An Advanced Software for the Geotechnical Design of Foundations
Prof. Dr. Nainan P.Kurian, Ph.D., D.Sc. Retd. Professor of Civil Engineering, IIT Madras.
More specifically, the aim of geotechnical design in the case of foundations, for example, a shallow foundation such as an individual footing, is to arrive at the plan dimensions in such a manner that it simultaneously satisfies the requirements pertaining to ‘bearing capacity’ and ‘settlement’ from the soil side.
On the other hand, the aim of structural design is to cater to the structural action developing in the foundation in the process of load transmission, the adequacy of which has been ensured in the geotechnical phase of design. By way of example, in the case of an SB software for Foundation individual footing, the structural action is ‘flexure’ under the concentrated column load and the distributed soil reaction. The result of structural design is the thickness of the footing and the quantum and location of reinforcement. In the case of a deep foundation such as a pile, the geotechnical design phase gives both the length and cross section, so much so, the role of structural design is confined to designing the reinforcement. So also is the case with a retaining structure, such as a cantilever retaining wall, in whose case the cross sectional dimensions are fully established in the geotechnical design phase, which normally takes the form of a stability analysis aimed at satisfying the various aspects of stability of the system.
(One may note in this connection that design for ‘bearing capacity’ and ‘settlement’ in the geotechnical design phase of a foundation is analogous to design for the ‘limit state of collapse’ and ‘deflection’ respectively, of a structural member such as a beam.)
Since any number of softwares are available at the moment for structural design of foundations, the same is not included in the softwares developed here which is therefore confined to geotechnical design, but taking it to a very sophisticated level consistent with the current state of the art as it pertains to geotechnical engineering and satisfying all the relevant codal requirements currently in force. As regards sophistication, it comes a long way from the age old method of determining plan dimensions by dividing the column load by a so called ‘fictitious’ bearing capacity, which is incorrect and inconsistent with the current status of geotechnical engineering as a distinct discipline of civil engineering.
The following sections describe the features of the advanced software which has been developed for the geotechnical design of shallow foundations, deep foundations and retaining structures, based on the principles and detailed procedures of design enunciated in the author’s book (Kurian 2005). Hence a reasonable acquaintance with the same is deemed as necessary to appreciate the usefulness and value of the software.
This software has been developed by Roy (2002) under the guidance of the author.
It is developed in “C” adopting the modular approach in PC environment which outputs the results in both textual and graphical (color) formats. It provides for the entry of data in an interactive mode.
The software executes the following designs:
The type of soil accommodated are: c, E , c– E and layered, with the limitation that in layered soil it designs only footings, and that too, if the thickness of the soil in the layer below the footing is sufficient to accommodate the bulb of pressure.
The design makes use of N-values (from the Standard Penetration Test) in the case of sand, and qµ (unconfined compressive strength), in clay. If q values are not available, the same are reduced empirically from the N-values. In the case of c- E soils, however, c and E values, determined from testing samples extracted at various depths, are directly used, which makes for greater accuracy in design. (This means, the latter can be used in c-soils and E –soils as well, which indeed will give more accurate results that can be obtained using qµ and N-values.
The CAD approach is more general and is happily free from many of the limitations of the manual approach. By way of example, a footing is designed for the nearest bore hole data – identified by coordinates. At each trial B, both ‘safe bearing pressure’ (sbp) and ‘settlement’ (S) are determined before moving on to the next trial value. Differential settlements are checked for all possible pairs, whatever the number of footings. (By way of example, if there are 30 footings the number of pairs, mathematically, is 435!) Such a formidable task will never be attempted manually. Iterations on B, L or D (depth of foundation) are carried out in regular steps (intervals), which can be chosen at will.
For given sets of loading and soil data, the program designs pier and pile alternatives – the latter as pile groups – in c,E and c-E soils, under three modes of load transfer, viz., end bearing, skin friction and end bearing-cum-skin friction. Thus in the output we have 9 pairs or 18 individual results, accompanied by the necessary illustrative graphics. To enable comparison – at the material level – the program outputs volume of concrete for the pier and pile alternatives, excluding the respective caps.
As regards pile groups, while group capacity is based on the individual capacity of the pile – both in terms of bearing capacity and settlement – in pure E-soil, it is based on the action of the soil prism defined by the pile group, in the case of c and c-E soils. (Kurian 2005: Sec. 5.1.4.2) In either case, the number of piles in the group is determined at the minimum spacing prescribed by the code.
For horizontal loads on the pile group in addition to vertical loads, a pile group analysis is carried out to determine the axial forces in the inclined and vertical piles in the group, as per (Kurian 2006: Sec. 11.1.3). In the same way the horizontal load a caisson of given data can sustain in the presence of a given vertical load is determined by its stability analysis, vide (Kurian 2006: Sec. 11.3.1).
