Predicting Structural Settlement

Humphrey Okereke – Coordinators Geotechnics, Fundamental Integrated Site Appraisal Services (FISAS) Limited, Abuja

All structures built on the earth crust are made up of two parts namely: the superstructure – the part above the ground, and the substructure – the part beneath the surface. The part beneath the earth surface is referred to as the foundation. It is the stability of the foundation that controls the general structural stability. The leaning tower of Pisa in Italy is an example of a structure with a defective foundation.

 

The main determinant of the stability of structures is referred to as a settlement. The settlement is the distortion or disruption of parts of a structure due to compression of its foundations under load. There are three types of settlements namely: (1) uniform settlement (2) tipping settlement and (3) Differential settlement. Settlement estimation is a major challenge that continues to face foundation engineers.

 

For the civil engineering structure to perform satisfactorily, the foundation must be able to support the loads transferred to it from the superstructure throughout the design life. In other words, the foundation must be safe against shear failure (i.e. mobilise adequate bearing capacity) in the soil, and they must not undergo excessive settlement.

 

Usually, shallow foundation designs are governed by settlement criterion rather than shear failure. Eurocode 7 allows maximum total settlement of 25mm for serviceability limit state of buildings occupied by humans, and up to 50mm for industrial buildings.

 

The values of foundation settlement can be predicted during the design stage. To make this prediction, the detailed geotechnical investigation must be carried out at the site of the proposed development. During this investigation, the geotechnical specialist uses appropriate methods to examine the sub-surface soil profile. The anomalous zones and any weak soils beneath the surface are identified. The effect that groundwater would have on the foundation is also estimated. In addition, representative soil samples are recovered and tested in the laboratory. The resultant data from in-situ and laboratory tests are then used to estimate the settlement at the site. When the estimated settlement exceeds the allowable limits, recommendations for soil improvement are provided.

 

The cost of conducting this needed geotechnical investigation is usually small when compared to the monumental economic loss that results when structures fail because of a faulty foundation.

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