BS EN 1997-2:2007 + UK National Annex sets the ground investigation framework for any rigid pavement design in the UK, but in Exeter the need for a properly sequenced approach becomes clear once you cross the M5 corridor. The city sits on a mix of Permian breccia, sandstone, and thick alluvial deposits along the River Exe floodplain — a combination that creates abrupt transitions in subgrade stiffness over very short distances. A pavement joint layout that works on the sandstone ridges east of the Cathedral will not perform the same way on the compressible silts near Marsh Barton. We have seen this repeatedly on industrial estate expansions where early cracking traced back to differential settlement the initial site investigation simply did not flag. For projects in Exeter, we tie the CBR road subgrade assessment directly into the concrete slab thickness design so the modulus of subgrade reaction is derived from in-situ testing, not assumed from a desk study. When the ground profile hints at deeper soft layers, we also run in-situ permeability tests to check whether excess pore pressures under cyclic loading could degrade support over time.
A rigid pavement is only as durable as the subgrade it rests on — in Exeter, that means verifying stiffness across floodplain transitions where 30 metres can change the support condition completely.
Scope of work in Exeter
Critical ground factors in Exeter
Exeter's post-war expansion pushed industrial development south and west into the River Exe and Alphin Brook floodplains, where much of today's logistics and distribution infrastructure now sits. The historical land use — a mix of water meadows, gravel workings, and later infill — left a legacy of made ground that varies from loose silty sand to pockets of organic clay. When a rigid pavement is placed over these deposits without engineered improvement, the first sign of trouble is usually corner cracking at the slab joints within the first two years of trafficking. The mechanism is straightforward: differential settlement under repeated axle loads concentrates bending stress at the joint edges, and once micro-cracking starts, water ingress accelerates the deterioration cycle. We approach this by mapping the thickness and composition of the made ground with dynamic probing before any concrete design is finalised. In areas where the alluvium exceeds 2 metres, we specify a cement-bound sub-base combined with a geogrid separation layer; this lifts the effective k-value into a range where the slab bending analysis in TRL 550 and the concrete society design charts remains valid for the design traffic loading.
Our services
We deliver the full chain from ground investigation through to concrete specification and joint layout for rigid pavements across Exeter. Each scope is tailored to the site geology and the operational loading the pavement will see over its design life.
Subgrade investigation and CBR profiling
Test pits and dynamic cone penetration along the pavement alignment, with laboratory CBR determination at formation level. We map stiffness transitions across the site and correlate results with the Exeter geological sheet 325 to anticipate soft zones before earthworks begin.
Concrete mix design and specification
Mix optimisation for flexural strength class F4.5–F5.5, with aggregate selection from local Devon sources. We specify air entrainment where exposure to freeze-thaw cycles is expected and confirm workability for slipform or fixed-form placement.
Joint layout and load transfer design
Contraction, expansion, and construction joint spacing based on slab thickness, subgrade modulus, and expected HGV axle loading. Dowel bar sizing and location per TRL 550, with saw-cut timing specified to avoid random cracking during early-age shrinkage.
Construction-phase QA and plate load verification
On-site plate load testing of the prepared subgrade and cement-bound sub-base to confirm the design k-value before concrete placement. We also verify joint saw-cut depth, dowel alignment, and concrete cube strength at 7 and 28 days.
Common questions
What is the typical cost range for a rigid pavement design package in Exeter?
For a full design package covering ground investigation, CBR profiling, concrete specification, joint layout, and construction-phase QA on a typical Exeter industrial site, the fee ranges from £1.670 to £4.940 depending on the pavement footprint area, number of test pit locations required, and whether stabilisation design for soft alluvial ground is included.
Which British Standards apply to rigid pavement design on industrial sites?
The key documents are BS EN 1997-2 for ground investigation, BS 5930:2015 for site investigation practice, BS 8500-1 for concrete specification, and TRL 550 for the structural design of jointed concrete pavements on industrial and distribution facilities. BS EN 13877-1:2023 covers materials and construction requirements specific to concrete pavements.
How deep should the subgrade investigation go for a rigid pavement in the Exeter area?
We typically investigate to at least 1.5 times the depth of influence below the formation level, which for a 200 mm slab on a 300 mm sub-base means probing to roughly 1.0–1.2 metres below finished subgrade. In floodplain areas near the River Exe where made ground or alluvium may be deeper, we extend investigation to 3.0 metres to capture the full compressible layer thickness.
What joint spacing do you recommend for rigid pavements subject to heavy forklift traffic?
For plain jointed concrete pavements under repeated heavy forklift loading — common in Exeter distribution centres — we generally specify square bays of 4.5 m to 5.0 m, with dowel bars at all contraction joints to maintain load transfer efficiency. The exact spacing depends on the slab thickness and the k-value confirmed by plate load testing on the prepared subgrade.
Can rigid pavement design accommodate ground improvement on soft Exeter alluvium?
Yes, and in many Exeter sites it is essential. We design a cement-bound sub-base layer, typically 150 mm to 200 mm thick, placed over a geogrid separation layer on the natural alluvium. This composite system raises the effective subgrade modulus to a level where the concrete slab thickness can remain within economic and practical limits while meeting the design fatigue life for the expected traffic.