Presentation on theme: "CHAPTER 4 SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTS "— Presentation transcript:
1 CHAPTER 4SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTSGROUND ANCHORS (or ANCHORAGES)1.Definition 2.Design 3.Corrosion protection4.Types 5.Materials6.Construction Testing*Drilling (or driving) *Capacity prediction*Tendon (manufacture & assembly) *Quality control*Anchor homing (installation) *Monitoring*Grouting*StressingDefinition: 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 nailsPrestressed anchors
2 Type A Type B Type C Type D 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 interfaceType 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 shearType 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 depositsType D-Tremie grouted holes-bells or underreams formed-firm to hard cohesive soilsOthers : Jet grouting , expand bodies , use of explosives , splitting of anchor bulpRock A or packer grouted AFor improving rock/grout bond also B
3 Grout/tendon or grout/encapsulation interface Anchorage categoryMinimum safety factorProof load factorTendonGround/grout interfaceGrout/tendon or grout/encapsulation interfaceTemporary 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*2.02.5* (if no tests)1.25Permanent 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.2.03.0#3.0#4.0 creep is expected3.0*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.Design of Anchors*Fixed anchor dimensions*Depth of embedment*Overall stability*Group effectsMain 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 deteriorationFig. Minimum safety factors recommended for design of individual Anchors
4 v’: average overburden effective pressure Ground-grout interface in cohesionless soilsGround-grout interface in cohesive soilsGround-grout interface in rock1.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 cm have been observed to be up to kN ( 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 resistanceL= 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 kPa in-situ stress fileds & dilation character.n kN/m in coarse sands and gravels, k>10-4 m/sec (10-2 cm/sec)n kN/m in fine to medium sands, k= m/sec ( cm/sec)A : the ratio of contact pressure at the fixed end anchor/soil interface to the effective overburden pressurev’: average overburden effective pressureL : 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) diameterEnlarged diameter =38-61 cmMaking use of bearing capacity theory an alternative solution is:Tf = A . v’ . . D . L . tan’ + B . . h . /4 . (D2-d2)
5 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 cm (or 1.5d-2d)For very dense sand D is reduced (18-20 cm) (1.2d-1.5d)Slenderness ratioh/D’2630343740151120437514391941741402581873139h/D=25B.C. component of the above ap. is difficult to assess.A values: (Pgrout<1000 kPa) for compact sandy gravel, ’=40o , A=1.7for compact sand, ’=35o , A=1.4
6 Type C AnchorsTheoretical predictions of load capacity are not reliable. Design curves are obtained from field (actual) load tests.In alluvium medium sandvariable deposits of sand & graveld=10-15 cm kN/m at kPakN/m at kPafixed anchor lengthPgrout500 kPa on averageRD TfWhen RD1RD2 if U1>U2 TfU1>TfU2Lfixed after 10m no increase in Tf.In kPa grout pressure range Tf increase is not muchUnit skin friction for sand 500 kPa maxsandy gravel 1000 kPa maxUnit skin friction N80 kPa – 350 kPa (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
7 Type A Anchors (Tremie grouted straight shaft) Similar to bored holesTf = . d . L . . cuTf : ultimate load capacityd : borehole diameterL : fixed anchor length : adhesion factor (stiff soils )cu : average undrained strength over the fixed anchor lengthType C Anchors-high grout pressures-with or without post-grouting-ultimate capacity can not be calculatedIc : consistency index m : natural moisture contentSkin friction m increases with increasing consistency & decreasing plasticity.In stiff clays (Ic= ) 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)
9 Tf= . D . L . cu + /4 . (D2-d2) . Nc . cub + . d . l . ca Side Shear End bearing on clay side shear along shaft lengthD : diameter of underreamL : length of fixed anchorcu : average undrained shear strength along fixed anchorNc : B.C. factor take qcub : undrained shear strength at the endl : the length of the shaft (m)ca : shaft adhesion cu (kPa)Reduction coefficients due to construction techniques underream geometry0.5 for open or sandfilled fissures in claydrilling – 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 kPa , not possible for cu<50 kPa), low plasticity PI<20Fixed anchor length in clay 3-10 m.Fixed anchor spacing m. minSpacing to any adjacent foundation/underground service 3 m. minDistance to surface foundation 5 m. minType D Anchors
12 Tf = . D . L . wet L : length of fixed anchor Fixed Anchor Design in RockType 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 distributionD : diameter of fixed anchorTf = . D . L . wet L : length of fixed anchorwet : ultimate bond or skin friction at rock/ grout interfaceIn weak & deformable rock stress concentrationsTendon/grout failures initiate grout/rock interface failuresStrong rock : 10 % of qu (wet limit = 400 kPa)Lfixed anchor : 3 m. minTable 24 p.131 BS8081 : Rock/Grout Bond valuesVery poor rocks : u1.5*102 – 2.5*102 kPa Marls3.5*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 : kPa (1.0 N/mm2)Clean crimped wire 1500 kPaClean strand or deformed bar kPaFor 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-situ2m where tendon homed & bonded under factory controlled conditions
13 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) removeAsteel 15% borehole area for multi unit tendons 20% borehole area for single unit tendonsEncapsulations For Rock Bolt Recommended by manufacturerAt grout/encapsulation interface max. ultimate bond 3 N/mm2 (3000 kPa) unless adequately provenFig. 11 BSEncapsulations 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.
