Positioning of Ground Anchors for avalanche Defence Cable Net System

Transcription

1 Positioning of Ground Anchors for avalanche Defence Cable Net System Vinay Chaudhary Snow and Avalanche Study Establishment, Sector 37 A, Chandigarh, India Abstract Proper layout and placement of Flexible cable net in the formation area play very important role in preventing avalanche initiation. These nets are effective when ground anchors are placed at proper position on inclined slope. This paper gives an overview of placement of ground anchors on inclined slope by using linear optimization of projection and parametric study. Reasonable agreement has been observed between linear optimization of projection and parametric study. Keywords: Flexible cable net, Position of uphill anchor, swivel post, Avalanche, hazard 1. INTRODUCTION: Flexible cable nets are less sensitive to creep movement of snow and impact of rock fall. Therefore, they are preferred for flexibility, adaptability to terrain configuration and also adjustment with the amount of the snow cover on the slope, time to time. Ball & Socket joint arrangement is provided at the base, which gives free movement to Swivel Post in all directions resulting in better flexibility and functioning as well. Position of ground anchors B & A (Fig. 1) along with tilting angle ( P ) at the base of Swivel post C play very important role for maintaining proper geometry, slackness, sag of net and gives proper flexibility to the whole net system from zero load upto the peak load. Angle of Flexible supporting surfaces 1 connecting lower edge of Net and upper fastenings of a net is generally maintained 30 o (= B ). In the recent past, Flexible cable nets were erected at an experimental site D-10, Banihal top in J&K which posed problems of dislocation and uprooting of shallow foundations. It is because of improper placement of ground anchors (Chaudhary, 2013) at A & B (see Fig. 1) in the field on inclined slope which created uneven length of slackness & sag of supporting plane and correspondingly exerting exorbitant tensile forces on few ground anchors. Therefore, unsymmetrical snow pressure is exerted over the continuous rows of nets, resulting into distortion of net geometry, uneven transfer of force, consequently, failure of structures. There is a need to work out position of ground anchors ( A & B ) at proper place on the inclined slope for individual structure (see Fig. 1). Initially, development of Snow nets was basically empirical by first Haefeli, 1954 static calculations for Snow net. These findings are still in use for the dimensioning of Cable net. Now a days, only few approaches have been made to improve the design of the snow nets system by Nicot et al, 2002 and Boutillier et al, Measurement of forces was done by Elisabeth, 2008 on the two net systems and compared the results between calculated and measured forces. He has recorded the change in geometry of supporting plane nets and tilting of the swivel poles for the assessment and interpretation of the effectiveness of the snow net systems exposed to different snow loads. Analysis was done by Matthias, 2008 between rectangular type and triangular type nets and inclinometer was attached on the post and nets to gather the information of change in geometry during winter. Findings from these nets should help to define an Austrian guidelines for snow nets. Design parameters for supporting structures were investigated by Tomas and Josef, 2008 under Icelandic conditions through measurement. 1

2 Figure 1: Side view of Flexible cable net showing position of ground anchors A and B A great deal of work has been carried out on the arrangement of structures in row(s) with respect to fracture lines and direction of snow pressure and lateral extension of structures along with advantages & disadvantages was highlighted 1. An overview of the effects of snow pressure, requirement for supporting structures and brief of foundation forces on Snow net were given by Margreth, S Attention is not much given to placement of ground anchors on inclined slope in case of for Flexible cable net system. Paper deals with placement of ground anchors in the field by using linear optimization of projection and parametric study in terms of computation of anchor force distribution on ground anchors by varying its position. 2. METHODOLOGY: Optimum efficacy of net system can be worked out by two criterion (i) maximum retaining capacity of Flexible net system and, (ii) anchor force distribution on ground anchors at B & A. 2.1 LINEAR OPTIMIZATION OF PROJECTION Flexible Net construction is simplified in 2 D model which is represented as a simple cable (Fig. 1). Worst condition is considered when net is fully buried with snow upto depth D k. Tangent to the Net TD is drawn perpendicular to ground slope from top of post for simplification (Nicot & Tacnet, 2002). Swivel post is tilted o ( P ) upside so as to give flexibility and stability to the whole net system. Position of ground anchors is computed considering when structure is filled with snow upto structures height. Projection CD is obtained on the basis of maximum retaining capacity of Flexible net system considering extreme snow depth. Projection CD is added and subtracted while computing ground anchor positions B & A in BTD & ATC (ref Eqn 1 & 2). Position of ground anchors is responsible for tightening & loosening of the net and transferring high or low magnitude of snow loads to the ground anchors. Position of uphill anchor ground is worked out carefully which decides flexibility of whole net system. This flexibility facilitates the adjustment of net with amount of snow load coming over it, time to time.position of uphill ground anchor depends on depth of snow cover (D k ), slope inclination of net ( N ) and slope inclination of post ( P ). Position of B can be worked out using simple trigonometry in the triangles BTD, CDT & ATC which is as under: 2

