INFLUENCE OF TSUNAMI INDUCED SCOURING ACTION ON FOUNDATION STABILITY OF COASTAL STRUCTURES

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 7, July 2018, pp , Article ID: IJCIET_09_07_093 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed INFLUENCE OF TSUNAMI INDUCED SCOURING ACTION ON FOUNDATION STABILITY OF COASTAL STRUCTURES AHL Swaroop 1 Research Scholar, AU, Visakhapatnam & Sr.Gr Assistant Professor, Gudlavalleru Engineering College, Gudlavalleru, AP India Sk.Yazdani Associate Professor, Department of Civil Engineering, Andhra University, Visakhapatnam, AP, India SRK Reddy Professor, Department of Civil Engineering, Gudlavalleru Engineering College, Gudlavalleru, AP, India ABSTRACT Power of tsunami has been realized through many failure examples witnessed during 1992 Nicaragua, 2004 Sumatra and 2011 Tohoku tsunami events. Besides many causes of structural failures due to hydrodynamic, impact and surge forces against tsunami attack, tsunamis also are capable of scouring the soils surrounding the footings of foundation structures and sometimes lead to stability failure of foundation. Depth of foundation of coastal structures is normally calculated considering the factors like bearing capacity of soil, foundation type, settlement and depth of water table. During tsunami events, besides sound static designs, foundation sometimes over topples due to scouring the surrounding soil. Hence, scouring phenomena is another important factor to determine the depth of foundation in the event of structure subjected to mega tsunami events. In the present investigation, the foundations of two typical coastal structures; one a school building and the other an elevated water tank are taken as examples and scour depths are obtained for different tsunami heights and bathymetry conditions. The results show that for lower grade beach slopes and when tsunami wave heights increase, the scour depths also increase. When the scour depth is greater than the depth of foundation, overturning effect may take place and sometimes may lead to the extent of collapse. The authors also suggested a new formula to calculate the scour depth of a coastal structure considering the additional parameters like beach slope, bathymetry conditions and distance of building from shore line editor@iaeme.com

2 AHL Swaroop, Sk.Yazdani and SRK Reddy Key words: Tsunami Wave, Scour Depth, Bathymetry, Column Size, Depth of Footing, Stability. Cite this Article: AHL Swaroop, Sk.Yazdani and SRK Reddy, Influence of Tsunami Induced Scouring Action on Foundation Stability of Coastal Structures, International Journal of Civil Engineering and Technology, 9(7), 2018, pp INTRODUCTION The mega tsunami events such as 2004 Indonesia tsunami and 2011 Tohoku tsunami and the resulting wide spread damage to coastal structures, reminded the scientists and researchers internationally, to identify and recognize the tsunami generation, propagation, inundation and coastal erosion effects while designing the structures against tsunamis. The exponential rate of increase in the activity of construction and infrastructure along the coastal regions, alerts the scientific community to identify the reliable failure modes and design methodologies for stability of foundation against scouring. The causes of failure against tsunami impact are mainly due to hydrodynamic forces, impact forces and surge forces and also due to erosion/scouring action.though there are some guidelines provided by FEMA [6] and authors like Harry Yeh [5]etal on obtaining the tsunami induced hydrodynamic and impact forces on the exposed parts of the structure above ground level, the failure of foundations due to tsunami induced scouring action is yet to be fully understood. In this paper, the authors have mainly focussed on the stability of foundations against tsunami induced scouring depths. Tsunamis are capable of creating destructive effects on buildings, roads & bridges and erosion failures to foundations of coastal structures. Kato etal [2] carried out field investigations on failure effects during the great 2011 Tohoku tsunami event and observed that 49% of failures were due to scouring action on foundation at the leeward slope.besides the tsunami forces that act on the structures, tsunamis also scour the cohesionless soils like sand, surrounding the footing and endanger its stability. In general, scouring occurs due to movement of granular sediments subject to shear forces induced by tsunami turbulent flow. High tsunami velocity generating high shear stresses on the seafloor, is the primary cause of scour. Particularly for non-cohesive soils like sand, the failure criterion indicates that scour failure occurs when the ratio of shear stress(τ) and the normal stress (σn) reaches greater than the tangent of angle of friction (ф) i.e when (τ/ σn) > tanф, failure due to scouring action occurs. During tsunami drawdown [3], the water level subsides and the pressure on the sediment bed decreases, creating a vertical pressure gradient within the sand and thus decreasing the effective stress. Hence, it is necessary to predict the scour depth near the footing in order to design the buildings to survive against high tsunami impacts.it has been established [1] that scour depths near square columns were slightly larger than the cylindrical columns.when a building with isolated footing of foundation, designed under gravity loads, is constructed in an erodible sea bed, extensive scour around the footing may reduce its stability against overturning and thus may lead to failure. Parameters such as tsunami speed, shape and dimensions of column/footing, inundation depth, angle of leeward slope and Froude number, mainly influence the scour depth. 2. PROBLEM CHOSEN In the present study, foundations of two typical coastal structures, one a school building with G+2 floors of size 32m x15m in plan with floor height 3.5m each and the other, an elevated water tank with slender staging and top heavy mass are chosen for obtaining the scour depths editor@iaeme.com

