II. Marco metodológico
3.1. Análisis descriptivos Tabla 11
Based on above discussion, the parameters which require particular attention in a soil report from the engineer are summarised as follows:
1.9.4.1 Field test
• Has SPT test been carried out?
The obvious intention is to find out N on which G value depends. This can also be utilised to cross check the field observed dynamic data.
• If SPT values are furnished are the observed data or corrections need to be done?
A point to be checked for field observed data as shown earlier needs to be corrected. While the soil consultant will do this correction during his own calculation of bearing capacity of soil for foundation recommendations, usually furnishes observed field data while furnishing the bore log detail in the report. So for your calculation this data needs to be corrected. If you are not too sure you can back calculate it from recommended φ value.
• Has Ground Water Table been established during boring?
Usually provided in a soil report but better to check for this has significant effect on the net vertical stress.
• Has any dynamic field test carried out?
◦ Block Vibration test
◦ Seismic cross hole test
One of the tests should be a part of the soil report. But do not take the values furnished sacrosanct. Back check with theoretical co-relation to establish if the order is close, if not you do have the right to ask your soil consultant why there is this discrepancy. There could be special geological condition which could result in such discrepancy and you should be clear about it.
• If the above tests are carried out, what is the strain range induced in the soil during
the test?
This is something usually not supplied by the soil consultant who usually would recommend a unique G value. This should not be acceptable to you.
You should clear it at the very outset when providing him the specification for Geotechnical investigation that this is an input you are looking for and it should be
a part of his report. It is more realistic to start with this value rather than guessing a theoretical value of 10−3/10−4%.
1.9.4.2 Laboratory tests
• Check Atterberg’s limit – gives values of liquid limit, plastic limit, plasticity
index etc.
Generally speaking37as a ritual, structural engineers/analysts ignore this topic. Our suggestion would be, do not disregard this for this is the basic data which gives you the first insight into the fact as to how the soil behaves. Moreover plasticity index being an important property it is all the more important that you should pay attention to this.
• Triaxial test gives values of c,φ and pre-consolidation history
Again given a back handed treatment by the analyst who without going through the test data would prefer to pick up the numbers which concerns him (caandφ). We
suggest go through the test and develop enough skill to interpret the pre-consolidation stress. Make sure to ask during enquiry to the consultant to supply this data. For Over Consolidation Ratio (OCR) plays a very important role in arriving at the correct value of Gdyn.
During interpretation if need be, seek help of a geotechnical specialist to make sure what has been understood is correct – this will save a lot of headache in the long run.
• Bulk density and void ratio of soil
• Grain distribution to check if the soil is gap graded, uniformly graded, or poorly
graded. Relative density of cohesion less soil is highly dependent on this.
1.10 EPILOGUE
The technology described in this chapter to our perception is still in its infancy and we are optimistic that with time and research that is being carried out all over the world, we shall be in a better position in future to predict more realistically the dynamic properties of soil which affect the response of structure.
Whatever we have presented here is what we believe is simple to apply, provides reasonably realistic results and practical for day to day design office practice.
There is hardly any comprehensive text which gives a defined picture on this issue. Most of the techniques developed herein are based on research papers (names furnished in the reference) and typical practices followed in some design offices38.
We urge the readers to go through these papers which we believe will give them further insight to the problem.
37 Exceptions are always there. . ..
The ideas presented in this chapter is to make the reader aware of the limitations prevalent with soil and also to caution him on the fact that without these values realistically estimated, the whole analysis related to dynamic soil structure interaction could become a questionable exercise.
So be aware and use your judicious best to furnish a meaningful design.
SUGGESTED READING39
1 Cohen, M. & Jennings, P., ‘Silent Boundary Methods For Transient Analysis’, Computa- tional Method in Transient Analysis – Computational Method in Mechanics, Vol. 1, North Holland.
2 Dasgupta, S.P. & Kameswara Rao, N.S.V.K. 1976, ‘Some finite element solutions in the dynamics of circular footings’, Proc 2nd International Conference on Numerical Methods
in Geomechanics, Blacksburg USA.
3 Dasgupta S.P. & Kameswara Rao, N.S.V.K. 1978, Dynamics of rectangular footings by Finite elements, Journal of GT Division ASCE, Vol. 104, No. 5.
4 Gazetas, G & Tassoulas, A.L. 1987, ‘Horizontal Stiffness of Arbitrarily shaped embedded foundation’, Journal of GT Division, ASCE, Vol. 113, No. 5.
5 Kameswar Rao, N.S.V. 1977, ‘Dynamic soil structure system – A Brief Review’, J. Struct.
Engg., India, Vol. 4.
6 Lysmer, J. & Kuhlemeyer, R.L. 1969, ‘Finite dynamic model of infinite media’, J.EM.Divn, ASCE, EM4.
7 Segol, G., Abel, J.F. & Lee, P.C.Y. 1975, ‘Finite element Mesh Gradation of surface waves’,
J. GT Division, ASCE, Vol. 101, GT 11.
8 Wolf, J.P. 1985, Dynamic Soil Structure Interaction, Prenctice-Hall Englewood Cliffs, NJ. 9 Wolf, J.P. 1988, Dynamic Soil Structure Interaction in Time Domain, Prenctice-Hall,
Englewood Cliffs, NJ.
10 Wolf, J.P. 1994, Foundation Vibration Analysis: Using Simple Physical Model, Prentice- Hall, Englewood-Cliffs, NJ.
11 Whitman, R.V. 1970, Soil Structure Interaction – Seismic design for Nuclear power plants, The MIT press, Cambridge, Massachusets.
39 This topic being relatively new, there are not much reference books (other than reference 8, 9 & 10) which deal this topic comprehensively. Many literatures though have mentioned the interaction effect in their work. The references suggested are thus mostly restricted to research papers, which we would request you to get hold of and rummage through patiently.