Foundation is an essential structural element connected to the ground, and it receives the load from the piers and abutments and transmits it to the foundation soil. Depending upon the foundation depth, and requirement of the project, different types of foundations are used in construction. Selection of foundations types is crucial in Geotechnical Engineering.
Selection of Foundations Types :
- Anticipated Discharge and corresponding maximum scour level in bridges.
- Position of groundwater table, liquefaction depth and swelling potential.
- Availability of suitable bearing strata under anticipated loads.
- Settlements of foundation strata under anticipated vertical loads.
- Availability of adequate uplift capacity if any under anticipated loads.
- Foundation type in the existing Structure at the vicinity, if any
Foundations are divided into two categories; which are grouped as under
- Shallow Foundation ( Depth < = Width ): Spread footing and Raft foundations
- Deep Foundation ( Depth > Width ): Pile foundation and Well foundation
A shallow foundation is a type of open foundation; it is suitable when the foundation soil has sufficient bearing capacity within a short depth below the existing ground level. They require enough plan area to transfer the heavy axial loads to the subsoil. Shallow foundations are also known as footings. Shallow foundations are chosen when the subsoil has moderately good strength, except the raft foundation, which is also useful in relatively weak soil. Raft foundations are under shallow foundation as they have shallow depth compared to the deep foundation, but the depth of the raft foundation is much larger than those of other types of shallow foundations.
Shallow Foundations Types:
Spread Footing (Width < 6.00 m):
The spread footing is a type of shallow foundation where the structural members used to support the column, pier and walls as well as transfer and equally distribute the load getting on the structure to the foundation soil. Spread footings are generally adopted for below-mentioned structure.
- Major and minor bridges
- Box and slab culverts (pre-cast and cast-in-situ)
- The headwall of RCC pipe culvert
- Retaining Wall along Road either on hillside or valley side
Raft Foundations (Width >= 6.00 m):
The raft foundation is known as mat foundation; which is a continuous slab covering the entire area and resting on the foundation soil, thereby supporting the structure and transmitting load to the soil strata. Raft Foundations are generally adopted for below-mentioned structure.
- Major and RCC Box Minor Bridges
- RCC Box and Slab Culverts
- RCC Box Structure for Vehicular Underpass (VUP)
- RCC Box Structure for Pedestrian/Cattle Underpasses (PUP / CUP)
Foundation Shape, Size and Depth :
The shape of the foundation varies based on the Site condition and Structural requirement.
The size of the foundation depends on sub-surface profile and properties, site condition, structural requirement etc.
Based on the sub-surface profile, structural requirement, position of GWT, scour level etc. as per IS:1904 & IRC:78: 2014
- In Soil: For bridges: minimum 2.00 m below scour level or protected bed level and for other types of structure minimum 2.00 m below original ground level (OGL).
- In Rock: Minimum 1.50 m below in soft rock and 0.600 m below in hard rock(>12.5 MPa)
Safe Allowable Load Bearing Capacity :
- Check against shear failure, total & differential permissible settlement
- For foundation in soil, completely disintegrated & highly weathered rock (IS: 6403, IS: 8009 (Part I) & IS:1904, IRC: 78: 2014)
- For foundation in rock other than completely disintegrated and highly weathered rock(IS: 12070)
NOTE: To satisfy the required design safe bearing capacity, ground treatment (if necessary) in the form of replacement of top weak soil with compacted granular soil (sand/gravel) blanket of the desired thickness or any other treatment like stone column etc. may be adopted below the proposed funding level of structures.
In case of weak soil near to the original ground surface, the bearing capacity is very less, and the foundation needed in such case is the deep foundation. A deep foundation is a foundation that transfers structural loads to the foundation soil strata farther down from the surface than a shallow type foundation does to a subsurface layer or a range of depths.
