1、 IEEE Guide to Installation of Foundations for Transmission Line Structures Sponsored by the Transmission and Distribution Committee IEEE 3 Park Avenue New York, NY 10016-5997 USA 15 February 2011 IEEE Power +1 978 750 8400. Permission to photocopy portions of any individual standard for educational
2、 classroom use can also be obtained through the Copyright Clearance Center. Introduction This introduction is not part of IEEE Std 977-2010, IEEE Guide to Installation of Foundations for Transmission Line Structures. This guide is one of several documents, covering all aspects of overhead transmissi
3、on line construction, that are being prepared by the Working Group on Construction of Overhead Lines of the Towers, Poles, and Conductors Subcommittee of the Transmission and Distribution Committee of the IEEE Power otherwise, the hole may have to be redrilled. Second, the horizontal stress controls
4、 the foundation side resistance. If it is allowed to relax, the side resistance, and therefore overall foundation capacity, will be reduced. 2.6 Construction influences on foundation performance The construction practices employed during foundation installation can influence the performance of the f
5、oundation in a very significant manner. In general, progressively “poorer” practices result in progressively lower capacities and higher displacements of the foundations. However, these practices can be controlled by the engineer. In fact, foundation performance can be improved by controlling the co
6、nstruction variables through a comprehensive construction specification and construction quality surveillance. 10 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations for Transmission Line Structures 2.6.1 Excavation For any type of foundation system r
7、equiring an excavation, two important rules should be followed. First, support any excavation that is questionable and, second, always minimize the amount of time that any excavation is left open. If these rules are not followed, soil degradation will occur and, in the extreme, the foundation capaci
8、ties can be reduced by a third or more (EPRI EL-3771 B18). 2.6.2 Backfill When either soil, concrete, or grout backfill is used, two simple rules should be followed to improve foundation performance. For the concrete or grout, the use of non-shrink or expanding cements should be considered, as compa
9、red with conventional cements that shrink, when the side resistance capacity dominates the foundation design. Side capacity increases of a third or more can result from the simple change from conventional to expanding cements. In a similar manner, any soil backfill should be well-compacted to develo
10、p a good foundation system. Uplift capacity increases of more than 100% are readily achievable by well-compacted backfill, in contrast with just “dumping” it into the excavation (EPRI EL-3771 B18). Dumping should never be allowed. 2.6.3 Adverse construction influences The following comments are of u
11、se in minimizing particular adverse construction influences. First, when patented systems are being used, every reasonable effort should be made to follow the manufacturers recommendations. The manufacturer has, by far, the most experience with the system involved and should have developed an optima
12、l installation procedure that maximizes capacity and minimizes displacements. Second, when piles are to be driven, reasonable hammer and driving criteria should be developed during design, not construction. Different hammers and different driving criteria will result in different capacities. These n
13、eed to be specified, within reasonable limits, for a proper design. Third, when it is necessary to minimize the displacements of anchor foundations, pre-stressing should be considered. 2.6.4 Human factors The last issue to consider, which probably is the most important of all, is the human issue tha
14、t establishes the job attitude and, therefore, the quality of the final product. This issue applies equally to the owner, the design engineer, and the contractor. It consists of three parts: competence, cooperation, and communication. If these three Cs are maintained and followed through an entire p
15、roject, there should be no major foundation “surprises.” However, if one or more of the three Cs is ignored, problems may occur. 2.7 Applicable codes and standards Numerous codes and standards are applicable to transmission line structures. Perhaps the best known is the National Electrical Safety Co
16、de (NESC),2which contains safety rules for the design, installation, and maintenance of transmission line systems. Some states have similar safety codes. Contained within these codes are minimum specified loading conditions for wind, ice, and construction loads. However, foundation construction is r
17、arely addressed in these documents. 2The NESC is available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http:/standards.ieee.org/). 11 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations f
