This is something that all geotechnical engineers should know but it is surprising how many do not! Just a brief overview of how the Mohr – Coulomb strength criterion came about.
The Mohr – Coulomb criterion is the outcome of inspiration of two great men, Otto Mohr born on 1835 and passed away on 1918 and Charles-Augustin de Coulomb born on 1736 and passed away on 1806.
The two men never coexisted but their brilliant minds contributed significantly in the scientific knowledge. The combination of two hypotheses gave us the Mohr – Coulomb failure surface.
Chronologically, Coulomb was involved in military defense works (how much knowledge have we gained due to war!) trying to built higher walls for the French. In order to investigate why taller walls than usual were failing and try to built them to stand, he wanted to understand the lateral earth pressure against retaining walls and the shear strength of soils. He devised a shear strength test and observed (at that time, with his tests) that soil shear strength was composed of one parameter that was stress – independent named cohesion (c) and one that was stress – dependent, similar to friction of sliding solid bodies named angle of internal friction (φ). Probably he executed shear strength tests and found for different normal stresses (σ) different shear stresses (τ). By plotting these data on a (τ-σ) diagram he obtained the straight line denoted by the equation τ=c+σ.tan(φ) as can be seen in the next figure.
Mohr (1900) proposed a criterion for the failure of materials on a plane which has a unique function with the normal stress on that plane of failure. The equation for that was τ=f(σ) where τ is the shear strength and σ the normal stress on the plane. With the use of the Mohr circles which is a two dimensional graphical representation of the state of stress at a point and the circumference of the circle is the locus of points that represent the state of stress on individual planes the Mohr failure envelope was proposed. The Mohr envelope was a line tangent to the maximum possible circles at different stresses and no circle could have part of it above that tangent curved line. (figure 2).
It is not known (Holtz et al, 1981) who first combined both theories but combining the Mohr failure criterion with the Coulomb equation gave a straight line tangent (to most of the Mohr circles) and the Mohr – Coulomb strength criterion was born (figure 3).
Holtz R. D., Kovacs W. D., (1981). “An Introduction to Geotechnical Engineering”, Prentice Hall.
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Today I was reading on the digital wire of ENR.com about the costly mistake of the highway 520 Bridge Pontoons. Cracks start to appear and the long term durability is questioned. The article states that “the state chose to design the pontoons itself on a fast track (rather than delegate that responsibility to contractors) as a strategy to attract lower bids…”. In my mind came also the article from International Business Times about the problems and collapses of new Chinese infrastructure. Is this new fast track project approach responsible for poor engineering?
Is the time pressure and the lowest budget to blame for serious engineering mistakes? The challenging economic environment is pressing away from serious engineering judgment and step by step design and construction, to all at once. Designs are constructed from automated drawings with serious mistakes before the calculations have been completed. Thinking time, quality control and checking are practically omitted even though this is not admitted. Is this the way to go? Should we stop and rethink and try to persuade decision makers and money driven decisions that in the end you are not saving money?
Additional information regarding the fix of the bridge pontoons can be found here
An expressway tunnel in central Japan whose ceiling collapsed in December, killing nine, became fully operational Friday after a 68-day hiatus, its operator Central Nippon Expressway Co. said.
The nearly 5-kilometer-long Sasago Tunnel in Yamanashi Prefecture became fully usable again ahead of the previously targeted reopening date later this month as restoration work was hastened ahead of the upcoming long weekend when traffic is expected to increase.
The Ministry of Land, Infrastructure, Transport and Tourism gave its go-ahead on the grounds that all safety issues have been cleared.
Some bereaved relatives of the victims are taking issue, however, with the road operator’s policy to prioritize the tunnel reopening before the cause of the fatal accident has been fully clarified.
The tunnel restarted partial operation on Dec. 29 after tunnel collapse with half of the lanes reopened to traffic.
Tunnel collapse can happen for a number of reasons such as: inadequate ground investigation, shallow ground conditions, inadequate support measures, cost optimization, inexperienced contractors, inadequate supervision, delays of excavation and support erection.
In recent history numerous headlines present tunnel collapses in major cities. One can ask why so many failures? I would like to provide some insight regarding this issue.
The following reasons can produce a single cause or combined effect that can result in ground tunnel collapse during tunnel construction in urban areas:
- Inadequate ground investigation due to condense building construction.