Piles are acted like columns which can be precast or cast-in-situ by nature. Piles support the structure by the skin friction between the pile surface and the surrounding soil and end bearing force if such resistance is available to provide the bearing force. Accordingly, they are designated as frictional and end bearing piles. They are usually provided in a group with a pile cap at the top through which loads of the superstructure are transferred to the piles.
Type of Pile Foundations:
- RCC bored cast-in-situ: For soil, completely disintegrated and highly weathered rock
- RCC Bored and Rock Socketed cast-in-situ: For rock, other than completely disintegrated and highly weathered rock
- Driven piles: Precast RCC piles, sheet piles, timber piles
The shape, Size and Depth of Pile:
The shape of the pile is generally circular type
As per IRC: 78: 2014, The size of piles varies from 1.00m to 1.20 m diameter.
Depth of Pile :
- Based on the sub-surface profile, structural load requirement, scour level etc. fulfilling minimum depth beyond “Depth of Fixity” in accordance with IS: 2911 (Part-I/Sec-II): 2010 & IRC: 78: 2014.
- For socket length in rock, IS: 14593 and IRC: 78: 2014
Spacing of Pile :
As per IS: 2911 (Part-I/Sec-II) : 2010, IS : 14593 : 1998 and IRC: 78 : 2014.
- For frictional pile in the soil – Minimum 3.00 times of pile diameter
- For end-bearing pile in the soil – Minimum 2.50 times of pile diameter
- For rock socketed pile – Minimum 2.00 times of pile diameter
Safe Load Carrying Capacity :
Vertical / axial compression, uplift and lateral load
- Check against shear failure, total and differential permissible settlement
- For foundation in soil, completely disintegrated and highly weathered rock (Ref: IS: 2911 (Part-I/Sec-II): 2010, IS: 8009 (Part II), IRC: 78: 2014)
- For foundation in rock other than completely disintegrated and highly weathered rock (Ref: IS: 14593: 1998)
- Negative friction or down-drag (Ref: IRC: 78: 2014)
Well foundation is a type of deep foundation; provide a solid and massive foundation for heavy loads as against a cluster of piles which are slender and weak individually. Wells are hollow and have a large moment of inertia with the minimum cross-sectional area. They can resist large horizontal and vertical loads even when the unsupported length is high on scouring river. Well foundation is being used for hundreds of years as deep foundations for important bridges, buildings and other structures
The shape, Size and Depth of Well :
Different shapes of wells that are commonly used.
Minimum 3.00 m diameter of dredge-hole as per IRC: 78: 2014
Depth of Well :
Based on the sub-surface profile, structural load requirement, scour level etc. in accordance with IRC: 78: 2014
- In Soil: Minimum grip of 1/3rd of maximum scour depth below design scour level
- In Rock: Should be evenly seated all around the periphery on the Sound rock (i.e. devoid of fissures, cavities, weathered zone, likely extent of erosion etc.
- A “Sump (Shear Key)” of 300 mm in hard rock and 600 mm in the soft rock inside the well by chiselling/blasting
- The diameter of “Sump” maybe 1.5 to 2.0 m less than the inner dredge-hole subject to a minimum size of 1.5 m and 1.5 m anchored in rock with 25 mm dia HYSD dowel bars through 65 mm dia grouted borehole
- Safe allowable load-bearing capacity: similar as shallow (open) foundation only from end bearing and no side friction
Information to be Checked:
- Borehole Location plan with Borehole top Co-ordinates and Level
- Borehole / Drillhole Log Profile with Top RL, Depth and Description of Foundation Layers with SPT (N) value
- NOTE on Required Safe Load Bearing Capacity (SBC) separately for Pier and Abutment
- NOTE and Detailing of Ground Treatment, if any for required SBC
- ALL the above should be included in GAD and Detail Drawings of Pier and Abutment Foundation
- Detailed Geotechnical Investigation Data and its Engineering Assessment with Design Calculation including Foundation Design Recommendation in Consolidated Report Form for each Structure