18、or Transmission Line Structures The following lists, generically, the most applicable codes and standards for the foundations of transmission line structures: a) Steel designAmerican Institute of Steel Construction (AISC) Code b) Concrete designAmerican Concrete Institute (ACI) Code c) IEEE Guide fo
19、r Transmission Structure Foundation Design and TestingIEEE and ASCE d) Material performance and installationAmerican Society for Testing and Materials (ASTM) Standards e) Foundation design criteriaAmerican Society of Civil Engineers (ASCE) standards for driven piles and drilled shafts that are being
20、 developed, plus manufacturers guidelines f) Installation criteriaAssociation of Drilled Shaft Contractors (ADSC) guidelines, manufacturers guidelines, owner guidelines g) Safety practicesOccupational Safety and Health Administration (OSHA) rules, plus those associ-ated with item a) through item e)
21、above Finally, it must be remembered that the periodic issue of these codes, standards, guidelines, etc. reflects advances in the state-of-the-art, as well as accumulated experiences. Therefore, they should all be considered as recommended criteria that must be adapted to the necessary local conditi
22、ons and experiences. 12 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations for Transmission Line Structures 3. Spread foundations 3.1 Introduction This clause reviews the various types of spread foundations that are used commonly for electrical trans
23、mis-sion line structures and the basic procedures to install these foundations. The following will not describe the construction procedures in detail, but will only review each step and some of the factors that must be addressed in the construction of this type of foundation. 3.2 Types of spread fou
24、ndations Spread foundations, shown in Figure 5, typically consist of a buried rectangular or square pad with a “leg-stub” or “stub angle” column connecting the foundation to the tower body. The typical foundation depth-to-width ratio is between 1 and 3, with the maximum depth often limited to 4.56 m
25、 (1420 ft) because of construction equipment limitations. The foundation usually is set horizontally, with a leg-stub battered to the same slope as the tower legs. Steel or concrete, or a combination of both, usually are used for the foundation. On cast-in-place concrete pad and pier spread foundati
26、ons, a concrete pier or column is used instead of a stub angle. The basic types of spread foundations are as follows: a) Steel foundations 1) Pressed plate (single or multiple) 2) Grillage b) Concrete foundations 1) Cast-in-place 2) Precast c) Concrete encased grillage The connection between the spr
27、ead foundation and the leg stub is, most often, a bolted connection. Most steel foundation connections can be assembled simply and bolted together. Concrete foundations may have anchor bolts embedded in the concrete and a base plate or base shoe connection tying the leg stub to the foundation. Level
28、ing nuts and base plate/base shoe leveling grout often are necessary with this connection detail. 13 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations for Transmission Line Structures a)Single Pressed Plate b)Double Pressed Plate c)Grillage type d)P
29、ad and Pier type (permission from EPRI EL-2870 B17) Figure 5 Types of spread foundations 14 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations for Transmission Line Structures 3.3 Site plan outline A site plan, or a tower site construction layout pla
30、n, can serve as a basis for an informal discussion between the contractor and the owner, or it may be the basis for a formal signed document on how the foundations will be installed at a given site. The three basic parts of this outline would be as follows: a) Tower site information b) Tower foundat
31、ion construction survey c) Foundation site information 3.3.1 Tower site information The tower site information would include all the limitations and restrictions on the mobilization, operation, and demobilization of the equipment required to install a spread foundation. Some of the factors that woul
32、d need to be addressed would be as follows: a) Restrictions on points of entry to tower site b) Equipment limitations on site c) Underground and overhead utilities d) Existing structures on site e) Clearing restrictions f) Presence of surface water g) Environmental restrictions 3.3.2 Tower foundatio
33、n construction survey The tower foundation construction survey establishes the foundation center hub, reference hubs, elevations, and required depth of excavation. Before excavation can begin, the tower foundations must be marked (staked), and the depth of excavation must be computed. Ground staking
