NZ Geomechanics News

Questions from Chile

Below are a series of questions put to Laurie Wesley by Rafael Iglesies, with Laurie’s responses. The Editors thank Laurie for sharing these thoughts and insights, they make interesting reading!

1. In many parts of Chile we can easily find high-rise residual soil slopes right next to highways and roads. In multiple documents you comment on the importance of visual inspection in the stability analysis of this type of slopes. What are the most important indicators to observe in the field to assess the stability of these types of slopes? What visual indicators may warn us that the analyzed slope is susceptible to a landslide or to some type of damage? What is your assessment of the future of analytical procedures for evaluating residual soil slope stability?  

I doubt that analytical analysis will ever have more than a minor role in assessing the stability of slopes in residual soils for the following reasons:

  • Residual soils are seldom homogeneous so it is not possible to determine reliable shear strength parameters
  • The seepage state in residual soils usually changes with the seasons and the weather, and the worst condition can only be guessed at. It is possible to create a worst seepage state using a programme like SeepW and assuming the rain falls long enough to establish a steady state seepage condition. This is likely to be excessively conservative.
  • Careful visual inspection is always the most reliable guide, and examining the geology and aerial photographs, if available also provide useful information. 

The following is an interesting observation of Terzaghi:

“However, as soon as we pass from steel and concrete to earth, the omnipotence of theory ceases to exist. In the first place, the earth in its natural state is never uniform. Second, its properties are too complicated for rigorous theoretical treatment. Finally, even an approximate mathematical solution of some of the most common problems is extremely difficult” (Terzaghi, 1936).

Regarding evidence of instability or potential instability from visual inspection, it depends whether the slope is a natural hillside or a cut slope. If it is a natural slope it is the shape of the surface that indicates whether or not a slip has occurred in the past. If the slope is smooth without unexplained irregularities it is unlikely to have had instability in the past. If its surface is uneven with humps or hollows these are likely to have resulted from past slip movement. With cut slopes the situation is quite different. If a cut is being planned for a specific project, it is generally the case that some cuts have already been made in that area. Inspection of those cuts is then the most useful source of information with which to plan future cuts. The inspection does two things. Firstly, it tells you about the height and inclination of existing stable cuts. and secondly it provides a picture of the local geology. 

I have been involved with assessing the stability of cut slopes on roads in Malaysia and Indonesia. The only useful guidance on this issue at the time was an excellent paper written by a British geologist in 1968. (Bulman, 1968). The paper is based on inspections of a very large number of highway cuttings and provides guidelines for selecting slope angles in a range of geological materials. It appears that it is still the best available guide for highway planners in Malaysia. Indonesia’s soil types are very different from those in Malaysia, and a similar study would be very useful in Indonesia. In other words, studies of this sort are vastly more useful than analytical analysis. Unfortunately, universities do not encourage studies of this sort, as resulting papers are unlikely to be accepted by prestigious journals. 

Bullman, J.N. (1968) A survey of road cuttings in Western Malaysia. Proceedings First Southeast Asian Regional Conference on Soil Engineering, Bangkok, 1968

One last comment is that if I wished to improve my competence in assessing the stability of natural slopes, I would not be taking a course in constitutive modelling or any other form of theoretical analysis. Instead I would be taking some papers in geology, as understanding the geology of a site is always valuable for whatever project you are planning.

 2. Throughout your career as a geotechnical engineer, which has been the most challenging project you have ever worked on? Could you briefly explain the difficulties you had to face, and how you solved them?

In 1982 I was asked to go to Malaysia to produce a proposal for remedial work on a large number of cut slopes on a major highway between the capital, Kuala Lumpur, and a place called Karak. The four lane highway was not very old and had been built to a high geometric standards which meant that a large number of very high cut slopes were made, some as high as 70m. The material was mostly highly weathered granite soil at surface, and the usual deep weathering profile below that. I was given two weeks to examine all the major cuts, which I recall were over fifty, and produce my report. I was supplied with a car and driver, as well as a technician to hold one end of a tape plus an Abney level for measuring slope inclinations. I was young and fit in those days so had little difficulty climbing or “scrambling” to the top of the cuts. The photo below shows a typical cut slope about 70m high. This was a double cutting, so I took this photo from halfway up the cut on the opposite side of the road. 

A typical cut slope on the highway, showing severe erosion and a shallow surface slip

The photo clearly shows that the main problem was erosion rather than slope stability. I learnt from that job that weathered granite is normally highly erodible. So my problem was what to do about it. I soon made some Important observations. In particular it appeared that the lower part of each slope was behaving rather more like a rock than a soil. It appears that the designers made this assumption as the cuts were much steeper at their base than higher up. However, all the material seemed to be equally highly erodible. So I decided as follows

  • A judgement would have to be made as to the level below which the material could be treated as rock and above as soil 
  • The rock part could then be “tidied” up a bit by filling in the erosion channels and then faced with shotcrete to prevent further erosion.
  • The only feasible way the “soil” could be protected was to cover the face with vegetation. To do this the slope would need to be flattened to 1V to 2H or possible 1V to 1.5H. I was not knowledgeable enough in this area to make a firm judgement. 
  • A wide “bench” was to be made at the boundary between the rock and the soil, so that a large lined permanent cut off drain could be placed here.

