7 metre high King Post Retaining wall

[Nialloc]

I have been asked to check a proposal for a 7 metre high retaining wall in a gravelly clay. It is proposed to use a king post type wall with timber sleepers forming the lagging between the steel posts. The posts are proposed to be 7m above ground with an embedment of 3m. Without doing any calculations on the proposal, intuitively it would seem as if the embedment is too shallow and the posts would have to be very big steel sections. The timber sleepers would appear to be overloaded also. Overall I feel the concept is incorrect and may need revisions. All comments welcome and suggested sources of info!!

 

[PEinc]

7 meters is not too high for a soldier beam and lagging wall - as long as the wall is either braced or tiedback with ground anchors. Generally, the soldier beams (your posts) are spaced at about 2.4 meters on center. A wall 7 meters high usually has a single tier of bracing or tieback anchors unless it is supporting a large surcharge pressure. 3 meters of soldier beam embedment below grade in front of the wall is usually more than enough. The timber lagging boards (your sleepers) are usually 75 to 100 mm thick. It is generally not a good idea to support critical structures with soldier beam and lagging walls. They are too flexible and can allow structure settlements. Usually, soldier beam and timber lagging walls are used for temporary walls.

 

[MSEMan]

This is not an unusual form of construction anymore. I agree with PEinc that I would expect to see a row of soil anchors 3-4m from FGL. If the wall is intended to be permanent, I would also expect to see a permanent structural facing such as precast or CIP concrete.

In order to check the structure properly you will need to have detailed knowledge of the construction sequence so you can identify the critical cases to check.

Good luck.

 

[Ginger]

Agree with the two postings above. Using timber laggings as a facing will help to keep pore water pressures down but may not look too good with time. You may also need to address the design life of the timber members.

The king posts could be cast into larger diameter bored holes to help increase passive resistance at the toe, but you will definitely require tie-back anchors.

If in the UK a good design source is CIRIA report SP95