3 L U Vinay Chaudhary / International Journal of Modern Sciences and Engineering Technology (IJMSET) tan tan(180 ) P N Position of downhill ground anchor point depends on depth of snow cover (D k ), slope inclination of guy rope ( A ) and slope inclination of post ( P ). Position of A can be worked out by the expression as under: (1) L D tan tan A 2.2 PARAMETRIC STUDY P (2) Optimum force transferred to the ground anchors are obtained on the basis of parametric study by varying position of ground anchors. In present case, 3 D Net modeled in Hyperwork software and uniformly distributed load of snow 1.5 T/m 2 is applied on 3.0 m height of structure. In parametric study, impact of anchor forces transferred to ground anchors was assessed by shifting the ground anchors B up & down side (see Fig. 2) keeping position A and C stationary. Angle of net N is increased upto 125 o and decreased upto 115 o considering 120 o as a reference value whereas in case II (see Fig. 3), angle of guy rope A is increased upto 41 o and decreased upto 29 o keeping position B and C stationary and considered 34 o ( A ) as a reference point. Figure 2: Shifting of position of uphill anchor B up & down side wrt N = 120 o Figure 3: Shifting of position of downhill anchor A up & down side wrt A = 34 o 3

4 3. RESULTS: Optimum projection CD (see Fig.1) was obtained for the extreme snow depth considering CDT by using linear optimization of projection. In this specific case, study of 3 m high Flexible cable net, the values of different parameters have been taken as under: D K = 3.0 m, P = 75 o and N = 120 o. Put the above values in Eqn 1, position of uphill anchor B comes out to be 2.53 m (= L U ). Similarly, put values of D K = 3.0 m, P = 75 o and A = 34 o in Eqn 2, position of downhill anchor A comes out to be 3.64 m (= L D ). In parametric study, N is design variable and its range varies from 115 o to 125 o (see Fig. 2). Initial boundary condition is set 120 o and force is a design constraint, and analysis is carried out for each variation and results are shown in Fig. 4, 5 & 6. A is design variable in the present study and its range varies from 29 o to 41 o (see Fig. 3). Initial boundary condition is set 34 o and force is a design constraint, and analysis is carried out for each variation and results are shown in Fig. 7, 8 & 9. Figure 4: Variation in uphill anchor force by shifting N on up & down side Figure 5: Variation in downhill anchor force by shifting N on up & down side 4

5 Figure 6: Variation in swivel post force variation by shifting N on up & down side Figure 7: Variation in uphill anchor forces by varying the slope of guy rope A Figure 8: Variation in downhill anchor forces by varying the slope of guy rope A 5

6 Figure 9: Variation of Swivel post forces by varying the slope of guy rope A 4. DISCUSSIONS: Position of ground anchors ( B & A ) obtained from linear optimization of projection is computed as 2.53 m in uphill side and 3.64 m distance in downhill side from swivel post. Ground anchors should be made at desired location on inclined slope which help to maintain proper slackness and sag of the net, resulting into better flexibility and provide better functioning to the whole net system. Similarly, position of downhill anchors help to distribute the snow load to the down side of structures and hold the net system in position. In parametric study, anchor forces are computed at B and A considering the optimum distance (reference point) obtained from former method. Anchor force comes around kn at point B (see Fig. 5) and kn at point A (see Fig. 6) when ground anchors are placed at reference point. An anchor force at B is increasing upto 22 % (see Fig. 4) while stretching the net (120 o < N < 125 o ) and forces are reducing upto 18.6 % while loosening of net (115 o < N < 120 o ). An anchor force at A is increasing drastically upto 72 % (see Fig. 5) while stretching the net (120 o < N < 125 o ) and forces are reducing upto 30 % while loosening of net (115 o < N < 120 o ). Forces in swivel post is increasing and decreasing upto 34 % while stretching the net length (120 o < N < 125 o ) and loosening the net length (115 o < N < 120 o ). There is an insignificant change in anchor force at B (see Fig. 7) while changing the position of A or change the angle of guy rope A whereas anchor forces at A are increasing upto 14 % (see Fig. 8) while reducing the length of guy rope (34 o < A < 41 o ). Anchor forces at A are decreasing upto 9 % while increasing the length of guy rope (29 o < A < 34 o ). There is insignificant change upto 3.6 % (see Fig. 9) in the forces at swivel post while changing the angle of Guy rope (29 o < A < 41 o ). It is observed that optimum forces are generated on the ground anchor points placed at reference point and all ground anchors points are loaded to its design load. Instability in the net system is created due to over loading or under loading of ground anchors, resulting into distortion of net geometry and transfers the uneven forces to the ground anchors. To achieve a uniform safety level for all components of the structure (including foundations) with varying slope inclination, the angle of the triangle formed by lower end of net and ground surface should be kept constant (Margreth, 2007). 5. CONCLUSION: Ideal position of uphill anchors is 2.63 m whereas ideal position of downhill ground anchor is 3.14 m on upside from post. Parametric study shows the distribution of optimum anchor force on the ground anchors computed at reference point. Stretched snow net generates high magnitude of forces in the ground anchors whereas loose net generates lesser magnitude of forces, but, it may touch the inclined terrain. Role of ground anchor at position A is to hold the net system in position and there is a little variation of anchor forces observed by changing the position of ground anchor at position A. It is found that linear optimization of projection leads to predict the optimum position of ground anchors that is in reasonably good agreement with the values obtained from parametric study. 6

