R.SUNDARAVADIVELU Professor IIT Madras,Chennai - 36.

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1 Behaviour of Berthing Structure under Changing Slope in Seismic Condition - A Case Study K.MUTHUKKUMARAN Research Scholar Department of Ocean Engineering, R.SUNDARAVADIVELU Professor IIT Madras,Chennai S.R.GANDHI, Professor Department of Civil Engineering, IIT Madras, Chennai ABSTRACT Berthing structures are subjected to large lateral forces due to berthing and mooring vessels. The lateral forces are to be resisted by the vertical pile, raker pile or diaphragm wall. A study has been conducted on the earthquake damaged structures of cargo berths No 1 to 5 at Kandla port, situated in Gujarat. The original design slope for the cargo berths was 1V:3H. Over the course of time, due to siltation the design slope underneath has been changed to 1V: 1.5H. As it is not easy to dredge out the deposited clayey and silty material over the design slope, the design slope is not maintained. Hence the design slope with factor of safety more than1.5, has been changed to an unstable slope with factor of safety much less than 1.0. The vertical piles designed for vertical forces are subjected to lateral force to stabilize the slope of 1V:1.5H. The earthquake vibratory forces had provided an additional instability to the slope, thus paving the way for the failure of the vertical piles due to large magnitude of lateral forces due to mudslide during earthquake. Keywords: Berthing structures, slope, seismic force, critical slip surface, and lateral earth force. 1. INTRODUCTION In the mooring hours at 8:46 am on the day of 26th January 2001, a killer earthquake of the magnitude of 6.9 on the Richter scale lasting for about 90 seconds rocked Gujarat, resulting in the tremendous loss of human life and property. The Kandla port, a major port of Indian situated on West coast in Gujarat, suffered the maximum damages due to the devastating earthquake. A typical cross section of cargo berth 1 to 5 is shown in Fig.1. The actual design slope is 1V: 3H, over a period of time due to siltation the design slope is changed to 1V:1.5H. In the present study, both design slopes are analysed and the extra lateral load coming to the pile due to unstable slope are determinated. 2. GEOTECHNICAL DATA The geo-technical investigation carried out at the site reveals that there are considerable variations in the soil profile. Typical soil profile for berth 1 to 4 shows that the soil strata consists of soft clay till m, follows by silty clay with lenses up to m and then after sandy stiff clay for nearly 20 m depth and then followed by a 2m thick and layer over a hard clay layer. 3. DESIGN SLOPE OF 1V:3H The original design slope of 1V:3 H was stable by itself and hence no failure of slope was expected. The front vertical piles were designed to take only deck loads and were free from any lateral horizontal loads. Raker piles provided behind for the Transit shed took the lateral forces. Hence no mud slide force is considered for the design slope of IV: 3H, on the vertical piles.

2 4. EXISTING SLOPE OF 1V:1.5H The design slope of 1V:3H has been changed to 1V:1.5H due to the siltation and inability to dredge below the open piled berthing structure for cargo berth 1 to 5. During the earthquake more than 50% of total no of 3000 piles have cracked just below the tie beam at +3.09m level (Fig.1). After observing the damaged to piles, the bed slope was measured. A detailed study has been conducted on the effect of this changed slope on the stability of the berthing structures especially in the presence of the seismic vibrations during earthquake. 5. PRINCIPLES OF STABILITY CALCULATIONS There are numerous methods currently available for stability analysis. The majority of these methods are categorized as limit equilibrium methods. The basic assumption of limit equilibrium approach is that Coulomb's failure criterion is satisfied along the assumed failure surface. The assumed failure surface may be straight line, circular arc, logarithmic spiral or other irregular surface. Based on the field observations of slope failure, it is recognised as a cylindrical failure surface and is most commonly used in the computation of slope stability. It is assumed that the shear strength of each individual soil occurring in the cross-section may be represented by an expression in the form of Coulomb's empirical law: S = C+ σ tanφ (1) Where C = effective cohesion φ = effective friction angle In order to test the stability of the slope of a C-f soil. Trial slip circle is drawn, and the material above the assumed slip surface is divided into a convenient number of vertical strips or slices. Each slice is assumed to act independently as a column of soil of unit thickness and width 'b'. The weight 'W' of each slice is assumed to at its centre. The weight 'w' is resolved into normal (N) and tangential (T) components, the normal components will pass through the centre of rotation (O), and hence do not cause any driving moment on the slice. However, the tangential component (T) causes a driving moment. M D = T x r, (2) Where, r = radius of the slip circle If 'C' is the unit cohesion and DL is the curved length of each slice, then the resisting force, from Coulomb's equation is equal to: M R = C L + N tan φ (3) For the entire slip surface, Driving Moment M D = r ΣT Resisting moment M R = r [CΣ L + tanφσn] Where, ΣT = algebraic sum of all tangential components ΣN = Sum of all normal components Σ L = length of slip circle Hence, Factor of safety against sliding is, A number of trial slip circles are chosen and factor of safety of each is computed. The circle giving the minimum factor of safety is the critical slip circle for the slope. 5.1 Effect of Earthquake The vibratory earthquake waves may laterally move the shoreline slopes. The earthquake forces will add to the overturning moment in the failure of the slopes.

