Practice Periodical on Structural Design and Construction

While representing owners at projects where explosives are used to excavate rock or demolish concrete, engineers participate in the management of blasting risk when they (1) design projects; (2) develop contracting methods; (3) write blasting specifications; and (4) oversee fieldwork. In performing this work, the engineer assumes the responsibility of managing the many risks associated with blasting operations. Although rare, terrorist acts and incidents involving explosives create negative public perceptions. Despite these incidents, explosives are safely used to excavate rock at thousands of projects throughout the United States each year. This paper defines various forms of blasting risk, illustrates them with case histories, and offers practical guidelines for identifying and managing them.

Trenchless pipe replacement, or pipe bursting, is a construction method that provides an alternative for rehabilitating underground infrastructure with minimal disruption to surface traffic. It is recognized as the only method of trenchless pipe rehabilitation in which a buried pipe can be replaced with a completely new pipe that functions independently of the existing line and permits the diameter of the new line to be increased. This paper presents an overview of three bursting systems currently used in the North American industry with discussion on the anatomy of a pipe bursting project, sequencing, risks, and a comparison of project elements to traditional open trench methods of construction. This paper concludes with three case studies describing the application of pipe bursting on projects that extended the current operating envelope of the technology to illustrate project specific engineered solutions to particular rehabilitation requirements. Through the sharing of knowledge and experience, a greater understanding of the technology and its application may be achieved to promote trenchless pipe replacement as a viable alternative to open cut construction methods.

The Alabama Department of Transportation (ALDOT) employs thinner decks on their highway bridges than almost all other states and countries, and unfortunately many of these decks are exhibiting significant levels of cracking and premature deterioration. Up until about two years ago, ALDOT deck depths ranged from 6 1/4 in. (159 mm) to 7 3/4 in. (197 mm), with the depth depending primarily on the girder spacing. Since that time, ALDOT has increased their minimum deck thickness to 7 in. (178 mm); however, they still employ thinner decks than most other highway agencies. As a first step in investigating bridge deck thickness and related deck design parameters, a survey questionnaire was prepared and sent to all state Departments of Transportation. Results of the survey indicate that the most common of the current design deck thickness in the United States is 8 in. (203 mm). Most states (61%) design-in an extra 1/2 in. (13 mm) of deck thickness to allow for future grinding, and most states (67%) do not employ a wearing surface on their decks. Other results of the survey questionnaire are presented and discussed.

Structural damage due to permanent ground displacement is a common occurrence during seismic events. Light-frame structures, typical of residential construction, seem both particularly susceptible to and easily reinforced against moderate ground failure. Because the foundations of these structures typically receive little or no engineering attention, a spreadsheet capable of quickly checking the capacity of a foundation to resist anticipated lateral and vertical displacement might encourage residential designers to require more appropriate foundation cross sections. Unfortunately, a simple strengthening of the foundation elements is generally insufficient. Designs must also eliminate noncontinuous interior bearing supports, which are highly vulnerable to horizontal ground displacement. Site remediation, including the installation of free-draining base materials, may also be needed to enable placement of the footer at or near grade. Finally, we should strive for more efficient designs: smaller, more compact structures, with more redundant, lightly loaded foundation elements.