House extension foundations - Piling or raft?

rattwood

Hi

I am confused. I have had plans for my extension approved. The architect suggested a raft as there are trees along the boundary and the building will start 1 metre away. A builder came to give me a quote and suggested I go for piling thinking it would be cheaper than the raft. I asked the structural engineer and he agreed. Although the structural engineer also suggested getting a geotechnical engineer involved.

I just want to know will a raft suffice (which another builder is certain will be cheaper) or do I need piling? It seems like the structural engineers won't confirm unless I have the geotechincal report but that is another £1000.

I have had 2 quotes for piling which are coming in at around £30k. Could this really be the cheapest option? 1 of the quotes included 17 piles at 13m each. Is this necessary? The extension is replacing a double garage with a 2 storey extension, the footprint is 10.2m x 5.1m.

Apologies for all the questions but I am confused by it all and reluctant to spend any unnecessary money on reports if the foundations are going to cost £30k

Owain_Moneysaver

What plans have been approved? Planning plans, or construction plans for building regulations?

In typical new build conditions a raft will be quicker and cheaper, but having an extension is not the same as new build on virgin land, and for an extension you may have to pile as a raft would not be connected to the rest of the house. 13m piles does suggest you may have some interesting ground conditions.

You can either pay your structural engineer to design both a raft and a piled foundation, and then get quotes for both, or get him to design only one and get quotes for that. By doing both you may end up with a cheaper quote for foundation but of course have higher engineer's fees.

rattwood

Thanks for replying.

Planning plans have been approved. Building regs have also approved but have asked to see structural engineer plans for the foundations before we start.

I spoke to the structural engineer after receiving the approval from building regs and they have talked about a geotechnical survey and also gave me some companies to contact about piling. They haven't given clear guidance on exactly what I need and who can provide it. When I asked them about having a raft they haven't replied. It feels like they don't want to get involved in piling (even designing it) although they have asked to see the solution before we go ahead.

EachPenny

What kind of trees are they? The species and approximate height are important.

Also, do you know roughly what kind of sub-soil you have? Are you on clay, or is it sand or chalk?

EachPenny

rattwood wrote: »

I spoke to the structural engineer after receiving the approval from building regs and they have talked about a geotechnical survey and also gave me some companies to contact about piling. They haven't given clear guidance on exactly what I need and who can provide it. When I asked them about having a raft they haven't replied. It feels like they don't want to get involved in piling (even designing it) although they have asked to see the solution before we go ahead.

In many cases the piling contractor is responsible for designing the piling system. The structural engineer will specify the foundation loading and any specific issues about how the load is distributed. The piling contractor then works out the most cost efficient way of constructing the foundation to meet the specification.

Factors like the depth and diameter of the piles are important considerations. That affects the choice of piling equipment and how long the job takes. Smaller shallower piles can use less equipment, but if you need more of them to carry the loading then the time required to do the job increases. The piling contractor knows what equipment they will have available to do the job when it needs to be done, so can pick the overall optimum solution. The role of the structural engineer is then to check the contractors design to make sure it is correct.

To design the piles the contractor or engineer will need to know what kind of soils can be found at different depths below the building. The piles need to be deep enough to reach soil with sufficient bearing capacity and below a level where shrinkage/expansion of the soil is likely to occur. That can be surprisingly deep - if the best solution is a smaller number of larger diameter piles then a length of 13m is not that great.

Sometimes the structural engineer and/or a geotechnical engineer will be involved in the pile design, perhaps because of a unique requirement of the building. In that case the piling contractor might suggest an alternative approach, but otherwise will just charge appropriately to deliver what they've been asked to build. That could include the cost of hiring in equipment they don't already have, plus transport costs etc. Not cheap.

GDB2222

But isn’t there a concern here that everyone is covering their own backside? The piling company specifies 13m piles because that machine is idle at the moment, rather than because it’s the optimal solution?

Furts

A likely scenario is this ...you have been taken advantage of by your "architect". Either knowingly, or because they were incompetent. The first question at the first site discussion should have been "those trees will incur huge issues with your building costs, what is your budget and do you wish to go ahead with drawings then Planning?"

Nobody on this forum has seen your trees or soil, but a fundamental can be stated. A raft extension would be a bad move and unsuitable for your extension. Quite simply if the raft moves and settles in accordance with the trees, the seasons, and so on, it will split away from your existing home causing all sorts of cracking.

This in turn means your "architect" should have been advising piling from the first meeting with you.

Piling is the preferred option because it will go down to ground unaffected by the zone of influence of the trees. Do not be surprised by the depths - I have dealt with a piled basement that was going 25 metres below the ground, which meant 30 metres below the adjacent street level.

EachPenny

GDB2222 wrote: »

But isn’t there a concern here that everyone is covering their own backside?

In a way that is what you as a client want them to do though. Time after time on this forum you read about people trying to skimp or get away with stuff because they don't want to spend the money - usually because they have a limited budget and grand ideas. The boring practical things like foundations seem like the obvious things to skimp on, but only to those who don't appreciate the risks they are running.

The point Furts makes is the most important thing the OP can take on board - there may be cheaper ways of building the foundation, but if the result is the extension moves and cracks then the result will be many times more expensive to fix.

I would hesitate to say piling is essential without knowing more about the trees and the soil conditions, hence the first questions I asked. If the circumstances looked even remotely favourable then it might be worth going to a geotechnical engineer for an opinion about alternatives, but like Furts I'd say a piled solution is almost certainly going to be necessary in which case consulting a piling contractor is the next logical step.

GDB2222 wrote: »

The piling company specifies 13m piles because that machine is idle at the moment, rather than because it’s the optimal solution?

The optimal solution for the OP will be a foundation which works, for the least possible cost. If a reputable contractor doesn't think piling is necessary they will usually tell you as much because it is not in their interests to get involved in a job if before work begins the client finds out there is a much cheaper way of doing it.