Ground anchors incivil engineering: 1

Ground anchors constitute a specialapplication of prestressing in civil en-gineering, their use making it possible,for example, to excavate without brac-ing, to build retaining walls and otherstructures with economical anchorages,and to secure rock against movement.Rock and alluvium anchors, which arecovered by the term "ground anchors,"consist of a prestressing cable fittedwith a fixed anchorage, whose functionis to hold the cable securely in theground, and a movable anchor for ten-sioning the cable. A Swiss firm, Losin-ger 8t Co. S.A., have made a specializedstudy of the subject and as a resulthave produced the VSL anchors. Thefirst application of the VSL rock anchorwas ten years ago, though the alluvium

anchor is a more recent developmentand was first put to use five years ago.

The cable for the VSL rock anchorconsists of high-strength steel wireswhich are formed in the bond section ofthe anchor at one end of the cableby alternatively spreading and concen-trating, as shown in Fig. 1. Normally,five waves are formed over a length of9.8 ft (3 m) for the fixed anchorage,which transfers the anchoring force tothe grout and thus to the surroundingfoundation zone by bond and bearingpressure. Grouting is carried out severaldays before stressing, the stressingcable being protected by a sheath.

A movable anchorage is used forstressing the cable and this operationcan be carried out in any number ofstages. Once stressing is complete a

secondary grouting is done to protectthe prestressing steel against corrosion.If the rock anchor is to be stressedafter a longer interval than normal, orif for some reason the elasticity of thecable must be preserved, the wires areprotected against rust with bitumenor a plastic coating.

The alluvium anchors are fitted withstrands, which are free and extendfor the length of the bond. Lockingthese anchors to the surroundingground is done by injecting grout, andthe rest of the cable is protected by aplastic sheath against corrosion. Amovable anchorage is used for stressingthe cable after the grout has hardened,so that the operation can be carried outin any number of stages. Fig. 2 shows a completed alluvium anchor in place. Although the terms "rock anchor"

Drilling holes for rock anchors in the roof of the underground pumping station lor the ForcesMortices de I'Hongrin S.A., Montreux.

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Below lett: Fig 1, Rock anchor in position; Fin 2, Alluvium anchor in position; Below right:Placing rock anchors in the undergrund cavern for the pumped storage chamber.

Frg 1

Frg 2

and "alluvium anchor" refer to the ori-ginal applications of these two typesof ground anchor, they can be used inany type of ground capable of carryingload. The selection of the better typeand size depends upon the requirementsof the structure, load-carrying capacityof the ground, the drilling equipmentavailable and, of course, upon economicconsiderations.

Different types of movable anchor-ages are used for rock and alluviumanchors, the former being used for pre-stressing the wires and the latter forstressing the strands. The prestressingwires and strands are identical withthose used for prestressing concretestructures, ultimate tensile strengths for0.276-in (7-mm), 0.315-in (8-mm) and0.5-in (12.7-mm) dia wires being 108tons/in'170 kg/mm'), 102

tons/in'160

kg/mm') and 114 tons/in'180kg/mm') respectively. The rock anchor"normally have five coils in the wirespiral and the alluvium anchors eitherfour or five.

An important feature of these an-chors is that it is possible to check theload capacity of each one, so that it ispossible to determine the safety of astructure against the failure of theanchorage.

The fitting of the stressing anchoragesis done in two stages. The bearingplate and spiral are fixed to the form-work before concreting. The anchorheads, on the other hand, are fitted tothe stressing cable just before the final

stressing stage, and access to thestressing anchorage must be provideduntil all stressing and checking has beencompleted. Whether anchorages are

finally capped with concrete or leftopen and provided with protectionagainst corrosion depends upon operat-ing and aesthetic considerations. Re-

Ground Engineering 11

Left to right: Fig 3, Section through dam wall at Cluny; Fig 4, Penstock thrust blocks held by rock anchors at Hatch Hetchy Dam; Fig 5, Sectionof dock at the new Israel port of Eifat.

cesses can be made in the formworkso that the anchorages do not projectbeyond the concrete surface. Holes forthese anchors can be drilled with anytype of drilling equipment.

