grout

PVC (or similar) seath

drill hole

centralizer

anchor body (grouted)

grout

anchor plate

anchor head

tendonA

A

B

B

A-A B-B

if packer

centralizer

CHAPTER 5SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTS GROUND ANCHORS (or ANCHORAGES)1.Definition 2.Design 3.Corrosion protection4.Types 5.Materials6.Construction 7.Testing *Drilling (or driving) *Capacity prediction *Tendon (manufacture & assembly) *Quality control *Anchor homing (installation) *Monitoring *Grouting *Stressing Definition: An installation that is capable of transmitting an applied tensile load to a load bearing stratum.

Basic types: TEMPORARY anchors (usually life < 2 years)

PERMANENT anchors (life of the structure) Active and passive anchors v.s. bolts and nails

Prestressed anchors

There are four mechanisms of stress transfer from the fixed anchor zone to the surrounding ground (as functions of soil type and grouting procedure)

Type A-Tremie grouted (gravity)-may be lined or unlined-rock or very stiff to hard cohesive soils-depends on side shear at the ground/grout interface

Type B-low grout pressure (<1000 kPa)-lining tube or packer-diameter of fixed anchor length increased-permeates or fractured-weak fissured rocks & coarse granular alluvium & fine grained cohesionless soils (compaction grouting)-depends on side shear

Type C-high grout pressure (>2000 kPa)-lining tube or in-situ packer-fixed length is hydrofractured (grout root or fissures)-often secondary grout after primary through tube or manchette or grout tubes within the fixed length-fine cohesionless soils stiff cohesive deposits

Type D-Tremie grouted holes-bells or underreams formed-firm to hard cohesive soils

Others : Jet grouting , expand bodies , use of explosives , splitting of anchor bulp Rock....... A or packer grouted A For improving rock/grout bond also B

Design of Anchors*Fixed anchor dimensions*Depth of embedment*Overall stability*Group effects Main possibilities in failure of a single anchor-failure of ground/gout interface-failure of grout/tendon interface-failure of tendonSafety factors are considered.Other possibilities-displacement or excessive slippage of the anchor head-failure within ground supporting the anchorage-crushing or bursting of grout column around the tendon-gradual long-term deterioration Minimum safety factors recommended for design of individual Anchors

Anchorage category

Minimum safety factor

Proof load factor

Tendon

Ground/grout

interface

Grout/tendon or

grout/encapsulation interface

Temporary anchorages with a service life of say up to two years where,

although the consequences of failure are quite serious, there is no danger to public

safety without adequate warning e.g. retaining wall

tie-back.

1.602.5*

2.02.5*

2.5*

2.02.5* (if no tests)

1.25

Permanent anchorages and temporary anchorages where corrosion risk is

high and/or the consequences of failure are serious, e.g. main cables of a suspension bridge or as a reaction for lifting heavy

structural members.

2.0

3.0#

3.0#4.0 creep is expected

3.0*

2.03.0* (if no tests)

1.50

*Minimum value of 2.0 may be used if full scale field tests are available.#May need to be raised to 4.0 to limit ground creep.Note 1. In current practice the safety factor of an anchorage is the ratio of the ultimate load to design load.Table 2 above defines minimum safety factors at all the major component interfaces of an anchorage system.Note 2.Minimum safety factors for the ground/grout interface generally lie between 2.5 and 4.0. However, it is permissible to vary these, should full scale field tests (trial anchorage tests) provide sufficient additional information to permit a reduction.Note 3.The safety factors applied to the ground/grout interface are invariably higher compared with the tendon values, the additional magnitude representing a margin of uncertainty.

 1.Ground-grout interface in cohesionless soils2.Ground-grout interface in cohesive soils3.Ground-grout interface in rock

  1.GROUND-GROUT INTERFACE IN COHESIONLESS SOILSUsually Type B and C are used in sand.Ultimate capacity of anchors in sand with fixed lengths of 4-8 m and diameter 10-15 cm have been observed to be up to 1300-1400 kN (130-140 tons). These capasities can not be explained by usual soil mechanics computations.Among the factors that affect capacity:*Relative density, and degree of uniformity of the soil*Length and diameter of anchor (influences to lesser degree)*Method of grout injection & grout pressure used*Dilatancy in the soil*Drilling method & equipmentFor Type B , ultimate load capacity Tf (kN) (empirically)

Tf = L*n * tan’ ’= angle of shearing resistance L= fixed anchor length (m) n= the factor that takes account of -the drilling technique (rotary-percussive with water flush) -depth of overburden -fixed anchor diameter -grouting pressure in the range 30-1000 kPa in-situ stress fileds & dilation character.n400-600 kN/m in coarse sands and gravels, k>10-4 m/sec (10-2 cm/sec)n130-165 kN/m in fine to medium sands, k=10-4-10-6 m/sec (10-2-10-4 cm/sec) 

Enlarged diameter =38-61 cmMaking use of bearing capacity theory an alternative solution is: Tf = A . v’ . . D . L . tan’ + B . . h . /4 . (D2-d2)

A : the ratio of contact pressure at the fixed end anchor/soil interface to the effective overburden pressurev’: average overburden effective pressure

