Transcript Document

CHAPTER 4
SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTS
GROUND ANCHORS (or ANCHORAGES)
1.Definition
2.Design
4.Types
5.Materials
6.Construction
*Drilling (or driving)
*Tendon (manufacture & assembly)
*Anchor homing (installation)
*Grouting
*Stressing
3.Corrosion protection
7.Testing
*Capacity prediction
*Quality control
*Monitoring
Definition: An installation that is capable of transmitting an applied tensile load to a load bearing stratum.
centralizer
Basic types:
TEMPORARY anchors (usually life < 2
years)
if packer
B-B
A-A
PERMANENT anchors (life of the
structure)
tendon
A
anchor
head
anchor
plate
Active and passive anchors v.s. bolts and
nails
anchor body
(grouted)
A
Prestressed anchors
grout
B
drill hole
PVC (or
similar)
seath
grout
centralizer
B
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
Type B
-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
-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
Type D
-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
-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
Minimum safety factor
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 tendon
Safety 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
Fig. Minimum safety factors recommended
for design of individual Anchors
Anchorage category
Tend
on
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.
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.
1.60
2.0
Ground/
grout
interface
Grout/tendon
or
grout/encapsul
ation interface
2.5*
2.02.5*
2.5*
2.02.5* (if
no tests)
3.0#
3.0#4.0
creep is
expected
3.0*
2.03.0* (if
no tests)
Proof load
factor
1.25
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 soils
2.Ground-grout interface in cohesive soils
3.Ground-grout interface in rock
1.GROUND-GROUT INTERFACE IN COHESIONLESS SOILS
Usually 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 & equipment
For Type B , ultimate load capacity Tf (kN) (empirically)
Tf = L*n * tan’
’= angle of shearing resistance
L= fixed anchor length (m)
n400-600 kN/m in coarse sands and gravels, k>10-4 m/sec
n= the factor that takes account of
(10-2 cm/sec)
-the drilling technique (rotary-percussive with water flush)
n130-165 kN/m in fine to medium sands, k=10-4-10-6 m/sec
-depth of overburden
(10-2-10-4 cm/sec)
-fixed anchor diameter
-grouting pressure in the range 30-1000 kPa in-situ stress fileds & dilation character.
Enlarged diameter =38-61 cm
Making 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 pressure
v’: average overburden effective pressure
L : length of fixed anchor (m)
’: effective angle of shearing resistance
B : 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 anchor
d : 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.
Slenderness ratio
h/D
In coarse sand and gravel;
for d=10-15 cm D40-50 cm~3d to 4d, Pgrout<1000 kPa(10 atm)
In medium dense sand; permeation is limited,local compaction
for 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)
h/D=25
Berezantsev (1961)
150
100
Nq/B.C.
50
0
26
28
30
32
34
36
38
40
'
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
’
26
30
34
37
40
15
11
20
43
75
143
20
9
19
41
74
140
25
8
18
40
73
139
Type C Anchors
Theoretical 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
Pgrout500 kPa on average
RD
Tf
When RD1RD2 if U1>U2 TfU1>TfU2
Lfixed after 10m no increase in Tf.
In 500-5000 kPa grout pressure range Tf increase is not much
Unit skin friction for sand  500 kPa max
sandy gravel  1000 kPa max
Unit skin friction  N
80 kPa – 350 kPa  10 - 50
(Fujita et. Al. 78)
Fixed Anchor Design in Cohesive Soils
Load 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 grouting
ii.
using bells or underreams in the fixed anchor zone
iii.
