Clay soil, signs of erosion? 1

[sydshort]

Hello,
I am not a geotech, so forgive me if I use the wrong terms. I own a house constructed on fill material consisting of expansive clay soil with boulders ranging from very large to very small. The lot slopes from front to back. We have noticed soil cracking around the foundation on the back side, which increases every summer as the soil dries out. The foundation itself is also cracked in two places on the back side. There seems to be a water drainage problem in the winter months. Rain water accumulates in the front of the house, and on one side of the house at the bottom of a rail road tie retaining wall. I am also concerned about a cut made at the bottom of the slope for a narrow roadway that was not re-seeded. The cut has deteriorated due to construction traffic ie: running over, backing into. The soil is dry enough now that it crumbles to the touch. There is also cracking of the asphalt in the narrow roadway. Are any of these things individually or collectively signs of erosion or slope instability? If so, do you have any recommendations?

 

[emmgjld]

Everything you have described is normal for expansive (shrink/swell) clays which are undergoing wetting and drying cycles.

The clayey soils, when dry and 'cracked', are prone to erosion. A normal protection is to accomplish some landscaping which will minimize extreme soil moisture gain & loss. This must be accomplished with some thought. Your climate is a very big issue. What works in one area may not be advisable in another. A description of where you are and the general climate would be helpful. The 'tipsters' come from all over and will have a lot of advise.

The issue of slope stability requires a bit of specific information from you. The cracking tends to allow water entry, oftentimes quite deep. Fissuring and subsequent water entry to over 40 feet is not unusual in arid regions. This can be very, very bad. Water ponding and longterm infilltration is a concern for slope stability and also deleterious effects on your foundation. Inappropriate cuts and lack of slope control can be be a little or a big problem. Please describe the slope in a manner so we could sketch a cross-section. Include whatever you know or suspect about the groundwater.

 

[GeoPaveTraffic]

Your best bet would be to get an experienced geotechnical engineer from your local area, one not involved with the original construction or the road project, to look at your property. Much of what you describe could be either no problem at all or a very large problem depending on many things that are very difficult if not impossible to determine from a post.

That said, a few thing to look at
1. You indicate that the back foundation wall is cracked; describe the cracks - how they run, width of the crack at the top, middle and bottom.
2. If you have a deck with posts, are the posts verical or do the tilt down hill at the bottom? If they tilt, measure the amount of tilt on each post with a 4 foot level.
3. How long since you first noticed the soil and foundation cracks? What was the weather like for 6 months before? When did the road project start?
4. Do you know how much fill was placed on the back part of your lot? Under you house?
5. What part of the country are you in?

 

[sydshort]

