The traditional method involves pouring concrete into the bottom of the trench to a depth of around 300mm and then building up to floor level using bricks or blocks. In contrast, with trenchfill, as its name suggests, you just fill up the trench with readymixed concrete.
The first step is to lay the foundation of your house. In order to do a proper job, you need to construct two batter boards for each outside corner of the foundation. In order to build batter boards you need either 1×4 or 2×4 lumber, several nails and a hammer. Remember that the batter boards have to be rigid, otherwise while digging the foundation you might move them from the right position. In addition, you have to fasten squarely a crosspiece over the two stakes.
Digging Foundation Footings Digging proper foundation footings depends on several different things.
The presence of mature trees can do wonders for a building plot visually. But their roots can play havoc with your foundations. Because trees draw water in from their roots, the ground surrounding their root systems is prone to drying out and this creates instability. The bigger the tree and the closer it is to your foundations, the worse the problem tends to be. The problem is most acute in the clay soils common in southern England.
Being informed about the site and local problems will make you more prepared when tackling the foundations on your self build
Angular hardcore needs binding with a thin layer of sand to protect the polythene damp-proof membrane from puncturing and also to create a level bed for the insulation to sit on.
I will probably be digging the trench for my foundations by hand, over a length of time, to save money.
First, you have to mark the footprint of your foundation on the ground. In this way you will make sure the footings will be straight. However, we have to emphasize that you must read carefully the foundation plans, as to dig the foundation properly. Another aspect you have to take into account is to prepare a site, near the foundation, where to deposit the soil you excavate from the trenches.
Continue digging the foundation according to the plans, and make sure you deposit the soil near the construction site, as you will be needing a significant amount to fill in the foundation.
STEP 7: Cut 50 or 60 pieces of 1/4” or 3/8” rebar to 18 to 24 inch lengths. Use a cheap chop saw with an abrasive blade to cut rebar. Wear those safety glasses!
If its hard to get at all or even part of your foundations, you can make things a lot easier by hiring a concrete pump. This is basically an enormous hosepipe that comes on its own dedicated lorry. The readymix truck empties its contents in one end of the hosepipe and the pump operator can move the other end of the hosepipe around your foundations, thus doing away with the procession of wheelbarrows that usually accompanies a concrete delivery.
If the top of the rebar is below ground level a few inches, it is no problem. The primary concern is to be certain you maintain the target depth of concrete – as a minimum. If you end up with 16 inch deep footings the only result is that you buy more concrete and spend more money, and have a stronger house. These rebar stakes are to tell you how much concrete to put in the footings.
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You are then faced with a choice of how you fill this metre deep trench. There are basically two methods, often referred to as traditional footings and trenchfill.
Using deep drainage trenches for running in other services makes good sense. Gas, electric and water cables all have to be provided at your expense, so laying them in position now will save time and money later. Getting the supply companies to connect them when the house is complete can be a slow procedure, so get the job registered, priced and booked in well ahead to avoid delays later.
One of the nightmares facing groundworkers is what to do if the trenches collapse before the concrete is poured. It happens, often as a result of prolonged rain combined with particularly heavy ground. This is not usually a major problem on shallow trenches but it can be very dangerous if you are working down below waist height. You have to be very careful about such risks; if you have to dig down below one metre, you may have to shore up the trench walls to ensure everyone is safe.
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There are two basic methods of digging foundation footings – pick & shovel, or power equipment. If you get a backhoe, get one with a trenching bucket the width you want your trench. The buckets are interchangeable. Position the bucket so that it is on the ground line with one third outside and two thirds inside. You must keep about two thirds of the width inside the line, so your load will rest properly on the footing.
June 2008: The 600 mm trench is marked out for the foundations.
Trenchfill is quick and easy and its by far the commonest method now used. But it is expensive on materials using, as it does, around three times as much readymix as traditional footings do.
Next, you have to lay out the perimeter of your house, but you must ensure it is exactly over the location written in your construction permits. Even the slightest error will make your life miserable, so pay attention when installing the batter boards and laying out the foundation perimeter.