3 Average response acceleration coefficient (s a /g) for the site based on the appropriate natural period and damping of the structure shall be used for the calculation of the additional driving moment. The factor of safety of the slope in consideration will reduce further due to the additional driving moment. 5.2 Lateral Earth Force on Piles The piles of the berthing structure will offer lateral resistance by its dowel action to the lateral displacement of the slope, and hence, increase the stability of the slope. The lateral resistance offered by the pile i.e. lateral force on the pile due to the soil movement depends on the following factors: Magnitude of lateral soil movement - Depth of the sliding surface - Deformation behaviour of the sliding soil - Type of the pile and its boundary conditions - Buried length of pile below the sliding surface - Stiffness of the soil below the sliding surface The lateral resisting moment offered by the piles can be calculated using the expression given below: M = F 1 R 1 + F 2 R F n R n (5) Where, F 1, F 2, F n = lateral forces on the piles due to soil movements on the elements 1,2,.n. R 1, R 2, R n = perpendicular distances from the piles to the assumed centre of rotation. The lateral force on the pile due to the failure of the unstable slope depends on the relative displacements between the soil and pile. Earthquake forces will further add to the failure of the slopes. The factor of safety for the slope considering the earthquake effect also, can then be expressed by the expression. Where Sα = average response acceleration coefficient for g the site R = horizontal distance of the pile from centre of critical slip circle F = lateral force acting on the piles per m depth of pile embedded in failure slope. X = distance of piles from c.g. of critical slip circle Fig.2 shows the critical slope surface of failure for the 1V: 1.5H slope underneath cargo berths 1 to 5. Table 1 gives the details of calculations of the actual factor of safety for the critical failure slip circle without piles Table 2. shows the details of the resistance offered by the existing piles to the failure of the slope of 1V:1.5H. The earth force acting on the piles of the cargo berths 1 to 5 is then calculated by the above generalized formula given in equation 6. Earth force on the piles has been calculated by using average response acceleration coefficient of 0.15g and factor of safety of 1.5 for the critical slip surface. Seismic force on piles with 1V:3H = 27 kn/m Seismic force on piles with 1V:1.5H = 37.5 kn/m 6. ANALYSIS OF STRUCTURE The full structure is considered for the analysis, idealizing the piles and beam by beam elements. The piles are assumed to fixed as per IS: 2911(PartI/ Sec2) The Descretisation figure for the design slope of 1V: 3H is given in Fig.3 and for the existing slope is given in Fig.4. The first pile assumed to be hinge since the penetration of the pile below the sea bed is only 10d. In the analysis, the additional forces on the piles are considered for the existing slope of 1V:1.5H as

