Canadian Consulting Engineer

Geotechnical Considerations Come Too Late

May 1, 2009
By David Wood, David F. Wood Consulting

All large-scale construction projects have a full team of engineers from different disciplines, and over the last decades we have also seen the introduction of geotechnical engineering into the mix. H...

All large-scale construction projects have a full team of engineers from different disciplines, and over the last decades we have also seen the introduction of geotechnical engineering into the mix. However, as an engineering geologist with rock mechanics training, I have found that design concepts I need to understand are often not considered until relatively late in the design process. It appears to me that project leaders, who are commonly civil and structural engineers, sometimes do not understand the relevance of “ground” engineering to the project and may ignore critical components in the design process. This situation has been the subject of many discussions, and at a recent meeting of the Canadian Geotechnical Society (CGS) in Edmonton, it was said that there could well be a “hole” in the education of traditional engineers with regard to engineering geology.

Geotechnical engineering, in the broadest terms, refers to ground engineering, or engineering relating to foundations. Strictly speaking, geotechnical engineering is synonymous with soils engineering and most geotechnical engineers are, in fact, soil mechanics engineers.

Many undergraduate courses in civil engineering offer a preliminary understanding of geotechnical engineering, but civil engineers usually graduate without a solid base of expertise in the geotechnical field. The majority of engineers who specialize in geotechnical engineering do so through post-graduate education. So where does this leave us?

We have civil engineers and architects designing a project without a realistic understanding of the true impact that the ground conditions can play on the foundation design. Even when the design team uses the services of a geotechnical engineering consultant, the relevance of the engineering geology of the site is commonly not considered until very late in the design process, sometimes leaving the fundamental issues of anchoring a building or bridge to the bedrock until the initial phases of construction.

What can go wrong — an example

A design team is preparing for a “one-site” hospital to replace multiple facilities. The design specifically calls for elevator shafts that can ensure that access throughout the new building can be maintained even after a strong earthquake –perhaps even greater than the Canadian Building Code requires. The structural engineer is looking for foundation conditions and calls upon his geotechnical consultant for input. The most common design criterion in soils engineering is the “bearing capacity” of the foundation materials, so the structural engineer is looking for a value of bearing capacity to use in the design process.

Bearing capacity, however, refers to the ability of the ground to withstand a specific style of compressive loading in which the loads exceed the capacity. The foundations underneath the hospital elevator shafts are not going to be critically loaded in compression but in tension. In this example, the critical loading configuration is in uplift and so a different type of foundation condition has to be evaluated. As a rock engineer, I was asked to provide input since the designed anchorage system appeared to be inappropriate and non-constructible.

Since the design related predominantly to the pull-out resistance of the anchor bars, the anchorage design required evaluating the effects of bonding and shear strength of the complete system, including the foundation materials, the anchor bars and the proposed grouting medium. Reference to the Post-Tensioning Institute’s recommendations was considered far more appropriate to provide a workable solution than an evaluation of the bearing capacity, as had been requested. The principal engineering issue was to accurately determine the bond length so that the most appropriate anchor bar could be installed to resist the uplift forces. The design guidelines consulted are used to estimate the capacity of the system to transfer loads within the bond length, which is a function of the design load for the anchor, the surface area of the drill hole, and the working bond stress along the interface between the grout and the rock mass.

However, the average ultimate bond stress is affected by numerous other considerations, including some rock mass characteristics (shear strength, discontinuities, mineralization), drilling considerations (hole diameter, drilling method, hole cleaning), and grouting (grout consistency, strength, grouting procedures). So rather than develop a mathematical approach to the design, we used an empirical process that looks at relatively simple geological terms to describe different rock masses, and from that we were able to provide average ultimate bond stress values for the rock-to-grout contact.

So what started as a request from the structural engineer for a “bearing capacity” value to use as part of the foundation design was changed to a “bond length” value for the anchor design –an approach that was more accurate and appropriate. A considerable amount of design input had been exercised in completely the wrong direction because the design engineers were not aware of the importance of “rock engineering” at the site.

Not An Isolated Case

I have come across many other examples where relatively straightforward rock engineering would have been very useful early in the design phase. For example, bridge design along new four-lane corridors in Northeast Ontario has commonly been advanced without much understanding of the underlying Precambrian Canadian shield gneisses. Shopping centres are springing up all around, but the specific requirements for blasting often wait for a problem to develop before being discussed. And subdivision housing developments are usually planned in an office, on a flat desk, rather than in the field taking the natural ground into consideration.

It appears that more and more civil engineers are graduating with a limited perception of geotechnical (soils) or geo-mechanical (rock) engineering, and they do not understand the limitations that their lack of educational background might bring. Through the Engineering Geology division of the CGS we are hoping to re-establish education in this field for undergraduate engineering students, so that by the time they graduate they are at least aware of some of the pitfalls that can occur on large construction projects if the engineering geological components of the site are not considered early enough.

David Wood, P. Eng. is the principal rock engineering consultant for David F. Wood Consulting in Sudbury, Ontario. He is also current chair of the engineering geology division of the Canadian Geotechnical Society, and represents all the technical divisions on the CGS national executive board.


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