Existing underground structure under proposed building?
Yes-
Ground slabs on top of the existing underground structure should be suspended and should not be on grade.
Bored piles with sleeves and/or pile caps should be used. This will not create any undue stress or surcharge load on the existing foundation.
Diaphragm walls are preferred to be used in order to reduce vibration
Basement walls should be set back at least 500mm during excavation in order to reduce stress onto existing foundation in order to avoid imposing additional loading on the existing structures.
Soil Data
Sandstone-
Assume sandstone soil has hard bedrock with a bearing capacity of 3000kPa
Pad footing- Consideration of differential settlement if the foundation is on stiff clay. Footings cannot be used for fine sand soil.
When there is differential settlement, the designer should make the structure more flexible, so no damage will be done onto the structure due to settlement.
Weak soil stratum under layers of strong soil stratum-
Assume 30 degrees of load spreading.
Organic-
Soil disposal should not be frequent and soils should only be disposed at approved dump sites.
Clay-
There will be heaving caused by excavation and swelling, which is caused by changes in water table levels. The effects can be reduced by using an approximately 500mm compressible layer between the basement structure and the clay layer.
Ground Water Table?
GWT above or near proposed building structure (i.e. Ground Floor)-
Should not use ground bearing slabs
Provide waterproofing
Prefer to use bored piles
Provide dewatering for excavation
Should not excavate more than the water table since groundwater and dewatering will be problematic
Provide drained cavity floor
Reduce allowable bearing pressure for shallow footing design
Consider uplift in stability calculations and that uplift pressure is usually greater and a crticial case When there is high water table, a recharge well can be constructed adjacent to the structure for dewatering purposes. Piezometers and settlement markers should be installed to measure the changes and the effects of the water table levels.
No GWT-
Uplift can be ignored
GWT 5m below-
Uplift can be ignored
Differences in ground level?
Yes-
Retaining wall should be used for the steps to accommodate for foundation level changes in order to increase resistance to surcharge and water loads
Ground level with slopes?
Yes-Retaining walls should be constructed along the building perimeter in order to resist surcharge, soil, and water loads.
Consideration for uneven uplift forces should be used when some sections of basements are at or near the water table.
Open cut with 30◦ degrees
Yes-
Open cut can be used when there are no adjacent structures.
Open cut should be used because its is easy and inexpensive since no temporary support are required.
Note: When geological conditions are unknown, more GIs must be implemented and a trial pile should be constructed to find the site’s bearing capacity for initial design.
