Structural Engineer for Outbuilding and Pool

[phykell]

Hi,

 

I'm having an indoor pool built to replace an existing outdoor pool and while I think it should be a relatively straight-forward project, the structural engineer seems to disagree.

 

In addition to the cost of calculations for the roof and steels they want to drill several 5m boreholes to test ground conditions, in-situ geo-technical testing and soil samples for chemical testing.

I've recently had a water borehole dug out to 50m with a basic report (soil/ground type, water found, etc.) issued - I gave them a copy and they still said they wanted the above work done. 

I accept that they need to cover all bases due to professional liability, etc. but I've been through the mill with the LPA and all of their "required" reports, investigations and mitigations.

Am I now just being cynical or does that all sound reasonable?

Thanks for any advice/opinions offered.

[Nickfromwales]

2 hours ago, phykell said:

Hi,

 

I'm having an indoor pool built to replace an existing outdoor pool and while I think it should be a relatively straight-forward project, the structural engineer seems to disagree.

 

In addition to the cost of calculations for the roof and steels they want to drill several 5m boreholes to test ground conditions, in-situ geo-technical testing and soil samples for chemical testing.

I've recently had a water borehole dug out to 50m with a basic report (soil/ground type, water found, etc.) issued - I gave them a copy and they still said they wanted the above work done. 

I accept that they need to cover all bases due to professional liability, etc. but I've been through the mill with the LPA and all of their "required" reports, investigations and mitigations.

Am I now just being cynical or does that all sound reasonable?

Thanks for any advice/opinions offered.

Listen to your SE!

 

Anything short of their current proposal would be what worried me here, plus they won’t put even the hairs of their crown jewels onto the chopping block of assumption, why would / should they?

 

This one is absolutely fundamental, as the downward and outward forces at play here are significant to say the least.

 

If this is done even slightly poorly you risk losing the integrity of the house you’re putting it into.

 

Best not to tar them with the same brush…..

[Mr Punter]

The boreholes should not be too costly.  If they are good designers they may use the information from the geotechnical report for a more cost effective solution for the pool.

[saveasteading]

1 hour ago, phykell said:

it should be a relatively straight-forward project,

But it is not. It is a very big water tank, and if the ground moves a tiny bit, the slab and or walls could break and it gets very messy and expensive.

And you will be indoors now, not outside., so damage and repairs are x3.

There are a lot of failures of swimming pools and contractors have gone bust and Engineers had very expensive claims.

And some of these are for proper designs: the ground we live on is very mobile.

 

If the results of the boreholes are encouraging then you will save cost on the amount of concrete and of reinforcememnt.

If they show poor ground then it really is essential to know that and design to suit.

 

Is it still 27m long as previous posts? that is big and not to be dabbled with.

How much do you want it?

[phykell]

1 hour ago, Mr Punter said:

The boreholes should not be too costly.  If they are good designers they may use the information from the geotechnical report for a more cost effective solution for the pool.

For just the additional report on the ground composition, etc. it's just over £4k.

 

From the comments here and some "pseudo-research" on AI, it's not something I can or should avoid having done so looks like I just have to bite the bullet.

[phykell]

53 minutes ago, saveasteading said:

But it is not. It is a very big water tank, and if the ground moves a tiny bit, the slab and or walls could break and it gets very messy and expensive.

And you will be indoors now, not outside., so damage and repairs are x3.

There are a lot of failures of swimming pools and contractors have gone bust and Engineers had very expensive claims.

And some of these are for proper designs: the ground we live on is very mobile.

 

If the results of the boreholes are encouraging then you will save cost on the amount of concrete and of reinforcememnt.

If they show poor ground then it really is essential to know that and design to suit.

 

Is it still 27m long as previous posts? that is big and not to be dabbled with.

How much do you want it?

Understood 😎

Yes, it's the ~28m one that I had to take to appeal. The pool is approximately 11m x 4m with the rest for plant, changing area, and three-bay garage.

One day I'll update this place with the completed project, one fine day 👍

[Nickfromwales]

24 minutes ago, phykell said:

it's just over £4k.

This will be the best £4k you ever spend sir....  

 

21 minutes ago, phykell said:

One day I'll update this place with the completed project, one fine day 👍

Please do. I've done a few pools for clients over the years, next one is outdoors in ICF, and am ever curious (and keen to brush up where any opportunity presents itself).

[saveasteading]

12 hours ago, phykell said:

The pool is approximately 11m x 4m with the rest for plant, changing area, and three-bay garage.

I think I would design these as distinct units, so that they can articulate without cracking, as the loadings are very different, and an 11m pool structure will be less complex than 28m. The gaps can be filled in with flooring.

[phykell]

12 hours ago, Nickfromwales said:

Please do. I've done a few pools for clients over the years, next one is outdoors in ICF, and am ever curious (and keen to brush up where any opportunity presents itself).

