How to find the right type of foundation engineer?

[Great_scot_selfbuild]

A seemingly simple task. I know I need a structural engineer with a Civil / Geotechnical specialism, who has experience in designing gabion walls. Any recommendations on how to sift the internet to avoid wasting lots of time?

 

I'm in Farnborough, Hampshire.

 

Summary of the task: Driveway traverses a slope, but we can't dig into it because of tree roots. The design recommendation we've had is to use Gabion baskets to build up the lower side to create a level driveway surface (rather than lots of layers of 'Cellweb'. I've spoken to an experienced engineer familiar with gabion baskets and confirmed they have done this for other jobs (car parks etc...) so it is possible, but they're at the other end of the country and recommended finding a local engineer (who highlighted that the term 'structural engineer' covers very wide range and so I need someone with the right sub-specialism).

 

Thanks in advance...

 

Dave

[ToughButterCup]

Tanners - did a superb job for us. 

[Great_scot_selfbuild]

13 hours ago, ToughButterCup said:

Tanners - did a superb job for us. 

@ToughButterCup Can you please elaborate? Was it for a similar task (gabion / driveway)? Can you share any details?

Thanks

[Conor]

What are you retaining and how high?

[Gus Potter]

Gabions can look great. You can creat a structural butress in them if you want to design an Arctitectural seating area for example..

 

The quality of construction matters. cheep infill and badly placed = less perfomance.. time taken to lay the stone.. like a dry stone wall can increase the service life. You can use decorative stone on the outside.. grow plants on them and so on. Great habitat for wildlife too once they age a bit.

 

The structural design concept is a little different in that you expect them to move about quite a lot unlike say a concrete retaining wall which can't move quite as much.. well you hope not!. These things move which makes them flexible and forgiving and so long as you recognise that you are off to a good start.

 

The presence of the trees makes the design more fun!

 

I think some of the keys to this are:

 

1/ You need good ground information.. soil properties, water tables and hydraulic gradients etc.

 

2/ A good detailed topographical survey so you can see just how much you need to retain, the slope of the ground and the location and type of trees / vegitation.

 

3/ A good idea from you as the Client about how you want to finish the driveway and how often you anticipate maintaining it. The retained soil may move about so if you have a tarmac drive it may crack.

 

4/ Available drainage paths for draining the soil.. but then you need to watch what happens to the trees if you take their water source away or make them compete harder for water.

 

You can do a lot to help yourself here (as you are doing) by trying to get your head around the things we need to know and importantly why.. to enable a design that meets your expectations economically and also in terms of later maintenance. If you can gather some of this info then it will help you find the right SE for you.

Edited November 3 by Gus Potter

[Iceverge]

4 hours ago, Gus Potter said:

Great habitat for wildlife too once they age a bit.

 

What a great observation. 

 

I'm going to have to concoct some project that requires gabions now....

[Great_scot_selfbuild]

20 hours ago, Conor said:

What are you retaining and how high?

@Conor see OP - we're looking to create a level surface for the driveway which traverses a sloped area of ground. The gabion's would be topped with a cellweb layer and finishing topping, providing a free-draining driveway so the roots/area under the driveway still get water to them.

[Great_scot_selfbuild]

7 hours ago, Gus Potter said:

Gabions can look great. You can creat a structural butress in them if you want to design an Arctitectural seating area for example..

 

The quality of construction matters. cheep infill and badly placed = less perfomance.. time taken to lay the stone.. like a dry stone wall can increase the service life. You can use decorative stone on the outside.. grow plants on them and so on. Great habitat for wildlife too once they age a bit.

 

The structural design concept is a little different in that you expect them to move about quite a lot unlike say a concrete retaining wall which can't move quite as much.. well you hope not!. These things move which makes them flexible and forgiving and so long as you recognise that you are off to a good start.

 

The presence of the trees makes the design more fun!

 

I think some of the keys to this are:

 

1/ You need good ground information.. soil properties, water tables and hydraulic gradients etc.

 

2/ A good detailed topographical survey so you can see just how much you need to retain, the slope of the ground and the location and type of trees / vegitation.

