JAC LTD - Building Surveyors
Subsidence, Land Slip & Ground Heave
 
The soil in the south east of England consists predominately of boulder clay or London clay, which is an extremely shrinkable cohesive soil that is subject to levels of seasonal change.
 
Cracking In Buildings
 
Buildings built on shrinkable clay soils without adequate foundations are subject to various types of movement.
 
Most low rise domestic buildings suffer cracking of some description.
 
There are numerous types of cracks, most are as a direct result of small movements within and between the materials and elements used in the construction of the building/s or adjacent structures.
 
The most important factor is determining whether or not these cracks are historic or current in nature and if they warrant further investigation and or accurate identification by qualified specialists.
 
Subsidence, landslip and ground heave does warrant professional attention and regular monitoring, only then can accurate reinstatement specifications be prepared and specified.   
 
 
Subsidence
Definition   - The downward movement of a building displayed by cracking of the      superstructure.
 
Typical causes  - Can be attributed to mature trees in close proximity to the building/s, seasonal changes within shrinkable clay soils. 
 
 
Landslip
Definition - The immediate pronounced movement of land following structural failure of a bank or retaining wall.
 
Typical causes - Can be attributed to soil erosion, steep gradients, lack of movement joints in retaining walls and retaining wall failure due to poor design and or construction.
 
Ground Heave
Definition - The upward movement of a building displayed by cracking of the superstructure.
 
Typical cause - Can be attributed to recent removal of mature trees in close proximity to a building in clay soils, the soil then soaks up moisture causing the surrounding ground to swell, resulting in an upward movement and cracking of the superstructure.
 
 
Monitoring
 
 
Is the fixing of calibrated tell-tales, shot fired pins or triangular marker studs, fixed to the superstructure enabling accurate measurement and recording of any horizontal or vertical movement.
 
 
Monitoring is normally undertaken (usually for a period of 12 months) so that the full extent and direction of cracking can be identified and differentiated from seasonal variations and thermal movement, as the variety of movement caused can be considerable.
 
 
Trial Hole Excavations
The surveyor will not normally be expected to expose the foundations of a building during a building survey, however if the surveyor suspects faults below ground, additional investigations should be undertaken, to expose the foundations by means of a trial hole.
 
In some instances the assistance of a consulting engineer will be both recommended and required to oversee the works and organise both the site extraction of soil samples and analysis of the same in conjunction with other geological specialists.
 
 
Underpinning
Definition - Where a new foundation is constructed beneath an old foundation or a new foundation is formed at a lower level to take the place of an old foundation which has failed or is to be removed.
 
Purpose - To spread the load over a greater area or to carry the load down to a deeper level where the existing sub soil has a greater bearing capacity.
 
JAC LTD would be happy to advise clients on the most suitable method of underpinning, where required, in relation to their specific property defect needs on consultation.
 
 
Common Structural Defects
 
Substructure
  1. Foundation settlements due to clay shrinkage.  
  2. Foundation settlements due to subsoil erosion.
  3. Foundation settlements due to close proximity planting of trees.
  4. Foundation settlements due to the removal of existing close proximity mature trees.
  5. Structural settlements due to building on inadequate ground.
 
Superstructure
  1. Sinking porches build on inadequate foundations.
  2. Settlements of solid concrete floors in post war housing due to unconsolidated fill material. 
  3. Flexing of fully spanned timber ground floors due to excessive warping of joists. 
  4. Cracking and movement of block partitions resting on eccentrically loaded timber suspended floors.
  5. Over spanned floor joists in older buildings caused by removal of internal walls beneath. 
  6. Part removal of chimney breast/s and no supports inserted for over sailing breast/s at a higher level.
  7. Distortion of door architraves and floors on upper floors caused by inadequately supported point loads for MST’s or hot water cylinders.
 
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