In Dundee, many sites sit on weathered Old Red Sandstone or superficial till from the last glaciation. What we often find is a thin topsoil over a stiff, reddish-brown clayey silt that behaves very differently from transported soils. That residual mantle can be deceptive — it looks competent when dry but loses strength rapidly once excavated or saturated. Our lab team runs a full characterization sequence: moisture content, Atterberg limits, particle size distribution, and shear strength under remoulded conditions. Before we even classify the material, we always combine the index testing with an ensayo SPT to correlate blow counts against the actual fines content, because the visual description alone rarely tells the full story in these weathered profiles.
A residual soil that looks like stiff clay can turn into a low-strength slurry within hours of exposure to rain — characterisation catches that risk before the digger arrives.
Process overview
- Natural moisture content and density (oven-dry method, three replicates per horizon)
- Afterberg limits (BS 1377-2) to distinguish high-plasticity clay from sandy silt
- Direct shear box (BS 1377-7) on undisturbed and compacted samples
Local context
A housing development on the northern slope of Dundee Law needed a 4 m excavation for a retaining wall. The site investigation logged the top 2 m as firm clay, but our residual soil characterization revealed a collapsible structure — the material had a void ratio above 1.1 and a collapse potential of nearly 4% under wetting. Had the contractor benched the excavation based on the visual log alone, the batter would have failed during a heavy spring rain. We recommended a drenes verticales scheme to accelerate drainage before bulk earthworks, and the project stayed on schedule.
Reference standards
BS 5930:2015 – Code of practice for ground investigations, BS 1377-2:1990 – Atterberg limits, BS 1377-7:1990 – Direct shear test, Eurocode 7 (EN 1997-2:2007) – Ground investigation and testing
Additional services
Full Index Suite
Moisture content, Atterberg limits, particle size distribution, and specific gravity on every horizon — typically 5 to 8 samples per borehole.
Shear Strength Profiling
Direct shear and unconfined compression (UCS) on undisturbed block samples, plus remoulded strength for collapse potential assessment.
Chemical & Aggressivity Screening
pH, sulphate, chloride and organic content per BRE SD1 to classify the residual soil's aggressivity to buried concrete.
Typical parameters
Quick answers
Why is residual soil characterization different from alluvial soil testing in Dundee?
Residual soils retain the mineralogy and structural fabric of the parent rock — in Dundee that means quartz and feldspar from weathered sandstone. Unlike alluvial deposits, they have no transport history, so the particle arrangement is random and often metastable. Standard classification alone (plasticity chart) can misclassify a high-silt material as low-plasticity clay, which then behaves very differently under load.
What is the typical cost range for a residual soil characterization in Dundee?
For a standard residential site with 3 boreholes and a full index suite plus two direct shear tests, the cost is between £720 and £2,540 depending on sample count and the need for chemical screening. The range covers basic classification through to a detailed collapse-potential study.
How do you sample a residual soil without disturbing its structure?
We use thin-walled Shelby tubes pushed hydraulically at a constant rate of 10–15 mm/s, following BS 5930. For stiff or gravelly horizons we switch to Mazier rotary coring with a triple-tube barrel. All samples are sealed on site and transported upright to avoid moisture migration and structural damage before testing.
Which Dundee areas have the most problematic residual soils?
The slopes around Balgay Hill and the Law are notorious for high-plasticity clayey silt derived from weathered Old Red Sandstone. The material has moderate strength in dry conditions but loses up to 70% of its cohesion when saturated. Sites near the Dighty Burn also show collapsible behaviour in the upper 2 m due to leaching of the original carbonate cement.