 

3/ A good idea from you as the Client about how you want to finish the driveway and how often you anticipate maintaining it. The retained soil may move about so if you have a tarmac drive it may crack.

 

4/ Available drainage paths for draining the soil.. but then you need to watch what happens to the trees if you take their water source away or make them compete harder for water.

 

You can do a lot to help yourself here (as you are doing) by trying to get your head around the things we need to know and importantly why.. to enable a design that meets your expectations economically and also in terms of later maintenance. If you can gather some of this info then it will help you find the right SE for you.

@Gus Potter Thanks - it's good to know we're on the right track (we have just had borehole samples taken, SPTs conducted and soil analysis to take place over the next couple of weeks. We have a detailed topo map and the driveway will be topped with a fittleworth stone finish.

 

Can you advise how to find the right type of SE for designing gabions? (this is the bit I'm struggling with atm)

[Gus Potter]

On 04/11/2024 at 03:25, Great_scot_selfbuild said:

Can you advise how to find the right type of SE for designing gabions? (this is the bit I'm struggling with atm)

Hiya.

 

Hope this helps. Bit of theory first and a few general comments. Excuse the spelling and grammer please!

 

A gabion wall is fundamentally the same as what we would call a gravity retaining wall. Its weight does most of the work to stop it moving.

 

The difference is that gabion walls move about a lot more and often they are designed for a 20 -25 year life span rather than a 50 year life span which houses are designed for to meet mortgage requirements.. Now for all. If you truck about you'll see many more developments by big house builders where they have big high gaboin walls to retain gardens, the houses themselves are set back. Can you smell a rat?

 

The thing to take away from this is that gabions have a time and a place and much is to do with how much they move and their seviceable life.. before serious maintenance.

 

Ok how do we design a gabion wall..

 

1/ We need to know what it is holding back. @Great_scot_selfbuild it's your driveway. Now if its gravel then I would say.. how much do you want to spend.. do you mind if the gabions moves a bit and you need to level out the drive every 5 years? If you say I'm ok with that then I know I don't need to worry to much about movement (serviceability). I also need to know how long you expect it to last! The building regs can be a moot point here in terms of movement.. the regs aim to make sure it will still be safe!

 

To design a gabion wall we often apply the same principles as we would use for a concrete or brick gravity retaining wall.

 

Rule 1.. we need to make sure it does not tip over. Often walls have drainage behind but behind the drainage layer we also have a bit of sideways soil pressure. The drainage can avoid hydrostastic water pressure and having a "funny" effect on the soil.

 

Rule 2.. there needs to be enough of a key and friction load at the bottom of the gabions to stop them just sliding sideways. For all types of gravity retaining wall design we allow for a surchage load ( a load applied to the top of the ground) of roughly 1000kg per square meter on the drive way in this case.. thems the rules folks! Drianage or not this surchage load causes a sideways thrust on the wall.

 

Rule 3 / This is a biggy.. we need to watch put for a global failure (often called a slip circle) where the whole wall, driveway and soil above up the slope just takes off and moves down the slope. This can be a catastorphic failure and very dangerous. Some times to mitigate we need to drain the soil much further up the slope.. even then we are taking a risk. If you have challenging topography then I would need to know what is happening further up the hill. You may need to extend the scope of any site investigation.

 

Now most SE's should have a grasp of these basics. But its your vegitation, could be trees an local topography that needs nuancing. I see you are using a Fittleworth stone gabion fill and probably you SE will just design these gabions as having no intelocking effect.. so the design will be standard.

 

Theoretically the design can be realtively easy.. but for economic design.. that requires you to to identify what you expect and balance that with how much you want to spend on ground investigation.

 

To sum up.. I've given you a few tips. Have a chat with some SE's and say.. I've read this and that, here is what I would like to do and what do you suggest would be a good balance between soil investigation and practical common sense design? Once you can get a handle on what your expectations are in terms of long term perfomance of the gaboin wall then most SE's will be able to handle that kind of design.

 

What they will do is make sure is does not fail dangerously and if it moves about a bit, then you now know that .. and everyone is happy.

 

 

 

 

 

 

 

[ToughButterCup]

On 03/11/2024 at 23:57, Iceverge said:

...

