Sloping site stress

[Grian]

Hoping for general information regarding foundations on a sloping site.

 

I'd previously been worried about septic tank and soakaway as I have shallow soil and drains surrounding my site (thank you for responses to my question on this a couple of months ago). This week an experienced consultant visited and he has advised that I could locate the house on the highest point of my site and this would provide enough drop to avoid using a pump, plus there would also be enough soil depth in the lowest area for a low-profile tank and mounded soakaway without needing to peck out rock. I was so relieved to have a solution after months of worrying!! 

 

This solution moves the house back a few metres to the base of a steep bank and euphoria quickly turned to new concern when I looked at what I'd previously considered to be a negligible slope across the site. I was even more concerned after digging test pits in the region of the house corners - the greatest soil depth is at the low end (1000mm) and soil is shallowest at the highest corner (550mm), that can't be good.

 

I need to have a site survey done to map the levels and without that I realise my question is really vague, but I'd be grateful for any idea of what foundations on a sloping site entail. I am guessing the high point is scraped to rock and a retaining wall built around the lower parts to that same level. Then what fills the bit in between - rock type material (expensive on an island) or soil...? I'm hoping to use air-source heat pump und underfloor heating, I imagine this is relevant.

 

I'm attaching a pdf showing test pits and their depths, and a link to a shaky video https://youtu.be/2gyD1b7IR_0 which gives an idea of the site (I look in test pits relating to a previous and now discounted house position - this has been a long process already!).

 

Any input would be very very welcome! Until I have a site survey done and a response from builders my imagination is running wild!

 

Thank you, Kirsty

Site_Info_19th_Feb_2019.pdf

Edited February 20, 2019 by Grian

[ProDave]

It is all down to "what" soil goes down to 1000mm at the "deep end"

 

It someone in the past has piled it up with extra top soil to level the ground then that means more digging and deeper foundations at that end.  What do you hit at 1000mm?  If it's bedrock then you are laughing.

 

Your you tube link says "this video is unavailable"

[ToughButterCup]

18 minutes ago, Grian said:

[...]

I need to have a site survey done to map the levels and without that I realise my question is really vague, but I'd be grateful for any idea of what foundations on a sloping site entail.

[...]

 

Our site slopes. 

A proper site survey is what you need. It will repay you handsomely. Get that done and get back to us.

Loads of support here - I doubt whether you'll be facing a problem that hasn't been aired before on BH.

 

A slope can be a great advantage.

 

[Jeremy Harris]

I second the above.  I think our topographical survey cost about £450 and proved to be invaluable, as it was used as the base plan for every site drawing, even including the latest landscaping work we've been doing long after the house was finished.

 

That topo survey provided the info needed to work out excavation depth, quantity of spoil that needed to be removed (and hence price) as well as drainage runs etc.

[scottishjohn]

I have  a sloping site and this  was first thing my architect wanted before he would even do any work  on the plans 

[Grian]

Thanks for speedy replies.

 

I hit rock at the bottom of all the holes. It is agricultural land, rough grazing, so nothing done previously. Rock in some places is a little friable but solid and in others clearly smooth bedrock.

 

I will try to fix that video.

 

The intended house is a longhouse style (15.6 x 6m) and only one floor developed, but with attic trusses to allow for upstairs later if funds allow. If the costs of the levelling are really significant maybe I should be thinking of making a smaller footprint with bedrooms upstairs instead... as I said, endless speculation with no knowledge at all to curb my wildest imaginings!!

 

The architect who will draw the site plan is getting me quotes for the survey - a friend on nearby mainland paid £700 so I'm expecting this as a ballpark figure.

 

The waiting at each stage is excruciating! Budget is limited and once I finally have all the information, foundation design etc the builder will produce the figure which reveals whether I can go ahead or not. I wobble from optimism to dread on an hourly basis!

Edited February 20, 2019 by Grian

[Grian]

Video now accessible, probably of little use though, hard to know how I could make it so unclear and shaky! The base of the holes viewed are covered with soil as it was so long since I dug them but give an idea of soil type. The two photos show new holes dug yesterday. The shallow one is at the highest point, you can see smooth rock on the base. The other one was too taxing to fully dig out at this point but it is solid at the bottom, it is at the low end.

 

 

[scottishjohn]

15 minutes ago, Grian said:

The intended house is a longhouse style (15.6 x 6m) and only one floor developed, but with attic trusses to allow for upstairs later if funds allow. If the costs of the levelling are really significant maybe I should be thinking of making a smaller footprint with bedrooms upstairs instead... as I said, endless speculation with no knowledge at all to curb my wildest imaginings!!

