ground movement

3 main reasons for ground compressing, slipping, shearing or deforming (ground movement)

1. subsidence

2. settlement

3. change in water table level

4. proximity to trees

5. frost heave

subsidence

soil moves away from the structure, allows the foundation to move

settlement

the structure moves the soil through the application of the load

causes of manmade or natural subsidence

• mining/quarrying

• techtonic movements/earthquakes

causes of settlement

• overloading of the soil

• poor design of foundations

• poor workmanship on site

water table

the level of water in the ground below which the pore structure is totally saturated (sometimes have aquifers, water table is uppermost surface of underground waterway)

how does the lowering of the water table cause ground movement?

• less water saturation of soil alters bearing capacity (esp cohesive soils)

• reduced moisture content → volume decreases

• reduces upwards pressure on foundation

causes of lowering the water table

• pumping water from the ground

• insertion of land drainage

• excavation below water table

• roots of trees drawing water

causes of rise in the water tab;e

• prolonged periods of rain

• felling of trees

eluviation

the process by which material suspended in water is removed from one layer of soil to another by rainfall or chemical decomposition

what large distance can some tree species still affect foundations at?

up to 20m away

what were the old building regulations for structures near trees?

• distance to a single mature tree must be at least equal to height of tree

• distance to a group of trees must be at least equal to one and a half of the height of the tallest tree

frost heave

swelling of soil due to freezing of moisture in the ground

how can frost heave in cohesive soils destabilise structures?

• water in soil freezes to solid - expands and swells during change of state

• creates upwards pressure on foundation system

• develops structural instability over time

what is the minimum depth for clay and non-clay soils of a foundation to avoid frost heave?

clay: 0.75m or deeper

non-clay: 0.45m

what is the minimum depth below ground level for strip foundations? what is the unofficial standard?

minimum BR depth: 0.9m

unofficial: approx 1m

shear movement of soil

layers of soil moving diagonal to each other

what are the 3 ways settlement can vary?

1. degree of settlement

2. uniformity of settlement

3. differential settlement

differential settlement

settling at different rates, induces additional stresses on foundation system

why may differential settlement occur?

• poor design

  • soils have vastly different bearing capacities, some areas are overloaded