Designing Retaining Walls without Geotech Reports 1

[CANeng11]

As per my title, I'm wondering if any of you would design retaining walls without geotech reports? I find often with residential projects, no geotech exists and the client does not want to pay for one. Would you say no to doing the design, or if you would design it, what would you deem as conservative and reasonable assumptions for the design parameters? If you backfill with a granular material and install a weeping tile at the bottom of the wall, can you eliminate any hydrostatic pressure and just assume a saturated soil?

 

[JAE]

We occasionally do small (<8 ft. high) retaining walls without geotech reports - usually assume 1,500 psf soil bearing and 50 pcf active lateral and either do perforated drains to daylight or a series of weep holes (2" dia. PVC pipes) through the walls at 4 to 6 feet o.c.

For small residential - the contractors usually are surprised that they can't simply trench a "footer" about 18 inches wide and build off that....and they complain.

 

[JoelTXCive]

We do smaller ones without geotech too. Since the client doesn't want to pay for a geotech, they usually do not want to pay for a design either; so we just provide the TxDOT standard walls for them. Or we just point them to the TxDOT website and let them worry about.

Does your state highway department have stock wall plans? If so, I would use them because they are empirically proven to work.

 

[CANeng11]

I'm actually in Canada. Can you point me to an example DOT design? Do they provide their design assumptions?

 

[JoelTXCive]

Yes, they have some basic design assumptions on their sheets.

Link

http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/standard/bridge-e.htm#RETAININGWALLS

Download this file from the TxDOT website: "RW 1(L)C Low Ftg Pres, Design C Ret Walls" That is the low footing pressure retaining wall WITH a surcharge load applied.

Additional wall details are in this file: "RW 2 Ret Wall Miscellaneous Details" There are some assumptions shown in the general notes.

 

[Tomfh]

For residential and small commercial we assume the soil is bad. If we know the type of material or have geotechnical advice we use that.

 

[msquared48]

I agree with JAE, but would add 250 psf/ft passive and sliding coefficient no greater than 0.3

Mike McCann, PE, SE (WA, HI)

 

[kmart30]

Yes for smaller walls we use conservative soil values and note that its the contractor responsibility to achieve minimum compaction requirements per lift and for supporting soils. Its mostly sand around here and if they get into anything nasty they are going to have a hard time compacting and getting a density to pass. If they get good compaction then conservative soil values should be suffice. Obviously typical deleterious soil notes(muck clays, peat, trash, ect.) should also be included in your design notes but no one ever reads those anyways....Check out your states department of transportation soils and foundation manual for typical design values for different soil types.