Retaining Wall on Steep Slope

[jjeng2]

I have a 90' long retaining wall with a curved 90 degree bend(about 35 feet one dir, curved portion, 35' perpendicular dir). Both sides of the property slope away steeply. Basically we are taking the top of the slope and leveling it out to have a little patio extension. The wall will be about 5' high. About a foot out from the face of the wall, there is a steep(45 degrees and steeper) slope going about 30' down. The slope has been there a long time. It is filled with mature trees and bedrock outcroppings. I told the client we could design the wall but make no judgement on the stability of the slope. He would need to hire a geotech and spend a good amount of money for a slope stability analysis. Im working on the assumption that we will design the wall and it will not affect the stability of the slope below(i.e. if it was alwalys unstable, it will remain unstable, if it was alwalys stable, it will remain stable). This is where I need advice. The wall will have a 1 foot toe three feet deep. Can the sliding and bearing pressure from a 5' wall affect the overall slope stability? If the wall was to slide all the way out and "slice off" what was in front of it it would be about a 18" X 2 feet wide section of soil. Should I install tiebacks, a key? Anything special I need to consider?

 

[casimmons]

I would have the global stability analyzed. The slope may appear stable, but it is very steep and changing the configuration and additing some load to it with the retaining wall could change that. Also, be sure to consider the sloping ground below the wall with your bearing capacity and sliding calculations. What type of wall are you looking to design? I doubt you'd need tiebacks for a wall only 5 feet tall, but the slope is very steep. A geogrid reinforced segmental retaining wall would probably work.

 

[jjeng2]

Im going to use a poured concrete gravity wall. I dont see the bearing pressure being a problem as the location of the toe will have adequate soil behind it. Obviously im neglecting passive pressure on the front of the wall. I was a little worried about the sliding pressure causing instability and was thinking about using tiebacks to reduce that.

A landscaper came in and built a segmental block geogrid wall. During construction it failed during a rain storm. No problems with slope below though. This is the remediation of the problem.

 

[BigH]

jjeng2 - what general area is the project located? The fact that you will be putting in the footer at a depth of 3 ft, gives you a backset of at least 4 ft from the edge of the slope - the bearing would probably nor would I think that the "slope stability" would be greatly affected given the description of the materials and rock-outcroppings, etc. Geotechs do not always need to do "expensive" slope stability analyses - at least ones with years of experience in the area.
Given the above, I would contact a geotechnical type that you know in the area, discuss your problem and for a modest fee, he might be able to provide you the comfort level you need based on his experience and rules of thumbs for the area that are successful.

 

[jjeng2]

Project is in northern NJ. Im going to dig out some of my geptech books. As long as Im comfortable that the wall will not affect the stability, ill put enough disclaimers on it that ill be able to sleep at night.

 

[MSEMan]

There is no question that the imposition of the wall will decrease the global stability - you're adding load to the driving end of the slip circle and doing nothing to add to the restoring moment - simple mechanics.

A couple of pointers:

Don't use mass concrete - its too heavy and will make the stability problems worse. Use a modular block wall (allen block, rockwood, keystone, etc etc) or use railroad ties for facing with soil reinforcment. Considering that you are only going 5' high don't worry too much about going proprietary with the reinforcement you could use normal welded structural mesh (galvanised) if you wish.

Don't unnecessarily remove vegatation below the wall face - no doubt the roots are adding to the stability of the existing slope by creating a semi-coherent facing.

Examine the slope thoroughly especially at its steepest sections - this will give you a baseline for your assessment of likely soil characteristics.

Cut a few trial pits in the slope - this is relatively cheap and will tell you alot - I would do three minimum at the top and three at the middle and bottom of the slope.

Make sure you undertand where the water table is in relation to the toe of the slope.

Get a geotech mate to run a few sections for you on a slope stability program - probably no more than a day's work.

In sumary I think you are probably right - it will be OK. In 99% of cases adding a 5' wall to the top of a stable 30' slope won't destabilise it. However you could lose your license if I'm wrong so I'd cover at least the basics.

 

[pba]

jjeng,

"As long as Im comfortable that the wall will not affect the stability, ill put enough disclaimers on it that ill be able to sleep at night."

I'm sure you (absent mindedly) forgot to also state that you will ensure that the client is fully aware of the risk taken in proceeding without the necessary slope stability considerations.

It does sound as if you have this the wrong way round. Consideration of slope stability should give you parameters for use in the design of a suitable stable wall.

Just for the record - What is at the bottom of the slope? That is, if the patio slipped down the hill, who or what could it hit?

 

[jjeng2]

pba....the client is fully aware. I made it clear to him that we were approaching this as designing the wall only and the slope will remain in the condition it was in. I advised him it would be prudent to analyze the slope and it may be unstable but it also may have alwalys been so he needed to make that decision. There is nothing at the bottom of the slope. Just woods.

 

[GeoPaveTraffic]

jjeng2, You said "I made it clear to him that we were approaching this as designing the wall only and the slope will remain in the condition it was in."

This is not true, by constructing a wall and structure at the top of the slope you are in fact making the slope worse. If the slope is barley stable today, a condition that many natural slopes are in during periods of heavy rain fall or snow melt, it may fail the next time.

NO construction should take place on a slope without being properly analized. You can put all of the disclaimers in that you want, but it still want keep you out of court if the slope fails.

By the way, I assume you have discussed with the owner that the cost to repair and/or stabilize the slope if it starts moving would be many many times the cost of having it properly analized.

 

[pba]

I concur with GeoPaveTraffic...