Building Setback from an Existing Retaining Wall

[00Z]

I have a client that would like to build a new garage 2 feet back from an existing 4 foot tall retaining wall.

Since I have no information on the existing wall other than that it is a "block wall with pins connecting the blocks together" I want to avoid placing an addition surcharge on the wall from the building.

Currently the footing is proposed to be 2 feet deep. I am concerned that this may still place a surcharge on the the block wall. Am I being to cautious. Would you design the footing to be 3 feet deep, 4 feet?

See the attached sketch, and thanks in advance for your input. A couple of other quick points, there is no soils report, the block wall has no footing, the garage is single story and it is a gable end wall so the load from the wall is only around 205 plf.

 

[slabongrade]

sketch helped; a 3' deep grade beam there would be better than 2 feet. If the CMU wall has no footing, I would still
worry a little being only 2 feet away from it. Good thing if there is no 8' or 9' high brick load on the grade beam.
IMO you are not being overly cautious; w/o a footing, the CMU wall could move pretty easily.

 

[RHTPE]

This must not be in a frost-prone area. I love how some engineers draw a slab-on-grade + monolithic grade beam with vertical sides - must be really stiff soil in the area to permit cutting an excavation with vertical sides.

You might want to consider what can happen if the exterior face of your grade beam is a formed surface - to what does the contractor brace his forms? A 3' deep grade beam cast against a one-sided form will exert about 675 lbs of horizontal force per lineal foot. They will likely over-excavate to allow room for the forms - what is left between the new and the old?

If it were me, I would construct the side along the existing segmental retaining wall as a typical frost wall - a foundation wall formed on both sides sitting on a footing. This way there will be no lateral forces exerted when the concrete is placed.

Ralph
Structures Consulting
Northeast USA

 

[fattdad]

put your footing at 4 ft.

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

Or use forms and form ties for the footing = no lateral pressure on the wall. The slab does not have to be mnonolithically poured with the footing.

Mike McCann
MMC Engineering

 

[RHTPE]

That's kinda what I was trying to say Mike. Frost wall or non-monolithic slab turn-down. It's just easier to form the vertical portion if there's a footing to set the forms on.


Ralph
Structures Consulting
Northeast USA

 

[concretemasonry]

00Z -

Since the existing wall is apparently a Segmental Retaining Wall (SRW), it would be prudent if is actually an engineered wall. That type of wall will actually have a small set back at every course as an indicator visually. The supposed height of 4' puts it into a "gray" world. Various codes put the critical height of a wall that must be engineered at over 3', 4' or 5'.

That type of wall is NEVER built on a concrete footing, even for 20' to 30' heights. This is because all research for design is based on that and the wall is a flexible structure with small joints between unit, there is always some sort of shear resistant mechanism incorporated into the structures. Normally, the shear key is protection molded into the masonry units, but the wall is allowed to flex with weather and moisture or frost. Many engineered walls do not have geo-grid because of engineering requirements because of the soil borings/types or the engineer doing the design.

In some cases horizontal geo-grid fabric is used to reinforce the soil block behind the wall. If it is engineered, it would be wise to determine if there is geo-grid in the area to be excavated. Most 4' walls do not use geo-grid, but the soils types could require it. Engineers that design these soil restraining structures are not unique and there also some engineers that wear both a belt and suspenders. Probe or excavate to determine if there is geo-grid in place.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[00Z]

Thank you all for the advice. I will definitely show the footing being formed to avoid lateral pressure. I'll have the contractor check for a geo-grid too.