2 levels could  make it cheaper to build and better insulation levels ,less slab  to pay for 

a box is best shape in some ways .less roof area -heat goes up so heats upstairs.

and if no height restricitons --

Edited February 20, 2019 by scottishjohn

[scottishjohn]

maybe you have to go with  a proper sewage treatment system so you can discharge to a water course --then ground depth won,t be a problem ?

[Grian]

8 minutes ago, scottishjohn said:

maybe you have to go with  a proper sewage treatment system so you can discharge to a water course --then ground depth won,t be a problem ?

 

Thanks John, it has been considered. This seems to be the lesser of evils - unless I use this layout I need a pump, and even a biodisc type system would require rock pecking out elsewhere on the site. Probably more significantly, SEPA have agreed to this approach and there is no watercourse available that would satisfy them for discharge.

[scottishjohn]

3 minutes ago, Grian said:

 

Thanks John, it has been considered. This seems to be the lesser of evils - unless I use this layout I need a pump, and even a biodisc type system would require rock pecking out elsewhere on the site. Probably more significantly, SEPA have agreed to this approach and there is no watercourse available that would satisfy them for discharge.

its all money --but how much is that view worth??

do you own the big cliff behind you ----

[Grian]

11 minutes ago, scottishjohn said:

its all money --but how much is that view worth??

do you own the big cliff behind you ----

 

I own to the top of the steep bank behind.

 

The view? Priceless!!!

 

I know there are more spendy solutions - and what wonders could be done on this site if there was money to throw at it, but I am absolutely scraping together the budget for the lowest-cost approach. It is in a National Scenic Area so a further consideration is that the view imposes costs in terms fairly limiting constraints on design and siting. A pre-app has had a positive response for this style of house in more or less this position and I was advised to be as close to the bottom of the bank as possible. 

 

Edited February 20, 2019 by Grian

[scottishjohn]

3 minutes ago, Grian said:

 

I own to the top of the steep bank behind.

 

The view? Priceless!!!

 

I know there are more spendy solutions - and what wonders could be done on this site if there was money to throw at it, but I am absolutely scraping together the budget for the lowest-cost approach. It is in a National Scenic Area so a further consideration is that the view imposes costs in terms fairly limiting constraints on design and siting. A pre-app has had a positive response for this style of house in more or less this position as I was advised to be as close to the bottom of the bank as possible. 

 

hobbit home comes to mind

built into ground   if money allowed  LOl

 

just a glass wall peeking out of the banking

Edited February 20, 2019 by scottishjohn

[Grian]

1 minute ago, scottishjohn said:

hobbit home comes to mind

built into ground   if money allowed  LOl

 

just a glass wall peeking out of the banking

An architect neighbour a couple of miles along the road has done just that! Well, they are partly into a bank and it is extended over the top with a turf roof. It is well up a hill track and invisible from the road. Very clever job and all done themselves, a lifetimes work though. I checked the lottery numbers this morning just in case

[Ed Davies]

1 hour ago, Grian said:

low-profile tank and mounded soakaway without needing to peck out rock.

 

Pecking out for a treatment plant doesn't have to be expensive if you have a big digger on site for a couple of days anyway. Just needs one with the right hydraulic feed and long enough reach. The chaps who did my site access and prepared the house area did it in less than half an hour, just in passing while they had the appropriate digger to hand. It'll need excavating fully later but a smaller digger can do that now the rock's broken up.

[ProDave]

That site does not look bad at all.

 

It will take a decent digger a day or less to level the house site area.  aim for the floor level of the house to be the "average" of the present ground level, so in some places you lower the finished ground level by digging out, and in other places you raise the finished ground level by building up with what you have excavated.

 

Definitely go for a treatment plant, not a septic tank.  And choose one that works on the air blower principle like the Biopure, Conder, Vortex, Graff and probably several others.  Trust me, you do not want to be the one servicing a failure of some rotating mechanical parts down in the brown smelly stuff.

[Thedreamer]

2 hours ago, Grian said:

 

I own to the top of the steep bank behind.

 

The view? Priceless!!!

 

I know there are more spendy solutions - and what wonders could be done on this site if there was money to throw at it, but I am absolutely scraping together the budget for the lowest-cost approach. It is in a National Scenic Area so a further consideration is that the view imposes costs in terms fairly limiting constraints on design and siting. A pre-app has had a positive response for this style of house in more or less this position and I was advised to be as close to the bottom of the bank as possible. 

 

 

Hi @Grian

I'm currently self building on Skye, our planning process was straightforward we received no comments from individuals and organisations. I think what helped us was that we were building a three bedroom house with a fairly traditional design. I'm 32 so don't have the funds to build a breath taking architecturally designed house, with our build it was more important to make the most from the views in a cost effective manner. 