The length of bond necessary for ananchor can be estimated from soil sam-ples. As a simplification it can be as-sumed that the transfer of force to theground occurs along the surface of thecylindrical bond due to regularly dis-tributed shear stresses. The value ofthe shear stress for rock and cohesivesoils can be determined directly fromtests, the diameter of the drilled holeand that of the anchorage bond beingconsidered to be identical. The differentvalues given below which can be ob-tained for the bond strennth tp are theresults of tests carried out by theSwiss Federal Laboratory for the Test-ing of Materials (EMPA). The testingprocedure consisted of placing the coresample vertically in the middle of asteel form and filling the surroundingspace with grout. Seven days later thesample was pressed out and the bondstrength between it and grout measured.The figures obtained were as follow:—

Slightly weathered marlytertiary sandstone ... 92.4 6.5

Fine-grained slightlyweathered tertiary sand-stone .........159 11 2

Fine-grained tertiarysandstone ......596 41.9

Tertiary limestone ... 441 28.3Chalky sandstone ... 411 28.9Granitic gneiss with

biotite .........353 24.8The value of tp for cohesive soils

can be determined from simple com-pression tests, whereby tp is half ofthe compressive stress at rupture. Theadhesion is normally small so that theanchorage length has to be widened.

The conditions are quite different inuncohesive soils. Due to the grouting aconcrete mass is formed whose sizedepends upon the permeability of theground. The force is mainly transferredto the ground by skin friction and, usingdata from established tests it can beestimated, for instance, that a VSL rockanchor of 167-ton (170-tonne) stressingforce with a bond length of 16.4 ft.(5 m) in tertiary limestone has thesame factor of safety against pulling

out of a VSL alluvium anchor of 64 tons(65 tonnes) with a bond length of 13.1ft (4 m) in a semi-coarse gravelly soil.

The following is general practice in

planning works using anchors:—1. Determination of the sizes, directions

and positions of the forces to becounteracted.

2. Determination of the anchorage typeand establishment of the stability.

3. Selection and design of the anchors.4. Deciding the stressing programme.

These VSL ground anchors have beenused on a number of important con-tracts of various types, such as the un-derground cavern for a pumped storagestation for the project Forces Motricesde I'Hongrin SA, near Montreux, andthe U.S. World Trade Centre, wheresome 1,400 tie-backs are being installedand tensioned for a bentonite slurrywall, the capacity of tendons rangingfrom 98 to 295 tons (100 to 300tonnes) .

The underground cavern at Mon-treux has a length of 448 ft, with awidth of 108 ft, and a height varyingfrom 72 ft to 89 ft (136.5, 31 and 22to 27 m). The rock consists of lime-stone with intermediate layers of shaleof maximum thickness of 31.5 in (80cm). The strata is horizontal of averagehardness, is fissured and water has in-filtrated. Instead of a conventional con-crete arch roof, a combination of pre-stressed rock anchors and sprayed con-crete was used. Altogether 2,393 VSL

Cont plated work anchors showing mesh re-inforcement for sprayed concrete.

rock anchors have been used. Of these643 are 42.7 ft (13 m) long, including afixed length of 9.8 ft (3 m), the work-ing loads ranging from 113 to 138 tons(115 to 140 tonnes) . The remaining1,750 rock anchors are 13.1 ft (4 m)long, including 23.6 in (60 cm) for afixed length in polyester. The tendonswere stressed after eight hours. Thiswas followed with 5.9 in (15 cm) ofsprayed concrete.

Another project is the Cluny Dambeing carried out for the HydroelectricCommission of Tasmania, as part of theLower Derwent Power Development.This was built by direct labour, andthe drilling and prestressing of theground anchors was carried out by TheCementation Co. (Australia), Ltd. In all78 cables were used of 300 tons (305tonnes) capacity and varying from 92ft to 112 ft (28 to 34 m) in length. Theproject comprises a dam and hydro-electric power station combined, thedam being of the gravity type for thenon-overflow abutments and having aprestressed spillway for over half of itslength. The dam is founded on doleriterock and the prestressing anchor cablesextend from 35 ft to 55 ft (10.7 to16.8 m) into the rock. A section of thedam wall is shown in Fig. 3.

A recently completed job of rather adifferent kind is the use of rock anchorsfor anchoring penstock thrust blocks,as shown in Fig. 4. This was for theHetch Hetchy Water Supply projectfor the City of San Francisco. The en-gineers were Sverdrup & Parcel andAssociates, the holes were drilled byBoyles Brothers Drilling, of Auburn,and the prestressing was done by theL.R. Yegge Co. of Los Gatos. In thiscase 35 rock anchors varying from 80ft to 100 ft (24.4 to 30.5 m), and 24rock anchors varying from 60 ft to 80ft (18.3 to 24.4 m) were used.

The Israel Ports Authority recentlycompleted a new port at Eilat, where adock 1,739 ft (530 m) long has beenbuilt with berthing space for three30,000-ton ~ (Fig. 5). The super-structure rests on precast pre-tensionedpiles, the pile caps being prestressedwith VSL cable of 123 tons (125tonnes) capacity. The precast deckslabs are transversely prestressed by acable of 88.6 tons (90 tonnes) capacityevery 7.1 ft (2.17 m).

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