Presentation on theme: "CHAPTER 4 SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTS GROUND ANCHORS (or ANCHORAGES) 1.Definition2.Design3.Corrosion protection 4.Types5.Materials."— Presentation transcript:
CHAPTER 4 SUPPORTING ELEMENTS : GROUND ANCHORS AND STRUTS GROUND ANCHORS (or ANCHORAGES) 1.Definition2.Design3.Corrosion protection 4.Types5.Materials 6.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 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 Minimum safety factor Proof load factor Tend on Ground/ grout interface Grout/tendon or grout/encapsul ation 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 * 2.0 2.5 * 2.5 * 2.0 2.5 * (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 # 3.0 # 4.0 creep is expected 3.0 * 2.0 3.0 * (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 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 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 & equipment For 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 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. (D 2 -d 2 ) 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 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)
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 D 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 cm (or 1.5d-2d) For very dense sand D is reduced (18-20 cm) (1.2d-1.5d) Slenderness ratio h/D ’’ h/D=25 B.C. component of the above ap. is difficult to assess. A values: (Pgrout<1000 kPa) for compact sandy gravel, ’=40 o, A=1.7 for compact sand, ’=35 o, A=1.4
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 kN/mat 1000 kPa kN/m at 2500 kPa fixed anchor length Pgrout 500 kPa on average R D Tf When R D1 R D2 if U 1 >U 2 Tf U1 >Tf U2 L fixed after 10m no increase in Tf. In 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 (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 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= ) 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. . c u Tf : ultimate load capacity d : borehole diameter L : fixed anchor length : adhesion factor (stiff soils ) c u : average undrained strength over the fixed anchor length
Type D Anchors Tf= . D. L. c u + /4. (D 2 -d 2 ). Nc. c ub + . d. l. c a Side Shear End bearing on clay side shear along shaft length D : diameter of underream L : length of fixed anchor c u : average undrained shear strength along fixed anchor Nc : B.C. factor take q c ub : undrained shear strength at the end l : the length of the shaft (m) c a : shaft adhesion c u (kPa) Reduction coefficients 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 c u >90 kPa (also problemmatic for kPa, not possible for c u <50 kPa), low plasticity PI<20 Fixed anchor length in clay 3-10 m. Fixed anchor spacing 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 D : diameter of fixed anchor Tf = . D. L. wet 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 q u ( wet limit = 400 kPa) L fixed anchor : 3 m. min Table 24 p.131 BS8081 : 1989 Rock/Grout Bond values Very poor rocks : u 1.5*10 2 – 2.5*10 2 kPa Marls 3.5*10 2 kPa Shale 3.7*10 2 kPa Soft sandstone+shales (working 1-1.4*10 2 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/mm 2 ) Clean crimped wire1500 kPa Clean strand or deformed bar 2000 kPa For min grout compressive strength of 30 N/mm 2 (30000 kPa, 300 kg/cm 2 ) 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/mm 2 (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 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 between 0.40 & 0.70 there are applications.Higher values in sandy alluvial deposits. -There are limits for total sulphate content (4% (m/m) SO 3 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 AdmixturesChemicalOptimum dosage of % cement by weight AcceleratorCaCl 2 Calcium chloride1-2Accelerates set & hardening RetarderCalcium lignosulfonate Also increases fluidity May affect set strengths Tartaric acid Sugar FludifierCalcium lignosulfonate Entrains air Detergent0.5 ExpanderAluminum powder 15% expansion AntibleedCellulose ether Equiv. to 0.5% of water Entrains air Aluminum 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.
-Recommended unconfined comp. Strength of grout : 30 MPa 7 days 40 MPa 28 days -Bleeding of (tendon bonding) grout at 20 o 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 Nom. Dia. mm (Ult. Load) Specified characteristic strength (kN) Nom. Steel Area mm 2 Non-alloy steel wire 7-wire strand 7-wire drawn strand Low alloy steel bar Grade 1030/835 Grade 1230/ Stainless steel Wire Bar
Anchor Head Stressing head + bearing plate (anchor plate) Tendon is anchoredtendon 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 5 o from the axial position with no effect on ultimate capacity. 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. 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 2.5 o except in case of closely spaced design + staggered design. o 10 o to facilitate grouting Overall drill hole deviations 1/30 (1.9 o ) 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 typeb)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 temperature4.quantity of grout injected 2.age of constituents5.tests conducted etc. 3.grouting pressure 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 categoryClass of protection TemporaryTemporary anchors without protection Temporary anchors with single protection Temporary anchors with double protection PermanentPermanent 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 anchors1.25 Permanent anchors1.50 Short duration : To save time and money
1. Proof load-Time Data Temporary Anchors Load increment (%Tw) Permanent Anchors Load increment (%Tw) Minimum Period of Observations (min) 1 st load cycle * 2 nd load cycle1 st load cycle2 nd load cycle * ** * This cycle may include deformations due to wedge ‘pull-in’; bearing plate settlement, initial fixed anchor displacement. ** Take 5 min readings.
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%T w ) If the tests fail diagnosis a.Grout-tendon b.Grout-soil c.? 2. Apparent Free Tendon Length A t : steel cross-section E s : elastic modulus of steel X e : elastic displacement of the tendon (disp. Monitored at proof load-disp. at datum load i.e. 10%T w ) During destressing stage of 2 nd cycle. T : Tproof-10%T w I.Apparent free tendon length should not be less than 90% of actual free tendon length in design II.Apparent free tendon length should not be more than actual (intended) free length+50%of tendon bond length III.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 Period of Observation Permissible loss of load (% initial residual load) Permissible displacement (% of elastic extension e of tendon at initial residual load) Min% Using load cell 0.5% Accuracy If lift-off check 5% accuracy or more ( 1 day) ( 3 days) ( 10 days) 88
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 criteriona. 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) 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 Initial residual load x apparent free tendon length e = area of tendon x elastic modulus of tendon