 

Our self build is on a slope and clay, this can make the foundations a lot more expensive. Unless your very close to a quarry the cost of importing fill for access and building up the levels

can be a lot. 

 

On our croft and we had access to 'rotten rock' we were able to extract around 1000 tonnes out of the ground using a borrow pit. We then filled this pit with the clay  that was dug out of the ground before putting the turfs on top.

 

Our ground floor is a suspended timber floor if you compare this to standard slab this is not going to be cost effective or the most simplest on a flat/easy site, but once you are dealing with slopes and importing fill it becomes more cost effective. You could also consider block and beam however from my experience these are rare in the Hebrides.

 

Might be worth asking a local digger driver what is below the surface, any recent builds close to your plot?

 

Not sure if you have lived locally or moving to the area but with most builds in the Hebrides local knowledge can be very important. 

 

I only just realised now that we are building in a National Scenic Area as well! 

 

[Grian]

Thanks for further input.

 

Under a previous planner my parents had a struggle getting permission just adjacent to me when they wanted to move from a tenanted house onto their own land, and other's locally have had difficulty. However I've had a positive response to a pre-app so am confident if I conform to what is asked I should be ok. I've been advised that from a building perspective a simple rectangle is the cheapest way to go and fortunately the 'longhouse' style has become a local norm for new houses.

 

I've tended to see a septic tank as the default as that is what I have grown up with as the norm, with the only maintenance being emptying from time to time. 

 

In places certainly the underlaying substrate is bedrock and the guy who does the majority of digger work hereabouts said that he had come across similar elsewhere. Perversely, about 70m away my mum's standard onion tank was easily installed by digging out some boulders and rotten rock.

 

Unfortunately our source of rotten rock has run out so I will have to buy in any infill / hard standing that is needed.

 

@Thedreamer is your build ongoing then? Good luck if so. Sounds as though you are in the same boat with great views and strict budget.

 

@ProDave your words are very reassuring! I made a crude attempt to measure the surface-level difference in height from front to back of the footprint of the house and I concluded it could be in the region of 900mm, and then there is the difference in soil depth...

 

I have dug so many holes trying to figure out what can go where that the site is more hole than land now!

 

 

 

 

Edited February 20, 2019 by Grian

[Thedreamer]

30 minutes ago, Grian said:

Thanks for further input.

 

Under a previous planner my parents had a struggle getting permission just adjacent to me when they wanted to move from a tenanted house onto their own land, and other's locally have had difficulty. However I've had a positive response to a pre-app so am confident if I conform to what is asked I should be ok. I've been advised that from a building perspective a simple rectangle is the cheapest way to go and fortunately the 'longhouse' style has become a local norm for new houses.

 

I've tended to see a septic tank as the default as that is what I have grown up with as the norm, with the only maintenance being emptying from time to time. 

 

In places certainly the underlaying substrate is bedrock and the guy who does the majority of digger work hereabouts said that he had come across similar elsewhere. Perversely, about 70m away my mum's standard onion tank was easily installed by digging out some boulders and rotten rock.

 

Unfortunately our source of rotten rock has run out so I will have to buy in any infill / hard standing that is needed.

 

@Thedreamer is your build ongoing then? Good luck if so. Sounds as though you are in the same boat with great views and strict budget.

 

@ProDave your words are very reassuring! I made a crude attempt to measure the surface-level difference in height from front to back of the footprint of the house and I concluded it could be in the region of 900mm, and then there is the difference in soil depth...

 

I have dug so many holes trying to figure out what can go where that the site is more hole than land now!

 

 

 

 

 

Shame about the exhaustion of rotten rock on your site.

 

Unfortunately most folk ain't wanting their crofts dug up even if you offer a few £££.

 

We could have built in an easier location on the croft but if you want privacy, good views and shelter you got make that initial investment to turn the site from agricultural ground to a house site. 

 

The hardest part was getting the services in and forming a long access. The investment made at this stage was only ever going to add value, although it was disheartening when funds depleted as we did this work. 

 

Yes here is the blog. It's been a long old ten years of saving, waiting and just standing on the site dreaming if it would ever happen. 

 

 

[ProDave]

The sloping ground really does not seem an issue. In the simplest form you did the footings until you hit hard ground. sure the footings may be deeper at one end but it is easy to make them stepped and build up from there.

 

Choosing a septic tank will prevent you from connecting to a watercourse, you would have to accommodate a soakaway somewhere. It really would choose a treatment plant, there is very little extra cost and they are so much better in so many ways.