1. Investigating the soil
When new roads are planned for adoption or improvements are proposed for existing public roads, it is crucial to conduct thorough investigations beneath the soil, known as a ground investigation. This ensures that the designs shall meet our detail designs.
To investigate the various layers of the soil, exploratory holes should be spaced between 50m and 100m adjacent to the line of the proposed carriageway and encounter all the subgrade types on the site.
Adequate samples of each soil type shall be collected from suitable depths and tested at their natural moisture content.
Understanding the strength of the subgrade
It is important to understand the strength of the soil layer the road is built on, known as the subgrade, to ensure the road is built to the correct depth to prevent deformation.
The California Bearing Ratio (CBR) determines the strength of the subgrade and shall be tested in a laboratory in accordance with BS 1377:1990 Part 4 Clause 7.2.4.4 Method 5.
If a Sustainable Drainage System (SuDS) is proposed alongside or beneath the new road, the CBR shall be tested in a laboratory in accordance with BS 1377:1990 Part 4 clause 7.3 with any swelling of the samples recorded and reported.
Samples for CBR testing shall be taken from the level the road is built on, known as the formation level, to 0.5m below, except where there are different soil types and the weakest soil type shall be sampled and tested.
Additional exploratory holes should be undertaken where ground conditions are variable; particularly where they are weak. In-situ and laboratory testing shall be undertaken as appropriate.
The lowest CBR test result shall determine the proposed construction thickness of the new road which may differ for each individual legal agreement.
If the soil is not tested to determine the CBR in accordance with the above, it shall be assumed, subject to an appropriate investigation for suitability, to be less than 1.5% and the new road designed accordingly.
The strength of cohesive or clayey soils could vary as they are susceptible to moisture, therefore the maximum acceptable design CBR on cohesive soils shall be 3% for low trafficked roads and 2% for high trafficked roads, as defined in the Road build tables.
The density of the soil type in its natural state, known as the field dry density, in accordance with BS 1377:1990 Part 9 clause 2.2 shall not be less than 90% of the maximum dry density of the same soil type as determined by the vibrating hammer test in BS EN 13286-4.
Surface water soakaway testing
To determine the rate at which surface water soaks away into the soil, known as infiltration, testing shall be carried out at the proposed location, depth and head of water when draining the new road. Testing shall be carried out at a minimum of two locations and should be no more than 150m apart.
Infiltration testing shall be carried out in accordance with the Bettess Report (1996) Infiltration drainage manual of good practice CIRIA R156, which is based on the design approach in BRE Digest 365 (1991).
Test pits are to be filled three times and should achieve 75% empty for each test. The lowest infiltration rate result shall be used when applying the appropriate factor of safety (as shown in Table 25.2 CIRIA SuDS Manual) for calculating the required hydraulic design of infiltration systems.
For effective infiltration, the soil shall be permeable and unsaturated.
Ground water level
To ensure the water in the ground does not overwhelm drainage systems through SuDS features, the highest recorded ground water level shall be a minimum of 1m below the underside of the proposed SuDS feature.
The highest ground water level shall be determined by recorded evidence over a six month wet winter period.
If high groundwater level is noted before or during construction, monitoring wells might be requested to provide further detail on fluctuating ground water levels.
Polluted land
If the land that the new road and footpath is to be built on was previously used as an industrial development including farming, for storing chemicals and disposing of waste, or has no known history, it is classed as Brownfield.
Brownfield land has a risk of containing chemicals within the soil which are harmful to people and the environment from rainwater absorbing the pollutants and draining into the ground.
Therefore Brownfield land shall be subject to contamination testing and an environmental risk assessment. Records should be maintained to enable the verification of the details of the works undertaken. During construction of the road and footpaths, should any contamination be encountered, it shall be investigated and reported to us.
Chemical content that is destructive to concrete shall be assessed in accordance with BRE Special Digest 1:2005 ‘Concrete in Aggressive Ground’.
Contaminated land which is to be removed and replaced with fill materials shall be designed and constructed in accordance with the Specification For Highway Works under the guidance of, and signed off by a Chartered Geotechnical Engineer or Chartered Civil Engineer with specific experience in the design and supervision of earthworks.
Where SuDS features are proposed alongside and under new roads, consideration shall be made where there is contamination and fill material to ensure an appropriate design reduces the risk to watercourses, groundwater and the new road construction.
Reporting ground investigation results
Investigation and testing results along with a summary section shall be submitted to us in the form of a Ground Investigation Report and shall include the following:
- Design of the proposed new road and footpath with associated drainage.
- Description of the land including:
- Topography of the land
- Boundaries around the land
- Access to the land
- Vegetation
- Water features
- Watercourses and ditches
- Surface and groundwater flows
- Any other important information such as structures.
- Geology of the land including the expected and known materials it contains.
- History of the land including:
- Past and present Ordnance Survey maps.
- Reference to known previous developments.
- Known flooding incidents and consequences.
- Made ground or fill material which is not naturally occurring including the material type and depth. This material shall be removed from beneath the new road.
- Investigation results including trial pits and exploratory holes and particle size distributions and PH values for the soil.
- Testing results shall include:
- Description of testing processes.
- Details of laboratory and accreditation status for each test.
- CBR test results.
- Soakaway test results.
- Classification as either Brownfield or Greenfield. A brownfield shall be supported by contamination test results, an environmental risk assessment and chemical content assessment.