Click for next page ( 4


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 3
3 Details of Type II Reinforcements and plastic sheaths, and a trumpet head assembly surrounds the reinforcements behind the bearing plate. Double corrosion Type II reinforcements include ground anchors and rock protection is recommended for ground anchors in aggressive bolts. Key features of these systems, summarized in Table 1, are ground conditions and permanent installations. Products on described in this section. More complete details of these rein- the market today offer systems that comply with the current forcements, including descriptions of components, materials, standards. However, many of the older installations do not installation details, and performance issues can be found in incorporate details that meet today's standards, or may have USACOE (1980), International Federation for Prestressing been installed without any corrosion protection beyond the (FIP) (1986), NCHRP Web Document 27 (D'Appolonia et al., passivation of the grouted portion of the tensioned elements. 2001), Kendorski (2003), and Sabatini et al. (1999). Rock bolts are installed with either mechanical anchorages Tensioned elements of the system include bar and strand (e.g., expansion shell, split wedge), or are grouted into rock components. The steel grade and level of prestress employed using portland cement or resin grout. The anchorage may in these systems are relevant to the type of corrosion problems either be concentrated near the end point of a mechanical that may occur, and prediction of service life. Bar elements device or by the short length of grout near the end of the bolt; are available in a variety of steel grades ranging from Grade or the bolt may be fully grouted with the pullout resistance 60 to 160. Strand elements are manufactured from Grade 250 distributed along the length of the bonded zone. and 270 high-strength steel and generally consist of seven wire Older style rock bolts with mechanical anchorages may have strands with six wires wrapped around a seventh wire called no corrosion protection. Portland cement or resin grouted the "king wire." Wire tension systems using the button head rock bolts are surrounded by grout, but the bolts heads are anchorage of BBRV (Birkinmaier, Brandistini, Ros, and Vogt) often not encapsulated. There is also the possibility of voids and Prescon have also been used, but are not as popular as along the grouted length. Rock bolt installations may also be strands. These systems use a set of parallel wires, rather than similar to ground anchors with a free length and a bonded strands, as reinforcing elements. zone, but trumpet head assemblies are not always installed, Ground anchors include an anchored or "bonded" zone leaving the area behind the head of the rock bolt exposed. and a free length or "unbonded" zone. The bonded zone is anchored to the soil or rock with cement grout. Current guidance documents [Post Tensioning Institute (PTI), 2004; Durability and Performance Issues Sabatini et al., 1999] recommend incorporating corrosion for Earth Reinforcements protection measures into the design of ground anchors. Cor- Durability of earth reinforcements is controlled by backfill rosion protection measures include the use of coatings, protec- characteristics, site conditions, climate, steel type (galvanized tive sheaths, grouting, encapsulation, and electrical isolation. or not), and details of project construction and in-service Use of portland cement-based grout provides limited corro- operations. Weatherby (1982), FIP (1986), Briaud et al. (1998), sion protection as a barrier, and by fostering a passive film layer D'Appolonia et al. (2001), Withiam et al. (2002), and Elias due to its high alkalinity. et al. (2009) describe factors that contribute to corrosion poten- Recent installations employ Class I or Class II corrosion pro- tial of earth reinforcements and measurement of the relevant tection systems as recommended by PTI (2004). For Class I electrochemical parameters for soils and groundwater. In protection the anchor is encapsulated (often referred to as general, "minimum" resistivity (min), pH, chemical compo- double corrosion protection) and, for Class II, the anchor is sition including the presence of organics, porosity, and ground- protected by grout (often referred to as single corrosion pro- water level are the factors that most affect the corrosiveness of tection). The free lengths of the anchors are protected by grease the underground environment. Generally, ground conditions Table 1. Summary of Type II reinforcements. Type of Metal Tendon Anchorage Corrosion Tensioned Systems Type Type Protection Ground anchors Strands or Cement grout More recent permanent installations use bars in bonded Class I or Class II Protection (PTI, zone 2004); older systems may have no protection other than grout cover. Rock bolts Usually Mechanical, Epoxy coating, bars, but resin grout, or galvanized, could be cement grout grout cover, strands older installations may have none