The Wise Man Built his Bridge on the Sand? What to do when there is no rock

Carrie Denesha, ‘00
Professional Geologist II, Kansas Department of Transportation

There are three main types of bridge foundation design utilized in today’s construction practices; drilled shafts, spread footings and piling. The first two are common when bedrock is near the surface. However, in much of the central and southern U.S. bedrock is in excess of 50 feet and it makes foundations in rock not economically feasible. Under these circumstances piling becomes the most cost effective and easiest to construct foundation option. Piling, such as pipe pile or H-pile is driven into the ground by a hammer and the resulting end bearing and/or side friction supports the superstructure. The amount of support or bearing capacity needed by each piece of pile is determined by the design engineer. When bridges are constructed in central and western Kansas, PDA testing (Pile Driving Analyzer) is required to verify that the planned length of pile will be enough to support the bridge. The PDA is comprised of strain gauges, accelerometers, and the PDA pak (hardware/software computer combo).
The strain gauges and accelerometers are attached to the pile and with each blow of the hammer dynamic measurements are taken by the PDA. Each blow creates a stress wave that travels through the pile and is then partially reflected back to the gauges. The calculations derived from this wave can tell the user the soil resistance, capacity of the pile, stresses on the pile, and performance of the driving equipment during the testing. Once the total bearing needed on the pile is reached, driving may cease. A re-strike can be preformed on a pile hours to several days after the initial drive, this confirms the capacity of the pile and often demonstrates a property called soil set up in a static state. During driving the pore pressure increases within the soil which loosens the soil similar to liquefaction, once the soil returns to its natural state, the pile can be given a few more blows, this nearly always results in an increased bearing capacity in Kansas. Using previous projects as examples, we are able to predict how much soil set up we may get and drive the piles to a shorter length resulting in substantial savings.