cement grout & gravel injection into augered holes
Type A Anchors (Tremie grouted straight shaft)
Similar to bored holes
Tf =  . d . L .  . cu
Tf : ultimate load capacity
d : borehole diameter
L : fixed anchor length
 : adhesion factor (stiff soils 0.35-0.4)
cu : average undrained strength over the fixed anchor length
Type C Anchors
-high grout pressures
-with or without post-grouting
-ultimate capacity can not be calculated
IC 
LL  m
LL  PL
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 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 underream
L : length of fixed anchor
cu : average undrained shear strength along fixed anchor
Nc : B.C. factor take q
cub : 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. min
Spacing to any adjacent foundation/underground service  3 m. min
Distance 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 A
Ass : Uniform bond distribution
Tf =  . D . L . wet
D : diameter of fixed anchor
L : length of fixed anchor
wet : ultimate bond or skin friction at rock/ grout interface
In weak & deformable rock  stress concentrations
Tendon/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 values
Very poor rocks : u1.5*102 – 2.5*102 kPa  Marls
3.5*102 kPa  Shale
3.7*102 kPa  Soft sandstone+shales (working 1-1.4*102 kPa)
Grout/Tendon Interface
Grout is in tension like the tendon. Not similar to reinforced concrete.
Ass : Uniform ultimate bond stress
Limits recommended.
Clean plain wire or plain bar : 1000 kPa (1.0 N/mm2)
Clean crimped wire1500 kPa
Clean 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 other
If 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 manufacturer
At 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.
Fig.11
Materials
Cement
Ordinary 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 substances
sulphate <0.1%
chloride ions 500mg/1 liter
Admixtures
Chemical
Optimum dosage of %
cement by weight
Accelerator
CaCl2 Calcium chloride
1-2
Retarder
Calcium lignosulfonate
0.2-0.5
Tartaric acid
0.1-0.5
Sugar
0.1-0.5
Calcium lignosulfonate
0.2-0.3
Detergent
0.5
Expander
Aluminum powder
0.005-0.02
15% expansion
Antibleed
Cellulose ether
0.2-0.3
Aluminum sulphate
20
Equiv. to 0.5% of water
Entrains air
Fludifier
Accelerates set &
hardening
Also increases fluidity
May affect set
strengths
Entrains air
Excess water results in bleeding of the mix and low strength, as well as greater shrinkage and lower durability
of the hardened grout.
Compressive strength (MPa)
50
40
30
20
10
0
0.3
0.4
0.5
0.6
W/C ratio
0.7
0.8
0.9
-Recommended unconfined comp. Strength of grout : 30 MPa 7 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 Grouts
Resins: 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 steel
Non-alloy steel wire
7-wire strand
7-wire drawn strand
Low alloy steel bar
Grade 1030/835
Grade 1230/1080
Stainless steel
Wire
Bar
Nom. Dia.
mm
(Ult. Load) Specified
characteristic strength (kN)
Nom. Steel Area
mm2
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
26.5
568
552
32
830
804
36
1048
1018
40
1300
1257
25
600
491
32
990
804
36
1252
1018
7
44.3
38.5
25
491
491
32
804
804
40
1257
1257
Anchor Head
Stressing head + bearing plate (anchor plate)
Tendon is anchored
tendon force is transmitted to the structure
Head should be designed to permit the tendon to be stressed and anchored at any force up to 80% of the characteristic tendon
strength and should permit force adjustment up or down during the initial stressing phase.
should be restressable (load adjustments 10 possible)
should be detensionable
should permit an angular deviation of 5o from the axial position with no effect on ultimate capacity.
Construction
1.Method of drilling (with or without flushing)
2.Tendon installation
3.Grouting system
Time period of the above operations (1,2&3) may influence the capacity of the anchorage.
Drilling
Any drilling procedure that can supply a stable hole free of obstructions may be used.
minimum disturbance or disturbance most beneficial to capacity
Care should be taken not to use high pressures with any flushing in order to minimize the risk of hydrofracture particularly in buit-up
areas.(Open return within BH is desirable.)
Drill hole entry point: 75 mm accuracy.
O.C. soils and several hours waiting  check swelling tolerance
 o drill up tolerance is 2.5o except in case of closely spaced design + staggered design.
 o 10o to facilitate grouting
Overall drill hole deviations 1/30 (1.9o) should be anticipated.
