Foundations for a timber build

[Nininog]

Hi everyone,

We’re in the process of a new build for a business on our land and have been granted planning permission for a brick and block L Shaped one storey. 

The problem is that we had a soil investigation done because we have protected trees near the build and the engineer has recommended piled foundations. We’ve had a quote for these and they’re knocking on £50,000! Which is out of our budget and not viable. 
So we’re pondering on a timber frame building instead and hoping the foundations would mean we’re able to put a concrete slab down instead?

Any suggestions or ideas would be appreciated!

 Thanks

[George]

What size is the building? £50k may be excessive or may be cheap!

 

Mini piles and ground beams is not an unusual foundation system so you should get three quotes to get a true feel for the cost. There are many factors when pricing work and one data point is not enough. 

 

If the issue is the roots of the trees then you will probably require some sort of pile.

 

If the issue is heave then a raft (the concrete slab) may also require significant groundwork.

 

Unfortunately spend on foundations is the most unsatisfying part of a build. But it is one of the most important to get right. 

[Kelvin]

Our whole groundworks is £50k for timber kit plus large garage. That includes all drainage, treatment plant, retaining wall, longish drive to give you some idea. 

[nod]

Some SEs prefer mini piles others hate them and would rather deep

We were surrounded by hundreds of trees on our previous build and on clay We used deep strip foundations Around two meters and Clay board to help with the heave 

Came in at around a third of the cost of piling 

[Dave Jones]

screwpiles can be cheaper depending on the depth traditional piling needs to go down. Also worth getting another SE opinion.

 

https://www.screwpilesltd.co.uk/

 

 

[Dreadnaught]

I have an L-shaped bungalow and have 26x screwpiles because of tree roots. And then a concrete raft foundation atop. I can provide the benefit of my experience if desired.

[Iceverge]

Most lightly not unusual in your location. 

 

Contact some local groundworks companies and they'll soon give you chapter and verse. 

 

 

[Gus Potter]

On 13/02/2023 at 15:58, Nininog said:

The problem is that we had a soil investigation done because we have protected trees near the build and the engineer has recommended piled foundations.

Building near protected trees can be quite a challenge which is fascinating if you are an Arborist or say an SE.. less exciting if you are the one footing the bill and on a budget.

 

Building near trees on BH crops up a lot and loads of folk on BH have made some great comments / given good advice. One common problem is that of soil shrinkage when the trees suck the water out the ground, another when you cut them down and the soil starts to recover and swells up. This recovering process can take a few years at times in certain clays soils.

 

But the reality is that sometimes the tree protection orders (TPO's) can significantly impact the design and if you don't play by the rules you could face a big fine and get your job stopped.

 

Ok lets look on the bright side.. who doesn't like some good looking trees round about?

 

That said with a bit of careful thought it can work.. it doesn't always need to be a show stopper.

 

If you are the SE you need to first embrace the challenge. Here are some things that require consideration and how you progress a design.

 

The starting point is to look at the tree. What kind is it or do you have more than one also maybe of different species?

 

Roughly some trees have a tap root that goes down deep.. can be right into fractured rock below. This root collects deep water and provides structural stability. Closer to the surface it has thicker lateral roots that follow the nutrient source in the soil. These also provide structural stability to the tree as they get a grip of a wider lump of soil that acts partly as ballast. Thin (fine to very fine) roots grow from these roots and do the grunt of the collecting nutrients and water in the soil. Some trees don't have a tap root though.

 

The first thing you have to look at is that if you run machinery over the ground you compact it and that can cause a lot of damage to the fine root system.. the soil compaction stops the fine roots from regrowing and this could kill the tree... not right away but you can weaken the tree and it is thus more exposed to disease.

 

Next is that if you put a building over part of the root zone you can reduce air infiltration into the ground, exclude water.. as you have taken it away in the gutters and the worms will move off or die.

 

The above often results in the authorities drawing a big circle around the tree and saying.. no heavy machinery.. no building.. no cutting strip founds or drain / service tracks pretty much no nothing in this circle., oh and you need to put a fence round the root protection zone to show compliance.

 

However, sometimes if you spend some time investigating the tree and species you can understand how and where the roots grow for that type of tree. Next is to look carefully at the ground. Tree roots don't grow in a perfect circle around the tree.. they follow the water and nutrients. Once they know they are stable their aim is to propagate so why spend time and energy growing unwanted structural roots.

 

This can be a key.. if you know more about the soil type, the nutrient bearing layers, where the roots are growing and where they need to grow in the future you are on the right track.

