Tag Archives: site investigation

Geotechnical investigation data, always not enough?

 

IAEG XII congress
IAEG XII congress

This is a very controversial topic in which a straightforward answer is not possible. In this entry I would like to tackle some issues related to our own profession since we are responsiblefor the “acceptable” amount of investigation.

Recently I attended the IAEG 2014 (Engineering Geology) conference in Torino. In this conference numerous interesting topics of engineering geology and geotechnical investigation were covered. It was very interesting to note that in many cases a general conclusion was that not enough geotechnical investigation was executed prior to a geotechnical related failures.DSC_0527 geotechnical investigation

 

In conversations regarding the site investigation of a project it is very common to hear that “I would like additional investigation but the Client will not provide the funding” or that “the project finance does not allow for more or additional investigation, you have to do with what you have” etc. What do we do in such situations? We do what we have been taught as engineers to do, we overcome the problem. This means that either we accept a larger portion of liability, we either allocate the liability with statements like “additional investigation is warranted during construction” or we design very conservatively or all of the above. In any case, the design is based on limited information and it could go either way.

In many situations, due to the experience of the geotechnical designer or due to very conservative design assumptions no problems are manifested during construction or operation. But sometimes things go terribly wrong and somebody needs to take the blame, leading to long lasting litigations.

Is something wrong with the current practice? Everybody admires the great engineering attitude when nothing goes wrong and with limited investigation the project is completed. Even more, some of us proudly state “I saved so much by reducing the geotechnical investigation” but all this immediately changes when something goes wrong.

Maybe we should start thinking more as doctors? I don’t think anybody has gone with a medical situation and stated to the doctor that “I think you are asking too much medical testing” or “I don’t think an ultrasonic is warranted for my abdominal pain, cant you prescribe some conservative medicine that will make me better without doing all these expensive testing?” I would really like to see the face of the doctor hearing such negotiations. So why are we accepting such negotiations ?

SPT field testing

Today I came across a very interesting paper regarding site investigation with SPT and CPT. In this paper written by J. D. Rogers, the historical development of SPT from its creation from Charles R. Gow, to its modification and standardization by K. Terzaghi and A. Casagrande and its standardized use today are presented.

The road to SPT corrections and the reasoning behind them is provided while a critical evaluation of these corrections is presented. Some pitfalls in the execution and evaluation of the results can be found in this paper. Practitioners will find it to be a good review regarding SPT and new geotechnical engineers can understand the limitations of the methods.

Some issues that are not covered in detail regarding SPT testing and in my opinion are very important are the operator’s (driller) proficiency and the appropriate supervise. This is of paramount importance and I would like to share some insight.

I was present at an SPT execution in a hot summer day around 2:00 pm, the test had started while the geologist in charge and I were evaluating some outcrop geology not too far from the drill. As we started approaching the drill (the driller could not see as yet) I observed a very lazy move in the way the rope was tightened in the rotating drum.

The SPT hammer (a safety donut type, picture 2) was not traveling the appropriate height before it was released for its free fall. Even when it was released the driller was somehow holding very loosely the rope so the energy of impact was even more reduced. As it is easily understood the SPT blow count would be significantly higher than that required for the specific conditions if the test was executed correctly.

Another time I observed a driller (probably wanted to impress his supervisor) executing the test in a very quick pace. During this quick pace he was lifting the donut hammer and not releasing it as quickly needed for that pace. The safety donut was hitting the rod in the upper end (picture 3), displacing it upwards from the ground. It is easily understood that SPT blow counts again would not be correct.

Automated SPT execution equipment are available that could eliminate these issues but they are not a standard practice due to cost and other operational issues. Even if such equipment were used, other problems could arise during SPT execution.

It is very important to understand that SPT values are numbers with high scatter, even for very homogeneous materials, when evaluating ground properties, great care and judgment must be executed. Corrections can be found and applied but human errors are not so easy to predict or account for.