Residential Foundation Distress Issues

[theCorkster]

I'm looking at working with a client that has foundation performance issues. The single-story wood-frame structure consists of continuous foundations with short stem walls that step down a natural slope on all four sides (northwest corner near original site grade, remaining corners lower with southeast corner lowest).

The foundation is cracked in several locations. There is a crack on each side of the foundations descending the slope near the middle of the wall section that are about 1/4 inch wide. The lower foundation also descends and has about five 1/4 inch wide cracks in the down slope section of the foundation. Cracks tend to be wider at the top of the stem wall. Two of the cracks on the lowest corner show about 1/4 inch lateral displacement as well as longitudinal displacement.

My initial impression is that the lower corner of the structure was constructed on either fill or loose alluvium causing that corner of the structure to settle causing the distress.

My concern is that this settlement has pulled the structure down slope (based on the width of longitudinal and lateral displacement of cracks) and has completely destroyed the continuity of the foundation.

I'm planning on having the foundations chipped at the cracks to expose and check reinforcement continuity (or lack of).

Any thoughts or comments?

 

[oldestguy]

I'd not do any fixing until there has been a few borings and possibly lab tests. No point in doing anything in the way of fixing without full knowledge of conditions. A lot also depends on the budget as to allowable on site investigations, etc. . As to getting help here, photos help us a lot. Any other info that may not seem relevant really may be relevant. Many a post here is is incomplete early on and we only find important factors well into the discussions here. Be as complete as you can with info for us. I see by your history here I may be preaching to the choir.

 

[charliealphabravo]

How old is the house and show some photographs of the condition if you can. Is there backfill behind the foundation wall or is it a crawlspace?

This sounds like it could be a global slope stability or perhaps a local footing settlement issue. Step footings can be more susceptible to settlement due to the obvious difficulty of maintaining soil density near the steps during construction. Steps should not exceed 24" vertical and footing base should not be sloped. Poor control of surface runoff is also a potential cause.

 

[CANPRO]

Corkster, please see sketch below - do the walls step down, footing step, or both? If the walls step, that is an automatic red flag for me. If the foundation wall steps down, then at best the wood floor is bracing the top of the wall at the highest portion of the wall. Everywhere else, you essentially have a cantilevered concrete retaining wall and almost 100% chance this residential foundation is not designed to act as such. In my area, this is standard practice, and clear violation of the National Building Code.

I can't quite picture the crack pattern you describe...are there any cracks where I have them drawn in my sketch?

 

[oldestguy]

In looking at your history of comments and questions on this forum showing significant experience, I am wondering there may be a thought that you know what to do here,but perhaps we might come up with something you have not thought of. In that light I've at times run into mystifying problems and the cause was a difficult to detrmine. In this case, I am asking a few questions that may or may not apply here. Water table variations and surface water entering unwanted places and causing erosion, etc. How about trees and their effects, such as taking water and clays shrink? Old sewers in the area collapsing partially. Problems with original concrete quality, as placed in cold weather. Any info on the reputation of the original builder? A do-it-yourself project?

 

[theCorkster]

Thanks all for looking at this.

Site geology/soils; The structure is located in weakly indurated sandstone/siltstone/pebble-gravel conglomerate (geologic description). The area has been historically eroded resulting in ridges and canyons. The weak rock is near surface, usually covered with relative thin soil colluvium (thicknesses of one to three feet depending on location on slope). The colluvium is typically soft/loose and porous and is not suitable for supporting structural loads without re-engineering to the standards for engineered fill. The underlying weather bedrock can support substantial loads (5 ksf) with negligible settlement.

The structure was built in 1978; public records regarding site development are non-existent. At that time (and today)structures were/are supported on foundations stepping down the slopes that are embedded into the weathered rock beneath the colluvium. Plans indicate foundation embedment of 12 inches. However, even today, some contractors/developer complain about embedding the foundations this deeply.

A sketch of the structure and elevation on the east side of the structure is attached.

I noted in the elevation section that "lateral / downslope vertical displacement of about 1/4 inch." The cracks are about 1/4 inch wide in the longitudinal direction, with two of the cracks exhibiting about 1/4" lateral, down slope displacement.

Given these conditions, I suspect that the foundation in the southeast corner settled. It is also possible that the structure also moved toward the south east, only resisted by the western foundation.

Given site constraints and the local geology we'll explore the soil/rock conditions with shallow, manually excavated excavations on the south and east sides. We'll also map and expose reinforcing steel at several of the cracks to check for continuity.

 

[oldestguy]

So far great. However link to sketch didn't open for me. Can the sketch be inserted here? Any info on when cracks were noticed? Any earth quakes in the history? Does the local rock show any changes in time possibly affected by exposure to air, water? How about concrete just shrinking due to excess W/C at placement.

 

[charliealphabravo]

For subsidence issues I usually do a relative floor elevation map of each level to give me an idea of what is going on and set a baseline for future movement. In the past I used a standard water level but we just got a high precision altimeter that I am pretty impressed with. It works on the water-level principal but is precise to a tenth of an inch on standard precision mode and 0.005" on high precision mode.