Ground Anchor(1)

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Prestressed Ground Anchors

Construction Technology 463

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(Based on Notes by Mohamed Shahin 2008)

Content

• Function of Ground Anchors

• Application to Civil Engineering

• Anchor Groups and Types

• Stability and Design of Ground Anchors

• Worked Example

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Function of Ground Anchors• Ground anchor is a way to improve the

-structure system

• Ground anchor system consists of a steeltendon inserted into ground throughborehole in any direction. The system

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-stressing the tendon, which is anchored bya specially formed anchorage zone.

Function of Ground AnchorsGround surface

Prestressing

force

Anchored

structure

Primary

groutLoad distribution

plate

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• Main components of ground anchor: Anchor head, Free length and Fixed length



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Application to Civil Engineering• The use of ground anchorages can be traced back to the

last century. It was originally used to support canalan s a ong on on- rm ng am ra way n y

Frazer.

• First most impressive application was by Coyne in 1934on the strengthening of the Cheurfas dam in Algeria,using 1000-ton capacity anchor @ 3.5m to stabilise thegravity dam made of masonry!

• Since then, development has been made on drilling

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, , ,techniques and field-test and evaluation methods.

• The system is designed, installed and monitored tosustain its performance and to provide protection againstcorrosion attack.

Application to Civil Engineering

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Anchor head details

In-situ example of prestressed ground anchor



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Use of Single Bore Multiple Anchors in Natchez, Mississippi where high load anchors

founded in loess provided overall stability of soil nailed slope (Civil Engineering ASCE

December 1997) http://www.sbmasystems.com/anchorman/pdfs/GATP12.pdf

Anchor Groups

Anchorages can be grouped into 3 main

ca egor es n erms o groun

terminology at the site:

1. Soil anchors (70-80% of the market);2. Rock anchors (10-20% of the market); and

3. Marine anchors 10% of the market

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Soil AnchorsSupport retention systems in deep excavations.

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Tieback of deep excavation Retaining walls support

Soil Anchors

Stabilise foundation slabs subjected to uplift caused bygroundwater or heave.

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Hold down structures subjected to hydrostatic uplift



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Soil Anchors

Preconsolidate unstable soils to increase soil bearingcapacity.

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Preconsolidation of soft soils

Soil Anchors

Provide reactions for pile load tests.

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Providing reactions for pile load tests



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Soil Anchors

Compensate and balance the effect of overturning forces inpower transmission towers, large dams, television mastsand bridge abutments.

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Anchoring of footings Anchoring of dams

Rock Anchors:

Protect and stabilise rock formations and slopes

18Stabilisation of rock slopes

http://www.waterpowermagazine.com/graphic.asp?sc=2048373&seq=2



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Rock Anchors

Support underground rock cavities in tunnelling and mineshafts, where anchors replace timber and steel supports.

19Stabilisation of tunnel openings in rock

Marine Anchors:Protect oil jetties and coastal structures.

Protect river embankments and navigation canals.

Stabilise reclaimed areas

Strengthen sea and fluvial facilities

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Note

Difficulties in applying the ground anchoring

ec n que w e encoun ere , or

example, where the ground is not entirely

suitable for load transfer from the tendon,

or where aggressive materials exist but

remained undetected.

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Anchorage Types

• Temporary and permanent anchors;

• Active and passive anchors; and

• Method of load transfer.

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Temporary and permanent

anchors; Temporary anchors are

devices of a temporary Permanent anchors are

devices which willnature that will becomeuseless and inoperativebeyond a certain time,which is usually less than18 months. During thisperiod it is highly unlikelythat a corrosion rocess

maintain the stability of astructure on a permanentbasis. Permanentanchors must functionlonger than 18 months,and corrosion protectionand monitorin are

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of adverse magnitude willbe initiated; hencecorrosion protection is notrequired.

mandatory.

Active and passive anchors;

Active anchors are

“ ”

• Passive anchors (also

“ ”

that apply initial force

to the structure thus

supported, and will

persist with time

unless the structure

anchors) are not

prestressed and

respond to loading

only when the

structure thus

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undergoes

displacement relative

to the anchor itself.

supported begins to

move.



