Minimizing heel for basement retaining wall

[CressD]

I am designing a basement with an entrance via exterior stairs that go down 8 feet. The bottom of the basement is within seasonal high groundwater mark, so would expect hydrostatic pressure. The side with the stairs is close to the property line, and a cantilevered retaining wall with a 4 foot heel would bring the edge of the heel within 1 foot of the property line.
Any suggestions on design alternatives that would reduce the length of the heel to minimize how far toward the property line we need to excavate? What is the absolute minimum heel length that would be needed?
Some context: No constraints on toe length; the stairs to the basement will be concrete; the landing area at the bottom of the stairs does not necessarily have to be open from above in case installing top support there might help.

 

[KootK]

1) Span the wall horizontally.

2) Cast an external girt / beam thing on the top of the wall. Consider the implications of this for the envelope and for any potential frost / clay heave however.

3) Design a retaining wall that is predominately toe at the expense of the heel. This is often possible, if less efficient than a conventional design. Keep an eye on the rebar detailing in the joint if you go this route as, below a certain amount of heel, this strategy can make the joint design more onerous.

4) Heel-less retaining walls can be constructed on piles in more extreme situations.

 

[GeorgeTheCivilEngineer]

This is from a UK perspective but I imagine the laws/limitations are similar. Think about constructability - if you can't batter back over the property line in order to build the basement wall then you will need some form of piled temporary works - especially if groundwater is a concern. That may make going for a piled solution directly for the permanent works a better overall choice.

 

[driftLimiter]

I have some experience with @GeorgeTheCivilEngineer's recommendation. The only problem with that I have seen is that beyond the property line may be part of the public right of way, or another person's property and we can't guarantee that the piling would remain permanent.

One that we did in our area was at least acknowledged by the adjacent property owner as being a permanent support.

 

[CressD]

@KootK thanks for these ideas.

For suggstions 1-2, the areaway is 20 feet in length and the stairs are 12 feet in length, how close would the spans/beams needs to be in order for us to be able to use a standard basement wall with normal footings rather than retaining wall? Practically, the last 10 feet of the stairs could not have beams above them -- would that make your solution a non-starter or can we rely on the support from the concrete steps?

Suggestion 3 about a predominantly toe solution would be great. Any idea what the minimum heel length might be in that case. Could I get it down to 1-2 feet?

@GeorgeTheCivilEngineer @driftLimiter I'd love to go with a piled solution. But, it's a tight urban area and the property line is next to the home of a neighbor who is not disposed to be friendly. I worry that the disruption from a piled solution might generate a lot of ill will.

 

[skeletron]

In terms of reducing lateral soil pressure, an option is using Geofoam or some other lightweight backfill behind the wall. Perhaps that is worth looking into?

 

[CressD]

@skeletron, you bring up an brilliant option -- thank you! Will look into it. One issue I can think of is that because this basement is well within seasonal high groundwater, we'd have to be mindful of the bouyancy of the geofoam when thinking about how much traditional fill might need to go on top.

 

[skeletron]

@CressD: Yeah. That was a concern of mine. I haven't had to deal with buoyant pressure and anticipate that would be the one drawback for reducing the heel or the weight on the heel, in general. Perhaps the backfill option could be combined with a thicker footing just for weight purposes. You'd have to poke around with what works on your site.