After drilling full length and thoroughly flushed out to remove any loose material  probe into the hole.
Even cased drillingprobe whether e.g. saturated silts and fine sands moved inside.
1 m overdrill is the trick in such cases.
Drilling –tendon installation- grouting in the same day otherwise delays  ground deterioration in especially O.C. fissured clays and
marls.
a)changes in soil type
b)drilling rates
c)water levels
d)flushing losses or gains stoppages
must be recorded.
Tendon Installation (Homing)
 Tendons should be stored indoors in clean dry conditions.
 If they are outdoors, should be stacked off the ground and be completely covered by a waterproof branda/tarpaulin (air
circulation and avoid condensation)
 Tendons should not be dragged through surface soil and handled with care (carried by someway)
 Minimum grout cover in BH’s :
centralizers min 10 mm
between centralizers min 5 mm
1-3 m spacing
 Spacers for multiunit tendons : min 5 mm spacing
 Sleeve or nose cone at the bottom of tendon to reduce the risk of BH damage during homing.
 Check that there is;
-no damage to tendons, components
-no corrosion
If the assembly is more than 200 kg, use mechanical systems otherwise damage may occur.
(Funneled entry pipe may be sometimes used to ease the homing operation.)
Another check : Take out the tendon and inspect what has happened! Centralizers, spacers, smear of clay, damage, distortion,
etc.
Grouting
Grouting performs one or more of the following functions:
i)
To form the fixed anchor in order that the applied load may be transferred from the tendon to the surrounding
soil.
ii)
To augment the protection of the tendon against corrosion
iii)
To strengthen the soil immediately adjacent to the fixed anchor in order to enchance anchorage capacity
In permeable soils the loss of grout over the fixed anchor length should be checked observing the controlled grout flow
coupled with a back pressure. The efficiency of fixed anchor grouting can be finally checked by monitoring the response of
the soil to further injection when back pressure should be quickly restored.
In general if the grout volume exceeds 3 times the BH volume for injection pressures less than total overburden pressure,
then general void filling is indicated which is beyond routine anchor construction.
Preparation of grout :
*Weigh dry mass of cement, water (lt)
*Mechanical mixing at least 2 minutes (homogeneous mix)
*Thereafter the grout should be kept in continuous movement e.g. slow agitation in a storage tank.
As soon as practicable after mixing, the grout should be pumped to its final position. Do not use after its initial
setting time.
High speed colloidal mixers (1000 rpm) and paddle mixers (150 rpm) are used. High speed mixers are preferable.
Pumps should be -of the positive displacement type.
-capable of exerting discharge pressures of at least 1000 kPa.
Rotary screw (constant pressure) or reciprocating ram and piston (fluctuating) pumps are acceptable in practice.
Before grouting all air in the pump and line should be expelled and suction circuit of the pump should be airtight.
During grouting, the level of grout in the supply tank should not be drawn down below the crown of the exit pipe, as otherwise
air will be injected.
An injection pressure of 20 kPa/m depth of ground is common in practice.Where high pressures that could
hydrofracture the ground are permitted careful monitoring of grout pressure and quantity over the fixed anchor
length is recommended.
If on completion of grouting, the fluid grout remains adjacent to the anchored structure then the shaft grout
should be flushed back 1 to 2 m to avoid a strut effect during stressing.
Quality control to grout prior to injection :
-initial fluidity by flow cone or flow trough
-density by mud balance
-bleed by 1000 ml graduated cylinder (75 mm diameter)
Record
1.Air temperature
2.age of constituents
3.grouting pressure
4.quantity of grout injected
5.tests conducted etc.
Anchor Head
After final grouting and testing, cutting of the tendon should be done by disc-cutter (without head).
Projected tendons should be protected against accidental damage.
Stressing
Stressing is required to fulfill two functions, namely;
i)
To tension the tendon and to anchor it at its secure load
ii)
To ascertain and record the behaviour of the anchor so that it can be compared with the behaviour of
control anchors, subjected to on-site suitability tests.