L : length of fixed anchor (m)’: effective angle of shearing resistanceB : bearing capacity factor equivalent to Nq/1.4 : unit weight of soil overburden (’ below gwt)h : depth of overburden to the top of fixed anchor (m)D : diameter of fixed anchord : nominal anchor (shaft) diameter

This equation includes the effect of side shear and end bearing.-D is estimated from grout intake.-Porosity of the soil is also influencial.In coarse sand and gravel; for d=10-15 cm D40-50 cm~3d to 4d Pgrout<1000 kPa(10 atm) In medium dense sand; permeation is limited,local compactionfor d=10-15 cm , Pgrout<1000 kPa D 20-25 cm (or 1.5d-2d) For very dense sand D is reduced (18-20 cm) (1.2d-1.5d)

Slenderness ratioh/D

’

26 30 34 37 40

15 11 20 43 75 143

20 9 19 41 74 140

25 8 18 40 73 139

Berezantsev (1961)

0

50

100

150

26 28 30 32 34 36 38 40

'

Nq/B.C.

h/D=25

B.C. component of the above ap. is difficult to assess. A values: (Pgrout<1000 kPa) for compact sandy gravel, ’=40o , A=1.7

for compact sand, ’=35o , A=1.4

Type C AnchorsTheoretical predictions of load capacity are not reliable. Design curves are obtained from field (actual) load tests. In alluvium medium sand

variable deposits of sand & gravel

d=10-15 cm 90-130 kN/m at 1000 kPa 190-240 kN/m at 2500 kPa

 

fixed anchor length

 Pgrout500 kPa on averageRD Tf

When RD1RD2 if U1>U2 TfU1>TfU2

Lfixed after 10m no increase in Tf.

 In 500-5000 kPa grout pressure range Tf increase is not muchUnit skin friction for sand 500 kPa max Sandy gravel 1000 kPa max Unit skin friction N80 kPa – 350 kPa 10 - 50 (Fujita et. Al. 78) 

Fixed Anchor Design in Cohesive SoilsLoad capacity of anchors in clays is low.Application of low grouting pressure & use of casing tubes may be beneficial to the capacity. (without hydrofracturing the fissure penetration of grout can increase the skin friction values.) Load capacity can be improved;i.                     using high pressure groutingii.                   using bells or underreams in the fixed anchor zoneiii. cement grout & gravel injection into augered holes

Type C Anchors-high grout pressures-with or without post-grouting-ultimate capacity can not be calculated    Ic : consistency index m : natural moisture content Skin friction m increases with increasing consistency & decreasing plasticity.

In stiff clays (Ic=0.8-1.0) of medium to high plasticity the lowest m range is 30–80 kPa & in sandy silts of medium plasticity & very stiff to

hard consistency (Ic=1.25) high values (m >400 kPa) have been recorded.

 Post grouting increases m of stiff clays by 25% to 50%. Greatest improvements have been recorded in stiff clays of medium to high plasticity

(from 120 kPa to 300 kPa)

Type A Anchors (Tremie grouted straight shaft)Similar to bored holes 

Tf = . d . L . . cu

 Tf : ultimate load capacity d : borehole diameterL : fixed anchor length : adhesion factor (stiff soils 0.35-0.4)cu : average undrained strength over the fixed anchor length

LL

LC PL

mLI

Type D Anchors

Tf= . D . L . cu + /4 . (D2-d2) . Nc . cub + . d . l . ca Side Shear End bearing on clay side shear along shaft length D : diameter of underreamL : length of fixed anchorcu : average undrained shear strength along fixed anchor

Nc : B.C. factor take qcub : undrained shear strength at the end

l : the length of the shaft (m)ca : shaft adhesion 0.3-0.35 cu (kPa)

 Reduction coefficients 0.75-0.95 due to construction techniques underream geometry 0.5 for open or sandfilled fissures in clay drilling – underreaming – grouting time is very important. Even few hoırs may be critical. (Because of softening) Underreaming is suitable for clays cu>90 kPa (also problemmatic for 60-70 kPa , not possible for cu<50 kPa), low plasticity PI<20

Fixed anchor length in clay 3-10 m.Fixed anchor spacing 1.5-2 m. minSpacing to any adjacent foundation/underground service 3 m. minDistance to surface foundation 5 m. min

Fixed Anchor Design in Rock Type A to type D can be all applied in rock but straight shaft tremie or packer grouted type A is more popular in practice. Type B (low pressure grouting) to enchange rock/grout bond or to increase rock/grout interface area.Type C proving & site suitability tests are required. 