Why not go for a raft foundation, no need to dig footings and save on the cost of concrete and labour.
STEP 16: Place the corner pieces in the corners, one to each run of rebar and tie them in place, at least two ties on each side of the corner. All the rebar should be in the “top of the bottom third” of the height of the trench. So, a 12 inch deep trench means the rebar should be about 4 inches off the bottom.
All prices and estimates quoted correct as of September 2003.
How to dig footings
If there concrete stamps on the site of your foundation, you will have to break them with a jackhammer. This is the easiest and most efficient method to break concrete, but you can also use a sledge-hammer. It will do the same thing, but you have to put more effort and it is time consuming, that is why we kindly recommend you to rent a jack hammer for a day or so.
To check the existing building for square (see pic 3), I ran a string line along the end of the building A - B, extending it to point C. B - C being 4 meters. I measured 3 meters along the back wall of the house and made a mark at D. If my existing building was square, D - C, according to Mr Pythagoras, should be 5 meters. However, D - C measured around 4.89 meters, meaning angle A,B,D was greater than 90 degrees.
Some local ordinances require that the bottom of the footing trench step rather than slope. Your engineer or inspector can advise you on this.
Work attentively and with patience, as any mistake you make will amplify when building the actual house. Strange as it might seem, there are many persons who think that the foundation is not important, as it’s not visible. Nothing can be more wrong, so make sure you build the foundation according to the plans, as your safety is at stake.
Just use a laser level or a water level to make visible marks on four posts installed at each corner of the foundation. Next, use a tape measurement to see the distance from the marks to the bottom of your foundation trenches. If the four depths are equal, it means the corners of your foundation are level. If the depths are different, you have to adjust them accordingly.
Hi I am about to construct a single storey wooden framed office in my garden. The dinensions will be 7 metres by 12 metres and a height to the top of the pitch of 4.2 metres. I would like some advice on the depths and types of foundation that you would recomend. Thank you very much Mr D L Smith
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Any ideas for how to keep the trench in good condition, i.e. no crumbling sides, turning to muddy mess etc.
There are two approaches to this risk. One is the cautious one, that involves making as many investigations as possible beforehand often this involves considerable expense in itself. Cautious builders dig trial holes and have them inspected by structural engineers who write reports recommending certain courses of action.
On a standard foundation job, you excavate the foundation trenches to a depth of about a metre. The actual depth is something the building inspector will rule on but if there are no unforeseen problems, a metre is generally accepted as a good depth.
When digging a foundation, it important to make sure its bottom is perfectly level. There are cases in which the site is not level, so it is hard to tell by eye if the bottom of the foundation is perfectly level on the whole surface. Nevertheless, we will show you a techniques which works in any situation.
Water in the bottom of the trench is another perennial problem. Usually you can get rid of most of it using a mechanical pump but sometimes a building inspector will ask for the trench bottom to be flattened off again because, when wet, they can get very chewed up.
To hire a concrete pump, allow around £150-200 per session. They pump a full load (6m³) in 20 minutes, three times quicker than three men barrowing might do. If using a pump be sure to let the readymix supplier know, because the mix design is wetter and the throughput of lorries is much faster than on a normal job. Its also a little more expensive. Some lorries have pumps on board, ask.
If you over order you can’t return what you don’t need. Not only do you have to pay for it but you also have to find somewhere on site to dispose of it; think about this beforehand there may be the odd spot which could usefully use some concrete.
Hi. Talk to your council inspector about day joints in the concrete. This is where you concrete in sections that you have dug out . When you pour the concrete you insert rebar into the wet concrete stop end leaving you with a structural joint for your next pour . This will remove the problems associated with leaving open trenches. But you must consult you inspector prior to this as it will depend on your ground condiions
The other approach is more cavalier – don’t worry about the foundations until you open up the ground. In theory, the cavalier approach shouldn’t cost you any more – in fact it will save you the investigation costs. But in practice, if you are unlucky and you meet problems, it is likely to delay progress substantially and in an unplanned way. This is a cost in itself.