A Detailed Look At How To Lay A Building’s Foundation


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Allowing any complacency in the construction of foundations create significant room for errors that have a building’s structural wrong from the ground up. However, the mistakes may not necessarily have to do with structural integrity, but it can also lead to extra costs for the construction project.

The Types Of Foundations

•    Trench Fill Foundations

The foundation takes away the need for laying bricks below ground thus cutting time and labor. That is why it is a favorite for many self-builders and large-scale construction operators. The base of the foundations is made of concrete that’s poured to a thickness that is 150mm from the ground surface. It reduces the labor and time needed thus the project can be of the ground quicker than expected.

The load at the bottom gains some added support from the sides of the trench. But the inclusion of trenches means that the ground has to be stable so that the sides of the trench can bear the load. Trench fill foundations are ideal for grounds with compacted clay and chalk soil.

•    Strip Foundations

Strip foundations tend to have a wider base than the trench fill yet they use less concrete because of a thin layer of concrete. The soil conditions govern how thick the concrete foundation will be, but the typical thickness is around 300mm. Other factors that influence the thickness of the concrete of a strip foundation include building’s design and the number of masonry courses needed for the walls up to the damp-proof course.

If the construction site is a sloped gradient, the foundations should be level. To achieve this, you need to use plywood and retaining pegs to shutter across the steps you need to create that overlap the width of the concreted trench.

Strip foundations are an ideal choice for soft ground soils such as sand because they are designed to spread the load of the building thus increasing its stability.

There are also other forms of foundations such as screw pile foundations and micropile foundations for other more limited spaces or specific civil engineering uses.

Building Control And Home Warranty Inspections

During the project, you are expected to keep the building control officer updated of the progress at certain stages and wait for them to inspect and give approvals to continue. The best way of meeting this requirement is to let the contractor overseeing the project notify the officer. Your role in this is to check and confirm that work is at a particular stage and inspection have been done and the necessary approvals to continue are given.

Most of the local authorities or oversight organizations have no issues with inspection schedules booked over the phone, fax, or email. They most require that they are given at least a day’s notice. The insurance warranty inspectors also have the same request and approach to the delivery of their mandate.

The following are the schedules for building control inspections for groundworks:

•    A Two days’ notice done in writing for the commencement of work

•    One day’s notice via email, phone or fax for foundation excavations

•    One day’s notice by email, fax, or phone for oversite preparations

•    One day’s notice via phone, email or fax for laying of drains in trenches

•    One day’s notice by fax, email or phone for drains testing

How To Get It Right When Laying Foundations

Ensuring that you get everything right from the get-go is not a crucial thing to you. It also is something that the local planning authority take a keen interest in to ensure that the building is erected in the position and design as shown on the approved plans. The planning enforcement officers can determine the measures based on the dimensions of your blueprints to find out if your home is positioned where it should even if the dimensions are not detailed in the foundation.

The levels of the boundaries and the distances between each have a vital role in the planning aspect. They should be concise to avoid it over-shading and overlooking the neighbors’ buildings. As such, it is your responsibility, and that of your contractor, to ensure that everything is in the right position and distance before driving pegs into the ground and placing the chalk lines so that digging can commence.

It is also a good idea to perform GPR surveys of the area where you will be digging to ensure that there are no issues with the immediate area or public utilities. These surveys can illuminate potential issues that can cause thousands of dollars to put right eventually.

When setting the foundations, mark the center line to avoid confusion. For the walls, you will use pegs and string lines to mark where they will be position after concreting the foundations.

1.    Breaking Ground And Digging The Site:    Foundation Depth

If the site is on a stony or rock grounds, then the foundation can be shallow. A site with “shrinkable” soils will require the ground to be dug a bit deeper depending on the soil. The depth is deeper for lose grounds with loose earth such as sand that with those with compacted soils such as clay.

The construction control officer will guide you on required depths when they inspect the site to allow you to start digging and after you have dug the trenches. You will need to mark the level for the top of concrete, which you will do by hammering pegs into the sides of the trenches.

2.    Pouring The Concrete

Once you are done with digging the trenches for the foundation and they are inspected, the next stage is to bring in the concrete. If you are to do this using a ready-mix lorry, ensure it has an impeded access to the site. If the access is a bit restricted, then you can use a pump to deliver the ready-mix concrete to the trenches. In such a case, you should be sure of such an outcome so that you can book the pump in advance to avoid the inconveniences associated with delays. Hand-rake the concrete after pouring it in to ensure it is level against the pegs.

