ANALYSIS, DESIGN AND CONSTRUCTION OF TIE-BACK ANCHOR FOR COFFER DAM

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 6, November-December 2016, pp , Article ID: IJCIET_07_06_040 Available online at ISSN Print: and ISSN Online: IAEME Publication ANALYSIS, DESIGN AND CONSTRUCTION OF TIE-BACK ANCHOR FOR COFFER DAM H.P. Santhosh Assistant Professor, Department of Civil Engineering, MSRUAS, Bengaluru, Karnataka. India R.K. Chethan Gowda Assistant Professor, Department of Civil Engineering, MSRUAS, Bengaluru, Karnataka, India Dr. H.M. Rajashekhara Swamy Professor & Head, Department of Civil Engineering, MSRUAS, Bengaluru, Karnataka, India ABSTRACT The concept of a rock anchor system is to create an internally stable mass of soil and also to stabilize the cofferdam that will resist external forces at an adequate level of serviceability. The analysis and design of rock anchor concentrates on achieving secure against a range of potential failure conditions of cofferdam. The design should limit movements of the soil and the cofferdam while providing a practical and economical basis for construction. The design should consider the mobilization of resistance by both anchors and cofferdam elements in response to loads applied to the cofferdam particularly sea water [3]. This paper deals with the analysis, design and installation of cement grouted rock/ground anchors and anchored systems. The analysis, design and construction procedure have been demonstrated through a series of numerical analyse sand a parametric study has also been carried out based on different anchorage capacity. It is concluded that 150 ton anchor capacity is effective in controlling horizontal displacement of cofferdam at different levels. Also this paper deals with the step by step analysis, design and construction procedure implemented at construction site. Key words: Coffer dam, Pre-stressed Anchor, Grout, Bond length, Free length Cite this Article: H.P. Santhosh, R.K. Chethan Gowda and Dr. H.M. Rajashekhara Swamy, Analysis, Design and Construction of Tie-Back Anchor for Coffer Dam. International Journal of Civil Engineering and Technology, 7(6), 2016, pp INTRODUCTION The concept of a rock anchor system is to create an internally stable mass of soil and also to stabilize the cofferdam that will resist external forces at an adequate level of serviceability. The analysis and design of rock anchor walls concentrates on achieving a final constructed cofferdam that is secure against a range of potential failure conditions [6]. The design should limit movements of the soil and the cofferdam while editor@iaeme.com

2 H.P. Santhosh, R.K. Chethan Gowda and Dr. H.M. Rajashekhara Swamy providing a practical and economical basis for construction. The design should consider the mobilization of resistance by both anchors and cofferdam elements in response to loads applied to the cofferdam. A rock anchor is a long, high capacity reinforcement comprising a tube fixed into rock and tensioned to a predetermined load. Rock anchors are used widely to improve the stability and load bearing characteristics of a rock mass [4]. Cable rock anchors utilize bundles of steel wires to form a fixed anchorage depth. In general the anchor length should under no circumstances should be less than either 2.5m or the free length less than 5m. This paper deals mainly, but not exclusively, with the analysis and design of large anchors of the type shown in Fig. 1 and with their application in cofferdam engineering, particularly for concrete gravity, buttress and arch dams using STAAD Pro. Figure 1 Brief Details of Rock Anchor 2. DESIGN BASICS For designing 137 Ton capacity Rock Anchor Taking total stress losses to 15% the anchor is designed for= = Tons. The following are to be evaluated Number of 15.2mm dia strands Bond length of anchor 3. MATERIAL PROPERTIES 3.1. Pre-stressing Strands 15.2 mm diameter seven wire pre-stressed strands Table 1 Design parameters assumed in calculations Components Assumed Parameters Remarks Bond strength at Soil/Grout interface Bond strength at rout/strand interface Ultimate bond stress=0.4n/mm 2 Ultimate bond stress=0.4n/mm 2 As per BS 8081/ IS: As per BS 8081/ IS: editor@iaeme.com

3 Analysis, Design and Construction of Tie-Back Anchor for Coffer Dam Table 2 Safety factors used in design Components Required F.O.S Remarks Strand 2 As per BS 8081/ IS: Rock/Grout Interface 2 As per BS 8081/ IS: Grout/Strand Interface 2 As per BS 8081/ IS: 4. DESIGN FOR TON ANCHOR FORCE 4.1. Design Parameters Working load = Ton Free length =variable Drill hole dia. =150mm 4.2. Strand Properties 7 wires pre-stressing strands conforming to ASTM standards Diameter =15.2 mm Nominal Area=181mm 2 Ultimate strength =260kN=26.0 Ton 4.3. Calculations Number of strands required = h = Bond length required at Ground/Rock Interface = = ~12 " #. $ = " =16.399~ editor@iaeme.com

