Compliance with BS 5930:2015 and Eurocode 7 (BS EN 1997-2:2007) is the starting point for any ground investigation in the UK, and in Reading, the varied superficial geology makes the grain size analysis a non-negotiable step. The town sits on a transition zone between the chalk of the Chilterns and the alluvial deposits of the Thames and Kennet floodplains. A single borehole log might capture silty gravels, then fine sands, then stiff clay within a few metres. The combined sieve and hydrometer test is what lets us plot a complete particle size distribution curve, which directly feeds into soil classification under BS 5930 and determines drainage behaviour. When a site investigation takes place near the Oracle or in the redevelopment corridors south of the M4, we often pair the test pits programme with targeted sampling for the lab analysis, because the shallow strata here can change laterally over very short distances.
The transition from the London Clay to the River Terrace Deposits across Reading produces particle size curves that define the entire earthworks strategy.
Methodology and scope
Reading’s geology presents a classic southern England sequence. We find the London Clay Formation underlying much of the town, but the drift geology, especially the River Terrace Deposits and alluvium along the Kennet Valley, introduces coarse sands and gravels that behave very differently from the fine-grained clay. The hydrometer analysis becomes essential for the Claygate Member transitions, where silt content can vary from 20 to 60 percent and significantly affects permeability. In contrast, the terrace gravels often require the complete sieve stack, from 75 mm down to 63 microns, to establish the grading curve for drainage design. More than a routine classification, the grain size distribution influences the choice of earthworks specification and is a key input for the soil compaction control parameters during construction. The lab team works with the shift from cohesive to granular behaviour that defines so many Reading sites.
Common questions
Why does the London Clay in Reading require a hydrometer test if it looks like stiff clay?
The London Clay in this area, particularly within the Claygate Member transitions, often contains significant silt and fine sand laminations that are not apparent from visual inspection. The hydrometer sedimentation analysis per BS 1377-2 quantifies this silt fraction, which directly controls the permeability and frost susceptibility of the soil. A classification based only on the stiff appearance can lead to drainage design errors.
How much does a grain size analysis cost for a Reading site?
A combined sieve and hydrometer test typically costs between £80 and £140 per sample, depending on whether the full sedimentation analysis is required and the number of sieve sizes specified. The exact cost depends on the sample condition and the turnaround time needed for the project schedule.
What sample size is needed for the grain size analysis?
For gravelly soils common in the River Terrace Deposits around Reading, we need around 2 kg of material to run the full sieve stack. For the finer London Clay samples, 500 g is usually sufficient for the hydrometer analysis, but we always recommend taking a larger bag sample to allow for repeat testing if the grading curve shows unexpected results.