Hello again,
Thanks for the replies. Is there a way of posting pictures, sketches, house/plat plans? You know, a picture says a thousand words...I could have spared a few words in this post! I live in Estacada, Oregon (SE of Portland)in the foothills of the Cascade Range. The weather here can be quite extreme at times. Heavy winter rains can be continuous for days at a time. We often experience thunder storms producing hale and heavy downpours. A week ago we experienced 100+ degree weather. Today it is sprinkling and forecast to be in the low 70's. Yesterday it was sunny in the mid 80's. The cracks in the foundation are 1/8-1/4" wide, run vertical, and seem to widest at the top of the foundation. One runs along a foundation vent. The other is between two vents. The foundation is 4' deep, but only 2'is visible. Both cracks are within 15' of the back (downhill) corners of the foundation, on each side wall, not the back wall itself. The foundation was placed on 9/18/01. We noticed the first (widest crack) the following spring of 2002, which was when we first started noticing the cracks in the soil. The road below the back slope was crude and overgrown with weeds, at the time our home was constructed. Within the last year and a half construction began on homes on the other side of the road below us. The road was widened (cut in areas along the bottom of the slope) graded and graveled to provide access to the newer homes. Pavement was placed after the storm system was installed, I am guessing within the last 7-8 months. Our foundation drain is connected into the storm system in the road below. According to the neighbor across the street, whose home was built a couple of years before ours, the majority of our lot is fill material. In 2004, the deck on the back side was constructed. Deck posts are vertical and placed in sonotubes. One post in the middle of the structure leans slightly downhill. The others appear to remain vertical. I will do my best to describe how the property slopes. For reference purposes, the front of the home faces the south and the rear faces north. The neighborhood is stair stepped. The home across the street from us (south) is at the highest top step, so to speak. This home was constructed on native soil and is near the highest elevation of the slope, as the driveway slopes downhill from the house toward the street. The street runs east to west and slopes downward to the east. Our southeast and northeast corner property lines are lower in elevation, than the southwest and northwest corner property lines. Our driveway slopes downward from the street toward our home. If the street in front of our house would be considered the 2nd step, then our lot would be the 3rd step. The road below us on the back (north) side would be the 4th step. There is an 3' tall, L-shaped railroad tie retaining wall that wraps around the southwest corner of the home. The shortest part of the "L", on the south wall of the home, is a little over 8' from the foundation. The longer part of the "L" is +/-15' from the west foundation wall, and runs toward the north, ending +/-12' from the edge or top of the back slope. The railroad tie wall was constructed in the spring of 2002. There is a
4 1/2" plastic or pvc, perforated drain pipe installed at the bottom of the retaining wall, which exits at the northwest edge of the slope, longest end of the "L". We doubt this drain was installed properly as, we have never noted water running out of the end, even during heavy rain or when standing water was present. The ponding appears between the bottom of the retaining wall and the foundation. The property on the west side of ours is higher in elevation, hence the point of the railroad tie wall. I am guessing the non-functioning drain may play a role in the drainage problem. There are many plants and trees in the front, south yard and on the west, above the retaining wall. There are also plants along the foundation on the south and west sides, as well as the northern corner areas, not covered by the deck. There are pavers with low growing plants inbetween (stepables)in the area from the retaining wall to the plants along the foundation on the south/southwest corner. On the west side their was sod laid between the retaining wall and the foundation plants. The sod area has not been healthy since the year it was laid, and has been invaded by clover and dandelions. The sod wraps around the northwest corner of the home, and toward the northeast corner, at the top of the slope, even under the deck. The back northern slope had been hydro-seeded well before the home was built (pre-2001). Black erosion fabric was placed when the foundation was excavated, and buried when the backfill occured around the back foundation. Some of the fabric is still visible below the top of the slope at the northwest corner, and could not be removed by pulling...we tried. The foundation drain exits on the northeast corner, and was connected to the storm system in the road below within the last year. Until then, it projected out approximately mid point on the slope. Originally the northeast foundation corner was about 11' from the top edge of the slope. While connecting the foundation drain to the storm system (within the last year), a 2' wide trench was excavated from the top to bottom of the slope. After backfilling the trench, the northeast foundation corner is now 7' from the edge of the slope. We have also noted soil deterioration at the the top of the slope, where the trench was excavated and backfilled, but not re-seeded. The northwest foundation corner is +/-12 to 13'from the top edge of the slope. The deck posts installed on the slope are 16' from the foundation, and the deck runs 30'along the foundation, from east to west. I am not sure if this means anything, but a copy of a letter written by the engineering firm to the City before the foundation was placed, states the rear (northern) slope, which falls below these lots was stabilized at an approximate declivity of 1.5 to 2.0. horizontal to one vertical. The cut at the bottom of the slope is below our property line on the northwest corner. At the highest point it is +/-3 to 4'tall, and deterioration is nearing our property line. It is our understanding that this cut area is property of the homeowners who live in the house below ours. The Developer originally told us that he was going to put a small retaining wall below the slope. Then he said it would be more like a curb. The City tells us that the developer recently stated he plans on not doing either. We have considered installing 3-4' high block walls in a stair step pattern along the slope on our property. But do not want to invest the time/money to do this if the cut area will continue to erode at the bottom of the slope and undermine the stability of the majority of the slope, which falls on our property. I have considered hiring a geotechnical engineer, since I do not have much faith in the original engineering firm that is paid by the developer. I will save that story for another post. Let me know if you need more info or clarification. Thanks again for your replies.

 

[GeoPaveTraffic]

Thanks for the information, you paint a good picture of the situation. One thing I did not ask for was the total height of the slope; say from the road below to the back of your foundation, then to the front of your foundation then to the street in front of you house.

However, even with that it is going to be very difficult to get real advice on your potential problem here. You need to find a local geotechnical engineer that you feel comfortable with. Maybe one of the others who monitors this area can provide a recommendation or you could check with ASFE (American Society of Foundation(?) Engineers)for a list of local members.

As I recall, local conditions there can be quite variable and without knowing a fair amount about the soils it really isn't possible to give much information.

Based on the cracks you described, I believe it is likely you have some differential settement of the back wall where the fill is thickest and not a slope stability problem. At a minimum you should buy a couple of crack gauges and install them on the cracks to see if and/or how they are moving with time. You should be able to find some acrilic gauges that read to 1 mm on line for less than $20 each. Then just read them on a consistent and regular bais. At a minimum this may give a little piece of mind while you look for an engineer.

 

[sydshort]

GeoPaveTraffic,
The height from the road below our home to the top of our back foundation is +/-15 to 20'. In the front, height is +/-3 to 4' from the top of the southwest foundation corner corner to the street. The top of the southeast foundation corner is actually lower than street level, less than 1'. Regarding the crack gauges, are those to be installed on the foundation or in the soil? How often should they be read? every week? every month? By the way, I was reminded about the driveway for the neighbor on the upper west side of our home. His driveway also slopes toward his house, and is located at the southwest corner property line. I believe there may be run off from his driveway to our property as well. So if you are trying to find the highest point of the street for water drainage purposes, I would add another foot or two to the front southwest corner foundation to street height. I will check the into finding a local geotech that is on the ASFE list. Thanks.

 

[sydshort]

GeoPaveTraffic,
Correction...I meant to say that the top of the southeast foundation corner is HIGHER, not lower than street level by less than 1'. Please excuse my goof!

 

[GeoPaveTraffic]

The crack gauges should be placed on the foundation wall. Each gauge is actually two pieces, one is mounted on each side of the crack. I would suggest reading them weekly for the first month or so. If there is little or no movement then change to monthly or every other week readings.