Pile Foundations - Driven or Augured Piling Solution

[Steady_eddie]

Hi All, 

 

I was hoping that I would be able to get some friendly advice which would help us decide on a sensible solution for the foundation design on our self build.

 

During the planning phase we had an investigative ground survey, The report which was very extensive involving 6 piles drilled to various depths of 4.6 to 6.9m over the site, the soil was then tested for quality and the holes then monitored and tested for subsidence and water content over a 12 month period.   The results of the test were that construction could be permitted but it was highlighted that some ground movement to the top 3.5m of superficial soil had moved down the slope during the test (6mm during the year of this test).

 

In regards to foundation design the report summary recommended re-enforced deep trench foundations or a re-enforced concrete raft with a lightweight timber frame structure.

 

After getting our planning approved we approached a structural engineer who reviewed our plans and report and also spoke with the ground investigation company to discuss the report before visiting the site to assess the project. Their recommendation was to having a piling solution due to the area and the work involved.

For reference the property is a 10m x 10m square, and we are intending to go brick and block now we are having a piled foundation. 

 

Our site is sloping around 14-20 degrees, but the build will be on level ground so we will be cutting into the slope and installing a king post retaining wall behind the house to hold back the soil. There is already a log retaining in place which we will replace with something much better. It has held without moving for 10 years but as it's softwood it's now rotting away.

 

The plot is also in a world heritage site with a history of coal mining so the ground is generally poor in our county. Planners and engineers are very wary in this historic area due to many period properties subsiding due to the ground conditions, sloping sites and traditional construction methods used in the older buildings.

 

On the recommendation of the SE we approached several piling contractors for advice and quotes. The first company recommended and quoted for 35x 150mm driven piles driven to a depth of 5m. We were very happy so we booked him, he then supplied us with his structural engineering design but last week called up to say he couldn't do the work as he had spoken to another contracter who had worked in the area and said we needed augured piles rather than driven which was something he didn't do. 

 

I then went back out looking for a new piling firm and spoke with another contracter who was familiar with the area, he had actually done the piling for a build in the same street 10 years prior. He recommended that augured piles would be needed due to the ground conditions but his quote never transpired which was a shame, likely due to being so busy.

 

I then spoke with another company, this company seemed like a really good team, really helpful, loads of positive reviews and a great website with lots of videos and photos of previous work. After sending them all the information they quoted for roughly (35x 150mm driven piles driven to a depth of 4m) , the quote was really competitive and I called them to discuss it. I mentioned that another piling contractor had recommended augured piling and said driven was not an option for this site.  His reply was that they felt that the project really didn't need it? They also said following a site visit their structural engineer would make another SE plan for us and before that they would send me an additional quote which included heave protection and augured piles so at least I had a cost for both. The revised quote showed pre-augured piles down 2m. But didn't mentioned fully augured? Just augured.

 

I  then approached another company, after looking at the ground report and plans they said that driven piles would not be suitable and that I was going to need a more substantial piling solution. They quoted for 18x 300m augured piles to a depth of 12m. This was around £10k more than the driven piles quotes I had received from others but it's also the most heavy duty solution for the job.

 

It seems we are going for a piled solution for our self-build but with all the different information we have we are having a hard time deciding on the most sensible choice, it would be great if anyone with experience in this field could give us some advice on what way to go.

The information we have summarised:

1. Quote for 5m 150mm driven piles - But then said he couldn't do the job

2. Said we needed augured piles, driven no good - Quote never received.

3. Quote for 4m 150mm driven piles, but did an additional quote for pre augered piles with driven should it be need once's SE calcs done.

4. Quote for 12m 300mm driven piles. 

 

Many thanks,

Edd

[Alan Ambrose]

Ah illuminating and very difficult for you.

 

It shows very well how even though we generally think engineering contains a large dose of science - how 4 different 'engineers' can come up with 4 different solutions depending on how gung ho / risk averse they are.

 

Are none of the SEs / piling companies recommending driving until a particular load can be reliably supported - maybe with an increased depth / safety factor than normal as the ground seems a bit 'inconsistent'? At some point the safety factor / refusal point should be high enough that the SE is comfortable.

 

Maybe a contingent deal where you pay for piling by the hour or metre until the SE is comfortable? ATM you're putting all the risk on the contractor / their insurer and understandably that's making them nervous enough that they're a bit reluctant to do the job. What's the warranty arrangement - that's where some other part of the risk will go?

 

Alan

[Alan Ambrose]

I guess one option is just to go low risk and ante-up for the maximum belt-and-braces solution.

 

Alan 

[Roundtuit]

Sorry, I don't know enough about piling to add anything useful, but piles are usually designed to transfer load vertically to load-bearing strata.  What you appear to have is a plot that's sliding down hill, which perhaps may potentially put lateral stress on the piles.  Personally, I think your SE should be designing the foundations and then you can put the approved design out to tender.

[Jilly]

I had to have augured 6m piles for our extension and found a local firm to do it and the calculations at a reasonable cost. The quotes varied from £14 to £30K, so keep an open mind. Someone who knows the local ground is a good idea.

 

Self building can throw up these scary challenges all the way through.