Basement Design and BS 8102

[Vinny7]

Hi all,

I have been asked to look at an existing semi-basement (retains max. 2.8m). This was built about 5yrs ago but has not yet been passed by the local authority. I have been asked to prove the basement structure as constructed is adequate. It was build using blockwork with an external land drain at base level, tanking applied to the wall externally and a drained cavity. It will be quite difficult to prove the wall is adequate and I am wondering if I have to also include a head of water behind the wall at 0.7m below ground level (3/4 retained Ht.) as per BS 8102 - 3.4B, has anyone any thoughts or comments?

 

[Vinny7]

Any thoughts?

 

[patswfc]

I think that you could get away with not including the water pressure if you have a granular layer behind the wall as well as a drain at the bottom, which should prevent a build up of water pressure behind the wall.

However i would never design (i know its not your design, your doing an assessment) a wall on this assumption as you need to be sure that the drainage system will be adequately maintained throughout the life of the structure otherwise water could build up creating additional pressures and possible instability.

As you say 8102 states that for basements upto 4m you have to design for basement depth*3/4 head of water pressure. However 8007 Cl 2.2.3.2 (regarding flotation) states that you may reduce water uplift forces by using a drainage system, thus i dont see why you could not apply the same principle for the lateral force on the walls (as stated already i wouldnt do this for a new design as i couldnt be sure the owner of the building would maintain the external drainage system).

Do you know at what level standing groundwater is located and the ground condtions?

If its groundwater is below the basement and the material is permeable then you have a few more reasons to justify ignoring the water pressures, if this is required to make the wall work.

Out of interest how thick is the wall. I think a 2.8m blockwork retaining wall would need to be thick to be able to resist the forces.
As you have said it is proving difficult to show the adequacy of the wall. Unless it is quite thick at the base it may still fail when ignoring the water pressure.

Good Luck

 

[Vinny7]

Hi PAtswfc,

In answer to your question the architect designed/drawn wall is 680mm thick at the base with an additional drained cavity and inner leaf( small vertical load on this). At the moment the wall is inadquate even taking acount of the loads over. I have also checked the wall as propped at each floor slab level but this may be hard to justify with the cavity and the wall is still failing.

Have you ever come across a similar situation or can you suggest suitable remedial work?

I have been looking at grouting the cavity (don't think this would be effective without rebar), providing picture frames between opposite walls to act as butteresses (bridges cavity and requires floor and wall to be broken back) and simply digging out behind and forming a lean mix gravity retaining wall behind (possibly taking account of the origional wall as part of this.

 

[valleyboy]

Hi Vinny

In what aspect is the wall failing - overall stability, ultimate design of the stem, or both?

Another problem you may have is the lateral earth pressures - technically a basement wall should be designed to resist at rest earth pressure rather than active.

Is is possible to excavate at the rear of the wall and replace either with Lytag or self-supporting lightweight fill to reduce lateral loads?

 

[Vinny7]

It is actually the wall stem that is failing, either by inner face tension or outer face when propped.

I think the gravity concrete retaining wall behind the origional may be the most practical and cheaper option.