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BUILDINGS
Shallow Foundations
• rafts
• pads
• strips
Raft
Pads
Deep Foundations
• piles
What type of foundations to build depends on:
• depth to rockhead
• depth to water table
• geological structures
• rock properties
Buildings
So – the factors geologists assess before constructing a large structure should include:
• Jointing
• Dip of bedding
• Faults
• Cleavage
• Depth to water table
• Depth to rockhead
• Weathering
• Rock properties
• Radon gas concentrations
A
B
C
Leaning Tower of Pisa
Foundations
Variable, soft, silty deposits - unconsolidated
Very soft clays - consolidated
Dense, consolidated sandstones
Water table
10m
40m
Clay deposits are prone to settlement under load
Saturated sediments compress & subside under load
Load causes water to be squeezed out
Very weak sediment. Buildings need high foundation strength
Depth to water table affects the stability of the foundation material by affecting the pore water pressure
soil
bedrock
rockhead
Seismic Survey - Depth to rockhead
?Unconsolidated, porous & compressible
Consolidated, rigid & less compressible
Seismic waves speed = slow
Seismic waves speed = fast
drift
bedrock
rockhead
Seismic Survey - Depth to rockhead
?
Seismic source
Seismic recording stations
drift
bedrock
rockhead
Seismic Survey - Depth to rockhead
direct seismic wave
refracted seismic wave
?
drift
bedrock
rockhead
time
distance form seismic source
Arrival time of direct waves
Arrival time of refracted waves
Seismic Survey - Depth to rockhead
drift
bedrock
rockhead
time
distance form seismic source
Arrival time of direct waves
Arrival time of refracted waves
Seismic Survey - Depth to rockheadPoint of transformation
Point of transformation
5km
V2 = 5km/s
V1 = 3km/s
Electrical Resistivity Survey
- Depth to water table
Transmitting electrodes
Receiving electrodes
Wenner array of 4 electrodes
• Speed of electrical transmission increases with increasing water content.• Higher water table means reduced electrical resistivity.
Radon Gas
• Colourless
• Odourless
• Tasteless
• Radioactive gas
GRANITE
Radon
Produced by radioactive decay of 238 Uranium in granite
If concentrations build up it can cause lung cancer10 – 15,000 deaths in USA & Canada every year
Radon Gas pathways
HOUSE
BASEMENT
Joints & cracks in rocks
Pore spaces in rocks & soils
Pipes & other services
Concentrations build up in unventilated rooms & spaces
Radon Gas
RADON GAS