Stressing operation means:
1. Fitting of the jack assembly on to the anchor head.
2. The loading or unloading of the anchor including cyclic loading where specified.
3. Complete removal of the jack assembly from the anchor head.
Experienced crew is essential. Calibration is essential.Apart from pressure gauges on the jacks load cells are recommended like
in case of pile testing.Jacks must be calibrated at least every year.
Accuracy < 0.5 %
Loading-unloading friction hysterisis should be determined in the tests.
Load cells should be calibrated after every 200 stressings or after 60 days in use whichever is more frequent. If complementary
pressure gauges used simultaneously indicate no significant variation  calibration interval is up to 1 year.
Pressure gauges : calibrate after every 100 stressings or after every 30 days (whichever is more frequent)
On every contract specify method of tensioning to be used and the sequence of stressing (and level)
No tendon should be stressed beyond 80% of the characteristic strength.
Grout should reach 30 MPa strength.
In sensitive soils (clay,marl etc.) number of days before stressing may be longer.
After stressing this load will be the readings for future readings, then perform check-lift load measurement.
Provide safety during stressing.
Corrosion Protection
There are cases of corrosion (localized)failures (35 in the literature)
*All permanent anchors
*All temporary anchors exposed to aggressive conditions
should be protected.
Degree of protection depends on:
1) Consequence of failure
2) Aggressivity of the environment
3) Cost of protection
4) ...
Overall protection is required.
Anchor category
Class of protection
Temporary
Temporary anchors without protection
Temporary anchors with single protection
Temporary anchors with double protection
Permanent
Permanent anchors with single protection
Permanent anchors with double protection
Table. Proposed classes of protection
-Purpose of outer barrier is to protect inner barrier against the possibility of damage during handling and placement.
-Protective systems should aim to exclude a moist gaseous athmosphere around the metal by totally enclosing it within an
impervious covering or sheath.
-Cement grout injected in-situ to bond the tendon to the soil does not constitute a part of a protective system. (differential
strains, cracks etc.)
Non-hardening fluid materials such as greases also have limitations such as corrosion protection media;
Because;
i) Fluids are susceptible to drying out (usually accompanied by shrinkage and change in chemical properties)
ii) Fluids are liable to leakage
iii) Fluids having virtually no shear strength are easily displayed and removed from the tendon or metal pieces.
iv) Their long-term chemical stability not known with confidence.
These aspects require that non-hardening materials are themselves protected or contained by a moisture proof robust form of
sheathing which must itself be resistant to corrosion.
Nevertheless, non-hardening fluids such as grease fullfill an essential role in corrosion protection systems, in that
1..They act as a filler to exclude atmosphere from the surface of a steel tendon, create the correct electrochemical environment
and reduce friction in the free length. Also used on anchor head.
2..Use of thicker metal sections for the tendon is not a solution because corrosion does not uniformly operate.
Protective Systems
There is a variety of protective coatings or coverings.
The principles of protection are the same for all parts of the anchorage (details are different) : tendon bond length, free
length and anchor head
Free Length
Inject solidifying fluids to enclose tendon or by pre-applied coatings or combination of both.
Protective system should permit reasonably unhibited extension of the tendon during stressing, and thereafter, if the
anchor is restressable.
Greased and sheathed tendons are a popular solution in such circumstances.
No metallic coatings are recommended.
Bond Length
Requires the same degree of protection and transmits high tendon stresses to the ground.
Strength and Deformability Characteristics
No creep and no cracking is desired in bond length.
Epoxy and polyester resins may be used in encapsulations.
cementitious grouts are cheaper.
Stress\strain behaviour of resins and plastic duets (compatibility) must be considered.
for effective load transfer ducts are corrugated
Restressibility should be possible.