Type AAss : Uniform bond distribution

D : diameter of fixed anchorTf = . D . L . wet L : length of fixed anchor

wet : ultimate bond or skin friction at rock/ grout interface

 In weak & deformable rock stress concentrationsTendon/grout failures initiate grout/rock interface failures Strong rock : 10 % of qu (wet limit = 400 kPa)

Lfixed anchor : 3 m. min

Table 24 p.131 BS8081 : 1989 Rock/Grout Bond valuesVery poor rocks : u1.5*102 – 2.5*102 kPa Marls

3.5*102 kPa Shale3.7*102 kPa Soft sandstone+shales (working 1-1.4*102 kPa)

Grout/Tendon InterfaceGrout is in tension like the tendon. Not similar to reinforced concrete.Ass : Uniform ultimate bond stressLimits recommended.Clean plain wire or plain bar : 1000 kPa (1.0 N/mm2)Clean crimped wire 1500 kPaClean strand or deformed bar 2000 kPa For min grout compressive strength of 30 N/mm2 (30000 kPa , 300 kg/cm2) prior to stressing.Min bond length : 3m where tendon homed & bonded in-situ 2m where tendon homed & bonded under factory controlled conditions

Bond strength can be significantly affected by the surface condition of the tendon, particularly when loose & lubricant materials are present at the interface : loose rust, soil, paint, grease, soap or otherIf protected (protected oils or greases) remove Asteel 15% borehole area for multi unit tendons 20% borehole area for single unit tendons

Encapsulations For Rock Bolt Recommended by manufacturerAt grout/encapsulation interface max. ultimate bond 3 N/mm2 (3000 kPa) unless adequately proven Fig. 11 BS Encapsulations generally take the form of single or multi-unit tendons grouted with a single corrugated duet or within two concentric ducts which effectively protect the tendon bond length against corrosion. Encapsulation length 2m min (whole length for underreamed fixed lengths)Strands tests to investigate the strand/grout force to be transferred to encapsulation/grout interface.

Materials

CementOrdinary Portland cement is generally used.It should be fresh (at most 1-month old) and should be kept in ideal storage (damp free/not over hut) conditions. (Partial dehydration or carbonation can lead to particle agglomeration & reduction in postmix hydration.)If there is a risk of chemical attack, sulphate resisting Portland cement should be used. Use of high alumina cement is restricted. (only <6 months, reaction anchors) -Suitable water/cement (W/C) ratio is between 0.40-0.45 between 0.40 & 0.70 there are applications.Higher values in sandy alluvial deposits.-There are limits for total sulphate content (4% (m/m) SO3 of cement in grout)

total chloride content of the grout from all sources (0.1% (mm) of cement)-Fillers (inert) : fine sand, limestone dust, ground quartz Not common.-Mixing water : Generally if drinkable suitable

no oil, organic matter, deterious substancessulphate <0.1%chloride ions 500mg/1 liter

Admixtures Chemical Optimum dosage of % cement by weight

 

Accelerator CaCl2 Calcium chloride 1-2 Accelerates set & hardening

Retarder Calcium lignosulfonate 0.2-0.5Also increases fluidityMay affect set strengths

Tartaric acid 0.1-0.5

Sugar 0.1-0.5

Fludifier Calcium lignosulfonate 0.2-0.3   Entrains airDetergent 0.5

Expander Aluminum powder 0.005-0.02 15% expansion

Antibleed Cellulose ether 0.2-0.3 Equiv. to 0.5% of waterEntrains airAluminum sulphate 20

Excess water results in bleeding of the mix and low strength, as well as greater shrinkage and lower durability of the hardened grout.

0

10

20

30

40

50

0.3 0.4 0.5 0.6 0.7 0.8 0.9

W/C ratio

Co

mp

res

siv

e s

tre

ng

th (

MP

a)

-Recommended unconfined comp. Strength of grout : 30 MPa 70 days 40 MPa 28 days

-Bleeding of (tendon bonding) grout at 20o C should generally 2% (4% at most) of volume 3 hrs. after mixing.

Higher values may be allowed in gravels etc.

Resinous GroutsResins: Epoxy & polyster resins are most commonly used in capsules (rock bolting), fixed anchor protection encapsulations.

Follow manufacturer’s recommendations (mix time, setting time, filler’s strength etc.)

Stronger than cement grout > 75 kPa in compression >15 kPa in tension(Full scale tests needed.)

TENDON

Tendons usually consist of steel bar, strand or wire either singly or in groups. For soil anchors. Typical data for prestressing steel that may be used in tendon design is shown in the following table: (In the following page)

For high strength steels above the loss of prestress due to relaxation is small. (Relaxation: loss of prestress load at the same strain)  Under normal circumstances working loads should not exceed 62.5% & 50% of the characteristic strength of the tendon for temporary and permanent works, respectively.

To distribute load to the soil more uniformly, strands of different length are sometimes used within the fixed anchor zone. When these strands are stressed simultaneously displacements at the anchor head are the same for all strands, and thus the strains and hence stresses differ in individual strands.

Type of steelNom. Dia.

mm(Ult. Load) Specified

characteristic strength (kN)Nom. Steel Area mm2

Non-alloy steel wire7-wire strand

 

 7-wire drawn strand

7.0 60.4 38.5

12.9 186 100

15.2 232 139

15.7 265 150

12.7 209 112

15.2 300 165

18.0 380 223

Low alloy steel barGrade 1030/835

   

Grade 1230/1080

     

26.5 568 552

32 830 804

36 1048 1018

40 1300 1257

25 600 491

32 990 804

36 1252 1018

Stainless steelWireBar

     

7 44.3 38.5

25 491 491

32 804 804

40 1257 1257