3 Influence of Tsunami Induced Scouring Action on Foundation Stability of Coastal Structures under different tsunami heights and bathymetry conditions. The depths of foundation provided for school building and water tank are 2.0m and 1.5m respectively.the structures are assumed at a distance of 100m and 150m from the sea coast for different beach slopes of :50 and 1: METHODOLOGY Even though various empirical formulae were established by different authors[4], Breuser s formula and formula developed by Colorado State university (CSU) proven to be more reliable for obtaining the scour around bridge piers. Breuser s Formula d s=1.35k ib 0.7 d 0.3 (1) Where ds-scour depth, d-tsunami water depth b-width of column/footing in tsunami flow direction Colorado State University(CSU) Method: This method has introduced Froude Number (Fr) in predicting local scour depth and the equation is d s=2.0k if r 0.43 (b/d) 0.65 (2) In the present study, by using the CSU formula and the FEMA guidelines, a simple formula for scour depth is established for loose soils considering run up height, distance of the structure from shore, beach slope and width of the column as input values and is presented below The results thus obtained are compared with the adopted foundation depth values and can be known whether the structure is safe against scouring or not as in equation (3). d s=2.78(r-si) 0.35 b 0.65 (3) where, ds= Scour depth in meters R= Run up height in meters b= Width of the structure in meters at which the tsunami wave hits. i= Beach landward slope S= Distance of the structure from shore line in meters RESULTS: Using the above modified formula, scour depths are obtained for various run up heights and the beach slopes when the two chosen structures are located at different distances from shore and the results are presented in the table below editor@iaeme.com

4 AHL Swaroop, Sk.Yazdani and SRK Reddy Table 1 Scour depth values (in meters) of the two structures for different run up heights Scour depth values (in meters) of the two structures for different run up heights Parameter School Building Water tank Beach Slope(i) 1 in 50 1 in in 50 1 in 100 Distance(s) in metres R=3m R=6m R=9m R=12m Fig 1 Scour and Foundation depths at different distances from shore for a school beach slope1:50 Fig. 2 Scour and Foundation depths at different distances from shore for a school beach slope1: editor@iaeme.com

5 Influence of Tsunami Induced Scouring Action on Foundation Stability of Coastal Structures Fig 3 Scour and Foundation depths at different distances from shore for a Water beach slope1:50 Fig. 4 Scour and Foundation depths at different distances from shore for a Water beach slope1:100 CONCLUSIONS As the run up height increases the scour depth also increases The scour depth decreases when the structure is far from the shore for same beach slope. The influence of scouring is observed more in flexible structures than rigid structures editor@iaeme.com

6 AHL Swaroop, Sk.Yazdani and SRK Reddy It is observed from results that the school building is stable and safe against scouring as the structure is G+2 storey school building with minimum column width. However if the column width increases then the influence of scouring will also increases which may leads to overturning effect. It is observed from results that the water tank is stable upto 6m tsunami run up heights and unstable thereafter. In general water tanks are safe against tsunamis compared to the buildings as there is provision for passage of water; but if the end columns are failed due to scouring, stability against overturning may reduce and finally may lead to collapse of structure. Scouring action also depends upon the shape of column / footing and also type of granular sediment surrounding the footing. REFERENCES [1] RavindraJayaratne A Predictive Model for Scour Depth of Coastal Building Failures due to Tsunamis International conference on Costal Engineering PP-1-8 [2] Fuminori Kato1 Mechanisms of Coastal Dike Failure Induced by the Great East Japan Earthquake Tsunami Proceedings of the 30th International Conference on Coastal Engineering, ASCE, [3] Jeremy D. Bricker a, Mathew Francis b & Akihiko Nakayama Scour depths near coastal structures due to the 2011 Tohoku Tsunami Journal of Hydraulic Research Vol.50 No.6 (2012) PP [4] RadiantaTriatmadja Scouring Around Coastal Structures Due to Tsunami Surge in 6th Annual International Workshop & Expo on Sumatera Tsunami Disaster & Recovery 2011 TS [5] Harry Yeh and Wenwen, Li Tsunami Scour and Sedimentation Fourth International Conference on Scour and Erosion, November 2008-PP [6] FEMA, Guidelines for Design of Structures for Vertical Evacuation from Tsunamis, FEMA P646 Publication, Applied Technology Council (2008). [7] Christopher Koutitas and TheophanisKarambas A Computational Approach to design Codes for Tsunami resisting Coastal Structures ISET Journal of Earthquake Technology, Paper No. 461, Vol. 42, No. 4, December 2005, pp [8] Kent Inagaki, Eizo Nakaza, Carolyn Schaab and Satoshi Tanaka, Distribution Characteristics of Pine Trees along the Coast of Sendai City Swept away by the 2011 Tohoku Region Tsunami. International Journal of Civil Engineering and Technology, 7(6), 2016, pp [9] Mohammed Sirajuddin, Dr D. Rajya Lakshmi, Dr Syed Abdul Sattar and Nafisur Rahman, Fuzzy Logic The Fascinating Logic Behind Artificial Computational Intelligence International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 3, 2013, pp , ISSN Print: , ISSN Online: [10] Harry Yeh, Susan Tonkin Mechanisms of Scour Induced by Tsunami Run up Report US National Science Founation (CMS ) and Public Works Research Institute, the Japan Ministry of Construction editor@iaeme.com