The folks at Hammer & Steel Inc. (www.hammersteel.com) offer guidance on a common question - whether or not one should pre-drill when driving sheet pile. Here is a list of conditions they say may justify pre-drilling.

• Ground conditions are hard enough to slow production or cause sheets to fan out.
• Ground conditions are too hard for thin sheets, and pre-drilling is cheaper than using thicker sheets.
• Using a larger vibro hammer could drive the sheets in, but may damage the interlocks or tear the top of the sheets.
• Ground layers are sloped to the extent that sheets want to follow and not stay straight.
  

If these conditions exist, pre-drilling around the interlocks can have many benefits:

• A reduction in the surface friction and/or tip resistance.
• An increase in production rates.
• A reduction (or avoidance) in damage to sheet piles.
• A straighter pile wall as pre-drilled holes act as guides.
• The ability to see what is in the ground rather than relying on a bore log report.

OSHA Cracks Down on Crane Safety

ADSC: The International Association of Foundation Drilling has issued a safety alert that warns of rigorous OSHA enforcement of its crane regulations. There has been a rash of crane (both mobile and tower type) accidents lately, and OSHA has begun taking steps to ensure the construction industry is working safely when using cranes on its projects. A Regional Emphasis Program recently was put into effect in a handful of states in the south, and it is likely that these vigorous enforcement programs will spread across the country. Make sure your company is up on the latest regulations by going to www.osha.gov, and checking out the section covering regulations relating to cranes, derricks, hoists, elevators and conveyors.

Getting Started

A review of structure-specific and site-specific conditions is necessary for any foundation design. The first step in the entire process is to determine the general structural requirements for the foundation. Some important considerations:

• Project type (new bridge, replacement bridge, bridge widening, retaining wall, noise wall, sign or light standard, etc.). Continuous flight Auger (CFA) piles may be considered for any of these.
• Construction sequencing - either phased construction or all at once. Neither condition either precludes or favors CFA piles.
• General structure layout and approach grades.
• Surficial site characteristics. A stable working platform is required for CFA pile construction.
• Special design events such as seismic, scour, vessel impact, etc. These factors should be considered in planning the site investigation, and can have a significant effect on the selection of CFA piles.
• Possible modifications to the structure that may be desirable for the site under consideration.
• Approximate foundation loads and limitations on deformation.

In general, the components of the site characterization include a review of the site geology and foundation experience in the area, followed by a carefully planned and executed subsurface exploration program. Usually, the consideration of CFA piles does not require specialized investigation techniques differing from those used for driven pile foundations. Important considerations for CFA piles include the general site stratigraphy and soil classification, the depth and characteristics of the most likely bearing formation, ground water conditions, variability, and the presence and extent of unusual geologic features such as solution cavities, boulders, lenses or layers of hard rock.

The use of cone penetration testing (CPT) generally is considered to be particularly well suited for design procedures used for CFA piles, but especially for drilled displacement (DD) piles. CPT soundings provide a continuous record of a strength measurement that correlates well with CFA and DD pile performance, and they generally can be performed in soils where CFA or DD piles are to be considered. It also is a very cost-effective tool compared to conventional borings for sounding a large area. Where conditions are such that CFA piles may be considered as a viable foundation alternates, the use of CPT soundings is recommended and encouraged as a part of the exploration program.

Source: U.S. Department of Transportation's Geotechnical Engineering Circular No. 8, "Design and Construction of Continuous Flight Auger Piles." Photo courtesy of McDonnell Piling & Foundations Ltd.

The Ben C. Gerwick Award Banquet for Innovation in Design and Construction of Marine Works, a benefit for the Deep Foundations Institute Educational Trust Scholarship Program, takes place Saturday, Sept. 20, at the University of California, Berkeley.

Ben Gerwick represented the highest level of engineering excellence both personally and professionally. The phenomenal achievements of his 62-year career include world-class innovations in marine foundations.

The 2008 award winner is Menck GmbH, being recognized for its deep water hammer system for offshore deep foundation construction.

For complete details on the event, telephone 973-423-4030.

On the Job

A Top-down Challenge

Hub Foundation Co., Harvard, Mass., installed 44 core beams in shafts as big as 10 feet in diameter and over 125 feet deep for the Blackfan Research Center in Boston's hospital district. Some of the core beams weighed up to 100,000 pounds to facilitate the project's top-down construction. Hub's 200-ton crane, the Liebherr 895, and an APE King Kong Vibratory Hammer, were used to install the 40-foot long casings.

Hub Foundation was founded in 1966 by Frank Maxwell. His son, Jim, joined him in 1970, and took over management of the company in 1981. In its four-decade history, Hub has completed nearly 800 contracts under diverse - and sometimes difficult - conditions. Over the years, Hub has evolved from one of New England's largest pile driving outfits to become a leader in the drilled shaft market in and around Boston.

Featured Product of the Month

Con-Tech Systems has introduced its new Obermann VS 63-20-D Diesel Hydraulic Grouting Unit. This unit is used for the grouting and post-grouting of anchors and micropiles. The VS 63-20-D unit is said to be fuel-efficient, quiet and designed for high performance. It has a blue tooth wireless connection (optional) to PDA and/or laptop for flow and pressure data evaluation and protocol printing. For more information, visit www.contechsystems.com.

Check out Site Prep, a sister publication of The Foundations Report. Site Prep focuses on the challenges and opportunities of site preparation, including site clearing, earth moving, utilities and environmental activities. Go to www.siteprepmag.com, where you can subscribe to the magazine as well as its monthly newsletter, Industry Dirt.