FAILURE OF SHALLOW FOUNDATION & PREVENTIVE MEASURES

The major causes of foundation failures are as follow

1. Unequal settlement of sub soil
2. Unequal settlement of masonry
3. Presence of expansive soil below foundation
4. Presence of mature trees
5. Seasonal changes

1. UNEQUAL SETTLEMENT OF SUB SOIL

This is one of the most important reasons for foundation failure. Unequal settlement causes cracks in the foundation and ultimately leads to failure of the whole structure.

Unequal settlement of the foundation generally occurs for the following reasons

• The sub soil may not be uniform throughout the site. This means if some part of the site consist of compressible soil (such as clay) and some part consist of hard stratum (such as rock).
• The load coming from the structure may not be uniform
• It may occur also on building constructed on sloping ground.

REMEDIES

• The foundation should be designed in such a way that the pressure on soil on different portions of the structure and corresponding differential settlement is reduced to minimum.
• Loads from the super structure should be within limits of safe bearing capacity of soil.
• Foundation should be constructed with right type of material to avoid disintegration of foundation due to presence of harmful chemical f of soil or water
• Foundation should be so proportioned that center of gravity of load from the structure coincides with the center of gravity of the foundation.

2. UNEQUAL SETTLEMENT OF MASONRY

Generally the foundation for a building wall is constructed with brick masonry or stone masonry. Different types of mortars are used for bonding of these masonry units. During construction when the mortar has not set properly, if subjected to excessive loads, it starts to shrink. This shrinkage of mortar may cause unequal settlement of masonry and thereby cause cracks in wall.

REMEDIES

• Workable mortar should be used during construction. Workability of mortar is measure by performing consistency test of mortar. Mortar used in construction should not very lean or very stiff.
• The maximum height of construction for a wall in a day should be limited to 1.5 m.
• Masonry should be cured for at least 10 days. This will help the mortar joints to attain its required strength.

3. PRESENCE OF EXPANSIVE SOILS BELOW FOUNDATION

Presence of expansive soils (such as black cotton soil) has the tendency to show volumetric changes with the change in moisture content. These types of soils shrink and swell excessively and thereby cause differential settlement of foundation.

REMEDIES

• The maximum load on black cotton soil should be limited to 5 tonnes/m2. If there is a chance for water to come in contact with foundation, then the load should be limited to 9 tonnes/m2.
• Foundation should be placed at a depth where the cracks cease to extend. The minimum depth of foundation should be at least 1.5 m.
• Try to avoid direct contact of black cotton soil with foundation material. This can be achieved by making wider trenches for foundation and filling spaces on either side of the foundation masonry with sand or morroum.

4. SEASONAL CHANGES

• In rainy season, a part of the rain water finds its way to move into ground. This water bring with it some form of salt. This salt when come in contact with foundation material causes it to deteriorate.
• Heavy rain fall may result in erosion or scour of soil and thereby affect foundation
• Seasonal changes also changes the depth of water level which result in swelling and shrinkage of expansive soil. This can cause cracks in foundation.

REMEDIES

• The depth of foundation should be so chosen, that the adverse effect of seasonal changes are minimized.
• Provide adequate drainage facility if there is a chance of water level rising up to foundation level.
• Always use dense cement concrete or stone masonry where there is a chance of sulphate attack to foundation.
• Provide proper slope at ground level, near wall surface, to allow the rain water flow away from the wall.

5. PRESENCE OF MATURE TREES

Where clay soils contain trees the problem is more severe. Trees and heavy vegetation draw a considerable amount of water from the ground during the growing season. A mature poplar takes up as much as 1000 litres of water per week. In long hot summers with little or no rainfall the tree will continue to draw moisture out of the ground and the clay will shrink. This, of course, is in addition to the seasonal drying mentioned above. If buildings are sited near individual or groups of trees serious cracking in the walls can occur as a result of ground movement.

Where trees have been removed from clay soils the opposite problem occurs. As the ground slowly regains moisture it will expand and this can continue for a period of up to 10 years. The pressure that dry clay develops when reabsorbing moisture is likely to be greater than that imposed by the building load and upward movement of the structure will occur. If houses are built on the site before this ground expansion is complete, cracking will occur in the walls and foundations; the swelling will be uneven because it will be concentrated around the removed tree.

REMEDIES

• To prevent this movement from affecting strip foundations they must be deeper than the tree roots. An alternative, of course, is to site the buildings well clear of the trees.
• It is considered reasonable to keep the foundation at least the height (H) of the tree away from a mature single tree and about 0.5H from rows of trees.
• For mature trees the minimum safe distance should preferably 15 m and for young trees, the distance should be suitably increased.