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Re: Stepped Retaining Walls into Mountain Side

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gerard
I am going to take a stab at this knowing I might not have all the details. 
>I amwondering if the native soils are removed anyway due to the cuts for the
>residence down to and into the bedrock, that the earth pressures with be
>present at all. This is rock essentially being retained. 
Even in bedrock, whenever you place a back fill behind a wall you will get
a active pressure on the wall.  In addition, the top soil strata need to be
confined otherwise you will have sliding on the hill side.  Additionally,
most soil's engineers and perhaps engineering geologist will insist on a 5
feet of day light distance form the face of slope to regard any benifit
from passive soil pressue.  Further, you need to find out if the rock
condition has any anamolies like fractured bedrock and dipping planes to
see if the bedrock is favorable in your case or actually hurt your design.
I have had instances where fractured bedrock required an active pressure of
70  pcf smiply due to the fact when the fractured planes get wet the
slippage plane became defined as rock slippage condition. 

One condition you might want to investigate is using buttressed walls, but
you still have to achieve a 1.5 factor of safety against sliding.

Soil nail walls, or anchored slope facing, is a good practice if the
building is away from the wall and you are not surcharging building
loads,vertical and lateral to the slped wall system.  furhter , you need to
consider the unbonded zone in the tie backs or anchor system, which mean on
most cases, longer than anticipated depth for anchors and thus a drilling
and equipment issue on hilside.
Grade beams and caisson system is what I am most familiar with on hillside.
 the system is expensive and the relative depth of caisson and their
ability to attract loads due to their embedment length is certainly a
consideration in design.  Essentially, a down hill moment frame action with
seismic detailing.

One more thought, you need to really look at the drainage issue and
waterproofing behind wall elements abutting a living space due to moisture
and water intrusion problems.  I am sure you are aware of that.  I am not
aware of a good reference other than foundation textbooks.  In most cases
this information must be drawn from site investigation and geotechnical
engineer's recommendations.

HtH.
Samir Ghosn, PE
Harris & Associates
At 02:33 PM 1/8/2003 -0800, you wrote:
>      I have been asked to come up with a better design for a new residence
>cut into a hillside/mountain. The foundation is the focus of the review and
>my recommendations.  In summary, the house is 4 levels, where each level is
>sort of split in half (i.e. one floor has a half flight of steps up to the
>next floor level) …. Probably a little confusing. Basically a tiered
>cut into the mountain.  Anyway, the original engineer used cantilever
>retaining walls at the hillside and they range in height from 5 ft to 14 ft
>for the stem. The site is underlain in bedrock and the soils engineer
>called for drilled piers and grade beams for the foundation. The engineer
>did this where the house foundation sits on top of the soil, but used
>traditional cantilevered retaining walls on spread footings where it is cut
>into the mountain side.  In reviewing the calculations, the engineer seems
>to have designed these walls considering them as independent walls with the
>soil pressures from the soils report. However, it does not appear that he
>has accounted for the upper walls surcharging the stem of the lower walls
>as the upper wall bear on the retained soil of the lower walls.  The
>contracting is barking about his design because he is required to provide a
>5 foot deep continuous key at the bottom of the wall to resist sliding.
>Since this is in bedrock, understandably he is questioning this.  I am
>wondering if the native soils are removed anyway due to the cuts for the
>residence down to and into the bedrock, that the earth pressures with be
>present at all. This is rock essentially being retained. I am also
>wondering if it is a good idea to just provide a concrete wall inclined
>with the cut of the hillside and use rock anchors to basically form off the
>hillside. The house was designed with a crawlspace achieve by using a
>ledger at the retaining walls and sitting on top of the grade beams at the
>toe of the house.  If anyone could chime in with some suggestions and
>perhaps a good resource on hillside foundations I would appreciate it. 
>Thanks,  -gerard  Santa Clara, CA       


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