4 10kN/m for normal condition and 27kN/m under earthquake condition. 7. COMPARISON OF RESULTS The analysis of the structures for the cargo berths 1 to 5 Kandla port is performed for the various load combinations by using structural analysis software STAAD - PRO Table 3 shows the maximum bending moments and shear force acting on the piles for both design and existing slopes. It shows that the bending moments on all the piles for the structure with existing slope of 1V: 1.5H is much more as compared to that with design slope of 1V: 3H. The moment in the front vertical piles are almost same, whereas, the moments are more for raker piles. The shear forces are much higher for the front vertical piles in structure with existing slope of 1V: 1.5H, as compared to that for design slope, whereas, for raker piles increase in shear force with change of slope is less. Analysis has also been carried out to know the forces and moments in the structure for pre-earthquake, during earthquake and post-earthquake conditions. For pre-earthquake conditions the load on the piles is calculated due to the instability of slope. For earthquake condition the load on the piles is calculated due to the instability of slope and earthquake accelerating the failure of slope. For post-earthquake conditions both effect of instability of slope and seismic force are not considered, as the mass of the earth had already been slided down during seismic vibrations. The result shows that the moments in the structure during earthquake condition increase by 58% to that for pre-earthquake condition. The moments and forces are less for pre-earthquake condition. Table 4 gives a comparative study of the effect of slope on the overall failure of the structure. Forces and moments in the structure due to earth loads from the instability of the slope are almost equal to the forces calculated with the combination of both earth load and seismic loads. 8. CONCLUSION (i) The earth force due to the failure of the unstable slope as the main reason for the damage to the berthing structure of cargo berth 1 to 5. (ii) Earthquake forces have only initiated the failure of slope along critical slip surface. (iii) The analysis result for structure with design conditions of slope 1V:3H, shows that the behind raker piles are taking the maximum forces as compared to the front vertical piles. (iv) The slope underneath the structural changes to an unstable slope of 1V:1.5H, the front vertical piles are subjected to maximum lateral FORCES, WHICH LEAD TO THE FAILURE. REFERENCES Dunlop P. and Duncan J.X. (1970) "Development of Failure around Excavated Slopes", Journal of Soil Mechanics and Foundation Division, ASCE, 96(2), Hankel D.J. (1957) "Investigation of Two Long Term Failure in London Clay Slopes at Wood Green and Mortbolt" Proceedings 4th International Conference on Soil and Finite Element, London, England. Hassiotis S. Charean J.L. and Gunarature M. (1997) "Design Method for Stabilization of Slopes with Piles", Journal of Geo-technical and Geo-environment Engineering, April 1997, Vol 123, Vol 4, PP Kuppuswamy T. and Buslov A. (1987) "Elastic Creep Analysis of Laterally Loaded Piles", Journal of Geotechnical Engineering Division, ASCE, 113(4), Seetharam Raju S. (1991) "Lateral Forces on Pile due to Soil Movements in Soft Marine Clay Slopes and Slope Stabilization using Piles", M.S. Thesis submitted in Ocean Engineering Centre, IIT Madras. Skempton A.W. (1969) "Long Term Stability of Clay Slopes" 4th Ranking Lecture, Geotechnique, 1412, Tomio Ito and Tamotsu Matsui (1975) "Method to estimate lateral force acting on stabilizing pile" Journal of Japanese Society of Soil Mechanics and Foundation Engineering, Vol 15, No.4 Dec1975, Towhata I. and AI-Hussaini T.M. (1988) "Lateral Loads on Offshore Structure exerted by Mudflow", Journal of Japanese Society Journal of Japanese Society 28(37),

5 Table 1: Detail for the Calculation of Factor of Safety for Critical Failure Slip Cricle Table 2: Calculations of Resistance Offered by Piles to the Failure of Slope

6 Table 3: Maximum Bending Moments and Maximum Shear Force in the Piles of the Existing Berthing Structure for Various Load Combinations

7 Table 4: Effect of Earth Force on the Existing Structure Bending Moment (kn-m) Fig.1: Typical cross sectional details for cargo berths 1 to 5

8 Fig.2: Critical slip surface for slope and Pile dowel action Fig.3: Descretisation diagram for modal analysis with slope IV:3H

9 Fig.4: Descretisation diagram for modal analysis with slope IV:5H

Reinforced Soil Structures Reinforced Soil Walls. Prof K. Rajagopal Department of Civil Engineering IIT Madras, Chennai

Geosynthetics and Reinforced Soil Structures Reinforced Soil Walls continued Prof K. Rajagopal Department of Civil Engineering IIT Madras, Chennai e-mail: gopalkr@iitm.ac.inac in Outline of the Lecture