7 Ground anchors of Snow nets are made in the site at Banihal top (J&K) which is constructed at the same distance as computed from both the methods. Thereafter, there is no failure observed and it is working efficiently. ACKNOWLEDGEMENTS: Author would like to express gratitude to Sh Ashwaghosha Ganju, Director SASE for his guidance and valuable comments. NOMENCLATURE D K Depth of snow cover A Angle of guy rope with inclined terrain P Angle of Post with inclined terrain N S N Angle of net with inclined terrain Snow pressure parallel to the slope G Weight of the snow prism R Resultant force L U Position of uphill anchor L D Position of downhill anchor I N Supporting plane length of net R Angle between resultant force and snow pressure parallel to slope Angle between post and net Slope inclination of inclined terrain B Cable end bearing angle at B Cable end bearing angle at T T 6. REFERENCES: [1].Chaudhary,V. and Mathur, P. Composite avalanche control scheme developed for the Lower Himalayan zone: A case history. CRS&T 39, pp , [2].Boutillier, B., F. Nicot, J. Meyssonnier, O Gangliardini and F. Darve.,. Interaction between a snowpack and a snow net system. In daily publication of the International congress Interpraevent, Riva del Garda, Trento, pp 59-70, [3].Haefeli, R. Proposals for design and analysis of nets [Vorschläge zur Konstruktion und Berechnung von Netzwerken], Swiss Federal Institute for Forest, Snow and Landscape Research SLF, Davos, External Report Nr [4].Nicot, F., M. Gay & J.M. Tacnet. Interaction between a snow mantel and a flexible structure: a new method to design avalanche nets. Cold Region Science and Technology 34, 67-84, [5].Margreth, S.. Defense structures in avalanche starting zones. Federal Office for the Environment, Bern; WSL Swiss Federal Institute for Snow and Avalanches Research SLF, Davos. Technical guideline as an aid to enforcement. Environment in Practice no [6].Margreth, S. New Technical guideline on Snow supporting structures in Avalanche Starting Zones, In Proceedings of the International Snow Science Workshop 2008, Whistler, BC, CAN [8].Matthias, G. Comprehensive snow net project Hafelekar/Innsbruck, International Symposium on Mitigative Measures against Snow Avalanches, Egilsstaðir, Iceland, 19-23pp BUWAL / WSL [9].Tomas, J and Josef Hopt., Loading of supporting structures under Icelandic conditions. The type of structures and structural requirements in future projects. Results of a field experiment in Siglufjörur. International Symposium on Mitigative Measures against Snow Avalanches, Egilsstaðir, Iceland, , AUTHOR S BRIEF BIOGRAPHY: Vinay Chaudhary: He is working in Snow and Avalanche Study Establishment (SASE) based in Chandigarh since He is engaged in developing formation zone control structure schemes for more than 20 sites in Indian Himalayas. Presently, he is working for developing Micropile technology for formation zone control structures in diverse type of terrain in Indian Himalayas. Successful trials on Test piles were conducted in avalanche prone sites near Manali. His papers more than 20 were published in various esteemed reputable national and International Journals. He is a Member of Various Professional Bodies. He received so many prestigious awards. 7