- Piers are single unit supports carrying heavy loads and transferring them to deep-seated layers of soil or rock.
- They are also termed as large diameter bored piles, foundation cylinders or vertical shafts requiring boring or open excavation.
Types of Piers:
It can be classified as:
- Hand excavated piers
- Drilled piers
- Pipe piers
- Hand excavated piers:
- Hand excavated piers require dewatering by pumping initially.
- Then as excavation progresses, the sides are supported by precast rings, steel cylinders or timber planks etc.
- After reaching desired depth, the shaft is filled with concrete and side supports may be withdrawn or kept in place.
- Drilled piers:
- In this, excavation is speeded up mechanically using power augers, bucket drills or chopping bits.
- Diameter of drilled shaft may range from 0.6m to 3m. Bottom of pier is enlarged at base and socketed in rock.
- Casing or bentonite slurry may be used to prevent collapse of soil or inflow of water into borehole.
- Drilled pier can replace a group of piles and also offers large lateral resistance but they cannot be installed without a bearing strata and underwater excavation becomes very expensive.
- Pipe piers:
- Pipe piers can have diameter from 0.6- 2 m and are made of prestressed concrete or steel sections.
- Hollow shafts may be used to reduce the self weight.
- Casing is required in soft soils to prevent collapse of soils.
- It may be withdrawn or left with pier to increase pier capacity.
- Chopping bits or chiseling device is used for excavation in hard layers or in soil with boulders.
Methods of Installation of Pier:
- It is constructed either by open excavation and then cast-in-situ in bore holes, or are sunk into the ground by hand excavation or boring with large diameter power augers.
- They are carried through soft layers of soil having low bearing capacity to rest on the stiffer layers of soil or rock.
- They are spreaded at base by under reaming device or by hand excavation in large shafts.
- The shaft excavation may need bracing or lining for greater depths. Soils at the bottom can be inspected and collected for field testing as diameter of the shaft is large enough to allow inspection.
- Excavation of shaft in non-cohesive soils is comparatively difficult below the ground water table as the soil may cave in with inflow of water and cause obstruction to the pier installation.
- They are mostly used for tall multi-storeyed building, towers, highway interchanges etc.
- Shore protection works and large water front structures can have such foundations.