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Prestressed Rock
Anchors
Prestressed Rock Anchor
A prestressed rock anchor is a high strength steel tendon, fitted with a stressing anchorage at one end and a means permitting force transfer to the grout and rock on the other end. The rock anchor tendon is inserted into a prepared hole of suitable length and diameter, fixed to the rock and prestressed to a specified force.
Applications
Ground anchors are used in civil engineering for the following applications: To resist lateral thrust on retaining walls and
in-situ diaphragm walls, For stabilizing of slopes and land slides, To resist uplift on basements and other
foundations, To strengthen masonry and concrete
dams, and For testing of large diameter piles.
Applications Include
Retaining walls Tower bases Concrete structures Concrete form hardware Tunnels Mines Dams Bridges Slope stabilization Rock fall protection
Typical Details of Prestressed Rock Anchor
Fixed Length - The length of the anchor which is grouted in, from which the pullout capacity of anchor is achieved.
Free Length - The part of the anchor which is not bonded to the surrounding area and is free to elongate.
Terminology
Necessary Data
a) Service life of anchor (temporary or permanent);b) Design load per anchor; andc) Soil investigation for following factors:
i) Complete borehole log indicating types of soil and rock encountered with respect to depth. The depth of penetration into rock with core drilling should be minimum 10 m
ii) Un drained shear strength and bulk density at different depths
iii)Shear strength and unit weight of rockiv) Sulphate and chloride contents in soil as well as ground
waterv) Permeability of rock and fissurs -pervious zones water table
and artesian head if any recorded on boreholes
Method of Construction
Drilling
Drilling Through Overburden - Drilling through overburden is normally carried out by suitable equipment. For keeping the side stable, either temporary easing is provided or bentonite mud circulation is used. The size of the hole depends upon the capacity of the anchor. In case of inclined bores use of casing tube shall be obligatory.
Drilling Through Rock - Drilling through rock is carried out by using either rotary method with water flush or using pneumatic percussion method with air and/or water flush.
Percussion Drilling
Water Proofing of Hole
After drilling through rock a water test is carried out.
The anchorage length (fixed length) for all permanent anchors has to be tested for water-proofing to avoid corrosion and if water loss is found to be excessive the hole is grouted.
The grout is then re-drilled and water test repeated and the whole procedure is repeated till satisfactory lugeon value is obtained.
Lugeon: Lugeon is defined as flow of water in liters/minutes/meter length of the test section at a pressure of 10 kg/cm2.
Outlet
Inlet
Pressure Gauge
Arrangement for Water Test
Fabrication of Anchors
Anchors can be shop fabricated or fabricated on site depending upon the construction requirements.
Anchors shall be free of dirt, detrimental dust or any other deleterious substance. Anchors shall be handled and protected prior to installation to avoid corrosion or physical damage.
Fabrication of Rock Anchor
Required number of strands is to be cut to the required length. Cutting of cable is to be derived as Fixed length + Free length + extra 1m (for stressing)
Strands are cleaned thoroughly and oil on the surface is removed. 1st coat of epoxy paint is to applied immediately with the help of brush. After 24hrs 2nd coat of epoxy paint is to be applied on strands. Quartz sand is to be sprinkle on fix length of strands when 3rd coat is tacky.
Free length of strand is to be cleaned with the help of emery paper. Place the plain flexible HDPE pipe, 2mm thick on free length portion.
The pre-stressing strands, greased for the free length portion should be enclosed in HDPE sleeve, 2mm thick encasing individual strand.
Fixed length of the cable is then provided corrugated HDPE sheathing 2mm thick.
Centralizers fabricated from plastic or steel, are provided to ensure min 5mm grout cover to anchor in fix length.
Fix shoe with the help of brazing at bottom of cable. Then tie spacer at 1m c/c through out the length of cable.
Spacers & Centralizers
Spacers: The purpose of a spacer is to help insure that grout surrounds each strand for corrosion protection and for bond strength development. Designers should specify the desired distance between spacers (typically 7'-10').
Centralizers are placed over the assembled strand bundle in order to maintain the required spacing between the anchor and the borehole so that an adequate thickness of grout (minimum 0.5") surrounds the anchor. A wide variety of centralizers are available depending upon the anchor type.
Lowering Anchors
Anchors are placed in accordance with type of anchors.
Suitable guide system and temporary fixing of the anchor is required to avoid movement of anchor during grouting.
Grout tubes are checked to ensure that they are free.
Suitable spacers are also provided when required to ensure that anchor assembly does not get entangled.
Fixed Length Grouting
After the anchor is lowered, the fixed length of the anchor is grouted.
Grouting is carried out under pressure (@ 4-8 kg/cm2) by fixing a packer at the top of the fixed length or as necessary in accordance with the type of anchor.
Normally thickest possible grouting (0.5 water cement ratio) is adopted for primary grouting.
Adequate care is required so that the free length of the anchor remains free to elongate.
Stressing
Stressing is carried out after 21 days of grouting when, it attains the required strength.
Depending on the different types of anchors used, details of the stressing jack vary.
The anchor is stressed for about 10 percent of the load and elongation measurements taken beyond this range.
This takes care of any seating errors. Anchor is subsequently stressed to 10 percent excess
load over the design and elongation noted. After noting the elongation, the anchor is locked either
to design load or part of the design load depending on the requirements.
Setup for Stressing
Jack – K 200
12 Strands of 12.7 mm
dia
Pressure Application
Elongation Measurement
Stressing of inclined Rock Anchor
Case study
Project: Construction of Office Complex for Canara Bank at Bandra Kurla Complex, Mumbai
Client : Canara BankConsultant : STUPPMC : Gherzi Eastern Ltd.Contractor : NBCC Ltd.Sub Contractor: Freyssinet Presstressed
Concrete Co. Ltd.
