CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES जॉर्डनो ג׳ורדנו ጆርዳኖ جوردانو จอร์แดโน่ JORDANO Ιορδάνοςஜார்டானோ The foundations may fail due to the following reasons:
Settlement of the foundation structures is a problem that people have faced ever since they first started constructing. The size of a foundation structure settlement is primarily dependent on the weight of the structure and the deformation characteristics of foundation soil. When analyzing this problem, it is necessary to distinguish between the concepts of uniform and differential settlements.
Settlement of the foundation soil is a vertical displacement of the soil surface caused by the workload of the building. During and after the building construction, settlement of the foundation structure is considered normal and acceptable to a certain extent. However, a potential problem occurs with differential settlements.
Differential settlements are uneven foundation settlements that can be the result of numerous causes.
Figure1. Types of settlement source: - InterNACHI Figure2. Leaning Tower of Pisa (left); the geological profile of the soil at location (right) ይህን ህንፃ 9ኛ ክፍል ሳለን physics ላይ ስለ free fall ስንማር እናስታውሰዋለን።
Unequal settlement of the sub-soil may lead to cracks in the structural components and rotation therefore
1.2 Unequal settlement of sub-soil may be due to
I. Non-uniform nature of sub-soil throughout the foundation
II. Unequal load distribution of the soil strata, and
III. Eccentric loading
IV. Soil of different lithological characteristics in the horizontal direction (different compressibility and soil compressibility beneath different parts of the foundation structure).
V. Drying of soil surface layers.
VI. The proximity of trees with large roots.
VII. Piping leaks, sewer drainage etc.
VIII. Excavations near the structure.
IX. Different dimensions and depth of structure foundation.
X. Vibration
Fig3. Some possible causes of differential settlements source:-geoexperts
Figure4. Differential settlements and cracks caused by different deformation characteristics of soil foundation Source: - geoexperts Leaving out extreme cases like the Leaning Tower of Pisa, the most common sign of differential settlements is the occurrence of cracks on the structure at an angle of 45˚, usually around the openings (doors, windows). If there is a shift in the soil beneath the structure, or if one part of the structure has foundations on rock and another part on clay or embankment, the result is a change of the state of stress that is transmitted from the foundation to the structure, creating cracks in the walls.
It is important to note that these are not the same as the cracks of a mesh shape that do not pose a threat to the stability of the structure and occur due to poorly conducted or dilapidated plaster. Vertical, horizontal or sloping cracks are more characteristic for the edges of structures and point to potential differential settlements.
figure5. Cracks on the object wall caused by differential settlements ይህ የምትመለከቱት ህንፃ ቦሌ ወሎ ሰፈር የሚገኝ ሲሆን እንደምታዩት ተሰንጥቋል figure6. Cracks on the object wall caused by differential settlements source:- geoexpert Geotechnical exploration and investigation works are key in the foundation remediation in order to find the cause of differential settlements. The optimal method of foundation structure remediation is defined based on the specific cause. The foundation remediation can be realized by applying several technologies, the most commonly used being:
Considering the possible causes of differential settlements and their impact on the overall stability of structures, it is necessary to keep them in mind when designing the foundation and landscaping, in order to reduce their effect to an acceptable level.
I. resting the foundation on rigid strata, such as rock or hard soil,
II. proper design of the base of footing, so that it can resist cracking
III. limiting the pressure in the soil, and
IV. Avoiding eccentric loading.
As stated earlier, foundation includes the portion of the structure which is below ground level. This portion of masonry, situated between the ground level and concrete footing (base) has mortar joints which may either shrink or compress, leading to unequal settlement of masonry. Due to this, the superstructure will also have cracks. This could be checked by
I. using mortar of proper strength
II. using thin mortar joints
III. Restricting the height of masonry to 1 m per day if lime mortar is used and 1.5 meters per day if cement mortar is used, and
IV. Properly watering the masonry.
This is one of the major causes of failures of footings on cohesive soil, where the sub-soil water level fluctuates. When water table drops down, shrinkage of sub-soil takes place. Due to this, there is lack of sub-soil support to the footings which crack, resulting in the cracks in the building.
During upward movement of moisture, the soil (especially if it is expansive) swells resulting in high swelling pressure. If the foundation and superstructure is unable to resist the swelling pressure, cracks are induced.
Use of pile foundations where the soil is shrinkable, so that forces are transferred to the hard strata or rock. Taking the foundation levels down to avoid foundation on shrinkable soils. The vegetation is removed from the construction site and its roots are removed. Any cavity due to roots of vegetation shall be compacted and filled with concrete. Presence of any mining areas needs to be inspected and professional help shall be taken while construction new buildings in such areas. The walls transmitting the load to the foundation may be subjected to lateral pressure or thrust from a pitched roof or an arch or wind action. Due to this, the foundation will be subjected to a moment (or resultant eccentric load). If the foundation has not been designed for such a situation, it may fail by either overturning or by generation of tensile stresses on one side and high compressive stresses on the other side of the footing.
This is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads, especially at locations where the ground is sloping. Such a situation may also arise in granular soils where a big pit is excavated in the near vicinity of the foundation. Due to such movement, excessive settlements take place, or the structure may even collapse. If such a situation exists, sheet piles should be driven to prevent the lateral movement or escape of the soil.
Sometimes, small trees, shrubs or hedge is grown very near to the wall. The roots of these shrubs absorb moisture from the foundation soil, resulting in reduction of their voids and even weathering. Due to this the ground near the wall depresses down. If the roots penetrates below the level of footing, settlements may increase, resulting in foundation cracks.
The behavior of foundation may be adversely affected due to atmospheric agents such as sun, wind, and rains. If the depth of foundation is shallow, moisture movements due to rains or drought may cause trouble. If the building lies in a low lying area, foundation may even be scoured. If the water remains stagnant near the foundation, it will remain constantly damp, resulting in the decrease in the strength of footing or foundation wall. Hence it is always recommended to provide suitable plinth protection along the external walls by
I. filling back the foundation trenches with good soil and compacting it
II. Providing gentle ground slope away from the wall and
III. Providing a narrow, sloping strip of impervious material (such as of lime or lean cement concrete) along the exterior walls.
Source: - geoexperts
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