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Anchor type according to load

transfer

F r e e

z o n e

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F i x i n g

z o n

e

Anchor type according to load

transfer Type A is a straight shaft cylindrical hole of uniform diameter filled

with grout. This type is suitable for rock as well as very stiff to hard.

Type B is cylindrical but enlarged at the fixing length by groutinjected under low pressure. In this process, the actual effectivediameter of the fixing zone is increased with minimum disturbance tothe surrounding soil. This type is suitable for soft fissured rock andcoarse soils.

Type C is similar to Type B except that the grout in the fixing zone isinjected under high pressure, forcing the grout to penetrate the soil

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irregularly and thereby increasing the anchor resistance to thetensile forces. This type is suitable primarily for cohesionless soils.

Type D is a cylindrical enlarged at one or more positions along itslength by means of a special cutting device. This type is commonfor stiff to hard cohesive soils.



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A borehole (usually between 75 to 150 mm in

diameter) is drilled, using rotary machine. If the soil

is weak, use casing.

Installation of Ground Anchors

.

The anchor is fixed to the ground by grout injected

(under controlled) pressure from a grout pipe

attached to the tendon and the casing is withdrawn.

After routin and when the rout has hardened, the

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anchor is prestressing.

Finally the anchor is locked off (fixed) in the tendon head.

Installation sequence of Type B ground anchor (after Hanna, 1982)

Centralizers are used to make the

(a)

tendon centrally located in the borehole and

thus ensures a uniform grout cover in the

fixed length.

Spacers (Figure 6b), made of steel or plastic,are used in both the free and fixing zones to

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(a) Centralizer details; (b) Spacer details

(after Xanthakos, 1991)

(b)

maintain anchor components parallel and in their correct alignment, and thus prevent contact friction.



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Stability and Design of Ground Anchors

An anchor can fail or becomeinoperatable in one of thefollowing modes (Xanthakos,

In reality, it is unlikely that all theprevious modes of failure willoccur simultaneously, hence,

1991):

1. By bond failure (slippage) at thetendon-grout interface.

2. By shear failure along the contactsurface of the grout and ground.

3. By failure within soil or rocksupporting the anchorage.

4. By structural failure (rupture or

usual anchor practice is to designthe anchor based on potentialfailure modes (usually failuremodes 1 to 4) under anappropriate factor of safety,consistent with the actual knownstrength or the associated degreeof risk.

The desi n of round anchors

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sectioning) of the steel tendon.5. By crushing of the grout columnaround the tendon.

6. By displacement or excessiveslippage of the anchor head.

requires the calculation of thefollowing items:

• Angle of anchor inclination• Fixing anchor length; and

• Free anchor length.

• The inclination angle of anchors has a more favourableeffect if the structures are founded on a substratum with a

Angle of Inclination

ower ang e o r c on.

• The optimum angle of inclination should be chosen sothat the minimum anchoring force is obtained.

10

12

14

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c e ( M N )

30Effect of angle of inclination on anchoring force for different friction angles

α

0

2

4

6

8

0 10 20 30 40 50 60 70 80 90

Angle of inclination

A n c h o r i n g f o r

φ = 28.8o

φ = 33o

φ = 36.9o

φ = 38.7o



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Angle of Inclination• In cases where the stability against sliding and

overturnin is re uired e. . dams the o timum an le of . . ,

inclination for securing the structure against sliding does

not usually coincide with that for protection against

overturning. In such cases, the angle of inclination

should be chosen to secure both cases.

• For example, if securing the structure against sliding will

re uire lar er restressin force than that re uired to

prevent overturing, the angle of inclination is thus theone that is governed by the safety against sliding.

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Fixing Length for Type (A) Anchors

The fixing length, L, can be calculated by fulfilling the following two equations(assuming uniform distribution of bond stresses along the fixing length):

P

d nL FS

tg

grout tendon

τπ=−

1and P

d L FS

sg

grout soil

τπ=

−

2

where,

FS = factor of safety (usually between 2 to 3);

n = number of anchors (in case of more than one anchor in a borehole);

d 1 = diameter of anchor (diameter of strands × number of strands);

d 2 = diameter of borehole;τ sg = bond strength between the grout and surrounding soil;

τ tg = bond strength between the anchor (or tendon) and grout; and

P = anchoring force.