Front View
Design for Rock Anchor
Design Load: Design load is a assign load on anchor after allowance for all losses. In this case the design load is 50 T.
High Tensile StrandNos of high tensile strand required for 1.75 x 50 T capacity of anchor:Minimum breaking load of 12.7mm dia HT strand = 18737KgFactor of safety for tension in strand = 1.5(min)U.T.S. of cable = 18737/1.5
= 12.49 TNo of HT strand required for 55 T Design load =87.5/12.49
= 7 nosHence use 12 K13 Anchorage System i.e. 7 nos of 12.7mm dia HT Strand for each anchor.
Continue…
Fixed Length Capacity of anchor = 55 T (with losses)
Diameter of Hole = 110mmBond stress between strata & grout = 2.9 kg/cm2
Fixed length = 55000/(3.14 x 11 x 2.9)= 548.8 cm~ 550.0 cm
Free length Minimum 5.0 m (As per IS-10270 : 1982)
Total Length of Anchor Fixed + Free = 10.5 m
Estimate for Rock Anchors works
Sr. No.
Description of ItemEstimated Quantity
Unit Rate (Rs.) Amount (Rs.)
1Designing, providing and installing working permanent vertical rock anchors
a) 50 T net design capacity 60 nos 38000 2280000
b) 75 T net design capacity 10 nos 47500 475000
2Designing, providing and installing non-working vertical rock anchors
a)Performance test for 50 T design (net) capacity of the pre-stressed anchor
1 no 75000 75000
b)Load test to failure for 50 T design (net) capacity of the pre-stressed anchor
1 no 75000 75000
Total Amount (Rs.) 2905000
Water Test: Observation Table
Sr. No.Applied
Pressure (MN/m2)
Water meter reading (ltr)
Time (minute)
Water loss (ltrs/min/m
tr)
Lugeon Value (water
loss/pressure)
Average Lugeon ValueInitial
ReadingFinal
Reading
1 0.05 981.3 981.5 5 0.007 0.15
0.15
2 0.10 982.1 982.4 5 0.011 0.11
3 0.15 983.4 984.0 5 0.022 0.15
4 0.10 985.6 985.9 5 0.011 0.11
5 0.05 986.3 986.6 5 0.011 0.22
Water Test
Conclusion:Water loss is to be finding out in
terms of Lugeons. The lugeon value achieved is 0.15 which is less than 3.0 as per IS: 10270-1982. Hence the test was passed.
Pullout Test
Objective: To study the performance of tendon
anchorage assembly under the application of pre-stressing force.
To observe for any premature failure – any permanent deformation and abnormal behavior during application of load.
To measure the elongation of tendons during loading.
Test Setup and Procedure
The loading mechanism consist of 12 nos HT strand 12.7mm dia, Freyssinet K-200 Jack and electrically operated high pressure pump.
The electrically operated high pressure pump was equipped with calibrated pressure gauge and least count of 5 kg/ cm2.
The stressing equipment was placed over the rock anchor tendon in such a manner that the jack, bearing plates and stressing anchorage are axially aligned with the tendon and the tendon was centered within the equipment.
The pullout test was performed by incrementally loading the ground anchor in accordance with the schedule given in the attached format.
Test Setup and Procedure
The load was raised from one increment to another, immediately after recording the ground anchor movement up to the failure of anchor.
The anchor failure may occur due to following Anchor Grout Bond failure HT strand failure
The load shall be monitored with a pressure gauge.
After stressing the plot of ground anchor movement versus load for each increment in the failure test is as under.
Elongation v/s Pressure Graph
0 50 100 150 200 250 300 350 400 4500
20
40
60
80
100
120
140
160
81
96
110113118
127133
140149
Pressure in kg/cm2
Elo
ng
ati
on
in m
m
Conclusion
Anchor failed at 400 kg/cm2 pressure. The failure occurs due to grout bond failure.
No failure of the strand or distortion of anchorage was observed after dismantling the test set-up.
Research Paper
Rock Anchors for Dams, The National Research Project: The
Evolution of Post-tensioned Anchors on Hydropower Dams
By Dr. Donald A. Bruce, Geosystems, L.P., USA and John S. Wolfhope, P.E., Freese and
Nichols, Inc., USA
Abstract
Rock anchors provide a cost-effective and low maintenance solution for strengthening hydropower dams.
Prestressed rock anchors have been used to stabilize dams and appurtenant structures in North America for over three decades.
The goal of the National Research Program on Rock Anchors for Dams is to advance the awareness and understanding of the use of post-tensioned anchors throughout the dam and hydropower industries.
The prime objectives are to: a) produce a definitive and detailed list of all the North American dam anchor projects, b) trace the evolution of practice via an analysis of codes and specifications, and c) to project the market for post-tensioned anchors on large dams and hydropower facilities.
Over 390 case studies have been collected and studied.
The research approach for the National Research Program was as follows: Conduct Survey of dam anchoring practice. Collect all published books and technical
papers. Develop an Internet-based interactive
database repository of information related to anchoring of dams.
Review the evolution of North American anchoring practice and technical guidance documents.
Characterize the market for prestressed rock anchors in dams.
Histogram of Anchored Dams
Conclusions
• Prestressed rock anchors have been used successfully over the past thirty years on nearly four hundred dams and hydropower facilities in North America.
• Prestressed rock anchors provide a cost-effective, environmentally acceptable low maintenance solution for rehabilitating dams to meet modern design standards.
• North American practice has evolved substantially over the past four decades through emphasis on codes, technical specifications, and improvements in construction techniques. Particular attention has been paid to corrosion protection.
• Post-tensioned anchors have a long history of being successfully applied on FERC regulated hydropower facilities in a wide range of geometries and capacities.
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