i) Unequal settlement of the sub- soil: Masonry should be raised uniformly over the whole area. A slow progress of masonry work makes stronger joints and has more uniform settlement.
ii) Unequal distribution of the weight of the structure on the foundations due to eccentricity of loads. In continuous wall foundations reinforcement should be provided whenever an abrupt change in magnitude of load or variation in ground support may occur.
iii) Horizontal movement of the earth adjoining the structure. This is effective in the case of clayey soils and black cotton soils. Such soil become soft and swell when wet losing their bearing power considerably and shrink and crack when dry.
iv) Atmospheric Action: Rain and the sun are the main agents with the change of seasons. Rise and fall of the sub soil water level, increasing and decreasing the moisture content which is especially effective when the underground water is near the surface , or in damp soils overlaying a layer of porous material like sand; the sub soil shrinks or expands causing cracks. Soakage of the rain water in the sub soil also produces a number of changes as above and sometimes brings in salts which react chemically on the lime and bricks in the foundations and cause them to disintegrate. Underground open drains should be provided to drain out the excess water when the subsoil water level rises. Method has been explained in detail in the section on “Roads and Highways.” Deep foundations with sides (of the trenches) well filled and consolidated with good slope away from the walls given to the ground surface will help against rains. A plinth protection of about 60 to 90cm width with concrete or flat brick flooring will give further protection. Cement or Hydraulic lime should be used with stone or over-burnt Bricks up to the plinth level in damp locations.
v) Transpiration of trees and Shrubs: This is a very important factor which is not generally considered. The drying effects of the transpiration of trees and shrubs are superimposed on the seasonal conditions mentioned above whenever the root system approaches the shallowly founded structures. The root system is isolated trees generally spread to a radius greater than the height of the tree and have been observed to cause significant drying of flat clay soil to a depth of 3m. the fast growing trees are especially dangerous and within 5 or 6 years the roots extend to a distance of 15 or 18 m and dry out the clay abnormally below the foundations of the nearest part of the house. Sometimes a permanent depression of the ground is produced during the early period of the rapid growth when the tree demands more water than this available on the ground. Abnormal spells of dry weather have much more serious effects. It has been found that the differential shrinkage below the foundations can be sufficient to produce cracking in the brickwork. Such cracking can be most common at the western and southern sides of the house since these parts receive more radiation from sun than the northern and eastern sides. The drying which progress from the out sides towards the inside edge of footing causes outward tilting of the walls and corners; in the worst cases. The outward movement of the wall may drag the upper floor and roof with it and cause cracking to spread throughout the interior of the house.
For overcoming the damaging effects of shrinkage the foundations should be taken sufficiently deep. A depth of 90cm is necessary to avoid cracking in normal brick houses away from fast growing trees. It has also been suggested that that fast growing and water seeking trees should not be planted within 18m (7.6m Minimum) of buildings.
Trees planted alongside roads have caused marked depressions along the edges of the roads and underneath cement concrete paths.
(Based on the research made at the building Research station, Watford, England.)
a) Excavation in the immediate vicinity which may cause a reduction in support to the structure.
b) Mining or Tunneling operations in the neighborhood.
c) Adjacent structures which may impose additional loads on the foundation strata or additional stresses in earthwork and supporting structures.
d) Vibrations and ground movements resulting from traffic, piling or explosions in the immediate vicinity.
e) Shrinkage of clay soil due to weather, transpiration of plants.
d) Lowering of the ground water level by pumping from wells may cause settlement of the ground surface over a wide area.
e) A rise in the ground water level may cause movement of the foundation strata.