Factsheet 9.1; Soil Heave
(Factsheet Updated 05th July 2000)
Causes of Soil Heave
Soil Heave can be caused by the following reasons:
- "Stress relief" due to the removal of soil from an excavation.
- An upward seepage of water resulting from a change in the water table.
- Swelling of the subsoils due to increases of water into the ground resulting from seasonal changes or from felling or dying trees.
- Frost action.
There are many plastic clays that swell considerably when water is added to them and then shrink with the loss of water. Foundations constructed on these clays are subjected to large uplifting forces caused by the swelling.
These forces will induce heaving, cracking, and break-up both of building foundations and slab-on-grade members.
As noted, an increase in moisture content causes clay to swell. The depth in a soil profile up to which periodic changes of moisture occur is usually referred to as the active zone. The depth of the active zone will vary depending on the location of the soil profile and can vary from 1.0m to 5.0m in Southeast England. The active-zone depth can be determined by plotting the liquidity index vs. the depth of the soil profile over several seasons.
MC - PL
Liquidity Index LI = ------------------
LL - PL
Where MC is the Natural Moisture Content, PL is the Plastic Limit and LL is the Liquid Limit of the soil at a particular depth.
A relationship between the moisture content and the liquid limit for swelling clays is given on Figure No. 1. The moisture content of the soil at its Liquid Limit is plotted against the Swell Index, Is, which is the Natural Moisture Content divided by LL moisture content.
The resulting point is plotted on the figure and should fall into one of the four swell bands where swell is <1%, 1 to 4%, 4 to 10%, and >10%.
Frost Susceptible Soils
Two potentially damaging effects are associated with frost action in soils; the expansion and lifting of the ground in winter (frost heaving and frost boiling), and the loss of bearing capacity during the spring thaw. Soils that display one or both of these manifestations are referred to as "frost susceptible". The problem of frost damage is widespread and occurs in temperate regions where there is seasonal soil freezing. The depth of frost is usually about 450 mm in Southeast England.
Frost susceptibility tends to be a feature of silty and sandy clays; that is, soils of low to medium plasticity. The table (below) from the Transport and Road Research Laboratory (1970) gives an indication of frost susceptibility and permeability with grading and plasticity index for preliminary identification.
|High Permeability||Granular < 10% finer than 75 microns.||Not susceptible|
|Intermediate Permeability||Granular > 10% finer than 75 microns.
Cohesive PI < 20%.
|Low Permeability||Cohesive PI >20%.||Not susceptible|
Migration of water and frost heaving are also influenced by the mineralogy of the clay fraction. Clay minerals with expandable structures are able to hold more water, but the water is relatively immobile compared with non-expandable clay minerals. Consequently, strong frost heaving is more likely to be associated with soils where the fines are devoid of montmorillonite and related minerals.