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Subsidence Index | Problematic Soils Index | Shrink/Swell Index | Diagnostic Characteristics | Geographic Occurrence | Mitigation | Key Contacts & Expert Advice | Photo Gallery | Essential References & Further Reading Effects of Ground Shrinkage or Swelling Houses
Clay How to Identify a Clay Soil
Classification of Shrink / Swell Hazard Clay
The modified plasticity is used as it takes into account
the larger grains that are not included in the liquid and plastic limit test,
that is particles 0.425 mm or larger. These larger grains dilute the effects
of shrinking and swelling hazard indicated by the plasticity index. Modified plasticity index = Plasticity
index x (%<0.425 mm/100) Plasticity index is related to the to shrinkage potential as given below
Table 1. Shrinkage potential classification
The effects of climate The effects of shrinkage and swelling in Britain were first brought to light as a major problem following the dry summer of 1947. Many more buildings were affected during the dry periods of 1975 76, 1989 90, 1995 - 1997 and 2004 - 2006.
Trees Trees and Ground Shrinkage Many of the shrink/swell problems in the UK are often a combination of the high shrinkage potential clay and trees. This is because trees generally take more water from the ground than a drought will and from greater depths, potentially from below foundation level. Trees and Ground Swelling or Heave Alternatively, if a tree is felled or dies, a clay soil will regain the moisture that has been removed by the tree. This clay will swell, distorting the foundations built above or near the building, and potentially damaging the foundations if they have not been designed to take the extra load or movement. Building Near Trees The National House Builder Council (NHBC) has produced a chapter on building near trees (1995). It contains the technical requirements and recommendations for building near trees, particularly in shrink/swell prone soils. The amount of water a tree will take from the ground (water demand) varies depending on the species of tree and how mature it is.
Table 2. Water demand of various common trees and the area affected by the tree Ground Movement
Cracks in Buildings
Why do Buildings Crack and When is it a Problem? Buildings move for a number of reasons and most of them are not serious and will not affect the structure but may be unsightly. An extensive survey of building defect by the Building Research Establishment (BRE) found that fewer than one in five were of structural importance.
Minor cracks are fairly common above lintels or the join between a bay and the main wall. The bay has less load on it and often settles less than the rest of the house.
If you are concerned about a crack in your house measure the width; this is best done with a ruler. In general it does not matter how long the crack is. The BRE have provided guidelines to classify the crack width and the damage.
Aesthetic class only affect the appearance, they may be unsightly but they do not affect the structure,
Serviceability class cracks may allow rain or wind in and could damage gas and water pipes,
Stability class cracks start to affect structural stability of the house.
The shape of the crack is also important, those that bulge or show side ways movement are more important them those that are flat. Long cracks away from windows and doors, especially if they go through bricks or stone are also of greater concern.
Houses can move quite a lot before they may need major work.
Reasons for houses to crack and the type of cracking
New Houses Thermal Movement
Cracks may be because of the seasonal expansion and contraction of walls from summer to winter. Typically, the cracks are vertical and of constant width, generally less than 2 mm wide and will open and close with the change in temperature. These cracks often occur at the junction between different materials for instance a solid wall and a partition wall.
Other common reasons for cracks to form
Lintel failure or movement
Many houses have movement that can be seen above a lintel or window, this is because of the movement or failure of the lintel. It also happens when old metal or wooden frame window are replaced with new upvc window. Other information Cracks http://www.residentsline.co.uk/pdf_files/Cracks.pdf http://www.oca-arb.co.uk/img/pdf/dissertation.pdf BRE. 1996. Cracks caused by foundation movement. BRE Press. ISBN: 1860810977
Subsidence Index | Problematic Soils Index | Shrink/Swell Index | Diagnostic Characteristics | Geographic Occurrence | Mitigation | Key Contacts & Expert Advice | Photo Gallery | Essential References & Further Reading |
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