Posted at 11.01.2018
The developers plan to develop a amount of new sites to provide low and medium rise luxury housing accommodation for private sale in the current economic market. Within the planning gain it could be essential to negotiate other accommodation.
You have been successfully appointed as the 'technical development manager' to co-ordinate the design and the construction of the new and proposed developments, alongside the after sale and maintenance of the new building stock.
Foundations are made to distribute all of the weight of any structure / building equally over the ground.
During the construction of an building the weight is spread over certain points such as piers and columns and this requires the foundations to be spread evenly under a building so that it reduces the weight of this building.
Foundations are either classified as shallow foundations or deep foundations.
Shallow foundations are used to transfer the load of your building to the soil at a rate closest to the ground of an building.
Deep foundations use piles and different types of piers and transfer the load to the soil a lot deeper from underneath of the building in comparison to that of shallow foundations.
Foundations must also comply with buildings regulations please see diagram overleaf.
Shallow foundations are pads, strips or rafts.
Shallow foundations are near the bottom surface and are significantly less than the width of the footing normally significantly less than 3m. That's where the surface loading or other surface conditions affect the bearing capacity (the stress on the soil) of the foundation and is called shallow.
Shallow foundations are being used when surface soils are strong and in a position to support the imposed loads. These types of foundation are not suitable in weak or highly compressible soils.
Deep foundations are Piers and caissons.
Deep foundations are those that are deep below the bottom surface so their bearing capacity will not be afflicted by any surface conditions, normally at depths of 3m below ground level.
Deep foundations are being used to transfer the strain to a deeper depth if unsuitable soils are found nearer the surface.
Piers are foundations used for the carrying of heavy structural loads and are built in situ in deep excavations.
Caissons are a deep foundation and are designed above ground level as soon as built is then sunk and used for excavating or the dredging of materials from within the caisson.
Foundations were created and built to construct buildings that will stand up to the forces of nature and these forces are:
Frost heave this is a common problem in sandy soils which is where in fact the water in the ground freezes and pushes upwards then expands and turns into ice.
The opposite of this problem is Drought and this can cause the shrinkage of the land and it is quite typical in clay soils.
These problems can affect the buildings foundations and generally will disturb and crack them. This can be easily avoided if you dig deep enough and in most cases at least two thirds of any meter will suffice.
These forces make a difference the structure of buildings so when constructing foundations care must be taken to avoid settlement or movement as this might cause damage to the building once constructed.
It is also essential to prevent against damage caused by the wind or atmospheric conditions.
Foundations are also called structural members and are exactly like giant beams. They carry loads like structural steel beams and their job is comparable to that of foundations.
Foundations can be constructed from a number of materials this is so the foundations can handle carrying the heavy load of a building.
The two types of load imposed on buildings are:
Loads imposed by wind.
These are long lasting non moving loads like the roof, walls and floors.
They can consist of whatever isn't long term such as furniture, people and snow loads.
Foundations can be produced from concrete, stone, concrete block, wood and steel however the best way to construct foundations is by using strong, durable, water-resistant materials.
The designing of foundations is a technical job and is performed by an Architect or a Structural Engineer they will properly size the foundations and specify the materials that it should be constructed.
Foundations can only be designed and constructed once a soil test is carried out.
The outcome of the test/s will reveal that which we can and can't build on our site and the kind of foundations we can lay.
This will also tell us approximately how much the development will definitely cost and exactly how long it should take to complete.
Once we have the results back from the soil sample and know what we can build. We'd need to get ready our site before any building construction work takes place.
Then our site would have to prepare yourself as this can be an important part of this process. This might include the clearing of trees and the putting of stakes to mark the boundaries of your site.
The clearing of trees is an important part of preparing our site as the roots from trees could damage the foundations as they grow, this is because of the massive amount water that trees removes of the land and around it. As this can change the total amount of the subsoil and when any large trees need to be removed we'd need to allow at least annually for the land to stay before any work is started on the foundations.
Please see overleaf a diagram on the effects due to shrinkage and ground swell.
The removal of around 300mm of the topsoil would need to be done as this part of the ground wouldn't be strong enough to aid the structure of the building.
The topsoil may also contain vegetation and plant growth that could damage the foundations and the structure of the building once built. This also really helps to even out the task surface and flatten the bottom.
The excavation and laying of foundations would include digging a hole for the foundations and setting and pouring a solid durable material in to the hole.
Then the trench would need to be dug as soon as it's been excavated care must be studied as the walls of the trench may fall in and the moisture in the soil may drain or dry out.
Consideration must be studied when digging trenches from enough time it takes to dig to the current weather conditions. The building of the walls in the trench means that wet weather may hamper the process if the trench floods, its walls may collapse and it's recommended that the task is completed during dry weather.
The digging of trenches can be carried out yourself or machine depending on size of the structure which is done by using the design drawings, leveling instruments and boning rods.
These tools are being used to verify and identify the depth of the trenches for the required foundations.
Below are the possible types of foundations we might use in the construction of the building projects.
Strip foundations (Deep Strip or Trench Fill, Traditional Shallow Strip and Wide Strip). Please see diagram of strip foundations overleaf.
