In the spring of 1998, my graduation date from the University of Florida was fast approaching and I had to get serious about finding a job. One employment ad mentioning two things: “geotechnical field testing” and “extensive international travel” caught my eye. I was skeptical of the first part, having specifically avoided all but the basic required undergrad soil mechanics classes on my way to a Master’s degree in offshore structural engineering. The second part, however, was quite appealing to a young, single grad student. And that is how I wound up three months later on a construction site in Singapore, standing next to a digital level with a clipboard, pushing a button every two minutes and jotting down the displayed elevation number.
The level was monitoring a barcode target which was fixed to a reference beam. The beam served as the attachment point for a number of displacement transducers monitoring a bi-directional static load test. The reference beam itself had to be monitored for displacement, because according to ASTM D1143 the beam supports had to be three diameters away from the test pile, which was 6 feet in diameter, resulting in a 36-foot span. Singapore’s tropical weather (blazing sunshine interchanged with monsoonal downpours) resulted in temperature changes that guaranteed thermally-induced movement of the beam. This movement of the supposedly “stable” reference had to be logged, to correct the apparent displacements of the transducers.
So there I was, having traveled halfway across the globe to write down a column numbers on a pad of paper, then transcribe them into a spreadsheet. Needless to say, it got old very quickly. I noticed the digital level had a little communication port, which got me thinking…
Fast forward a couple of years. I’d written a program to communicate with the level and log the data automatically. This elevated me from ‘the new guy’ to ‘the R&D nerd’ in the company. It made correcting for the reference beam’s movement easy, even in extreme circumstances. In one infamous incident, the engineer running an overnight test with a long-duration load had set the hydraulic pump to maintain pressure, which kept dropping as the test pile displaced, and then dozed off. He woke up to discover the pile had pushed upward enough to catch and physically lift the reference beam off its supports! Luckily the data acquisition software had recorded everything, so the only negative outcome was a bit of embarrassment.
Using the level to monitor the beam, and adding the data to the test analysis became universal procedure. The beam was now assumed to displace, and its displacement was simply subtracted from the transducers’ displacements during every test. The true reference was now the digital level, and the beam was just an intermediate step between it and the test pile. It took a nuclear engineer (Robert Simpson, LTC’s co-founder) to look at this situation and see what none of us civil engineers did: “Why don’t we just get rid of the beam, and monitor the test pile with the level directly?” Following the maxim that it’s sometimes easier to ask forgiveness than permission, we started doing just that on some tests, with great results. This eventually led to my first-ever technical publication (co-authored with Robert) on the error analysis of reference-beam versus direct top-of-pile monitoring. You can download it here.
It’s often assumed that the top-of-pile displacement monitoring is done using lasers. I think this may be mainly because ‘lasers’ sounds cool and hi-tech. However, our research with actual lasers had shown that field conditions (mainly atmospheric scattering) make laser measurement too imprecise for this application. The best results so far stem from use of optical digital levels, with digital image correlation (DIC) looking like a promising upcoming technology.
Mainly because of wanderlust, I lucked into a fantastic career of constantly improving the instrumentation and testing of foundations. The direct top-of-pile method has become the accepted standard for bi-directional tests, in no small part because it was too boring for the new guy to write down data by hand.
LTC