 

It may be that with a bit of time and effort you can show that the area of ground the tree needs to maintain health is away from where you want to build! .. and presto .. your project is much more back on track.

 

In reality what happens is you try and change the shape of the protection circle if you can and do a bit of clever SE / geotechnical design to reach a compromise that suits all.. even the tree.. if only they could talk?

 

If you want post more info. Don't despair!

[Nininog]

On 14/02/2023 at 16:53, Dreadnaught said:

I have an L-shaped bungalow and have 26x screwpiles because of tree roots. And then a concrete raft foundation atop. I can provide the benefit of my experience if desired.

Hi Dreadnaught

That sounds interesting. Non of the structural engineers have mentioned screwpiles with a raft. How did that work exactly? Was your raft insulated ? Did the screwpiles have to go to the same depth as conventional piles?

 

sorry for all the questions in advance 

nini

[Jilly]

I had 3 quotes for piles and one was double, so definitely get more quotes.

[Temp]

Did the site investigation involve a proper lab test?

 

We're on clay with a tree or two nearby and ended up with 1.5m deep trench foundations with expansion board linings.

 

Edit:..

 

Ah right I see they are protected trees so the use of piles is probably to protect the trees from the house not the house from the trees. I suspect they may not allow trench foundations for that reason.

Edited February 15, 2023 by Temp

[Temp]

https://www.abbeypynford.co.uk/wp-content/uploads/2020/03/Treesafe-Brochure.pdf

 

[Dreadnaught]

7 hours ago, Nininog said:

How did that work exactly? Was your raft insulated ? Did the screwpiles have to go to the same depth as conventional piles?

 

The screwpiles was one of easiest part of my project so far. My foundation was designed by a specialist company in Ireland that others have also used on this site. There were three parties involved with the screw piles themselves: the screw pile provider, who was in the neighbouring county, his screw pile engineer, who specified the size & required torque needed for each of the individual piles based on my ground survey report, and the screw pile installer recommended by the supplier.

 

The installation was done over two days by the installer using an excavator with a special head (he is the most skilled person I have ever seen using an excavator … he even used the excavator to load his tools in his side-door van at the end of the job!) and he noted the screw pile torque figures for each pile on a sheet of paper as he proceeded. I then received a copy of that report and, in turn, submitted it to building control so that they were satisfied.

 

After the installation of the screw piles, another team then arrived to shutter and cast the raft, which sites atop the top-plates of the screw piles. That raft was indeed insulated with thick PIR insulation. The design of the raft was such that it had extra thickening to form an integrated ring beam and cross members so it was not one consistent thickness across its entire span. All carefully designed with UFH and plumbing embedded within.

 

All in all, the following engineers were involved with the screw piles:

1) ground surveyor (I inherited this survey when I bought the plot)

2) foundations designers, in Ireland

3) specialist screw-pile designer in Suffolk

 

All in all, the process ran smoothly and, I thought, the screw piles were quite inexpensive at about 3.5% of my overall build budget.

Edited February 16, 2023 by Dreadnaught

[Hilldes]

On 16/02/2023 at 05:58, Dreadnaught said:

After the installation of the screw piles, another team then arrived to shutter and cast the raft, which sites atop the top-plates of the screw piles. That raft was indeed insulated with thick PIR insulation. The design of the raft was such that it had extra thickening to form an integrated ring beam and cross members so it was not one consistent thickness across its entire span. All carefully designed with UFH and plumbing embedded within.

I remember when I was looking for a piling solution a comment about investigating an insulated raft foundation with piles - it was you I think. I went for concrete piles with reinforced concrete ground beam, block foundation walls supporting and insulated beam and block floor. This has a number of challenges such as the need to ventilate the void under the beam and block floor and the depth of escalation required for the min 300mm depth of the void. Being yours is a raft so it sits on the ground and has no need for a ventilated void?

[Dreadnaught]

1 hour ago, Hilldes said:

Being yours is a raft so it sits on the ground and has no need for a ventilated void?

 

If have a void, but not ventilated. The void is for heave only. And is created using large crushable ground-heave protection panels (I used the Jablite version) made of polystyrene. They sit below the insulation, atop the compacted MOT sub-base.

[Nickfromwales]

This is what was used for heave on my current project.

 

https://www.google.com/shopping/product/1?q=cordek+heave+protection&prds=epd:18157365360563409597,eto:18157365360563409597_0,pid:18157365360563409597&sa=X&ved=0ahUKEwjbmqKwsaD9AhWRTcAKHWvhAnEQ9pwGCAU