Subsidence in Buildings.
If you suspect you have subsidence do not ignore it. Valuation surveyors get really nervous when they see cracks in buildings & will ask for a specialist to advise if they see something suspicious. There are many causes of subsidence it could be attributed to any of the following.
Buildings suffering from subsidence will develop cracks they will usually be diagonal and can appear almost anywhere, often they will be narrow at the bottom getting wider the further they go up the wall and may just appear at low level, restricted to climb vertically by a window or door opening.
Poorly designed or constructed foundations.
Concrete strip foundations. This type of foundation is the most popular choice in the UK. Concrete is poured into a prepared trench often no more than 1m deep. The structural walls of the building are built directly onto these strips of concrete. Foundations that are laid in poor quality load bearing soil such as clay (Clay has a high shrinkable quality ) in certain conditions it can shrink and swell by as much as 30%. This can cause foundations to crack under the pressure when the soil expands or collapse when the soil shrinks
Leaking water pipes and drains can erode soil from under the foundations causing potential for failure.
Trees by their nature absorb water from the ground some varieties more than others here are the worst offenders.
Oak, Elm, Eucalyptus, Poplars, and Willows.
If foundations are built close to trees special precautions should be taken to allow for the continuous soil shrinkage that can occur. This should result in deeper foundations being used or a alternate system such as piles. Removal of the trees by a specialist over a period of time is also another option, assuming the trees are not preserved by a tree preservation order (TPO) .
During hot summers (no worries for now then) the ground will give up its moisture through evaporation causing the soil to shrink. Trees also require moisture and both these combinations together will cause the soil to dry out to quite a depth. This results into high shrinkage of the soil and can cause foundations to become unstable and fracture. The concrete foundations will settle down onto the shrunken soil, and when the rains come in Winter, the ground will expand and push up the foundation thus compounding the problem. (Never underestimate the pressure of ground water)
Other reasons for cracking to the structure of your home.
Poorly constructed roofs
Pitched roofs need to be braced against spread. This is achieved by forming the roof like a 3 sided triangle. If a roof has not been well constructed the rafters can spread and apply pressure to the tops of walls – often seen by a bowing or cracking on the upper part of the wall.
In some cases walls are poorly constructed ie: the walls were built out of alignment. Its possible the incorrect amount of wall ties have been used. The metal ties hold the brick walls together either side of the cavity and hold the skins of brickwork together as one unit.
Wall tie failure. This is caused by wall ties having been incorrectly fitted.
Lateral restraint – Floors are supposed to be tied into to the walls by using galvanized steel straps- This now forms part of the building regulations. It’s possible for walls that have not been secured to move. It’s a relatively easy task to fit these straps, however it will be necessary to lift the floorboards.
Timber joinery can and will often shrink in summer and swell in winter especially if it’s not well protected from the weather. This can lead to cracks forming where timber abuts the brickwork.
Building materials can become defective with continual wetting and freezing causing flaking and cracking.
Lintels are a supporting beam that is used over openings forming windows and doors to support the structure above the opening. If an undersized lintel has been used or in some cases omitted it will lead to failure of the brickwork or structure above the opening.
Minor cracking (for Barbara)
Often people will confuse minor thermal cracks as a potential structural problem. All properties will suffer from continuous minor cracks. In the most part these are caused by thermal movement of indifferent materials. All materials expand and contract with thermal movement and the altering of moisture levels depending on atmospheric changes. First floor structures are in the most part constructed using timber and will move often causing hair line cracks where the ceiling abuts the walls- often this is obscured using some form of coving. This is just one example, problems like this can can occur pretty much anywhere that you have differential material movement. Often block work walls will suffer from minor cracking in the plaster finish. Often this is caused by plaster being applied directly to new very dry dusty blocks. Ideally blocks should be well wetted with a water and pva solution prior to being plastered. (however in most cases this does not happen). Over time the plaster can become detached from the wall and cause cracks. In serious cases (where if you tap the wall and hear a hollow sound) the plaster may need removing.
The remedy for subsidence
The term used in Construction & Building for Stabilising and strengthening the foundation of an existing structure
Unless you are building on solid rock all buildings need foundations. Most post 1920 houses are built using standard concrete strip foundations.
Prior to this simple spread foundations were used
First- A bit about- foundations.
Concrete strip foundations. (90% of all modern dwellings in the UK have this type of foundation. A trench typically 600mm wide by 1000mm deep (often deeper) is excavated to locate sub soil that has weight bearing properties. Once it has been inspected, it is filled with Concrete the minimum depth requirement of the concrete is usually 300mm. More often the trench is mass filled with concrete to just under ground level.
Spread foundations (Extensively used previous to 1920)
Consists of a shallow trench this was excavated in most cases was as little as 300mm deep, the brickwork was started straight from the soil however is was often started double the thickness of the wall for a few courses allowing the pressure of the completed wall to spread over more area. However, typically buildings constructed on this type of foundation were built using soft stock bricks bedded in lime mortar. This type of brickwork / foundation combination is very flexible compared with todays methods and will allow for movement.
There are lots of articles on this site that will help you understand more on foundations and soils etc- look for the links at the bottom.
Typically a series of pits are excavated 1m x 1m and deep enough to extend down past your existing foundations until a solid suitable base is found. The depth required, will usually be assessed by a structural engineer, who makes calculations based on drilled soil sampling or a series of trial holes.
The soil directly under the existing foundations is also removed.
This pit is then filled with concrete to within 50mm of the underside of the existing foundation. The small gap left is then dry packed with a dry mix of mortar that is rammed in under pressure once the concrete has set. If your wondering why- Concrete is poured wet and will shrink when cured. Dry packing doesn’t.
The pits are excavated & filled in an approved series. Obviously if you excavate too many before filling the house will likely fall down.
When complete all the pits will form a complete new foundation under the existing foundation.
Piled and beam underpinning. (usually for large buildings and extreme cases)
Piling – is the process of forming deep concrete friction piles with a diameter ranging from mini piles @ 150mm dia to huge -600mm diameter piles for supporting massive structures like bridges. They are bored deep into the ground deep enough to locate in very stable sub soils. In a domestic underpinning situation they are drilled both internally and externally. Pockets in the walls are knocked out and steel reinforced concrete beams are formed – connecting each pair of piles. These types of installation are always designed by a Structural engineer.
Pile and Knuckle system for domestic dwellings. (This system is carried out externally)
and is only suitable in some circumstances.
First holes are augered at precise pre calculated intervals and depth into the sub soil adjacent to the foundations.
Steel reinforcement is then placed into the hole.
The hole is then filled with high strength concrete.
A meter wide trench is then excavated to the expose the existing foundations.
Once the piles are formed. Holes are then broken out into the existing foundation.
A Concrete knuckle reinforced with steel is then cast into the hole -this connects the Knuckle to the pile head.
Once these are all completed some of the existing brickwork is removed thus transferring the pressure onto the new piles- (this brickwork is removed below the finished soil line so it is not visible.)
Repairing- cracking after underpinning has been completed-
In serious cases external sections of the brick work will have to be demolished and re-built. Internally specialist chemical resins together with steel renforcment are used to repair the defective areas.