Structural cracks are the cracks formed when the stresses or forces exceed the capacity of the structural member.
Structural cracks will endanger the safety of the building. These cracks occur due to Building settlement, Overloading, Construction mistakes, Design mistakes and Vibration.
I have already written a blog on how to identify a structural crack earlier and you can look at that here in another blog section.
In this blog I will discuss about reasons of formation of settlement cracks and remedial measures for the cracks formed due to building settlement.
What is soil settlement and when will it lead to cracking?
The vertical movement of foundation is known as settlement. This settlement can be equal all along the building or differential where one side or part settles more than the other.
Equal settlement does not cause serious problem if it does not exceed the limits. If it exceeds the limit, your floor may settle below the soil and functionally you may not be able to use it properly. However equal settlement is not that much structurally an issue.
Unequal settlement causes plaster to crack. Most of the times diagonally or vertically; originating from corners and widening towards the end. If the differential settlement is more, it can result in structural cracks too.
Development of cracks formed due to soil and foundation movement can be prevented by the methods mentioned below.
Selection of Proper foundation
The differential settlement should be kept within the permissible limit as given in the table of IS 1904.
Selection of design of foundation should be based on extensive soil exploration and testing. A geotechnical consultant/Soil consultant shall be consulted, and the soil report needs to be followed correctly. Any civil engineer needs to know some basics of soil mechanics. If you want to read about how you will decide the number of bore holes, see here.
Special arrangements should be made to safeguard drainage system and other connections.
Interface of the building and the neighboring site conditions and the existing buildings all around need to be studied and considered during design and construction.
Ensure that the soil is not compressive and if yes, ensure the pressure bulb do not get into the soft soil. I will be writing a separate blog for this some time with a soil report as an example. If you like to get this when it is ready, ensure you are in our mail list by joining here
Construction on filled up ground
Uneven ground should be properly filled and consolidated before commencing of any construction. Methods like preloading, sand drains, rollers etc. may be used for achieving quick compaction.
Sometimes for a project, the site team may do a mass excavation for ease of construction. If the footings are not in same level, this may result in some of the upper-level foundations being in filled up earth. You may not even realize this during the design and drawing development stage. So, a close co-ordination with site team is mandatory, if you are doing something like this.
Long Buildings
Use of long buildings should be avoided or wherever possible they should be divided into small lengths to account for unequal settlement.
Variations in Soil Strata
Placing a building on natural ground and partly on filled should be avoided as this cause differential settlement of foundation.
Difference in time of construction of adjoining parts of a structure
This causes differential settlement due to time difference in the different parts achieving their maximum settlement. Suitable measures should be taken in design phase to account this problem. Rigid raft foundation eliminates differential settlements.
Provision of deep basement
This method is useful in preventing excessive differential settlement between parts of a structure having different foundation loads. Deepest basement can be provided under heaviest part of structure.
Construction on shrinkable clay
Clays which shrink on drying and swell again when wet causes movement of foundations. Root of plants penetrates into the soil and dry it in summer. Shrinkage occurs horizontally as well as vertically. Hence, there is tendency for walls to be drawn outwards in addition to settling and cracks open between clay and silt of foundations. These cracks allow entry of water and soften the clay. During winter cracks are partially closed but due to accumulation of debris within these cracks they go on widening each summer.
Following precautions should be taken to prevent crack formation
Stabilization of soil with lime or thermal stabilization should be done
No trees should be planted near the building
It is preferable to remove all the roots while clearing the site itself
Isolated footing should be avoided. Combined footing or raft footing is preferred. Under reamed piles are most suited
Stagnation of water around the building should avoided by providing a sloping plinth protection slab.
Continuous beams should be provided at the plinth and lintel level
Flooring for non-load-bearing walls should be constructed to sufficient depths to avoid horizontal movements of basement filling.
Dynamic movements should be controlled in and around the building.
Expansion Joints
Vertical movement joints should be provided at changes in height of the building where differential settlements are likely to occur. Read this blog here for more information.
Structural software and analysis of Foundation and its significance
We discussed a lot of points related to crack development due to settlement of foundations. All the construction precaution that one need to take need to be taken. In addition to this, its important to attend to the analysis and design more carefully.
If you have a structural design tool like ETABS, staadpro, SAFE or STAAD Foundations, you can do an analysis and understand the behavior of foundations by capturing the soil properties in structural analysis of foundations. With available soil properties, it is possible to calculate the soil stiffness or the soil spring constant or the modulus of subgrade reaction of soil and then do a proper analysis to design the foundation.
For more important structures, even a reliability analysis shall be done by varying the subgrade modulus higher and lesser to capture any kind of unforeseen variation or deficiency in compaction. All this are covered in our course on foundation design and is explained in detail.
Summary
Cracks in concrete and buildings is always an issue. If these are structural cracks, then there are more severe issues like structural failure. Foundation related issues are a major reason for cracks in building. Therefore, it is important to ensure we know about the reasons of structural crack formation and adopt mitigation principles in structural analysis and design stage itself and ensure the construction aspects are also taken care.