The diagram shows my concept. The contractor my report was prepared for did not get the job, but the contractor who was given the job apparently did what my report recommended.  

My proposed remedial measures shown diagrammatically.

3. In 2019 you published the book: “The Bishop Method: The life and achievements of Professor Alan Bishop, soil mechanics pioneer”. What has been the reception of the community? Could you briefly tell us what we can learn by reading the book? How would you describe Professor Bishop’s influence on your career?

Re your first question, I have no categorical answer here as it depends on the person. Some people, including me, have a natural interest in the past, while other have no Interest at all. I have had very positive reviews of the book, mostly by soil mechanics people, but last year the publisher sent me a review by a structural engineer who did not know Bishop or me. I will send it as a separate attachment.

Re your second question you will learn that Bishop played a critical role in sorting out how to measure the shear strength of soil and how to apply the results of measurements to practical situations, especially the stability of slopes. In effect, he settled the questions about the shear strength of cohesive soils that Terzaghi had addressed but failed to answer. One reviewer (Mick Pender) commented that Part 2 of the Bishop biography would provide a good basis for a course in basic soil mechanics

Re your third question, Bishop’s lectures for the one year M.Sc(Eng) degree in 1964-65 straightened out my confused understanding of total stress and effective stress analysis. I did my undergraduate degree in the 1950s when Bishop was leading the way to providing a clear answer to this issue. Geotechnical engineers would benefit from reading his 1960 paper with Bjerrum:

Bishop and Bjerrum “The relevance of the triaxial test to the solution of stability problems” ASCE Research Conference on Shear Strength of Cohesive Soils, Boulder, 1960 

I should add also that reading my textbook “Fundamentals of Soil Mechanics for Sedimentary and Residual Soils” will, hopefully, be more use technically than reading the Bishop biography. At present I am helping my good friend, Omar Nunez. produce a Spanish edition of that book. Omar is doing the translating I am just helping with the figures. 

4. What are you currently working on, and how has been the last year considering the Covid situation?

I think the best answer is that I am currently working on enjoying being retired. On 2 Sept I will be 85 and my wife and I will celebrate our Diamond Wedding Anniversary (60 years).

However, I still get asked to do various consulting roles and, if they are interesting and in my field, I usually can’t resist taking them on. Having said that I remind myself always that there are more important things in life than geotechnical engineering, especially at my age. The Covid situation in New Zealand had a big effect during the 7 weeks of our severe lockdown in March-April last year. Since that time, we have lived normal lives except that we cannot travel overseas and people cannot visit New Zealand. 

5. Looking back, can you tell a specific moment or a turning point in your career that led you to become a Geotechnical Engineer? Is there any message or advice that you could give to the new generations of Chilean Geotechnical engineers?

Soon after graduating I went to Indonesia under a Volunteer Scheme to work for the Indonesian Government. I was assigned to their Institute for Soil and Highway Investigations. The Institute had been set up by the Dutch when Indonesia was The Dutch East Indies, and it was well set up with both laboratory and field testing equipment but when I went there in 1960 it was desperately short of staff. The result was that I played a key role in the institute for four years and I found the work so interesting that I settled for geotechnical engineering as a career. When I began work there, the only countries that used the CPT test were Holland, Belgium and Indonesia, so I learnt very quickly what a useful tool the test was. All the devices in use at that time were mechanical devices. Regarding advice to Chilean geotechnical engineers, I will include this in my answer to your Question 7. 

A CPT test being carried out for a new bridge in Sumatra Indonesia

6. In comparison with other areas of civil engineering (such as hydraulics, mechanical, or structural engineering), Geotechnical Engineering is a very young discipline. What would you say has been the most significant advancement or breakthrough in Geotechnical Engineering during the last 20 years?

My view is that the golden years in the development of Geotechnical Engineering were in the 1940s to the 1960s, when soil mechanics established a sound understanding of the shear strength of cohesive soils as described above in my comments about Bishop. I doubt that there are any dramatic breakthroughs still to be made except possibly in the issue of soil liquefaction during earthquakes. However, there have been steady and very useful advances made in construction methods. Reinforced earth, soil nailing, diaphragm walls, and piling methods come to mind. I suppose the advent of the computer has been beneficial to geotechnical engineering, but I have serious reservations about that. 