TESTING
There are three categories of anchor testing:
1. proving tests
2. on-site suitability tests (identical conditions similar to working loads)
3. on-site acceptance tests
1. Proving Tests
Several variables (fixed end length and others)
Thisi is a rigorous test program : Procedures\Soil conditions\Materials\Level of safeties all studied in detail (e.g.
grouting different ways)
2. On-site Suitability Tests
These tests are performed under identical conditions similar to working anchors. They are loaded in the same way
and at the same level.They are performed (In advance of main contract or on selected working anchors. Period of
monitoring should be sufficient to ensure that prestress or creep fluctuations stabilize.)
3. On-site Acceptance Tests
Every anchor should be tested;
Check transfer of load to fixed zone
Check capacity of anchor
Apply greater load than design load in shorter time
Compare with on-site suitability tests which are performed in longer time (long term behaviour.
Proof Load : Temporary anchors
Permanent anchors
Short duration : To save time and money
1.25
1.50
1. Proof load-Time Data
Temporary Anchors
Load increment (%Tw)
Permanent Anchors
Load increment (%Tw)
Minimum Period of
Observations (min)
1st load cycle*
2nd load cycle
1st load cycle
2nd load cycle*
10
10
10
10
1
50
50
50
50
1
100
100
100
100
1
125
125
150
150
15**
100
100
100
100
1
50
50
50
50
1
10
10
10
10
1
*This
cycle may include deformations due to wedge ‘pull-in’; bearing plate settlement, initial fixed anchor displacement.
5 min readings.
**Take
If proof load does not reduce more than 5% in 15 min (after allowing for any movement of the anchored structure)
Anchor is OK.
If not, two further proof load cycles; if fail again 5% criterion new load
Alternatively: Proof load can be maintained by jacking and the anchor head monitored after 15 min. in which case the
criterion is 5% Xe (elastic displacement of the tendon=Displacement monitored at proof load – displacement at datum
load i.e. 10%Tw)
If the tests fail diagnosis a.Grout-tendon
b.Grout-soil
c.?
2. Apparent Free Tendon Length
At .Es .X e
ApparentFr eeTendonLe ngth 
At : steel cross-section
T
Es : elastic modulus of steel
Xe : elastic displacement of the tendon (disp. Monitored at proof load-disp. at datum load i.e. 10%Tw) During
destressing stage of 2nd cycle.
T : Tproof-10%Tw
I.
II.
III.
Apparent free tendon length should not be less than 90% of actual free tendon length in design
Apparent free tendon length should not be more than actual (intended) free length+50%of tendon
bond length
Apparent free tendon length should not be more than 110% of the actual free tendon length
III is for
a.short encapsulated bond lengths
b.fully decoupled tendons with an end plate or nut
If outside the limits  Diagnose (Es may be 10% less in strands)
If behaving elastically may be considered OK. (i.e. when the lengths are near the criteria.)
3. Short Term Service Behaviour
Residual load criteria
Permissible loss of
load (% initial
residual load)
Period of Observation
Min
Using load cell
0.5% Accuracy
If lift-off check
5% accuracy or
more
Permissible
displacement (% of
elastic extension e
of tendon at initial
residual load)
%
%
5
1
1
15
2
2
50
3
3
150
4
4
500
5
5
1500 (1 day)
6
6
5000 (3 days)
7
7
15000 (10 days)
8
8
Using properly calibrated load cells and logging equipment residual load may be monitored at 5, 15, 50
mins.
If rate of load loss reduces to 1% or less per time interval after allowing for temperature, structural
movements, relaxation of the tendon  Anchor is OK.
If the rate is more than 1%  further readings up to 10 days
If does not satisfy the criterion
a. abandone and replace
b. reduce in capacity
c. subject to remedial stressing programme
Alternative to load monitoring  Displacement-Time data at the residual load at the specific
observation periods in the table.
Rate of displacement should reduce to 1% e or less per time interval. (resort to the table)
e =
Initial residual load x apparent free tendon length
area of tendon x elastic modulus of tendon
If prestress gains (more than 10% Tw each time) are recorded;
A) Insufficient anchor capacity or overall slope failure
B) Capacity  destress and provide additional support