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Considering that Geotechpedia project runs parallel to our geotechnical design work, your great response encourages us to keep going!
In my previous entry regarding the Yeager airport landslide where I hypothesize for a possible shear zone somewhere near the foundation, I got some interesting comments from fellow engineers. In this entry I would like to clarify some issues.
The hypothesis made about a shear zone was based on published information regarding intercalation of sandstone and shale and photograph observations of the area. A possible dormant landslide was hypothesized by other contributors in the ASCE group in LinkedIn. The model I presented was very simplified in order to test the possibility of the shear surface hypothesis. I will come back to the model with some more details, but I wouldn’t like to deviate from the failure discussion to a model discussion.
I stated previously that usually failures of such large magnitude are more complex and significant data and observations are required to understand the real mechanism. This I am sure will be dealt with by enquiry committees, forensic consultants etc.
I based my hypothesis considering that the Designers of the reinforced earth had done a good job, that the materials used were of appropriate quality and that something outside the structure was responsible or partially responsible for this failure. Many have commented regarding the reinforcement design and all are valuable comments which I will not reply but can be found here. Before a detail analysis and modeling of the failure is possible, additional hard data are required, such as shear strength properties of the actual soil, current tensile strength of the geogrid (as found in the landslide area), excavation at the toe of the slope etc. Until such data become available, only hypothesis can be made that may be completely wrong in the end.
The reason why I made the hypothesis of a shear zone is because it is based on previous information about older landslides in the area, because shale is notoriously tricky material when combined with sandstone and to broaden the possibilities of failure outside the earth structure.
Many times, failures are formed due to very thin weak layers which are very difficult to identify during ground investigation. Sometimes zones (or layers) of a couple of centimeters can be responsible for extended failures. Such zones many times are ignored, especially in very large structures. Consider a borehole of 50-100m with a low strength shale zone of a couple of centimeters, is it always possible to identify it? Even if not ignored, during investigation, a shale zone could appear strong and competent. In the following photograph a translational failure on a lignite mine can be observed. The failure took place on a clayey shale layer of couple of centimeters in a nearly horizontal stratification material. Observe the magnitude of the failure in relation to the huge bucket wheel excavators. Also observe the horizontal movement based on the misalignment of the conveyor belts. The slope inclinations before failure were very shallow, around 1:3 (V:H).
Coming to the slope stability model I used, it was only to validate the possibility of such a failure and not to model the actual reinforced earth slope and its failure. The parameters used were the ones provided by the Lostumbo 2010 presentation and instead of including the reinforcement, a cohesion was used to produce a factor of safety above 1.3 for a circular shear surface inside the soil reinforced structure. A shear surface was not included and a factor of safety above 1.3 was selected because it is assumed that the structure would have been designed above this FS. It is not the intention to assume a soil material with such cohesion. The initial calculation to validate the stability of the model before the failure surface is presented in the following figure.
But once again I hope the discussion will not deviate from the actual issue and get focused on the model. As R. Peck very elegantly noted “stability analyses are tools for the guidance of the investigator. They have their limitations with respect to evaluating the stability of existing dams [the paper was about dam failure] It is not meant that they should never be performed. However, the numerical values for the factor of safety should carry little if any weight in judging the actual safety of the structure with respect to catastrophic failure”. Peck was evaluating a dam failure, and he focused on other issues that play important roles in relation to failures. So in that context I (among others) proposed the lower shear failure or old landslide issue as part of the controlling factors. Hopefully soon we will have many additional hard data to address this issue.
Finally I would like to note that earth retaining structures are a very good solution to many situations and we should not be reluctant to use them because of such incidents. We should though learn about such failures and put all our effort to avoid them in the future.
R. Peck, (1998). “The Place of Stability Calculations in Evaluating the Safety of Existing Embankmnet Dams”, Civil Engineering Practice, Fall 1998.
Recent news and photographs present the spectacular slide that occurred in the Yeager Airport Expansion Runway 5. The slide occurred in the South slope which was among the highest if not the highest (~74m) reinforced earth slope in the US. The project had received the award of Excellence – TenCate Geosynthetics in 2007 International Achievement Awards. According to FHWA Manual the Yeager Airport in Charleston WV had been constructed as a massive earthwork in 1940’s. The mountainous conditions around the airport produced steeply dipping slopes to the Elk and Kanawha Rivers. In order to meet FAA Safety Standards runway 5 required a 150m extension in order to create an emergency stopping apron. The most cost effective solution was a 74m high 1H:1V reinforced steepened slope (RSS). The chosen solution is presented in Figure 1 taken from Lostumbo 2010.
As can be seen from Figure 1, most of the reinforced earth area is above the original ground and only a small part in the slope base is excavated in order to found the reinforced earth structure. Based on Lostumbo, 2010 over 100 borings were performed, with extensive laboratory testing and the final outcome was that the site consisted of primarily fill, colluvial and shallow rock. The material parameters used for the bearing soil zone were unit weight γ=22kN/m3, φ’=40ο and c’=0kPa. I would like to provide some speculations regarding this incident based on the available data found on line and photographs from the news and Google earth. I must point out that these are only speculations since no official data are available to me, nor the exact design or construction plans. These speculations are made just by observations and engineering imagination! I am sure that significant investigation will take place in the coming months and years which will produce the actual conditions and reasons for this spectacular failure. A picture from Google earth taken on 9/2005 presents the initiation of excavation for the construction of the reinforced earth slope.
A newer picture taken on 4/2006 presents the progress of works in which a part of the reinforced earth slope has been constructed.