 7. What do you think of the newest advancements in Geotechnical Engineering? Do you think we may be approaching to a turning point, especially regarding new technologies such as artificial intelligence (AI)? How do you predict or expect Geotechnical engineering will evolve?

This is a rather broad question, and I will answer it in general terms. I do not think we are at a turning point, or if we are it may well be the wrong turning point. Geotechnical engineering has changed greatly since I began my career. In short, geotechnical engineers today tend to be what I would call “textbook engineers” They solve problems by identifying the appropriate formula or method of analysis (or which computer programme to use) and putting it to use. As long as they have followed a learnt or standard method they will be satisfied with whatever answer they get. Textbook engineers are what they are because all of their learning has been from lectures and textbooks. 

On the other hand, there are what I will call “broad based or mature engineers” for want of a better term. Broad based engineers are what they are because their knowledge of soil behaviour has come from a combination of formal learning and first-hand experience. This first-hand experience may be simple visual observation, or handling soil samples, especially taking block samples. Broad based engineers will have a sufficiently good “feel” for their subject that they may well know (at least approximately) the answer before they make use of formulas and theoretical methods. What makes a “textbook” geotechnical engineer and a “broad based” geotechnical engineer is an interesting question. It may have some connection with genes, but much more likely it is the culture of their native country. 

Historically, New Zealand has been a very “classless” society in which everyone is more or less equal. This means most people are accustomed to doing manual work of some sort. For example, I built an additional room on to my house many years ago. I dug the trench for the foundations and mixed and poured the concrete and so on. At the same time I built a new garage for my car. When growing up my friends and I had bicycles, and we regularly took the wheel hubs apart to grease them. We were careful to count the ball bearings and make sure we put back the same number as we took out.These are just examples of the day to day culture of New Zealand, but what it means is that we obtained a good “feel” of the way the natural world works from our own experience of it.

In contrast, especially in many Asian countries, there is strong class consciousness, and the upper class is not involved in doing any manual things. The result is they gain little understanding of how the physical world works. It appeared to me while teaching at Auckland University that the only knowledge or understanding some overseas students had of soil behaviour was from text books or lecture notes, and they did not relate this knowledge to their own experience of the physical world around them. We should be clear on one thing — a prime challenge for geotechnical engineers is making judgements as to the extent to which theoretical concepts can be applied to the situations they are addressing. It really takes broad based engineers to have the ability to do this. 

Finally, some comments on computers. This is an all pervasive influence today that I never experienced during my education or while working as a geotechnical engineer. For the analysis of complex situations it may be a marvellous tool but for routine geotechnical issues I’m not sure that its positives outweigh its negatives. Sound geotechnical engineering cannot be done in front of a computer screen, although it is easy to think so. I have reviewed reports on the stability of natural slopes that rely entirely on pages of computer-generated printouts of slip circle analysis (with assumed strength parameters) but make no mention of what a visual inspection of the site has revealed. Pages of multi coloured slip circle printouts certainly impress clients with no knowledge of the subject but leave others cold. The idea that theoretical analysis of natural slopes can take the place of other non-analytical methods, especially careful visual inspection of the site, is ludicrous. I once heard of an American consulting company that forbade its new staff from using computers for the first several years of their employment. I thought that was a very sound idea.

Regarding advice to young geotechnical engineers I can offer the following. I think a clear grip of basic soil behaviour is an essential attribute. You may already have this, but to add to it you need to practice observation, which was one of Terzaghi’s dominant attributes, but is lost sight of today. You should observe behaviour in the field whenever the opportunity arises to do this. I don’t mean special opportunities, just the opportunities you come across incidentally in daily life. These include excavations, especially deep ones, trenches, and the cut slopes beside highways or even footpaths. Trenches are particularly useful because they are so plentiful these days — for installing or repairing (or re-repairing) services Also, you should use every opportunity to familiarise yourself with laboratory testing and spend time in the field observing drilling and sampling techniques and the execution of other in situ tests, especially SPT and CPT tests. Without such exposure you will not be in a position to judge the reliability of data coming from field or laboratory tests.

8. Covid-19 has changed the way classes are taught all over the world, from 5 years old kids to Ph.D. students. Some of the best universities in Europe have even gone so far as to replace Geotechnical field trips for a virtual format. What pros and cons do you see in replacing the face-to-face teaching method with online classes? Do you expect the teaching method to change forever once the Covid crisis ends?

That is an interesting question, and I think it will be a backward step if teaching continues to be online. I believe education should be an enjoyable and indeed a memorable experience, and I don’t see how this can be possible except by face to face contact between the teacher and the student. 

The photo is me (a younger version) teaching Indonesian engineers and technicians how to do Atterberg Limits. I don’t think I could do this effectively on-line.

Tags : #Residual soil#Slope stability#visual inspection

NZ Geomechanics News
laurie wesley
NZ Geomechanics News>Issue 102 - December 2021

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