- Continuous differentiation of shallow ground conditions due to manmade structures, faulty utilities seeping fresh water, sewage or storm water etc. Especially in shallow ground the mechanical properties of the soil may have been modified from one building block to another.
- Inadequate support method of excavation. Especially in difficult and complex ground condition with mixed face conditions (strong rock and soft soil are encountered on the tunnel face).
- Cost optimization of excavation and support. This can lead to reduced support measures or higher advance rates of excavation to meet deadlines and follow inappropriate scheduling.
- Cost optimization with selection of inappropriate tunnel boring machine (TBM) that can accommodate most of the encountered conditions but not all.
- Inexperienced contractors and design engineers with local conditions, especially for international projects.
- Inadequate supervision of construction works.
- Delays of excavation and support erection due to unforeseen conditions such as archeological discoveries, union strikes etc.
These are the most usual conditions (many other may exist) that can produce tunnel collapses. Unfortunately most of them are related to cost reduction or cost optimization. This is especially true in low bit contracts for investigation, design or construction.
Usually after a tunnel collapse has been formed and its causes are evaluated, it is found that
it could have easily been avoided. The most interesting outcome is that the cost of repairs usually is far more that the cost required for a better initial investigation, or design or construction!
On Monday afternoon (28/1/2013), three buildings collapsed after subsidence in Guangzhou. The incident happened near a metro tunnel construction site.
It is reported that metro workers spotted land subsidence near the project site and immediately the area was evacuated, hence no casualties have been reported. The subsidence area was about 10m deep and extended about 100m2.
It is also reported that the area is temporary stabilized by backfilling concrete into thesubsidence. The site is monitored for risk assessment.
Tunnel construction with a fast pace and lowered standards could lead to disaster. Land subsidence can be caused by a variety of factors. Tunneling in urban areas always includes careful consideration and monitoring of land movement.
Many Geotechnical Engineers probably have not even thought about it too much, but what is that motivates many of us to practice Geotechnical Engineering? Is it the money? I don’t think so. If you compare the salaries of different engineering disciplines we are not even in the middle. If you compare with other non engineering disciplines we are so far down the list! We are under Wedding Photographers if you can believe that. Here is an article from EngineeringSalary.com regarding this issue.
So what is motivating us?
I came across two very interesting articles, the first in Geo-Strata September/October 2012 issue and the second in Ground Engineering Vol 46, No 1, January 2013. The first was the “Thoughts form the President” by Phil King in which he describes how he felt when he visited Terzaghi’s gravesite and reminisced his career as a Geotechnical Engineer. The second article was about John Burland “Towering Achievements” which describes how Prof. Burland became a Geotechnical Engineer and his career path.
Both describe with great passion their careers as Geotechnical Engineers and what more geotechnical engineering is for them.
What we do in the end is more about our passion for Geotechnical Engineering and less (much less) for material gain, salary or recognition. Our work usually is not even visible, but can you build a great structure without solid foundations?
Francis Bacon once said that knowledge is power, these words have a profound meaning but in our days where vast amount of information is available we may want to reevaluate them. Is the vast information providing knowledge or distorts it?
In our days every question one has, he just needs to type it to a search engine and in milliseconds he will find one or multiple answers. Are any of these answers correct? Is the one asking the question qualified to evaluate the answers? Will he use any answer and then find out that he has mistakenly approached a subject? Think before you use is a good advice!
This is a very difficult subject for discussion, even philosophical one can say, but the issue is that we need to pay attention to the way information is used and evolved into knowledge. We think that information should be vastly available and easily comparable in order to produce appropriate conditions for knowledge. We want to produce such a platform where geotechnical information will be provided but at the same time it could be critically evaluated from the users and in the end rated for the benefit of knowledge.
We will try hard for that…
We would like to wish to all a happy and prosperous New Year. This year will be the starting point of our web site which will try to provide anyone involved with geotechnical engineering or geotechnical construction a vast amount of information. We will try hard to form a strong geotechnical community around geotechpedia in order to disseminate knowledge and information.
We will try to post regularly about new features in our site, technical and scientific information or opinions regarding the geotechnical field and interesting news related to geotechnical engineering. Our goal is to provide accurate and up to date information for the benefit of the geotechnical community…
… and welcome to Geotechpedia!
Geotechpedia, as we mention in our about page, is a continually growing database of assorted geotechnical engineering information, dedicated to anyone and everyone interested in geotechnical engineering.
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