Site Investigation

[Contraflexure74]

Hi,

I'm sure Engineers often ask this question. What is the best site investigation method to determine the actual bearing capacity of a soil for small 2 and 3 storey domestic projects? Any tricks of the trade?

 

[thaidavid40]

Depends on the situation. Do you know anything at all about the soils on that particular site? Is there other construction nearby to gain insight from? If a shallow investigation is appropriate, then hand-dug, or backhoe-dug, test pits can be helpful. And you can make steel probings to at least three feet below the bottom of those excavations. Where mobilization of a drill rig is not feasible or affordable, then a portable, handheld auger can provide information that could be helpful. All of these are pretty low budget. However, they probably won't completely substitute for a good old-fashioned drill rig providing you with samples down to deeper strata.
Dave

Thaidavid

 

[oldestguy]

The answer will be different from each geotech, since so many variables are involved, one being money available and balancing the cost of the investigation against savings that may come about because of the investigation AND analysis. I have commonly answered the question of "What number of borings do we need?". My answer is "At least one". As time goes on and more and more firms or individuals are in the picture, cost competition takes over. The more common technique for machine borings is the drilling of a test hole and driving a sampler, called the "Standard Penetration test", SPT. The firm I once was with used a 30" long sampler "spoon". and we were able to drive that in 2.5 ft. depth intervals to get a near full sampling with depth. However, with shorter "spoons" and a common depth of sampling interval sometimes as much as 5 feet, some soil depth segments are by passed. So, sometimes that lesser cost sampling method can result in unknown problems later. Chances are that the bearing pressure recommended might be shaded more conservatively under these circumstances. I suppose that is the same with any business when stiff competition is resent. You get what you pay for. So, my advice is going as cheap as possible has risks and generally increased job costs may result. My experience has been that once you save some one a major cost that might otherwise be recommended due to unknowns, they become steady clients and never question the cost thereafter. So what is the "best" way, I'd say go with experience and word of mouth reputation.

 

[Contraflexure74]

Hi guys,
Thanks for that. I've seen some experienced Structural engineers dig down circa 3ft, then get a heavy hand held bar and drive it into the ground. If they get good resistance then thumbs up, ground is good. Is there merit in this method?

 

[dhengr]

JohnFitzgerald74:
Sure, there is some merit to that method, just not much consistency, repeatability or ability to put some sort of a numbers on the results. That is, soil type, bearing strength, settlement potential, etc. are still basically unknown. You know the lower soil is kinda stiff and strong or not so much, but that’s about it. And, having done this method often enough the engineer might develop some feel for assumed bearing values which work, most of the time. He has removed the top couple feet of soil, any organic material and loose soil, which you never want under your found. in any case. Then, he has done some measure of penetration resistance of the soil which is an indication of soil strength and compaction, etc. But, he knows nothing of the bearing soil makeup, constituents, moisture, water level, etc., some of which can cause found. problems. If he would use the same .5" or .75" dia. rod every time, and drop a 20lb. weight 24" ever time, and count the number of blows per inch of penetration; then relate that to an assumed bearing pressure or successful found. design and performance, he could develop some log or history of bearing values vs. blow count. This is the same thing oldestguy can do with his split spoon sampling, except he gets actual soil samples in his process, which he can study and classify, and save for future testing. With years of experience and using the same equipment every time the GeoTech guys get very reliable results and soil design values. As thaidavid40 suggested you can, also/instead, dig a 6-8' deep pits at locations on the site and be able to study the virgin soil strata. With some experience some engineers become pretty proficient at making good judgements about the soil conditions and bearing cap’y. of the soil for small bldgs.