House's backwall as retention wall 4

[ANgK]

We are brainstorming for the client project. The lot has limited buildable land with a big hill in the back. The client wants to explore the option of cutting down the back hill which resulting in 20-25 ft wall.

One option is building the house with the 20ft concrete back wall flush against the cut hill and also act as retaining wall. Is pour concrete wall or ICF wall suitable for this purpose? Is soil nailing also required to stabilize the slope in addition to the concrete wall?


The other option would be shotcrete/ soil nailing retaining wall and 5 feet space between the wall and the back of the house. The house will be smaller and the wall will be higher, about 25 ft

In your opinion, which option would be more feasible and economical?

 

[oldestguy]

Before you make any decisions you need to know in detail what that hill consists of. The strength properties, water conditions,etc are essential for evaluating the current condition as well as predicting future conditions. It may take considerably more than sampling the exposed surfaces. I'd suggest involving a geotech engineer and making use of soil exploration and soil laboratory testing services.

 

[EireChch]

Impossible for anyone to provide advice with out seeing any soil profile or topography of the slope

 

[JoelTXCive]

Agree that you need soil properties for a wall like that.

25ft is a pretty good sized cantilevered retaining wall. Walls can certainly be built that high, but there might be significant deflections at the top even if you design for in situ (ko) earth pressures versus active pressures (ka).

This is fine for a stand alone wall, but when there is a house attached to the wall, there might be issues with the movement.

 

[ANgK]

Thanks!

I don't have the detail report yet but the preliminary result show silty sand and sandstone/siltstone bedrock in the area for the house. The hill has 12 inches clay soil on surface but there is silty sand soil below, similar to the building site

The slope on the surface of the hill is about 25% grade.

 

[oldestguy]

Roughly where in USA or other country it so we can guess at geology as a start. Maybe some pointers can be offered. Any geology mapping available to you?

 

[ANgK]

High desert Southwest. Arid, 7-8 inches of rain per year.
I look up the geo map and this is the info

description OLDER ALLUVIAL DEPOSITS
unit type Lithostratigraphic Unit
lithology Unconsolidated, Coarse-detrital
geologic history Miocene to Quaternary

 

[oldestguy]

I'd want to see a typical SPT type test boring from a cut bench about 50-60 feet or so higher than the proposed wall footing run down to 10 feet or more below footing elevation, all dependent on slope. The steeper the slope the higher the start. Of course topography mapped also. The object would provide info for a slope stability evaluation. One boring may not be sufficient depending on what is found.

 

[hokie66]

I would want to keep the retaining wall and the house separate. If it were a more substantial structure than a house, it might be feasible to use the building structure in resisting the embankment pressure, but not a house...

 

[ANgK]

How much is the typical cost for a shotcrete/soil nailing retaining wall of 20 ft height and 100ft width?

 

[oldestguy]

Before you get into any details, such as nailing, there may be the need to hold back a significant volume of soil that needs major facilities. These may be as significant as a major concrete wall held there by grouted in tied backs extending beyond a possible slip plane, perhaps 80 feet in. A stability evaluation is certainly needed to determine any such approach. With a sloping(hill) surface up hill from a "wall" the potential horizontal pressures can be significantly higher than for a level surface there.

 

[ANgK]

Thanks!

Assuming no complications, how much is the typical cost for a shotcrete/soil nailing retaining wall of 20 ft height and 100ft width?

 

[oldestguy]

OG again.

I am throwing out a thought that may or may not be taken up. Let's say the site hill up hill from the excavation has some form of "glue" or "temporary" adhesion that once disturbed results in a weaker shear strength test result. Thus any common sampling and testing method is likely to not show that added advantage. A common situation with partly saturated sand with temporary cohesion gives the same impression until dried. However,at the subject site the "glue" may be reasonably permanent and might not be taken advantage of with SPT estimates , etc..

I have run a few in-place direct shear tests that generally conform to ASTM-D4554.
Here is a link showing the test.

http://file.yizimg.com/175706/2012051620512314.pdf

In my case I used a back-hoe with bucket resting on a support, as the resistance to push a calibrated hydraulic vertical jack against (proving ring). The horizontal push was supplied by a hand jacked hydraulic cylinder with calibrated pressure gauge to read the push being applied. This set-up has been used to get on-site shear characteristics for design of excavation bracing, waste product disposal pile stability, etc. It takes some time and field equipment, but I'd prefer it to any lab test.

One may have to run three or more separate set-ups since any "gluing" factor (if there is any) will be destroyed by each test run. For the sandstone I have experience with there is a form of glue holding the sand grains together, sometimes quite weak.

 

[EireChch]

25ft is a big wall. I dont think you can soil nail and shotcrete in a silty sand. How can you cut the face vertical or near vertical, install soil nails and then shotcrete without the soil collpsing.

You need a top down construction method i think. A secant bored pile wall could be an option as this would be waterproof in the permanent condition. Drill the bored piles to depth and then excavate.

If you get a borehole on the slope and topo of the hill i would post it as you would get better responses. Someone may even do a prelim design for you if they are feeling generous.

 

[ANgK]

Thank you so your help and answers. I really appreciate it.
The hill was already partially excavated. There is an existing borrow pit there for more than 20 years. We hope to widening the pit and put the house there. The back-hoe cannot get to the top of the pit to sample because of the topography.

Here is the topography map with the house(red) and retaining wall (green)

Here are the photos of the actual pit and the wall

Thank you again for your help!

 

[oldestguy]

The topog and photos help. However for scale, what is the size of the house rectangle? More constrast in the topog map drawing would help.

My first reaction is that there is some cementing of the sand to allow a steep cut to stand, but it is not totally stable. It appears that between el. 765 and 790 a possible earlier much higher cut condition was present and since then some failing of that cut has occurred. Not a good sign.. The sloughed piles give the impression that what you see is result of deterioration of the earlier steep cut slopes.

A geotech would be able to tell more of what is there, but it appears to be highly weathered sandstone. If that is the case,any steep slope should be considered as not permanent and some day will erode and maybe fail with large volumes moving. Simple slope erosion resistance may not be sufficient here. Thus an experienced geotech would be mandatory on the job if it was me. Access to the slopes above the old pit it must be obtained so that a drill rig can do the boring there. If needed use a tractor mounted rig.

The direct shear test I described might well also provide good info about what is there. It could be run in a test pit at an elevation midway between top of final cut and floor grade, off to the side. The object would be to test the layers that also are behind the cut slope now there.

 

[ANgK]

Sorry for not including the scale. The red rectangle is about 60×80 feet

 

[ANgK]

What would be the option for the Retaining wall?

 

[ANgK]

BTW, the next door neighbor was already built and the site has similar elevation.

 

[oldestguy]

Your neighbor cut back the slope to a stable condition with a few small walls at the base. If you have to keep a near vertical wall, look at some of these photos with tied back walls. Simple soil nailing is not suitable for what it appears you want. One slope in your photos is about 32 degrees, typical angle of repose for some sands. Cutting back to that slope would work.

https://images.search.yahoo.com/yhs...ages/affiliate/deep_tie_back.jpg&action=close