15 MaterialsCementOrdinary 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 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 suitableno oil, organic matter, deterious substancessulphate <0.1%chloride ions 500mg/1 literAdmixturesChemicalOptimum dosage of % cement by weightAcceleratorCaCl2 Calcium chloride1-2Accelerates set & hardeningRetarderCalcium lignosulfonateAlso increases fluidityMay affect setstrengthsTartaric acidSugarFludifier Entrains airDetergent0.5ExpanderAluminum powder15% expansionAntibleedCellulose etherEquiv. to 0.5% of waterEntrains airAluminum sulphate20
16 Excess water results in bleeding of the mix and low strength, as well as greater shrinkage and lower durability of the hardened grout.
17 -Recommended unconfined comp. Strength of grout : 30 MPa 7 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.)TENDONTendons 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.
19 In such cases the stress in the shortest strand should limit the acceptable working load.If the design requires uniform stresses within the tendon, monostrand stressing is essential.Centralizers and spacers should be provided.Anchor HeadStressing head + bearing plate (anchor plate)Tendon is anchored tendon force is transmitted to the structureHead 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 detensionableshould permit an angular deviation of 5o from the axial position with no effect on ultimate capacity.Construction1.Method of drilling (with or without flushing)2.Tendon installation3.Grouting system Time period of the above operations (1,2&3) may influence the capacity of the anchorage.DrillingAny drilling procedure that can supply a stable hole free of obstructions may be used.minimum disturbance or disturbance most beneficial to capacityCare 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 toleranceo drill up tolerance is 2.5o except in case of closely spaced design + staggered design.o 10o to facilitate groutingOverall 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 drillingprobe 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 ratesc)water levels d)flushing losses or gains stoppagesmust be recorded.
20 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 mmbetween centralizers min 5 mm1-3 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 corrosionIf 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.GroutingGrouting 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 corrosioniii) To strengthen the soil immediately adjacent to the fixed anchor in order to enchance anchorage capacityIn 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.
21 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.
22 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)Record1.Air temperature 4.quantity of grout injected2.age of constituents 5.tests conducted etc.3.grouting pressureAnchor HeadAfter final grouting and testing, cutting of the tendon should be done by disc-cutter (without head).Projected tendons should be protected against accidental damage.StressingStressing is required to fulfill two functions, namely;i) To tension the tendon and to anchor it at its secure loadii) 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.
23 Experienced crew is essential. Calibration is essential 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.
24 Corrosion ProtectionThere are cases of corrosion (localized)failures (35 in the literature)*All permanent anchors*All temporary anchors exposed to aggressive conditionsshould be protected.Degree of protection depends on: 1) Consequence of failure2) Aggressivity of the environment3) Cost of protection4) ...Overall protection is required.Anchor categoryClass of protectionTemporaryTemporary anchors without protectionTemporary anchors with single protectionTemporary anchors with double protectionPermanentPermanent anchors with single protectionPermanent anchors with double protectionTable. Proposed classes of protection
25 -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 leakageiii) 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 that1..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.
26 Protective SystemsThere 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 headFree LengthInject 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 LengthRequires the same degree of protection and transmits high tendon stresses to the ground.Strength and Deformability CharacteristicsNo 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 corrugatedRestressibility should be possible.
27 TESTINGThere are three categories of anchor testing:1. proving tests2. on-site suitability tests (identical conditions similar to working loads)3. on-site acceptance tests1. Proving TestsSeveral 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 TestsThese 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 TestsEvery anchor should be tested;Check transfer of load to fixed zoneCheck capacity of anchorApply greater load than design load in shorter timeCompare with on-site suitability tests which are performed in longer time (long term behaviour.Proof Load : Temporary anchors 1.25Permanent anchors 1.50Short duration : To save time and money
28 1. Proof load-Time Data Temporary Anchors Load increment (%Tw) Permanent AnchorsMinimum Period ofObservations (min)1st load cycle*2nd load cycle1st load cycle2nd load cycle*1015010012515015***This cycle may include deformations due to wedge ‘pull-in’; bearing plate settlement, initial fixed anchor displacement.**Take 5 min readings.
29 2. Apparent Free Tendon Length 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 loadAlternatively: 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-tendonb.Grout-soilc.?2. Apparent Free Tendon LengthAt : steel cross-sectionEs : elastic modulus of steelXe : 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%TwApparent free tendon length should not be less than 90% of actual free tendon length in designApparent free tendon length should not be more than actual (intended) free length+50%of tendon bond lengthApparent free tendon length should not be more than 110% of the actual free tendon lengthIII is for a.short encapsulated bond lengthsb.fully decoupled tendons with an end plate or nutIf 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.)
30 3. Short Term Service Behaviour Residual load criteriaPeriod of ObservationPermissible loss of load (% initial residual load)Permissible displacement (% of elastic extension e of tendon at initial residual load)Min%Using load cell0.5% Accuracy51152503If lift-off check5% accuracy or more15045001500 (1 day)65000 (3 days)715000 (10 days)8
31 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 daysIf does not satisfy the criterion a. abandone and replaceb. reduce in capacityc. subject to remedial stressing programmeAlternative to load monitoring Displacement-Time data at the residual load at the specificobservation periods in the table.Rate of displacement should reduce to 1% e or less per time interval. (resort to the table)If prestress gains (more than 10% Tw each time) are recorded;A) Insufficient anchor capacity or overall slope failureB) Capacity destress and provide additional supportInitial residual load x apparent free tendon lengthe =area of tendon x elastic modulus of tendon