Geotechnical engineering is not an exact science
Stabroek News
December 4, 2001

Dear Editor,

I refer to the comments made by Mr. Ali and Mr. Ragwen in their letter [ please note: links provided by LOSP web site ] (30.ll.200l) regarding my thoughts on the breach of the conservancy dam. First, I must thank them for pointing out my error in the age of the dam. Likewise, I would like to point out that my name is Diyaljee and not Dyalgee. I was unaware that the dam was over 100 years old. I apologize for making this mistake. I was referring to the period of time after the breach, but should have been clearer on this issue. As a geotechnical engineer, I tend to use such an event as one important bench mark to evaluate the future behaviour of a structure at the location where a failure has occurred. I hope that you can appreciate my reasons for doing this.

As I alluded in my November 14 letter, geotechnical engineering is not an exact science. However, having said that, I must also emphasize that a considerable amount of engineering judgment is required to make satisfactory decisions. This judgment, of course, can only be appreciated by knowledgeable and dedicated practitioners in the geotechnical field. Again, as astute professionals, I am sure Messrs Alli and Ragwen can appreciate this. As you can appreciate, gentlemen, none of us knows the cause or causes for sure. We all have opinions and what must occur next is that we must consider these opinions and then begin the task of examining each of them despite how trivial they may appear. In the end, based on my meager experience, we may be hard pressed to find a definitive reason for failure from a geotechnical point of view. If you recall, I mentioned two areas (hydraulics and geotechnical engineering) that need to be addressed, as well as historic evidence and input from nearby residents. There are of course other factors as Mr. Dewar pointed out in his November 17 letter.

I am not sure, gentlemen, that you fully understood the context of my November 14 letter. This is rather unfortunate as my thoughts were not meant to imply a particular cause for the failure, nor to demonstrate that the dam could be raised, nor that a slip failure was the mode of failure. As I recall, another writer suggested a circular failure. That suggestion is not without reason, and a deep seated foundation failure is only one possible mode of failure that is generally evaluated when structures are placed on soft, compressible ground. Other forms can be slip failures of the upstream and downstream slopes of the dam. Yes, and indeed, piping may be a mode of failure, but I can also say that such failures are more prevalent with pervious soils. However, do not think that I am implying materials composed of gravel, sand and silt particle sizes only. Again, the micro and macrostructure of materials may very well dictate whether they behave as pervious or impervious materials. Now, gentlemen, you can now correctly imply, therefore, that this means that pegasse or peat or muskeg or clay soils can also be susceptible to piping. Perhaps you cannot. Think about it.

The terms muskeg and peat have been used interchangeably in engineering textbooks and in the engineering literature, but some academics and practitioners have endeavoured to show the distinctions. Suffice to say, broadly, that these are all soft, compressible materials of initial low shear strength and low bearing capacity. With this in mind I can imply, broadly, that pegasse and muskeg are of the same family. As a Guyanese, I know what pegasse is, and as a practicing geotechnical engineer in Canada I also know what muskeg is and, as well, how to identify and classify such soils and to broadly estimate their engineering characteristics, and hence behaviour.

I have never trivialized those materials but neither have I been ever afraid to take on the challenge to construct embankments on them. As you are aware, experience does not necessarily come with age by doing the same thing over and over, nor does it mean that experience is always synonymous with age, or with having been an engineer before others. What I do know is that I have engineered embankments on muskegs and have developed strategies for their successful construction over the most challenging deposits one can encounter. I have also written guidelines for construction of embankments on such soils, and have solved many failures associated with such constructions.

Please refer to one of my papers - "Geotextile Reinforcement of a Deep Muskeg Deposit" 3rd Canadian Symposium on Geosynthetics, held at Queen's University, Canada, October 1988. This paper was published by the Canadian Geotechnical Society. This paper addresses the construction of a 5m high embankment on a 4 to 6m deep deposit of muskeg. You will find details on instrumentation to monitor settlement, lateral movement, and porewater pressures caused by superimposed loading. These same types of instrumentation are used in dams. The net result was recommendations on how construction should be staged and the most suitable geotextile to use, amongst others. This height is by no means the maximum. I have engineered up to 20 m high embankment fills on such soils for bridge approaches on either side of main line railway tracks without any failure. I can also rise to your challenge of writing a manual, gentlemen, any day and any time.

In your call for a dam safety assessment, I will assume that you recognize that this assessment cannot be undertaken only by visual inspections and analyses, but that the need for planned instrumentation and its planned monitoring is implicit when one conceives the necessity and value of such a scheme.

If we are to contribute to the engineering problems that face our country and many such countries as a result of rapid infrastructure changes resulting from social and economic demands, or because of an aging infrastructure, then we must put our ideas and opinions to work. Nothing productive will be gained by taking pot shots at each other. That this occurs is unfortunate. As both of you are aware, we know each other well. I am no stranger to this game, my friends, and I can bowl the ball in any or all directions. Finally, as learned gentlemen, we can plan to continue this debate in another forum and I will be very delighted to do so. Let us not forget, however, that whether some opinions are considered vague or of no consequence, very often those are the missing links to the solution to many a problem in the civil engineering field.

Yours faithfully,

Vishnu Diyaljee, Ph. D, P.Eng