Settlement of buildings

Revision as of 10:04, 12 October 2015

Introduction

Settlement is the downward movement of the ground caused by a load exerting effective stresses. These are the forces that are effective in moving soil or causing displacements. Settlement is also used to describe the movement of the ground around an excavated space, such as that for tunnels, shafts, and 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 subsidence which is where the soil is unstable and sinks downward.

Causes of settlement

It is rare that the cause of settlement lies with the design. Instead, it is more common that damage is incurred by changes that occur within the foundation soils that surround and support the structure.

The more common causes of settlement may include:

• Weak bearing soils: Where soils are not capable of supporting the weight or bearing pressure exerted by a building’s foundation. This can often occur when foundations are designed based upon general guidelines rather than site-specific soil information, such as in the case of 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, which may be caused by maturing trees or leaking subfloor heating, ventilation, and air-conditioning (HVAC) systems, they have a tendency to shrink or contract.
• Maturing trees and vegetation: A general rule is that the diameter of a tree’s root system is at least as large as its canopy. Therefore, a common cause of settlement is root systems expanding from maturing trees and vegetation and drawing soil moisture from beneath a structure.
• 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

It is rare that settlement can affect the structure of the building. 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 lines or mechanical equipment may be adversely affected. 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.
• Measure ground and building movement where necessary.
• Investigate the existing conditions of buildings and how they may be impacted by further settlement.
• Footings may be designed to spreads the load over weak soils, thereby reducing potential foundation settlement.
• Use compensation grouting to treat the ground. This is a commonly-used technique on tunneling projects which works by injecting grout into the ground to firm up the area of settlement.

Find out more

Related articles on Designing Buildings Wiki

• Brownfield land.
• Contaminated land.
• Defects in brickwork.
• Defects in construction.
• Defects in stonework.
• Foundations.
• Ground heave.
• Ground conditions.
• Latent defects.
• Remedial works.
• Underpinning.

External references

Foundation Supportworks

Housemaster

NHBC