Steel mill foundations come under the spotlight

ThyssenKrupp Companhia Siderúrgica do Atlântico (CSA) is a large steel manufacturing complex currently under construction in the outskirts of Rio de Janeiro, Brazil.

The facility will consist of an integrated steel plant with production capacity of 5 million tonnes of steel, a thermoelectric plant with 490 MW generating capacity, and a port terminal that will receive imported coal and from where all the production of the plant will be shipped.

The complex covers an area of 9 million m2 by the Sepetiba Bay, and is a joint venture between TyssenKrupp Steel and Companhia Vale do Rio Doce (CVRD). It is the largest steel plant built in Brazil in the past 10 years, and will employ 18 thousand during construction and 3500 once in operation. The plant is expected to start production in December of 2009.

The foundations of the various structures include both driven piles and cast-in-place foundations, with diameters from 450 to 2000 mm. The foundation quality control was conducted by Brazil's Fugro In Situ Geotecnica, using equipment manufactured by Pile Dynamics, including its Crosshole Sonic Logging, Integrity Testing and Dynamic Load Tests.

Crosshole Sonic Logging (CSL) evaluates the quality of concrete in drilled shafts previously prepared for the test (by the installation of steel or PVC tubes during construction). This test was performed with the instrument Cross-Hole Analyzer (CHA).

During the test a transmitter inserted in one of the tubes sent a high frequency signal that was picked up by a receiver in another tube. These sensors (transmitter and receiver) were lowered to the bottom of the tubes, and scanned the entire length of the shaft. The resulting data was analyzed with the CHA and its software CHA-W.

Another type of integrity test was conducted with the Pile Integrity Tester ( PIT) to confirm the absence of cracks and voids in the shafts. This test, also known as Low Strain Dynamic Test, or Pulse Echo Test, is quick to perform, non destructive, and does not require preparation during construction (like CSL).

The test consists of placing one or two accelerometers on the foundation, and hitting the foundation with a hand held hammer. The signals from the accelerometer reveal any significant changes in cross sectional area that may exist along the shaft.

For load bearing capacity evaluation Dynamic Load Tests are a more practical and cost effective solution than traditional Static Load Tests. The execution of Dynamic Tests is relatively fast, and several tests may be performed in a single day.

Fugro In Situ conducted more than 900 dynamic load test, both on driven piles and on drilled shafts using the Pile Driving Analyzer (PDA), to which accelerometers and strain transducers were connected. The PDA software calculated the results of the test from the force and velocity signals obtained during impact by the accelerometers and strain transducers attached to the shaft.

For the driven piles, this was done during driving using the same hammer used for driving; for the non driven piles a drop weight was used. The data was further analyzed with the software CAPWAP, to calculate the capacity of both the driven and cast-in-place foundations (this analysis reveals the soil resistance distribution along the shaft and simulates a static load test).

The Dynamic Load Tests permitted the re-evaluation of the foundation design, taking into account the "setup" (increase in resistance with time, evaluated by a Dynamic Test performed during a re-strike some time after the initial drive). The Dynamic Load Test also evaluated the efficiency of the driving system utilized in this important project.