FEATURE

Ultra High Capacity and Totally Removable Tiebacks by Tony Barley* Director of SBMA Ltd and Mary Ellen Bruce* President of SBMA LLC UK and World Market USA

The installation of a multiple of individual anchor tendons within Figure 2: Analysis of load distribution and bond stress in very stiff by Woods a single tieback is gain- & Barkhordari (1997). ing popularity on a world-wide basis. Each individual tendon is referred jected to the same synchronized load over that of a conventional tieback to as a “unit anchor”, and each unit application during all loading stages. anchor with the same total fixed anchor has its own short fixed length Each individual unit exhibits its own length. Each additional unit anchor (8’ – 15’). As many as seven units performance behavior characteristics provides a proportional increase in may be installed in a single borehole throughout loading, and testing anchor capacity, such that efficient with unit fixed lengths at staggered results are compared to conventional total fixed lengths up to 60’ have acceptance test requirements. been used. A recently tested It is widely acknowledged that for anchor with four units each with 10 tensile anchors with long fixed foot lengths, (totalling 40’), has The installation of a mul- lengths (30’ to 40’), the elastic prop- achieved an enormous load of 1100 tiple of individual anchor erties of the tendon strongly influ- kips prior to failure of an individual ence the load transfer. The elastic unit. tendons within a single incompatibility between the tendon Unit anchors may be designed and tieback borehole is gain- and the grouted ground generally founded in differing strata - an ing popularity on a results in debonding progressing exclusive feature and a beneficial world-wide basis. along the grout/soil interface as load characteristic of the system. This is increased (Figure 2). This phe- promotes the system’s use in layered nomenon prevents the simultaneous soil strata, which are typically diffi- mobilization of the ultimate bond cult for designing conventional stress along the whole fixed length tiebacks. Each unit fixed length is locations. The principle is applied to and leads to inefficient utilization of designed on the basis of the known permanent anchors, temporary the available soil strength. This load or anticipated bond stresses at the anchors and removable anchors (See loss effect has been quantified as an grout to soil interface in the encoun- Figure 1). “efficiency factor,” which is used in tered strata. These bond capacities Since each unit anchor also has its design to account for very short fixed are typically higher in granular mate- own free length, and hence, its own lengths that mobilize the full avail- rials and lower in cohesive strata. load extension characteristics, each able bond (“skin ”), and the This multiple short length load unit requires its own hydraulic ram long fixed length mobilize only a transfer approach promotes a rela- which is seated on the communal percentage of that bond. As a result, tively uniform distribution of load bearing plate during testing. Each the simultaneous loading of multiple along the overall fixed length in all ram is coupled via a manifold to the short, fixed length units in a single strata. This contrasts with the high pressure (10,000 psi) hydraulic normal size diameter bore hole (5” to uneven load distribution known to power pack. This system ensures 7”) can, in total, achieve a consider- be exhibited along the fixed lengths that each ram and each unit are sub- able increase in pull out capacity (continued on page 36)

ADSC August 2005 Page 35 TIEBACKS Contd.

Figure 1: Unit anchor fixed lengths at staggered locations in a single borehole for permanent, temporary or removable mul- tiple anchors. Multiple of rams used for controlled testing.

of conventional tiebacks. In addition System (“SBMA”) system has been design load without reaching failure. to capacities being achieved, creep utilized extensively in the UK, Ire- Production multiple unit anchors losses demonstrated by multiple unit land, Austria and Singapore (over were installed in 2002 and are being 35,000 high load anchors) and is monitored annually. Tiebacks were gaining popularity in Australia, New installed by Gerace CC Inc., tendons Zealand and Spain. The system has were supplied by Lang Tendons Inc. In addition to capacities been used twice in the USA: *Geotechnica and Geosystems de- signed the Hodenpyl project and the being achieved, creep • At Natchez, Mississippi where the geotechnical engineer and reviewer losses demonstrated by tieback capacity of the multiple unit was Schnabel. multiple unit tiebacks are anchor far exceeded that achieved by conventional tiebacks in very fine In addition to permanent “double generally considerably (“”). Tiebacks were installed lower that those of con- protected” or temporary “SBMA”, by Haywood Baker, Inc.*, and ten- fully removable multiple anchors are ventional tiebacks. dons were provided by Lang Ten- also available, and demonstrate the dons, Inc.* same ultra high performance charac- • At Hodenpyl Dam, Michigan, pre- teristics. This system, also patented, viously installed conventional tie- utilizes a multiple of looped, fully tiebacks are generally considerably backs apparently exhibited high sheathed strands in a single bore lower that those of conventional creep losses resulting in unaccept- hole (Photo 1) and can guarantee tiebacks. able wall movement. A post grouted The Single Bore Multiple Anchor SBMA was tested to 2.7 times the (continued on page 38)

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www. kellytractor.com ¥ 561-683-2015 Ext.179 TIEBACKS Contd.

- Very dense silt (Hong Kong) - Hard silty clays (UK) - Dense clayey / (Spain) - Stiff clay with silt and part- ings (UK) - Upper units in gravel, lower units in chalk (UK) • 400 tons (900 kips) - Medium dense silty sand (UK), and highly weathered sandstone (Singapore) • 500 tons (1100 kips) - Dense fine sand with some gravel (Ireland).

When preliminary or sacrificial Photo 1: Looped strand system allows total strand removal. Strand sheathed anchors are tested to failure, ulti- and greased for the entire length. mate grout/soil bond stress is recorded for each individual SBMA and an extraordinarily wide data- removal of the entire length of the Ultra high capacity? The following base is thereby collected for tieback multiple of tendons after destressing. capacities have been demonstrated in design in a host of different soil con- Removability is becoming mandatory over 70 investigation anchors in dif- ditions. for temporary tiebacks installed in fering soil strata over a 10 year peri- The provision of ultra high capaci- several major cities: Berlin, London, od in various countries: ty (up to twice that of conventional Edinburgh, Manchester, Hong Kong, Perth and Singapore (Photo 2). • 350 tons (800 kips) (continued on page 39)

Page 38 ADSC September/October 2005 TIEBACKS Contd. anchors in controlled condi- tions) ensures that the use of this system can reduce the number of tiebacks, in a deep excavation with multiple rows of ties, by as much as half. This inevitably provides con- siderable direct financial ben- efits plus a time savings. A three month time savings was achieved in a deep excavation in Hong Kong when 500 SBMAs each with 450 kips design load were installed and removed during construction. For further information see: www.TheAnchorman.com www.SBMASystems.com Tony Barley and Mary Ellen Bruce are ADSC Technical Affiliate Members. Mary Ellen Photo 2: Over 2500 high capacity SBMA-type Removable Anchor Tiebacks have serves as the Secretariat for the been installed in Singapore where complete tendon removal is mandatory. International Society on Micropiles (ISM). (Editor) *Indicates ADSC Members. ■

ADSC September/October 2005 Page 39