Strip foundations are the most typical kind of foundations used it is because they can be strong, inexpensive to construct and can only just be utilized when the ground conditions are good.
The width of the foundation will be based upon the soil type, but are usually 450mm wide for single storey buildings and 600mm wide for two storey buildings and both will as a rule have a thickness greater than 200mm.
These types of foundations are being used to aid a line of loads like load bearing walls or if the row of columns require support and are ideal for 1 & 2 storey houses.
Strip foundations are created by using a concrete strip with a reinforced steel mesh as this will support the walls of the trench. The trench depth may differ but should be at least 1000mm with a width of 600mm. The concrete should then have the very least depth of 225mm. This is to avoid any harm to the foundations caused by any changes in the topsoil.
The trench would have to be excavated / dug until the clay has been found and the floor of the trench would have to be covered with at least 150mm of concrete.
This is employed when the load of the structure is high in relation to the weight bearing capacity of the subsoil.
Wide strip foundations are laid the same way as standard strip foundations except that the trench is a lot wider and reinforced concrete can be used.
Deep strip or known as Trench fill can be an example of a shallow foundation and it is constructed by digging a trench and then filling it with concrete or rubble. It is also used when firm or shrinkable clays are present.
These foundations have smaller narrower trenches and are filled with concrete to within two brick courses of the finished ground level as it's important to be accurate if the walls are to be central.
The costs of the materials used because of this type of foundation are more costly than the aforementioned but it requires less time to develop therefore labour costs below ground level are less expensive and the work is easier to do.
Deep strip foundations may also be reinforced and is definitely an option to wide strip foundations in soft clay subsoil and should be at least 400mm wide and 900mm deep.
Pile foundations can be used to support different kinds of structural loads and are being used when the ground is soft as well as for small structures. They are often used because adequate bearing capacities can not be bought at shallow enough depths to aid the structural loads.
Piled foundations are made up of a group of columns built and inserted in to the ground and this type of foundation was created to transmit the strain of a building further into the subsoil.
Pile foundations are used when the surface soils are incredibly soft and when designed consideration should be taken so that the pile caps are tied as well as beams or with a reinforced concrete slab. It is because they can work in tension and compression and the foundations can become an individual unit.
This kind of foundation is employed to support buildings where conditions of the subsoil has shrinkable clays or with a high water table.
They are also used when other types of foundations can not be used it is because the depth would be too deep and it wouldn't be economical to use.
Pile foundations are made up of some columns which take a seat on lots bearing layer of the soil up to 4m below the surface. Any deeper than this might be very costly and can't be used for small building projects.
Pile foundations would consist of scaffold poles or timber and the bottom end of the timber would have an iron shoe with a spot on.
This is done so that they can be easily driven into the ground and the most notable of the timber would also have an iron band round them. This would prevent the timber from splitting when their being driven in to the ground.
The digging of foundations would normally be done by machine normally by way of a crab particularly if there are large boulders in the ground. This would be done by hoisting much weight called a monkey.
The monkey is hoisted up in the air and released and then will fall with force and drives the pile into the ground.
The monkey can weigh anything from weights of 100kg to 1120kg and is normally dropped from heights of around 4. 5m to 14m.
The piles are then driven under all the walls or under the piers as that's where the weight of the building will be concentrated.
Please see diagram of pile foundations overleaf.
Pad foundations are suitable for most sub soils except loose sands, gravels and filled areas and are being used for buildings over five stories high.
This type of foundation is usually used for office blocks with a steel framed construction and are usually designed with reinforced concrete and where possible are normally square in plan.
Pad Foundations are designed to span in two directions with main bars and the pad is equal by any means points under the soil this is to avoid any unequal settlement.
This is performed by designing the centre of gravity of the foundations consistent with that of the imposed loads.
Raft foundations are the biggest type of foundations you can get and are designed and constructed to spread the load of a structure over the whole area. These are being used on soft or loose soils with a minimal bearing capacity therefore the loads can be spread over a larger area.
Raft foundations can be utilized when the loads are close together and are also well suited for areas where there may be lots of water as the soil bearing capacity is treated like water.
A raft foundation is made up of a concrete slab that may cover every one of the loaded area. It may all so be stiffened by ribs or beams which may be incorporated in to the foundation. They are also used to help reduce any differential settlement as the concrete slab will resist differential movements between loading positions.
Raft foundations require reinforced steel near the top of the raft as this can help with cracks and more reinforced steel is necessary at the bottom under the walls to resist any tensile stress.
In soils with low compression reinforcement will be needed at both top and bottom of the raft. The foundations would need to be extended by at least 300mm past the external walls to help protect the soil from possible frost and also to help spread the strain.
When constructing raft foundations on sand we'd need to use a down stand as this will help to drive back soil erosion. But when building on shrinkable clays the soil under the foundations should be protected by extending the edge of the raft between 125mm & 150mm at night external wall. That is to protect it from any shrinkage or expansion of the clay.
Source of information
Roy Chudley & Roger Greeno
Jack Stroud Foster
Information also provided by course lecturer Harish Patel