Topic B2Sub-structure groundworks

• Hazards

• Control of water

• Earthwork support

• Function of a foundation

• Understanding how foundations are detailed

Hazards associated with groundwork• Gas – exhaust fumes from petrol or diesel powered equipment such as

generators and compressors. Don’t put equipment near to the edge or in an excavation unless the fumes can be ducted away or the area can be safely ventilated.

• Collapse of the side of excavations – batter the sides to a safe angle or support sides with timber, sheeting or proprietary support systems.

• Movement of groundwater – sump pumping for temporary control of sub-soil and surface water during excavation and then land drainage for permanent control of sub-soil water.

• Confined space• Safe access and egress – provide good latter access.• People and vehicles falling in – put guard rails and toe boards around the

excavation. Keep vehicles away with brightly painted baulks or barriers. Use stop blocks so vehicles won’t over-run into the excavation when tipping material in.

• Avoiding services – look at plans to locate known services. Look out for signs of services such as valve covers or patches on road surfaces. Use locators to trace service pipes and mark ground when you find them.

• Collapse due to soil type

• Function of a foundation

• Safely transmit loads from the building to the sub-soil

• Allow even settlement

• Support the loads of the building for its lifespan

Strip and deep strip

Strip foundations are used where the soil is of good

bearing capacity. They are used in most types of low-rise

housing up to 3 storeys. A deep strip, sometimes known as a

trench fill is a more economical alternative when the foundation

is relatively narrow. It can reduce the amount of brickwork, blockwork and backfill needed

in the substructure, which reduces the costs.

Raft• Raft foundations spread

the load over the entire ground floor of the building. The ground floor slab becomes the foundation for the building. Rafts usually have to be reinforced with a steel mesh and layed on a damp-proof membrane.

• Rafts can be used on poor soils or where subsidence may be expected.

• They also give an early level working space during the construction phase of the building work.

Short bored piles Pile foundations transfer loads to much deeper subsoil levels that have much higher weight bearing capacity. They are concrete or steel columns that can

be driven into the ground using a pile driver (solid piles). Piles are arranged in patterns or grids and linked together using reinforced concrete ground beams. Bored piles are cast in situ. A cylindrical hole is bored with an auger and then

it is filled with concrete.

Engineering brickwork to DPC and cavity fill• Brickwork below the DPC will be engineering brick. This brick is fired

in the centre of the kiln so it is harder and less porous than ordinary brick.

• The damp-proof membrane stops moisture transfer by lapping the material to prevent capillary traction between sheets. Joints are taped to prevent water penetrating the membrane lap. The membrane is thick so it won’t get cut by aggregates or other sharp objects.

• The cavity below ground is filled with a weak concrete mix. This stops the cavity collapsing in on itself due to soil pressure and it also stops groundwater filling the cavity.