By Bob Koerner
Back in the early 1960s I was the very young resident engineer on a sand drain/surcharge project requiring the use of a large pile driving “rig” that was made even more massive than usual with leads hanging over the front and a compressor in the back for counterweight.
The first part of the project consisted of spreading a 3ft-thick (~ 1.0m) layer of sand called a “working blanket” over the 500ft (150m) x 200ft (60m) site. It was underlain by a 65ft (20m)-thick layer of saturated silty clay foundation soil. The purpose of all of this was to rapidly consolidate the soft foundation soil via expelled water from the sand drains for the construction of two large steel molasses storage tanks.
While the consolidation part made sense to me, the working blanket (which I thought was just to release the expelled water) was not obvious until the large crane crawled out from its assembly location to begin driving the sand drains. For each impact, everything around the rig vibrated. In particular, incremental turning movements of the crane shook the localized area like one was standing on a water bed!
The project itself was so formative for me that I wrote an article about it: (ref. Koerner, R. M., “A young engineer, molasses, and failed sand drains” Geosynthetics, Vol. 7, No. 5, 2009, pp. 24–31).
These thoughts came back to me when I was reading the short article in Foundation Drilling magazine (Nov./Dec., 2014) by James Finbow of Bauer Foundations Canada Inc.. His title “Working platforms? Of course I have one. By the way what is it?” reminded me of those days long ago. Jim shows us three awesome photos of inadequate/improper working platforms.
To geosynthetics oriented people, the solution for an adequate and proper working platform (working platform is a much better description of the function than working blanket) is obvious—geotextiles, geogrids, geocomposites, geomattresses, etc. All have key roles to fulfill in reinforcing such soft and unstable soil against the equipment-specific live loading system.
The above said, I see little in the way of geosynthetic design solutions specifically for working platforms in the technical literature (or even in promotional literature from geosynthetic manufacturers and design engineers) that suggest geosynthetics have the “key” toward safe and economical working platforms. Some special focus seems to be in order (and even desired) in this regard.
Bob Koerner, Ph.D., P.E., NAE, is director emeritus of the Geosynthetic Institute in Folsom, Pa., and is a member of Geosynthetics magazine’s Editorial Advisory Committee. GSI: +1 610 522 8440