Ground-Bearing Floors

The ground underneath a ground-bearing concrete floor slab should be adequately prepared using the recommended treatment. Get rid of any plant life on the site and remove the topsoil before you start digging.

Laying the hardcore should have it at a thickness of not less than 150mm or greater than 600mm depending on the planned aggregate. Then proceed to compact down the hardcore using heavy, flat, plate compactor and ensure you do this correctly to prevent the formation of settlement that later leads to the formation of cracks on the foundation slab.

The materials to use for the hardcore should be clean and can be stone, broken brick, or concrete that’s crushed to about small particles of 100mm in size. You can source the read-made, approved aggregates from various reputable merchants.

For a foundation being done a sloping site that has the oversite at a depth of more than 600mm, lean-mix concrete may be acceptable for layers with the hardcore. The lean-mix will compensate for the recommended difference thus aiding to prevent the needed to opt for a suspended floor type.

If you are to use a round stone fill, then exercise some caution to avoid pushing out the external walls. If the walls move further outwards, then the stone will rattle around and fail to compact as expected. Use 1,200-gauge polythene as a damp-proof membrane that you will lap and tape down ensuring it stretches over the walls to allow for any lapping of the damp-proof course that may arise later.

Insulation boards can be placed over the damp-proof membrane and a layer of polythene place added over it before pouring the concrete. The boards should be fixed around the external walls and cut to allow for an adequate encasing of the concrete slab.  In the case of angular hardcore, you should bind with a thin layer of sand so that you protect the damp-proof membrane from getting punctured when pouring the hardcore. The sand binding also helps to create a level insulation bed for the angular hardcore.

3.    Leveling The Concrete

When trying to level the concrete, you need to rake it out, and them tamp it using a straight piece of timber that’s the length of the distance between the wall on either side. When tamping the concrete, you will consolidate it, and this is what vibrating the concrete also does. The tamping should be done using a technique that includes a gentle sawing and tapping motion that makes the top surface of the concrete level.

4.    Planning The Drainage System

When laying the drainage system, you need to account of the necessary levels. For instance, the invert level that should be underneath the pipe is what sets the systems gradients. To achieve this, the trenches should be of the right depth and gradient so that the bedding does not take up huge amounts of concrete to support the drain pipes without affecting the drain flow.

The bedding should have a layer of gravel at a thickness of 100mm to 200mm and the same for the surrounding materials that will protect and support the pipes. The bed should be at a gradient of 1:40. Using plastic drains is ideal if the drain flow is to have a slight fall. The fall should be in the right way and set correctly by looking for at least half a bubble out when you place the spirit level. Do not rely on your eye when sizing the ground slopes, make use of spirit level to get the precise reading.

The following tips may prove helpful when laying the drainage pipes:

•    Place the pipes to an even gradient and ensure they are in straight lines with little to no bends.

•    Ensure there is a soil vent pipe at the head of the drain run.

•    Where there are bends, ensure you there are inspection chambers and rodding points. Also, make the necessary changes in the gradient.

•    The soil vent pipes and water closets should be connected to the inspection chambers.

•    Cover the pipes with around 100mm of gravel before backfilling the trench. Do your best to have this done within the shortest time possible to avoid leaving the trench open for an extended period.

Drainage Trenches To Run Services

A deep drainage trench can be used for other services. It is a cost-effective measure that makes sound sense. The water pipes, gas lines, and electrical cables can be run through the drainage trenches. It will help cut on the costs of digging individual ways for running these, and it also saves on labor and time. Moreover, having them installed after the house is complete is a slow, experience and labor procedure.

Testing The Drainage System

It is essential to test the drains before backfilling the trenches to that any changes that may be found necessary can be done. The testing can be done using air or water pressure. Check all joint to ensure they are watertight. The control officer may want to see the results of the tests to confirm all is well before the backfilling, and another test is done to confirm the same because the drains can be damaged or displaced if the backfilling is done carelessly.

Leave the drainage connections to the mains system for a later stage. However, if you opt to have this done at this stage, then you should have the manholes covered and the gulley traps filled with water. It will prevent the escape of foul smells from the drain and making the surroundings unbearable for the workers on site.

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