Building on Former Industrial Land in Rotherham

What is actually under a site here

Three things, in layers of varying age and quality, and a single plot can have all of them:

• Steelmaking legacy - slag, industrial fill, levelled works platforms, demolition material and buried foundations from structures that were removed at ground level and left below it
• Coalfield legacy - spoil and tips above ground, and shallow workings and shafts below it, with parts of the borough inside development high risk areas defined by the Mining Remediation Authority, formerly the Coal Authority
• Natural ground that changes three times across the borough: Coal Measures in the west and centre, the Magnesian Limestone escarpment running north to south, and Sherwood Sandstone further east - with alluvium along the Don and the Rother valley floors

Why made ground is a foundation problem rather than a subsidence one

This distinction decides which investigation you commission. A void beneath a building - a shallow working, a shaft - is a subsidence question: something may collapse. Made ground is a bearing and settlement question: the material consolidates unevenly under load.

Made ground's defining property is variability. Nobody placing it was engineering a foundation; they were disposing of material. Composition and compaction change across a single plot, sometimes dramatically, and two trial pits ten metres apart can find quite different things. Fill placed decades ago may still be consolidating, particularly where it was tipped rather than engineered and where water moves through it.

A site can of course have both problems, which is exactly why they are assessed as two questions rather than one.

Buried obstructions: the ones that wreck programmes

On former steelworks and colliery land the thing that most often derails a groundworks programme is not the fill - it is what is in it. Machine bases, buried foundations, culverts, tunnels, rail beds, tanks and services from structures demolished to ground level and no further.

None of that is exotic and none of it is a reason to avoid the land. It is a reason for the ground investigation to be looking for obstructions as well as characterising the soil, and for the programme to carry some contingency for what it finds. Discovering a mass concrete machine base under a proposed pad foundation in week two of groundworks is a very expensive way to learn about it.

The sequence that saves money

It is the same sequence on every site of this kind, and getting it out of order is the single most common avoidable cost we see in this borough.

• Desk study first - historic Ordnance Survey mapping is unusually powerful here, because it shows what stood on the site before, and the mining records cover the other half
• Establish the mining position where the site is in a high risk area, because it may require its own assessment and treatment
• Ground investigation designed to answer the actual questions: depth and variability of fill, bearing stratum, groundwater, and obstructions
• Then foundation design - to what has been found rather than to a standard detail
• Only then fix the layout and the levels, because on variable ground the foundation solution can legitimately influence where things sit

What the foundation options look like

There is no single answer, and that is the point. Depending on the depth and consistency of the fill and the loads involved, the design may dig through to a competent stratum, use a reinforced raft to span the variability, pile through it, or - where the fill is shallow and consistent - treat, or remove and replace it.

Piling is common on this ground and it brings its own site questions: obstructions again, and what the piling rig can actually reach. Those are worth resolving on paper rather than on site.

What does not work is a standard strip footing designed on an assumption. The most expensive outcome we see is foundations designed before anybody looked, then revisited with the trenches open and the programme already running.

A note on contamination

Ground of this history frequently raises contamination questions as well as engineering ones. That is a specialist environmental discipline rather than ours, and the two investigations are usually best commissioned together so one set of boreholes serves both.

We will tell you where the boundary of our advice is rather than stray across it. What we will do is design foundations for aggressive ground conditions where the testing identifies them, because that is a structural response to an environmental finding.