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Test Your Soils Before You Build

Soil Testing

If you have any worries that the soil on your building lot might present problems during construction, it is a good idea to have soil tests done. I’m not talking about digging test holes with a backhoe to look for boulders or other underground problems may. I usually do not recommend that because test holes like these only reveal what is in the test hole. A giant boulder might be lurking within inches of the test hole and you would never know it was there until you do the excavation for construction. There could easily be other obstacles beneath the surface you won’t find unless your test hole happened to hit them exactly. Digging test holes is a lot like playing the game of “Battleship.” Every shot is either a hit or a miss…and most of them are misses.

Granted, I have found some interesting things underground on construction sites. Once, we found a huge buried pile of sawdust. This was a building site in the woods. Apparently, that exact spot had been used as a temporary sawmill during logging operations. Of course, we had to dig out all of the sawdust and replace it with engineered fill before building the basement of the new house. On another site, we found about a dozen washing machines! They had been buried along with other junk.

You can dig test holes and hope you hit the problems. But usually it is a waste of money and may give you a false sense of security if you miss the problems. However, if you have reason to believe you may have overly wet or weak soils, test holes are a very good idea.

But what if your area is known for having bad soils, soils that are not strong enough to support your house or are overly wet and unstable? That’s when you should have your soil tested. I have a recent project where we discovered very wet soils. Some were so wet, the excavation machinery was getting stuck, even with tracks on the machines instead of tires. It was important to find out how deep the problem soils were and what we needed to do to correct the situation. You can’t build a house on soils that cannot support the weight of the house. If you try that, the house will slowly sink into the ground, usually unevenly, and you will get cracks in your foundation, unlevel floors, and many other problems.

To find out the extent of the problem, we hired a geotechnical engineer (soils engineer) to come and test the soils. Interestingly enough, he does not use a backhoe or other excavating equipment, he uses a steel probe rod and a hand auger. The probe rod is a steel rod with a pointed end and a handle. The engineer presses the rod into the ground until he finds resistance. His experience and knowledge let him know when he’s hit decent soil. He notes the depth of the good soil and determines how thick the layer of bad soil is that will have to be removed and replaced.

His hand auger is similar to his probe, except there is a digging tool at the end of the auger that’s about the size of a beer can. The engineer twists the auger as he pushes it into the ground. When he pulls the auger out of the ground, it brings a sample of the soil up with it. The engineer will examine the soil sample, press into it to see if it is sticky or loose, and even smell it. Most of the time, he can identify the soil type without taking it to the lab and he can determine its bearing capacity. Bearing capacity is how much weight the soil can support.

Soil tests like these cost only a few hundred dollars. That’s a small price to pay to be sure your house is sitting on soil that is strong enough to hold it.

Bill Hirsch

Bill Hirsch

4 Comments

  1. Donna on April 21, 2014 at 8:12 am

    We are thinking about building a house, but heard we have to make sure about soil, it will be next to a creek in the woods, maybe you can help

    • Bill Hirsch William Hirsch on April 24, 2014 at 9:22 am

      Donna – For a few hundred dollars you could have a soils engineer (a geotechnical engineer) come out and evaluate the soils on your site. He would use a hand auger and a probe to determine the soils type and the bearing capacity of the soil. The bearing capacity is a measure of how much weight the soils can support. If you have any worries about the soils, hiring a soils engineer is a worthwhile investment. One other word of caution. With a site next to a creek, you should check to see if you are in a wetlands zone or a flood plain. Both areas have significant restrictions on what you can build or even if you can build. If you have either of those conditions, you will get a first hand encounter with the long arm of government regulation. Good luck.

  2. Keena on December 16, 2014 at 10:27 am

    Regarding your comment about discovering sawdust under the ground. I am curious if you have heard of Osprey Links in Callander, Ontario. This ‘exclusive’ residential and golf course development was built on top of mounds of sawdust from a mill that operated right where these million dollar homes are built. Many of these homes…some new…are literally sinking into the sawdust below. Everyone in the small community in northern Ontario knows about it, and I am sure most wish they could sell their homes and others are blissfully unaware. I suppose no one has brought the issue to the media because the subdivision development corporation are ‘powerful’ people. I guess no one wants to de-value their property by raising the issue. I just feel so sad and frustrated because these people managed to build million $$ homes on hazardous waste. One engineer I spoke to called the area “the Love Canal of Canada”. This engineer was brought in to sample the soil on a property recently purchased. He discovered that there was 80 feet of sawdust below the property. The owner of this property went through financial ‘hell’ to get out of his purchase. So many of these homes have compromised basements because they were built on sawdust. I looked at the environmental assessment and NO WHERE is there any indication of sawdust noted. Everyone knows that Callander exists only because of the sawmills in the area. We all watch the municipal workers clear out the mounds of sawdust that is pushed up onto the public beaches each spring.

    HOW does a subdivision like this get built?

    and,

    WHY doesn’t anyone ever do anything?

    I love my community, but it sickens me that the rich and powerful can ‘house’ and develop on top of mounds of hazardous waste?

    • Bill Hirsch William Hirsch on December 27, 2014 at 9:13 am

      Keena – This is amazing. I hadn’t heard about this place. Ironically, I got your comment as I was going into a seminar on geotechnical engineering and soils testing. I read your comment to the group.

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