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