Cofferdams of circular and diaphragm type alternatives are designed with variable height of water in the cell. The design of the H-pile and lagging type cut support accounts for variable spacing of the H-pile along the alignment of the cut.
(The total length of the Program covering the three sections is 11926 lines, made up as: Shallow Foundations – 2937 lines, Deep Foundations – 5184 lines and Retaining Structures – 3805 lines.)
It is fervently hoped that designers make the best use of this software which will prove to be a useful, efficient and time-saving tool in their hands for the sound design of foundation systems in the future.
Note: The instructions given in the CD under ‘Read Me’ must be followed for the successful implementation of the software. Note that the Design File and the Library File (for graphics) carried by each folder pertaining to a specific design should be copied on the hard disk of your PC for the execution of the design and display of the corresponding graphics.
Introduction
There are two phases to the complete design of any foundation system, whether it is a foundation (shallow or deep), retaining structure or substructure. These are the ‘geotechnical design’ and the ‘structural design’ phases. The purpose of geotechnical design is to satisfy the geotechnical requirements, i.e., requirements from the soil side, in the case of foundations, and stability of the system in the case of retaining structures. In fact the structural part of design can be taken up only when the geotechnical phase of design is satisfactorily completed.More specifically, the aim of geotechnical design in the case of foundations, for example, a shallow foundation such as an individual footing, is to arrive at the plan dimensions in such a manner that it simultaneously satisfies the requirements pertaining to ‘bearing capacity’ and ‘settlement’ from the soil side.
On the other hand, the aim of structural design is to cater to the structural action developing in the foundation in the process of load transmission, the adequacy of which has been ensured in the geotechnical phase of design. By way of example, in the case of an SB software for Foundation individual footing, the structural action is ‘flexure’ under the concentrated column load and the distributed soil reaction. The result of structural design is the thickness of the footing and the quantum and location of reinforcement. In the case of a deep foundation such as a pile, the geotechnical design phase gives both the length and cross section, so much so, the role of structural design is confined to designing the reinforcement. So also is the case with a retaining structure, such as a cantilever retaining wall, in whose case the cross sectional dimensions are fully established in the geotechnical design phase, which normally takes the form of a stability analysis aimed at satisfying the various aspects of stability of the system.
(One may note in this connection that design for ‘bearing capacity’ and ‘settlement’ in the geotechnical design phase of a foundation is analogous to design for the ‘limit state of collapse’ and ‘deflection’ respectively, of a structural member such as a beam.)
Since any number of softwares are available at the moment for structural design of foundations, the same is not included in the softwares developed here which is therefore confined to geotechnical design, but taking it to a very sophisticated level consistent with the current state of the art as it pertains to geotechnical engineering and satisfying all the relevant codal requirements currently in force. As regards sophistication, it comes a long way from the age old method of determining plan dimensions by dividing the column load by a so called ‘fictitious’ bearing capacity, which is incorrect and inconsistent with the current status of geotechnical engineering as a distinct discipline of civil engineering.
The following sections describe the features of the advanced software which has been developed for the geotechnical design of shallow foundations, deep foundations and retaining structures, based on the principles and detailed procedures of design enunciated in the author’s book (Kurian 2005). Hence a reasonable acquaintance with the same is deemed as necessary to appreciate the usefulness and value of the software.
A. Shallow Foundations
Since the geotechnical design computations of shallow foundations are essentially iterative (Kurian 2005: Part I), it was thought ideal to develop a general purpose computer program which would take the tedium out of manual calculations, and at the same time give accurate designs to the user. An added appeal is that it enables, in particular the oldtimers among structural engineers, whose knowledge of geotechnical design is limited to dividing the column load by the so-called bearing capacity, to obtain accurate results by merely learning how to input the soil investigation data into the program.This software has been developed by Roy (2002) under the guidance of the author.
It is developed in “C” adopting the modular approach in PC environment which outputs the results in both textual and graphical (color) formats. It provides for the entry of data in an interactive mode.
The software executes the following designs:
- Independent footings
- Backfilled raft
- Compensated raft, in
- Clay
- Sand
- c– E soil
- Layered soil
The type of soil accommodated are: c, E , c– E and layered, with the limitation that in layered soil it designs only footings, and that too, if the thickness of the soil in the layer below the footing is sufficient to accommodate the bulb of pressure.
The design makes use of N-values (from the Standard Penetration Test) in the case of sand, and qµ (unconfined compressive strength), in clay. If q values are not available, the same are reduced empirically from the N-values. In the case of c- E soils, however, c and E values, determined from testing samples extracted at various depths, are directly used, which makes for greater accuracy in design. (This means, the latter can be used in c-soils and E –soils as well, which indeed will give more accurate results that can be obtained using qµ and N-values.
