Excavation at the Toe of a Natural Slope 3

[Silty]

Hi All,
My client wants to build a single house with walk-out basement at the toe of a slope. The house will be half on the sloping ground near the toe and half on flat zone at the toe. There is no water course and so on nearby.
The slope is a little bit steeper than 3H:1V, based on topo and my rough measurements. I visually assessed the slope and no sign of instability found. The slope is almost 7 m high.
I know that the excavation at the toe is unsafe and can trigger landslides,
but is there any way that he can build on that land?
He can not move the whole house to the flat area, there is no room for that.
Originally I thought maybe he can build on helical piles without excavation. Frost depth is 1.2 m in the area.
I appreciate your thoughts.
Thanks a lot

 

[oldestguy]

As a geotech engineer of course you know the "stuff" making up that slope and roughly at least know what pressures to design for. I look at how things function. Is there going to be a problem with rain or snow on the hill up from the house? Is this a cheap lot, but then will the extra costs for holding that ground, etc. make it not worth while? Your "frost depth" may or may not apply there depending on the frost susceptibility of that ground. We might bring up points useful or maybe not. I'd make up a list of "benefits" and another of "not so hots".

 

[msquared48]

This is just a retaining wall with a sloping backfill.

It's done all the time - with higher lateral pressures for the wall to resist.

Mike McCann, PE, SE (WA)

 

[EireChch]

msquared48 - agree. least amount of risk, Contig piled wall if no water. Depending on retained height, anchored.

 

[Silty]

Thank you for sharing your thoughts, when comparing to retaining walls, lets consider stages of construction:
During construction in case of retaining wall we first build the retaining wall then put the sloping backfill
in this case, we cut at the toe of sloping ground first. This may trigger landslides due to releasing/changing stress condition. If the natural slope is in marginally stable condition, the slope may fail during excavation phase.
Please advise.

 

[oldestguy]

Depending on what the soil is, a steep slope may be stable. It all depends on the value of cohesion, not ncessarily the friction angle. Protect for erosion and you may find no wall is needed. I've seen these stand "permanently" if erosion is cared for.

 

[EireChch]

You would install the contiguous wall then excavate. Stability should be fine then.

But you should check global stability too

 

[Silty]

Thank you oldestguy and ErieChch, great comments. I think I have clear view now ad can proceed.

 

[MTNClimber]

Houses are built on slopes all of the time. A top-down constructed wall is the safest but some residential contractors will risk it and go for a straight cut and get away with it. It just depends on the subsurface conditions.

 

[Mad Mike]

I hate to be a naysayer, but I'd like to add a couple of geotechnical comments;

1. The toe of a slope steeper than 3H:1V is by necessity a seepage zone, whether a drainage course exists at surface or not.
2. If this is a natural slope and has been stable over geological time, it is very likely that you have relatively competent rock in there- there may be serious limitations to contiguous piles, which may not be able to penetrate the rock to your final grade or if so, may need to be tied back.
3. If there is not competent rock in the slope, the global stability of the bank should be assessed by a competent person familiar with the local conditions.

I note these points just 2 weeks after watching a small housing complex in my area, approximately 20 units, be evacuated due to an incredible landslide. The houses were all sited at the toe of a ~40' high natural slope, which was cut at 2H:1V to accommodate the houses. It all looked fine and dandy, and stood stable for at least a decade until it failed. The relatively minor toe cut brought down the entire slope, the geology of which comprised a strongly cohesive diabase saprolite.

Many of these toe cuts are not inherently problematic, but the fact that you're a "Geotechnical" professional and haven't made a single mention of the geotechnical conditions, may very well be problematic.

All the best,
Mike

 

[Silty]

Thanks Mad Mike, great post. I will open another tread about increase in frequency of landslides in our area, please share your thoughts there as well. Thanks!