The whole build will be ICF including at least the back wall of the pool - possibly concrete for the other three walls. I was planning on SIPs but preferred ICF due to being more substantial (we get winds from adjoining field), water (housing a pool) and better thermal mass (reduce temperature fluctuations).

A couple of items that might interest you in particular is that the ICF pool will require an extra chamber to house the in-ground pool cover. There's not much room around the pool so the cover will be underground - not much of an issue for an outdoor one but it also looks a lot better than the above-ground covers. The other item is that we're looking at an undercroft using block/beam to give access to the pipework/lighting, rather than just embedding it all in the floor. 

[phykell]

3 minutes ago, saveasteading said:

I think I would design these as distinct units, so that they can articulate without cracking, as the loadings are very different, and an 11m pool structure will be less complex than 28m. The gaps can be filled in with flooring.

Thanks - I'll see what the structural engineer comes back with. I believe the initial plan just shows one huge concrete raft though.

[Nickfromwales]

5 hours ago, phykell said:

The whole build will be ICF including at least the back wall of the pool - possibly concrete for the other three walls. I was planning on SIPs but preferred ICF due to being more substantial (we get winds from adjoining field), water (housing a pool) and better thermal mass (reduce temperature fluctuations).

A couple of items that might interest you in particular is that the ICF pool will require an extra chamber to house the in-ground pool cover. There's not much room around the pool so the cover will be underground - not much of an issue for an outdoor one but it also looks a lot better than the above-ground covers. The other item is that we're looking at an undercroft using block/beam to give access to the pipework/lighting, rather than just embedding it all in the floor. 

I specified a ‘sump’ in the space under the 5x15m pool, in the basement, to house all the ‘pool paraphernalia”, so if anything failed the entire contents of the pool could be evacuated quickly.

 

I combined that with the drainage system from the basement, and did the pump set in triplicate for redundancy. Gave me and the SE a headache or 3!

 

I planned to use UFH pipes in the base and lower sides of the pool walls, to minimise the impact of the pool plant space needed. Was an interesting idea, and was received well, but apparently not very common in ‘usual’ pool builds; seemed ideal for one done in ICF though.

 

If you need a very good ICF guy who’s fluent with pools and basements etc, send me a PM. 👍

[markc]

@phykell all the above is great advice. Just to add, the deep bore report is great for water extraction but not much use for a pool, hence the 5m bores. With a above ground pool the mass (weight) of the pool and water are very important but with an in ground pool it’s very different, the mass off the soil you remove is generally much less than the big concrete tank and fill with water. When you empty the pool the ground wants to push the walls in and any water in the first few metres wants to float your pool out of the ground. If you have a high water table you will be looking at anchoring the pool down rather than supporting it.

Edited May 15 by markc

[phykell]

Update to this topic:

 

The survey took a long time and the SE took even longer to act on the results. Long story short, there will be a concrete slab under the pool, and the rest of the building's substructure will be perimeter foundation with a block/beam floor. 

 

My issue is that the builder I'd like to use is at odds with this - he's done lots of very large pools and associated buildings and is convinced that a concrete slab would suffice other than for a high risk of ground instability. My layman's view is that the slab is 28m and only 7m wide so it's a relatively narrow slip and if it was a slab, upwards ground movement would have a lot of surface area to push up against whereas the slab under the pool should be OK as it has a lot of weight pushing it back down.

 

The survey came back with ground firmness rated Stiff to Very Stiff apart from part of one section/borehole sample which was just firm. Nevertheless, the survey noted that either trench or strip foundations would be a suitable approach, and this would explain the SE's requirement for a block/beam floor. However, while I can see that a simple slab wouldn't suffice, why wouldn't an engineered raft (mat) foundation work?

I haven't had any suggestion from the engineers on what type of pool I can have but I've presumed they're working on the assumption it will be built in ICF. I'm guessing though, that a one-piece fibreglass (or even stainless steel/composite, etc.) pool is a non-starter - does that sound right?

[Mr Punter]

You will need a SE design for this.  If your preferred builder does not like the SE proposal he would need to come up with a counter proposal with reasoning, that can be put to your SE for consideration.  You could get a price for both.

[saveasteading]

Ask your builder  to check with his insurer, what cover he has for designing a swimming pool. None obviously. And a warranty?

 

Yes it could collapse or float when empty, or crack when full,  unless designed as a water tank. 2 layers of mesh and careful detailing AND careful construction.

 

The issue with stiff (which sounds good)  ground is that any clay expands and contracts a lot seasonally, and with considerable force. 

[Roger440]

Ive got nothing useful to add other than yoo must really like swimming!

 

The work and considerations required to stop a sewage treatment plant popping out of the ground caused me enough stess, but this is a whole other level.