Study of Well Foundation under Different Grip Length andLoading Conditions

Tomar, RituPost Graduate Student

e-mail: reettomar@gmail.com

Department of Civil Engineering Ujjain (M.P.)

ABSTRACTIn this work, behaviour of well foundation components are studied and for this circular well for foundation ofbridge is considered having same height and different stein thickness and different grip length. Analysis has beencarried out by using different plate thickness of well foundation for different loading conditions at different griplength below maximum scour level by using staad pro software. B.M. and Menbrane sterss are studied for differentload combinations on different types of well model case e.g. 5 models for full grip, 5 models for 5/6 grip and 5models for 2/3 grip length. B.M. and Membrane stress at bottom, middle and top plate are considered. Behaviouris studied in between the stein thickness v/s B.M. and stein thickness v/s Membrane stress for different grip lengthand different load combinations. By seeing the results, on increasing stein thickness, Membrane stress is reducedin all three plates. But B.M. is increased in same three plates. The behavior of membrane stress and B.M. has beenstudied also. case Vertical load +Horizontal load +Earth pressure is taken and studied for all three grip length soit is found that the bottom plate shows that the B.M. and M.S. increases with stein thickness. But in case of middleplate and top plate B.M. and M.S. increases up to a certain level and then starts decreasing. This may be probablydue to the restraining effect of bottom plate on middle plate and top plate. The investigation examines theimportance of steining thickness and grip length on behavior of well foundation subjected to different loading.

Indian Geotechnical Conference 2010, GEOtrendzDecember 1618, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTIONWell foundations are the costliest among all otherfoundations they are extensively used for bridges, marinestructures, abutment in lakes, rivers and seas, subjected tohuge vertical and horizontal forces.

The main aim of this study is to understand thebehaviour of well foundation components under differentloading conditions and at different thickness of stein.Results obtained from staad modeling were compared andsaw the variation of membrane stresse and bending momentat different scour level for different grip length and studythe effect of well cap on Well Foundation components.

2. DESIGN ASPECTS OF WELL FOUNDATIONThe main design aspects of well foundation for comparisonare the grip length, steining thickness and well cap.

A well foundation should be sunk below the maximumscour depth, such that there is adequate lateral stabilityand this embedded length is called the grip length. Thegrip length is required for developing sufficient passive

resistance to counteract the overturning moment due tohorizontal force at well top.

The steining thickness should be designed in such away that it is adequate for the stresses developed duringsinking and during installation.

Well cap transmits the load of the structure to thesteining. The shape of the well cap is normally kept thesame as the well with the possible overhang all around ofabout 150 mm.

Here we are taking the shape of the well is circular.Advantages of taking circular well are strength, simplicityin construction, ease in sinking, and minimum steiningthickness due to minimum flexural stresses in steining.

3. ANALYSIS AND PROCEDUREAnalysis of well foundation is done in three parts.

(i) Laterally loaded well i.e. Earth pressure(ii) Vertically loaded well

(iii) Horizontally loaded wellIn present study, some well foundation components

904 Ritu Tomar

are studied. The well is circular cylindrical shell in whichbottom of model is fixed and top is free. The well is studiedfor same height i.e 22 m. The well is studied for varyingstein thickness i.e 1.28m, 1.44m, 1.6m, 1.76m and 1.92m.From practical point of view, the vertical load andhorizontal load are taken in some range so here loads areassuming under that range and studied. Density of soil istaken as 18KN/m3 and passive earth pressure is calculatedat different level of well foundation.In case of without wellcap, load is applied on periphery of well foundation asvertical load, horizontal load and earth pressure for fullgrip, 5/6 grip and 2/3 grip length. According to thecondition, membrane stress and bending moment arerecorded at different level of well foundation i.e top plate,middle plate and bottom plate. Same study has been doneon well foundation with cap also.

4. RESULTS

Fig. 1: Membrane Stress v/s Stein Thickness

Fig. 2: Bending Moment v/s Stein Thickness

5. CONCLUSIONS(i) For all the three load cases and grip length

considered, minimum reduction in MembraneStress for bottom plate is 29% when stein thicknessincreased by 33%.

(ii) For all the three load cases and grip lengthconsidered, minimum increase in BendingMoment for bottom plate is 31% and maximumincrease in Bending Moment is 37% when steinthickness increased by 33%.

(iii) Membrane Stress decreases and Bending Momentincreases as well cap is attached to the top of thewell.

REFERENCESYadav, B.A. (1998), Computer Aided Analysis, Design and

Drafting of Well Foundations, M.Tech Thesis,Department of Civil Engineering. IIT Kanpur.

Tang and Rotter, Linear Elastic Stress and BifurcationAnalyses of Discretely Supported Perfect Cylinders.