The CAD approach is more general and is happily free from many of the limitations of the manual approach. By way of example, a footing is designed for the nearest bore hole data – identified by coordinates. At each trial B, both ‘safe bearing pressure’ (sbp) and ‘settlement’ (S) are determined before moving on to the next trial value. Differential settlements are checked for all possible pairs, whatever the number of footings. (By way of example, if there are 30 footings the number of pairs, mathematically, is 435!) Such a formidable task will never be attempted manually. Iterations on B, L or D (depth of foundation) are carried out in regular steps (intervals), which can be chosen at will.
B. Deep Foundations
The software executes the following designs:- Pier and pile group – c soil
- Pier and pile group - E soil
- Pier and pile group – c- E soil
- transferring load in
- End bearing
- Skin friction
- End bearing-cum-skin friction
- Batter piles – Pile group analysis
- Well foundation.
For given sets of loading and soil data, the program designs pier and pile alternatives – the latter as pile groups – in c,E and c-E soils, under three modes of load transfer, viz., end bearing, skin friction and end bearing-cum-skin friction. Thus in the output we have 9 pairs or 18 individual results, accompanied by the necessary illustrative graphics. To enable comparison – at the material level – the program outputs volume of concrete for the pier and pile alternatives, excluding the respective caps.
As regards pile groups, while group capacity is based on the individual capacity of the pile – both in terms of bearing capacity and settlement – in pure E-soil, it is based on the action of the soil prism defined by the pile group, in the case of c and c-E soils. (Kurian 2005: Sec. 5.1.4.2) In either case, the number of piles in the group is determined at the minimum spacing prescribed by the code.
For horizontal loads on the pile group in addition to vertical loads, a pile group analysis is carried out to determine the axial forces in the inclined and vertical piles in the group, as per (Kurian 2006: Sec. 11.1.3). In the same way the horizontal load a caisson of given data can sustain in the presence of a given vertical load is determined by its stability analysis, vide (Kurian 2006: Sec. 11.3.1).
C. Retaining Structures
The software executes the following designs:- R.C. cantilever retaining wall
- Cantilever sheet pile wall
- Anchored bulkhead – Free earth support
- Anchored bulkhead – Fixed earth support
- Cellular cofferdam
- Circular type
- Diaphragm type
- Cut support – H-pile and lagging system
Cofferdams of circular and diaphragm type alternatives are designed with variable height of water in the cell. The design of the H-pile and lagging type cut support accounts for variable spacing of the H-pile along the alignment of the cut.
(The total length of the Program covering the three sections is 11926 lines, made up as: Shallow Foundations – 2937 lines, Deep Foundations – 5184 lines and Retaining Structures – 3805 lines.)
Conclusion
In the present scenario where foundation design practice has not much advanced from the age old practice to which reference has been made earlier, it is highly desirable and necessary that designers use this advanced software tool in the interest of generating the right geotechnical design of their foundation. The software indeed takes the designer to highly sophisticated levels, leading to all round advancement of professional standards in relation to foundations, which – needless to say – are the backbones of all civil engineering structures.It is fervently hoped that designers make the best use of this software which will prove to be a useful, efficient and time-saving tool in their hands for the sound design of foundation systems in the future.
Acknowledgment
The above described software is available with every copy of the author’s book (Kurian 2005). It has been prepared with the active assistance of his former students, Mr. P.K.Roy [shallow foundations – (Roy 2002 )] and Mr. G. Kiran [deep foundations and retaining structures – (Kiran 2003] ) whose contributions are greatly admired and gratefully acknowledged by the author.Note: The instructions given in the CD under ‘Read Me’ must be followed for the successful implementation of the software. Note that the Design File and the Library File (for graphics) carried by each folder pertaining to a specific design should be copied on the hard disk of your PC for the execution of the design and display of the corresponding graphics.
References
- Kurian, N.P. (2005), Design of Foundation Systems: Principles and Practices, Third Edition, Narosa Publishing House, New Delhi, xxviii + 830 pp. with Design CD attached.
- Roy, P.K. (2002), “Development of a software for the geotechnical design of footings and rafts in cohesive and cohesionless soils,” M.Tech. Thesis, Department of Civil Engineering, Indian Institute of Technology, Madras.
- Kiran, G. (2003), “Development of a software for the geotechnical design of deep foundations and retaining structures and parametric studies using the software,” M.Tech. Thesis, Department of Civil Engineering, Indian Institute of Technology, Madras.
NBMCW September 2007
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