Assessing the Site: Understanding What Lies Beneath
Every successful paving project starts long before the first slab, block or sett is laid. The finished surface may get all the attention, but the ground beneath has a far greater influence on how the pavement performs over the coming years. Settlement, standing water, movement and even premature failure can often be traced back to problems that existed before construction began.
Taking time to assess the site properly helps identify these risks early. It allows the installer to understand ground conditions, drainage arrangements, existing construction and any factors that could affect the long-term stability of the pavement. For domestic paving projects, this stage is one of the most valuable investments of time you can make.
Start by Looking at the Existing Site
Before picking up a spade or excavator, spend time observing the area that will be paved.
Look for:
- Standing water after rainfall
- Areas of settlement or uneven ground
- Existing paving that has cracked or moved
- Signs of moss or algae growth
- Previous repairs or alterations
- Existing drainage features
- Look for indicator weeds, such as reeds (Phragmites australis) which is a clear sign of saturated ground
These clues often reveal problems that might otherwise go unnoticed until construction is underway. A patio that regularly holds water, for example, could indicate poor drainage, whilst areas of settlement may suggest weak ground or poorly compacted fill beneath the surface.
The aim is not simply to understand what the site looks like today, but how it behaves in different conditions.
Understand How Water Moves Across the Site
Drainage should be considered at the earliest stage of any paving project.
As part of the initial assessment, identify where water currently flows, where it accumulates, and how it leaves the site. Pay particular attention to:
- Low points
- Existing gullies and drainage channels
- Areas adjacent to buildings
- Changes in level across the site
- Signs of persistent dampness
Poor drainage affects more than just the appearance of the paving. Water trapped within the pavement structure can weaken foundation layers, encourage algae growth and reduce the lifespan of the installation.
Understanding existing drainage patterns allows suitable falls and drainage measures to be incorporated into the design before any excavation begins.
Consider Trees, Roots and Vegetation
Existing planting can have a significant impact on paving performance.
Tree roots may lift paving, shrubs such as some bamboo’s, weeds such as Japanese Knotweed (JK), disturb foundations or make excavation difficult, or extremely costly in the case ok JK. Invasive plants can create future maintenance problems if they are not properly addressed during construction. Root systems often extend much further than the visible canopy, so it is important to consider what may be happening below ground as well as above it.
Where mature trees are present, additional considerations may apply. Protected trees and trees within conservation areas may restrict how excavation can be carried out, and specialist solutions such as reduced-dig or no-dig construction may sometimes be required.
The objective is to identify anything that could affect the long-term structural integrity of the pavement and decide what measures are needed to manage the risk.
Dig Trial Holes
Visual inspection only tells part of the story.
A simple intrusive investigation is often needed to understand what lies beneath the surface. Trial holes provide valuable information about:
- Soil type
- Subgrade strength
- Existing foundations
- Made ground or fill material
- Buried construction debris
- Groundwater conditions
For sites up to 100 m², at least two trial holes should normally be excavated. Larger sites generally require at least one trial hole for every 50 m². Low points within the proposed paved area should always be included, as these are often where poor ground or drainage issues are discovered.
Unexpected materials such as topsoil, loose fill, buried rubble or standing water may indicate that additional excavation or redesign is necessary.
Check the Ground Can Support the Paving
The strength of the ground beneath the pavement has a direct impact on the thickness of the construction required above it.
The British Standard includes a practical site assessment based on footprint depth (boot test) in dry ground.
As a guide, placing your work boot on the subgrade will tell you:
- A footprint of up to 2 mm generally indicates stronger ground.
- A footprint of 2 mm to 4 mm suggests weaker but potentially acceptable ground.
- A footprint greater than 4 mm indicates ground that requires further assessment and a formal pavement design.
If suitable supporting material cannot be identified within approximately 500 mm of the surface, a more engineered design should be considered rather than relying on a standard domestic paving build-up.
Subgrade Assessment to Determine CBR
Subgrade assessment is a critical part of pavement construction, because the ground beneath the pavement ultimately determines how well the completed construction will perform. The subgrade should be exposed and assessed to identify the soil or material present, whether that is bedrock, compacted stony ground, granular material, sand, silt or clay, and to establish whether there are any soft, variable or waterlogged areas.
Assessment can include visual inspection, simple site methods such as the boot test, and where necessary more formal testing such as CBR or in-situ stiffness testing. It is important to understand the difference between strength and stiffness: strength is the subgrade’s ability to resist failure under load, whereas stiffness is its resistance to deformation or deflection. A subgrade can appear firm at the surface but still have insufficient stiffness to adequately support the pavement. Correct assessment allows the required foundation thickness, improvement or capping measures to be determined before construction begins.
Assess Existing Construction Carefully
Many paving projects involve replacing or extending existing paved areas rather than building on untouched ground.
Retaining parts of the existing construction can reduce excavation, waste and cost, but only if those materials are suitable.
Before deciding whether existing foundations can remain, consider:
- Layer thickness
- Structural condition
- Cracks and damage
- Areas of settlement
- Drainage performance
Existing layers should only be retained if they can provide support equivalent to the requirements of the new pavement. Thin, damaged or unstable materials should not simply be buried beneath a new surface.
Where suitable materials are removed, consideration should also be given to recycling them where practical.
Look Beyond Individual Areas
A common challenge identified during assessments is the interface between old and new construction.
Established foundations have generally completed most of their settlement, whereas new construction may continue to stabilise after installation. This can lead to height differences where old and new areas meet.
Understanding these potential movement differences before construction starts allows suitable adjustments to be incorporated into the design and levels.
Good Paving Starts with Good Information
The purpose of site assessment is not to create unnecessary work. It is to ensure decisions are based on evidence rather than assumptions.
By taking the time to understand drainage, vegetation, soil conditions and existing construction, landscapers/paviors can reduce risk, improve durability and select the most appropriate construction method for the site. A thorough assessment provides the information needed to build paving that performs as well beneath the surface as it does above it.