4 H.P. Santhosh, R.K. Chethan Gowda and Dr. H.M. Rajashekhara Swamy Table 3 Anchor Design Details [1] Sr. No. Description Quantity 1 No. of strands of 15.2 mm 12 Nos. 2 Bottom Cone-Length 30cm-dia 110mm 1 Nos. 3 Fixed length 17m 4 No of spacers in fixed length 14 Nos. 5 Free length 24m 6 Spacers in free length 8 Nos. 7 Separator Plate 1 Nos. 8 Sheathing Pipe-110mm dia. 23.5m 9 Thrust Plate 400x400x25 10 Bearing Plate 12T 15 dynamic prestress 11 Trumpet pipe with bursting reinforcement and top plate of 10mm of 400x400mm 1Nos. 5. GROUTING The purpose of grouting is to provide permanent bond in the fixed length of the tendon. This also helps in stabilizing drilled holes. Grout also fills up void spaces, expelling the water collection therein, if any Grout Properties [1] The cement grout is prepared based on a w/c ratio of 0.45 with 22.5 liters of water, 50kG of cement and admixtures will be added as per requirement 100mm cube samples shall be taken from the grout mix at the mixer for the subsequent test The cement to be used for grouting is OPC (Grade-43) obtained from approved manufacturer/supplier The admixture CONBEX 100/equivalent will be used as 225 gm per cement bag 6. CONSTRUCTION PROCEDURE OF ANCHORING FOR COFFER DAM On completion of cofferdam, temporary earth filling will be carried out around the cofferdam. The width of the temporary fill in the docks shall be 8m width at the top level with natural slope so that the Wagon driller can hae sufficient space to move and anchor the anchors Depending on the site conditions provide proper slope to reach the anchoring level from the top of the cofferdam to the anchor level. The Wagon driller will be positioned in the location and start the inclined boring at required level. Placing and fixing of anchors, as per specifications including grouting will be carried out. On completion of inclined anchoring, excavation of soil up to required level will be done for full length. Fig. 2 shows the sectional elevation of 150 Ton Tie-Back Anchor. The procedure adopted will be same for remaining anchors also. Care should be taken that excavation should not start before completion of anchorage. Table 4 shows the time cycle for anchorage works editor@iaeme.com

5 Analysis, Design and Construction of Tie-Back Anchor for Coffer Dam Sr. No. Figure Ton Tie-Back Anchor for Cofferdam Support Description Table 4 Time Cycle for Anchorage works Quantity unit Rate of Production Unit Wagon Driller Hrs 1 Survey, Shifting, & Positioning 1 1 day Drilling through diaphragm wall - 5 Mtrs 5 3 Placing of MS Pipe for free length 5 10 Mtrs 4 Rock drilling Mtrs 6 5 Primary Grouting 6 30 Mtrs 6 Re-drilling & Flushing - 30 Mtrs 5 7 Homing of Cables -- 8 Secondary Grouting Mtrs Total 16.5 Add 10% of Misc 1.65 Total time taken for 1 Anchor 19 Total time duration required for one Anchor (20hrs/day) 1 7. ANALYTICAL RESULTS Static analysis was performed for cofferdam using STAAD Pro software to obtain displacement response at different level. Fig. 3 shows the STAAD Pro modelling of pre-stressed anchor to the cofferdam. The comparative result of displacement versus various prestressed force forcofferdam is shown in Fig. no editor@iaeme.com

H.P. Santhosh, R.K. Chethan Gowda and Dr. H.M. Rajashekhara Swamy Figure 3 STAAD Pro modelling of pre-stressed anchor Figure 4 Variation of Displacement versus Pre-stressed Force 8.

6 H.P. Santhosh, R.K. Chethan Gowda and Dr. H.M. Rajashekhara Swamy Figure 3 STAAD Pro modelling of pre-stressed anchor Figure 4 Variation of Displacement versus Pre-stressed Force 8. RESULTS AND DISCUSSION The main application of geotechnical ground anchors is to prevent horizontal movement and ensure stability of retaining structures including sheet pile, bored concrete pile and steel tubular pile walls. In addition anchors are used to prevent additional movement or stabilize existing retaining walls, bridge abutments, diaphragm walls and also cofferdams [5]. Cofferdams provides economical and practical alternative to propping and all anchorages are post-tensioned to 110% working load. A series of numerical analysis and parametric study shows that 150 tonne anchor capacity is effective in controlling horizontal displacement of cofferdam at different levels. REFERENCE [1] IS:.Indian Standard Code of Practice for Prestressed Concrete, Bureau of Indian Standards, Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi [2] BS: Code of Practice for Ground Anchorages, British Standards Institution, London, England, [3] Santhosh H.P., Dr. H.M. Rajashekhara Swamy, and Dr. D.L. Prabhakara.(2014) Construction of Cofferdam-A Case Study, IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), e- ISSN: , P-ISSN: X, PP editor@iaeme.com

7 Analysis, Design and Construction of Tie-Back Anchor for Coffer Dam [4] Monali Bhakare and A. R. Rajan.(2015)Case Study of Active Rock Anchor in Basement, International Journal of Innovative Research in Science, Engineering and Technology, ISSN(Online) : , Vol. 4, Issue 3, March 2015 [5] Peter George Fookes and Michael Sweeney, Stabilization and control of local rock falls and degrading rock slopes, Quarterly Journal of Engineering Geology and Hydrology 1976, V.9 P [6] E.T. Brown. (2015) Rock engineering design of post-tensioned anchors for dams A review, Journal of Rock Mechanics and Geotechnical Engineering, Volume 7, Issue 1, February 2015, Pages [7] Mohammed Y. Fattah, Hasan H. Joni and Ahmed S. A. Al - Dulaimy, Bearing Capacity of Strip Footing Resting on Dune Sands Stabilized by Grouting with Lime Silica Fume Mix, International Journal of Civil Engineering an d Technology (IJCIET), 7( 2 ), 2016, pp [8] Akshayakumar V. Hanagodimath, Dr. H. M. Rajashekar Swamy and Harshad R. Parate, Project Performance in Real Time Construction Industry - A Case Study. International Journal of Civil Engineering and Technology (IJCIET), 7(5), 2016, pp editor@iaeme.com

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