I'm going to have to concoct some project that requires gabions now....

 

I've been continuously building for just long enough now for us ( well, HerInDoors) to want to change the gabions we installed 9 years ago. Annoying doesn't come close. 

 

The job of removing established gabion baskets was so much easier than I thought - shove two crowbars through the basket, digga, straps , snap shackles, normal shackles, a bit of Oomph - jobs a goodun.

 

4 gabions; several GCNs, a good few toads, a Jeremiah (Bullfrog), loads of snails eggs, lots (3?)  of frogs and a pissed off normal newt.

 

I should have remembered to classify our gabions as a hibernacula (newt refuge) for our  Ecological Mitigation Strategy. When almost constantly wet,  gabions are an excellent refuge for wildlife. 

Loads of  mosses in the gabions that remain. 

 

 

[Great_scot_selfbuild]

On 05/11/2024 at 23:11, Gus Potter said:

Hiya.

 

Hope this helps. Bit of theory first and a few general comments. Excuse the spelling and grammer please!

 

A gabion wall is fundamentally the same as what we would call a gravity retaining wall. Its weight does most of the work to stop it moving.

 

The difference is that gabion walls move about a lot more and often they are designed for a 20 -25 year life span rather than a 50 year life span which houses are designed for to meet mortgage requirements.. Now for all. If you truck about you'll see many more developments by big house builders where they have big high gaboin walls to retain gardens, the houses themselves are set back. Can you smell a rat?

 

The thing to take away from this is that gabions have a time and a place and much is to do with how much they move and their seviceable life.. before serious maintenance.

 

Ok how do we design a gabion wall..

 

1/ We need to know what it is holding back. @Great_scot_selfbuild it's your driveway. Now if its gravel then I would say.. how much do you want to spend.. do you mind if the gabions moves a bit and you need to level out the drive every 5 years? If you say I'm ok with that then I know I don't need to worry to much about movement (serviceability). I also need to know how long you expect it to last! The building regs can be a moot point here in terms of movement.. the regs aim to make sure it will still be safe!

 

To design a gabion wall we often apply the same principles as we would use for a concrete or brick gravity retaining wall.

 

Rule 1.. we need to make sure it does not tip over. Often walls have drainage behind but behind the drainage layer we also have a bit of sideways soil pressure. The drainage can avoid hydrostastic water pressure and having a "funny" effect on the soil.

 

Rule 2.. there needs to be enough of a key and friction load at the bottom of the gabions to stop them just sliding sideways. For all types of gravity retaining wall design we allow for a surchage load ( a load applied to the top of the ground) of roughly 1000kg per square meter on the drive way in this case.. thems the rules folks! Drianage or not this surchage load causes a sideways thrust on the wall.

 

Rule 3 / This is a biggy.. we need to watch put for a global failure (often called a slip circle) where the whole wall, driveway and soil above up the slope just takes off and moves down the slope. This can be a catastorphic failure and very dangerous. Some times to mitigate we need to drain the soil much further up the slope.. even then we are taking a risk. If you have challenging topography then I would need to know what is happening further up the hill. You may need to extend the scope of any site investigation.

 

Now most SE's should have a grasp of these basics. But its your vegitation, could be trees an local topography that needs nuancing. I see you are using a Fittleworth stone gabion fill and probably you SE will just design these gabions as having no intelocking effect.. so the design will be standard.

 

Theoretically the design can be realtively easy.. but for economic design.. that requires you to to identify what you expect and balance that with how much you want to spend on ground investigation.

 

To sum up.. I've given you a few tips. Have a chat with some SE's and say.. I've read this and that, here is what I would like to do and what do you suggest would be a good balance between soil investigation and practical common sense design? Once you can get a handle on what your expectations are in terms of long term perfomance of the gaboin wall then most SE's will be able to handle that kind of design.

 

What they will do is make sure is does not fail dangerously and if it moves about a bit, then you now know that .. and everyone is happy.

 

 

 

 

 

 

 

@Gus Potter fantastic response - really appreciate you taking the time to be comprehensive. Happily, I spoke to an SE this week familiar with designing gabions for just my sort of problem and even better his feedback was almost identical to your response above. We have a positive direction of travel!