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Question DetailsAsked on 7/5/2016

cement walkway is sinking next to garage. The walkway has been replaced twice. The sewer line has been camered ok

Garage floor was replaced once and side of garage floor that was removed had quite a void under it. At that time garage floor was replaced I don't know if sand or pea gravel filled the void. The city sewer line runs along the chain link fence at the back of my garage.(about 3 1/2 feet behind The chain link fence has also dropped at least a foot over 30 years. I have added 9 yards of soil to my yard because my yard looked concave. My yard did flood from the middle of garage back to the chain link fence with heavy rain. The grass between my garage sidewalk and my garden is always very green. I am having the cement walkway and dirt removed to garage footings to see if I can determine the problem. Who is the best person to help me?

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2 Answers


Could be sewer main or your sewer, or leaking water line or, if in sinkhole country a sinkhole - or if a pretty small area sinking (which does not sound like your case) unknown abandoned septic or fuel tank collapsing.

Personally, and especially if the depression in the yard/fence more of less followed along the sewer or water line, that would be my suspicion - and if you mean the sewer main (not just your branch to it) the entire thing might be city's problem to address and pay for the damages. Commonly the water main parallels the sewer line on the other side of the street (or one in middle, other on one side or the other) and a leaking water main (especially large city main) can wash out a lot of soil over time - look at the sinkholes videos on Youtube for examples of entire intersections or strerets collapsing and taking cars and the paving with them.

Have they run a camera (or done a walkthrough if large enough) to look for damage or signs of soil coming into the city line (assuming here when you said sewer line camerad OK you meant your line to the street) ? You might want to do this before the walkway and dirt removal, because they might pay to have that area fixed as part of their repair, if a line failure - especially since soil slumping or washouts or failed buried pipes are generally not covered by your homeowner's insurance (except in areas with sinkhole insurance), if a large enough bill to significantly exceed your deductible.

However, since you said the yard has slumped from the middle of garaqge back to the fence, sounds like it could be washout along your water or sewer line. I would get a free utility locate for all utilities and see if any line up with the depression.

Backyard flooding in the depression could be from the sewer backing up to the surface, or just accumulation of water because it is a low spot, so that would only be definitive if you saw water burbling to the surface.

In some soil types, and depending what type of backfill they put around the sewer or water line and whether it was properly compacted or not, you can get quite a washout from a broken pipe, especially in silty or fine sandy soils that wash out easily.

Guessing the middle of the garage is where the water and/or sewer line enters the house, and that your garden is near the fence, I am guessing the "concave" area of the yard where you put the 9 cy of dirt, if it aligned between garage and fence, lay over one or the other of your lines and you have a broken sewer line taking in dirt (and broken for quite a length apparently), or a washing out area around a line - commonly around the water line because sewer lines rarely have enough pressure to flow enough to wash out much unless the water (on larger lines) breaks through to the surface. Sewer lines tend generally (and yes there are exceptions) to form fairly small sinkholes unless the entire line is collapsing (like with old wood sewer lines), leaking water lines generally are what form long linear collapses as the water washes the dirt to a surface exposure or to a break in a sewer line where the water excapes, washing a lot of dirt with it.

Ditto for the green grass situation - likely aligned roughly over one line or the other - if moderately deep line (i.e. you get freezing ground at times) than the water line is a lot more likely candidate than the sewer line - generally sewer lines do not green up the grass unless leaking within 2-3 feet of the surface, whereas water lines (due to water pressure) can green up the grass 6 feet or more above the line.

Water lines - and significant sewer line washouts - you can commonly use a metal rod driven 6 inches or so into the ground along the marked line location (avoid the gas/electric/communications utilities you had marked) and a stethoscope held on the top to detect significant leaks. [you need to holdthe rod very lightly in gloves or have free-standing in the ground - grabbing it with your hand damps out the sound significantly]. Make sure nothing is running in the house (water flow or fans or pumps to make extraneous noise) and listen for water flow - and if your meter is on the far side of the depression check it for the meter moving with all faucets/toilets not running.

Water utility can also come (or you can use stethoscope if shallow) and listen at the curb box (water shutoff at street) to hear if water is running through the line to your house. Sewer harder to hear but if you run a lot of water in the house you can sometimes hear water gurgling much louder at a break than elsewhere along the line (which you might not hear if deeper rather than shallower).

If line locates and sewer/water line camera runs (city line and/or yours as applicable) do not show anything likely to correlate as a cause, then a Geotechnical (soils/ foundations/ groundwater expert) engineer would be the expert to call on to investigate, probably with a light drill to probe for where the wet zones and collapsing soil are, or sometimes with a small backhoe. Likely to start closer to $1000 all told then less and can run quite a bit more if the problem has to be "chased" a distance, so camera checks first to look for broken lines is usually cheaper as the first step. Not an Angies List category - google for Geotechnical or Soils or Civil Engineering firms in your area, then cross-check Angies List for any listing (likely under Architect or Structural Engineer if listed).

If the green area/depression does roughly align with sewer or water line, it may well be more economic to just dig it up and replace it (especially since it sounds like a fairly short run) than pay an engineer to locate the problem and then still have to dig at least part of it up to do a repair - especially if camera shows issues or you hear running water or the water pipe indicates a leak before the house.

Commonly, once you get a foot or so down in the green area you start seeing the source area as a wetter zone, so usually pretty easy to pin down the source and dig down to it. Note - if digging down to your water pipe check first where shutoff is, if there is one between you and the street - may be shallow enough to shut off yourself, or if deep frost area you may have to have the city utility come out and shut it off for you. Do NOT dig up a live waterline without the means to shut it off quickly if the flooding will impact houses or such.

Answered 4 years ago by LCD


I live in Michigan. What I know. The city sewer line runs horizontally four feet behind my garage. My home's sewer line runs parallel to my garage, 4 feet away. Their are no gas or water lines in my back yard. I had the sidewalk next to my gargage removed. I found sand and clay, no pea gravel. Tha clay is holding the water. Also, my 2 1/2 car garage was built in 1952 with a 18-24" rat wall only. In Michigan footings need to be 42" deep to stop heaving due to our frost line. The rat wall that is partially exposed looks like swiss cheese. The sidewalk on the other side of the garage has just started to sink as well. My plan is to have the gargage underpinned in sections and have a proper 42" footing dug and cemented. Part of the garage floor that is cracked will be saw cut out. If there is any voids under the garage floor, a cement slurry will be added to the under side of the garage floor.

Answered 4 years ago by MeowMom

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