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o ac eve s a y, e grea er eng rom e a ove equa ons s ou eused.

P d 2

d 1

τ sg

τ tg Anchor Grout

Fixing length, L

Determination of fixing anchor length



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Fixing Length for Type (B) Anchors

The fixing length, L, that accounts for FS between the tendon and grout canbe calculated as in Type (A) anchors, and L that accounts for FS between thesoil and grout is calculated as follows:

q D D D L sg )(2

2

2

11 −π

+τπ

P grout soil =−

where,

D1 = enlarged diameter of grout;

D2 = diameter of borehole;

τ sg = bond strength between soil and grout; and

q = bearing capacity resistance between soil and grout.

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P D1

d 1

τ sg

τ tg Anchor Grout

L

D2

q

q

Fixing Length for Type (D) Anchors

As before, L that accounts for FS between the tendon and grout can becalculated as in Type (A) anchors, and L that accounts for FS between thesoil and grout is calculated from L1 and L2 as follows:

q D D D L D L FS

sg s

rout soil

)(4

2

2

2

12211 −π

+τπ+τπ=−

where,

D1 = under-ream diameter of the borehole machine (auger);

D2 = shaft diameter of the borehole machine;

L1 = under-ream length;

L2 = shaft length (should be known for certain augers); and

τ s = shear strength of soil;

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τ s

P D1

d 1τ tg

L1

D2

q

q

L2

L

τ sg



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Notes:• All bond and shear strength (i.e., τtg,, τsg,, τs,,q ) are obtained from

experimental tests.

• Bond on rout stren th of 30 MPa

τtg = 1.0MPa for plain wire & bar tendons

= 2.0 MPa for strands & deformed bars

τsg = 10% unconfined compressive strength of rocks

= (0.6 – 1)Su for still Clays

• FS ≥ 2.0 for temporary work < 6 months

≥ 2.5 for temporary work between 6 to 18 months

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.

• Working stress

– Permanent anchors = 50% ultimate

– Temporary anchors = 65% ultimate

Free Anchor Length

As mentioned previously, the free anchor length is the distancebetween the anchor head and beginning of fixing length. It isrecommended that the fixing anchor length starts some distance x beyond the most critical slip surface.

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Definition of free anchor length



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References

Hanna, T. H. (1982). Foundations in tension:grounds anchors, Trans Tech Publications,

, .

Xanthakos, P. P. (1991). Ground anchors and anchored structures, John Wiley & Sons, N. Y.

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Revision: Stability of Anchored

Structures with Prestressing

• Rock Anchor γrock = density of rock

φ' = angle of friction

c' = cohesion factor of rock

T

α

θl

Wθd

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Disturbing force:

Resisting force:

Factor of safety against sliding = Resisting force

Disturbing force



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• Slope Stability (without tension crack and no

surcharge load)

T

α

θ

l

Wθh

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θ

Disturbing force:

Resisting force:


Disturbing force



• Slope Stability (with tension crack and no

surcharge load)

T

α

θ

l

Wθh

40

θ

Disturbing force:

Resisting force:


Disturbing force



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• Slope Stability (with surcharge load)

T

α

θ

l

Wθh

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θ

Disturbing force:

Resisting force:


Disturbing force

Revision: Stability of a dam

with vertical prestressing

φ = angle of friction

T

W

o

c = co es on

ca = adhesion

h

l

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Factor of safety against sliding:

Factor of safety against overturning:



Revision: Stability of a dam

with diagonal prestressing

φ = angle of frictionwater level

W

o

c = co es on

ca = adhesion

h

l

α

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T



α

Revision: Stability of a retaining

wall with tendon anchor

φ = angle of friction

Ground and water level

TWc1

o

c = co es on

ca = adhesion

h1

Wc2

h2

α

Ground and water level

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