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Settlement of buildings

Revision as of 10:18, 30 March 2017

1 Introduction
2 Causes of settlement
3 Signs of settlement
4 Responding to settlement
5 Find out more
- 5.1 Related articles on Designing Buildings Wiki
- 5.2 External references

Introduction

Settlement is the downward movement of the ground caused by a load consolidating the soil below it or causing displacement of the soil. Settlement often refers to the downward movement of the ground around an excavated space, such as that for tunnels, shafts, or basements.

It is usual for buildings to experience some degree of settlement within the first few years after construction. Although the extent to which this ground movement impacts upon buildings depends on several factors, which may include:

Existing soil conditions.
Methods of construction.
Size and depth of the construction works.
Type of structure, its condition, and its foundations.

The impact of settlement is opposite to the effect of heave which is the upward movement of the ground, and is different from subsidence which occurs where soil is unstable and sinks downward without any imposed load.

Causes of settlement

Typically, settlement occurs within the foundation soils that surround and support the structure.

The more common causes of settlement include:

Weak bearing soils

Where soils are not capable of supporting the weight or bearing pressure exerted by a building’s foundation. This can occur when foundations are designed based upon general guidelines rather than site-specific soil information, such as in the case of some residential construction projects.

Poor compaction

The leveling of a site prior to foundations being constructed often relies upon the placement of fill soils. Often these are imported from off-site locations and unless they are properly placed and compacted, may compress under loading and result in settlement of the structure.

Changes in moisture content

The soil’s ability to support the load of a foundation settlement may be reduced by changes in moisture content within the soil. If soils are saturated, clays and silts may be softened. If soils are dried-out they have a tendency to shrink or contract.

Maturing trees and vegetation

Therefore, a common cause of settlement is root systems expanding from maturing trees and vegetation and drawing soil moisture from beneath a structure. A general rule is that the diameter of a tree’s root system is at least as large as its canopy.

Soil consolidation

Applied loads force water out of clay soils which compress and result in downward movement or settlement of overlying structures. Settlement caused by consolidation may take a considerable amount of time to be ‘complete’.

Signs of settlement

In cases of minor settlement, which could be due to cyclic or seasonal movement, small hairline cracks may appear in plaster. In more serious cases, doors and windows may develop sticking problems, and plumbing and mechanical equipment may be adversely affected.

It is rare for settlement to affect the structure of the building. Whilst situations vary from site to site, a general rule is that a crack is considered to be of a structural nature if it approaches or exceeds a width of one-quarter inch.

Responding to settlement

There are certain steps that may be taken to prepare for, and respond adequately to, settlement:

Site investigations can be undertaken to try and find out how much settlement is likely and the potential effect on buildings.
Actual ground and building movement can be measured.
The existing conditions of buildings and how they may be impacted by further settlement can be assessed.
Footings may be designed to spread loads over weak soils, reducing potential foundation settlement.
Compensation grouting can be used to treat ground. This technique is a commonly-used on tunneling projects, where grout is injected into the ground to firm up the area of settlement.