Drainage consideration for retaining walls in sandy soil

[mrzift]

I am looking for some solutions to a retaining wall proposed along a boundary with a nearby existing structure on the retained side. There's no reported water table near the proposed 2m excavation but the area is subject to frequent flooding.
The soil is medium dense to dense sand.
The current option I am considering is a secant pile retaining wall which will be designed for hydrostatic pressure.
Also I intend to use near at rest conditions to minimise surface movement.

I've also given the option to use grout injection but this has not been well received.
Could you please give some tips or advice on this proposal.

 

[EireChch]

how far is the structure from your excavation?

I would think a secant piled wall would be very expensive for a 2m high wall. Would soldier piles or vibrationan less sheet pile wall not be a better option?

How exactly are you proposing to use grouting to create your 2m excavation?

Re drainage, you can add a weep holes to most wall types.

 

[mrzift]

The existing building is approximately ~200-300mm away.
For soldier piles how far would they need to be spaced considering the risk of damage if any soil caves in.

For grouting, I've seen builders use contractors who inject grout to stabilise sandy soil for short term excavation.

 

[PEinc]

This sounds more like a concrete pier underpinning project.

http://www.PeirceEngineering.com

 

[mrzift]

@PEinc
Understandable. Part of the structure does offset back 1m. I doubt we will have permission to enter the property at that part to underpin. What solution do you recommend in that case?

 

[PEinc]

Someone knowledgeable about underpinning and excavation support systems (but probably not a lawyer) should explain the situation and construction procedures to the owners of the adjacent building and the proposed building. An easement should be requested to allow work, either temporary or permanent, to be performed on the adjacent property. If granted, do the work as required to properly protect the adjacent property and workers. If not granted, you have two options: 1) move your new building farther away so as not to affect the existing building (rarely done) and 2) install an expensive, very stiff, conservatively designed, but possibly less protective, temporary, sheeting wall or permanent, secant pile wall - both of which would then need to be on your own side of the property line. Don't be afraid to offer some REASONABLE compensation in exchange for the easement. If installing a stiff sheeting system or secant pile wall, be aware that anything less than traditional concrete underpinning entails a greater level of damage risk. If underpinning is not allowed or if stiff sheeting is desired over underpinning due to cost, the owner of the new building must be responsible for any minor damages that could probably happen to the existing building. The owner's savings from not underpinning the building should be used to offset any required cosmetic or minor building damage repairs.

http://www.PeirceEngineering.com

 

[mrzift]

Thanks for the response.
Before you mentioned weep holes for a secant wall. What is a common spacing and diameter?

 

[dik]

Typically about 4' and about 4" dia with a rodent screen to prevent 'critters' from using them as the beginning of a burrow.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EireChch]

Although the retained height isnt massive, the closeness of the existing building makes this a serious job. There is potential for serious problems if not designed, managed and constructed correctly. I would strongly consider PEincs recommendations on underpinning.

 

[mrzift]

Thanks all for the input.