34、 includes establishing a reference point (RP) hub to the pit center (PC) for each foundation. The elevation of the RP hub is established, and the depth of cut from this hub is computed. The hub will be used during excavation to control the depth of the excavation. During the staking process, a PC st
35、ake and depth of cut at PC is established. The four corners of large exca-vations also should be staked. The dimensions used to establish these corners are the dimensions of the gril-lage or pressed plate foundation to be set, plus 300 mm (12 in) on all four sides. For cast in-place concrete foundat
36、ions, where the concrete is placed against native material, the corner dimensions should match the foundation excavation dimensions. 3.3.3 Foundation site information Foundation site information would include the following: a) Access to foundation sites b) Foundation assembly site c) Spoil pile mana
37、gement d) Erosion control measures Access to the foundation sites, and the sequence of excavating each foundation, must be planned to avoid 15 Copyright 2011 IEEE All rights reserved. IEEE Std 977-2010 IEEE Guide to Installation of Foundations for Transmission Line Structures undercutting other site
38、s. This planning is especially critical for steep slopes or tower sites with limited working space. Access limitations may require that only one spread foundation at a time be excavated, assembled, set, and backfilled. Many steel spread foundations, especially high-capacity grillage foundations, req
39、uire a large level area to assemble the foundation prior to setting. This area needs to be planned for. Large excavations often are required for spread foundations, which require a spoil pile management plan. This excavated material usu-ally is used for backfill. The organic topsoil and fines often
40、need to be separated so that they can be replaced as top soil and be used adjacent to the foundation. On steep slopes, this spoil pile must be restrained from sliding down the slope. This spoil pile also must be protected from wind or water erosion. During heavy rains, the backfill material needs to
41、 be covered to control the moisture content, within allowable limits, to ensure proper compaction. 3.4 Excavation The equipment and techniques that are used for excavation depend on the type of material encountered at the excavation site. When soil, loose or fractured rock, boulders, or any combinat
42、ions thereof are encountered, the excavation usually is done with a track-mounted or rubber-tired backhoe. When the terrain is so steep that the backhoe equipment cannot be used for excavation, it may be necessary to incorporate the use of specialized digging machines that have the capability of man
43、euvering and excavating on hill-side slopes in excess of 45. Another method is to use a drag line bucket and crane. Hand excavation also is used at difficult sites. Drilling and blasting may be required whenever machine digging alone cannot proceed because of the hard-ness of the material being exca
44、vated. It is common to run into successive layers of soil and rock that cannot be removed with soil excavators alone. In such cases, drilling and blasting the rock will be necessary. A rock-drilling machine must be brought in to drill holes for blasting. Typically, such drilling would be to the full
45、 depth of the excavation in materials that are stable. 3.4.1 Drilling and blasting precautions Inspection of the drilling and blasting activity should include monitoring to ensure that survey controls, such as tower center monuments, survey hubs, reference points, and benchmarks, are not distorted o
46、r lost. A distorted tower center monument could result in an incorrect setting. Precautions required during blasting should conform to applicable state and local codes and requirements. All personnel on the site should be aware of blasting hazards and procedures and must be warned properly prior to
47、the actual blasting. When constructing a new line parallel to an existing line, or near inhabited or environmentally sensitive areas, a “heave mat” should be placed over the blast area prior to blasting. This mat prevents or minimizes the danger of rocks, wood debris (when blasting to remove stumps)
48、, and/or lead- in wires being thrown into the conductors on the existing line, resulting in conductor damage or electrical outages, as well as other property damage. Parallel lines should be de-energized prior to blasting whenever possible. In no event should lead-in wires be allowed to be placed un
49、der energized conductors when blast-ing. Non-electrical and nonconventional electrical detonation methods are recommended near transmission lines. If conventional electrical detonation methods must be employed, there are specific precautions that can be practiced to minimize hazards to the blaster and transmission line. If rock is encountered during hand excavations on steep hillsides, small portable compressors and jackham-mers can be used to loosen the rock material. Excavated material can be removed from
