Norwich is situated largely on glacial drift deposits overlying chalk. Historically these deposits provided suitable grounding for foundations. Over time, in some areas the fine sediments which bind the larger gravel fragments together have been washed away, resulting in instability and collapse into solution features within the chalk.

In addition, there are a large number of mines and pits across the city where flint, chalk, sand and gravel have been excavated. Many of the backfilled pits, underground mine networks and natural solution features within the city are unmapped so their extent is unknown. As a result, a number of buildings and roads are situated over cavities; in times of prolonged heavy rainfall, water main leakage or heavy construction, subsidence events have been experienced.

Shrinkage or swelling of clay soils as a result of changes in moisture content is also a common cause of structural defects. This is largely influenced by the proximity of trees.

For almost 30 years Harrison Group Environmental has provided Norwich City Council with a range of site investigation services to enable assessment of the local geology in areas of subsidence. Problems range from minor cracking of internal plasterwork to separation of joined walls and, in extreme cases, complete collapse of structures and huge voids appearing beneath roads.

SCOPE OF WORK Harrison Group Environmental offers a range of services to provide the information necessary to analyse the cause and effects of subsidence. Investigation methods include:

• Testing of surface/foul drains and water mains for leaks
• Window sampling and dynamic penetration testing to investigate local geology and soil density
• Hand dug trial pits exposing existing foundations
• Cable percussive boreholes to investigate the deeper geology
• Chemical testing for concrete specification
• Geotechnical testing to classify soils
• Tree root analysis
• Factual / interpretative reports

The interpretative report will discuss:

• Cause of the subsidence
• Concrete specification
• Foundation recommendations
• Recommended remedial action which might be underpinning, cavity grouting, foundation improvement, or in the most extreme cases, demolition

Subsidence and settlement of soil is not a problem limited to areas of solution features or mines. Washout of fine sediments alone can cause subsidence of granular soils.

Harrison Group Environmental is able to offer these services to clients who wish to investigate the ground conditions associated with any form of subsidence or land collapse including slope failure.