentially volatile material. Laboratory analysis of samples revealed low levels of PAH contamination

Development of the remediation strategy and method statement (agreed with the Regulator), comprising a simple cover system over exposed contaminated material. We also provided validation and verification reports for the client, regulator, warrantee provider and homeowners

Assessment of a pocket of asbestos exposed during groundworks to the Control of Asbestos Regulations 2012. Development of the remediation method statement, incorporating asbestos risk in the few plots affected.

OUTCOME

The ground investigations and assessments were completed safely, within the timescales and programme budget and to the satisfaction of the consultants and the client. Hopkins Homes continued to request our involvement at critical stages during design and construction of the homes.

 

Gavin Houghton, Head of Technical Services at Hopkins Homes, said:

“Your investigations allowed a technically challenging site to reach an earthworks strategy conclusion, which facilitated a site, that has stood for many years, coming to fruition as a housing development and this is through the merit of the work of the Harrison’s team. From our experience on this and many other sites, we would recommend the Harrison Group for works of this nature.”