It is very interesting to note that the excavation and foundation of the earth structure did not go all the way to the base of the hill. It is possible that good foundation material (assuming rock) was found in some elevation and was considered appropriate for founding the structure. After all the large earth structure is placed on sound rock (mostly sandstone) with a compressive strength between 30-95MPa! Weathered sandstone from the borrow area had a friction angle between 38.9-39.6o. The placement of the reinforced earth structure on top of bedrock can be observed in figure 4. The bedrock is clearly visible in the back and some moisture can be observed a bit higher in the slope.
Is the “competent” foundation bedrock to blame?, is the design of the reinforced slope to blame? Is the construction practice? Is the intense rainfall? Usually many factors contribute to such a large failure, but at this point with very limited information I would like to focus on the bedrock conditions and the fill material placed on the bedrock.
Is a sound bedrock always appropriate for placing such a large structure? Based on the unconfined compressive strength I would say yes, but are other conditions at play here? Based on Huang et al, 2014 “the on-site geomorphology consisted of weathered sandstone underlain by sandstone and some shale.” and “The compressive strength of the rock foundation varied from 30MPa to 95MPa. The high bearing capacity of the underlying sandstone foundation and the high friction angle of the onsite weathered sandstone soil meant that the extent of the reinforced slope could be kept to a minimum, and maximum use could be made of the onsite soil.”
Notice the phrase “sandstone and some shale”. Could this simple phrase be the key for what happened? It is well known that even small intercalations of shale can produce enormous geotechnical problems as was the case of Landslide on No.3 Freeway in Taiwan (Duncan, 2013). The first reason is that shale materials have much lower compressive strength but more importantly considerably lower friction angle. Furthermore if not fractured, they present a very low permeability barrier. Usually water is seeping in the sandstone – shale interface, asymmetrically weathers the shale and also produces increased pore pressures in that interface. Could such an interface (or failure surface) had been formed in this case? The answer is, it may be possible and can be seen in the following very simple model in figure 5. (will not go into much detail about the model it is just an example of the possible formation of such a failure surface).
Now let’s go back to actual observations, figure 6 is a Google earth image taken on 3/2012. Please observe the stones between the slope and the road in the red circle. Then let’s go to Google street view in the same location and what we see is shown in figure 7. The layering of the bedrock is clearly visible, furthermore some form of fissure can be observed even with some horizontal movement one can argue based on this image. Could such a feature or a similar one in different elevation be the weakest link of this structure? Could it have been in marginal stability and all it took was some heavy rainfall that increased the pore pressures in this interface and initiated the slide? Food for thought until the actual investigation comes out and the real conditions that lead to instability, which I really hope are much more complex, can be addressed.
Duncan J. M. (2013). “Impact of time on the performance of reinforced slopes” Geo-Congress 2013.
This is a very controversial topic in which a straightforward answer is not possible. In this entry I would like to tackle some issues related to our own profession since we are responsiblefor the “acceptable” amount of investigation.
Recently I attended the IAEG 2014 (Engineering Geology) conference in Torino. In this conference numerous interesting topics of engineering geology and geotechnical investigation were covered. It was very interesting to note that in many cases a general conclusion was that not enough geotechnical investigation was executed prior to a geotechnical related failures.
In conversations regarding the site investigation of a project it is very common to hear that “I would like additional investigation but the Client will not provide the funding” or that “the project finance does not allow for more or additional investigation, you have to do with what you have” etc. What do we do in such situations? We do what we have been taught as engineers to do, we overcome the problem. This means that either we accept a larger portion of liability, we either allocate the liability with statements like “additional investigation is warranted during construction” or we design very conservatively or all of the above. In any case, the design is based on limited information and it could go either way.
In many situations, due to the experience of the geotechnical designer or due to very conservative design assumptions no problems are manifested during construction or operation. But sometimes things go terribly wrong and somebody needs to take the blame, leading to long lasting litigations.
Is something wrong with the current practice? Everybody admires the great engineering attitude when nothing goes wrong and with limited investigation the project is completed. Even more, some of us proudly state “I saved so much by reducing the geotechnical investigation” but all this immediately changes when something goes wrong.
Maybe we should start thinking more as doctors? I don’t think anybody has gone with a medical situation and stated to the doctor that “I think you are asking too much medical testing” or “I don’t think an ultrasonic is warranted for my abdominal pain, cant you prescribe some conservative medicine that will make me better without doing all these expensive testing?” I would really like to see the face of the doctor hearing such negotiations. So why are we accepting such negotiations ?
The March / April Geo-Strata was almost entirely dedicated to the GAM (Geotechnical Asset Management) for transportation systems. It was very interesting to see how this concept is evolving in the broader field of Geotechnical Engineering and transportation infrastructures.
This Geo-Strata feature is worth reading if you are interested in the future of Asset management in relation to infrastructure projects and geotechnical involvement.
I would like to focus a bit on the issue of geotechnical monitoring. As Thompson et al (GeoStrata, 2014) very elegantly observe, we have all sorts of sensors and warning lights in our cars, which enable both us and the car dealer technicians to identify a future problem as early as possible. If treated early, this problem can be resolved at a minimum cost. If left untreated, however, it could cost us our very life or even other people’s lives should a terrible car accident occur.
Car engine sensors, or airplane sensors or even elevator sensors are mandatory and nobody really argues over whether they should be installed or not. Nobody goes to a car dealership and tries to reduce the vehicle price by arguing that he does not really need the engine sensors because he can visually check his engine once in a while…
Can you imagine an airplane company, saying that in order to reduce operating costs it will remove the black boxes?
So why is it so easy to eliminate geotechnical monitoring instruments from geotechnical projects or so difficult to persuade the owners of the importance of the use of such instruments and information? I am sure that there is not even a single geotechnical engineer who does not have examples of struggling to enforce the use of some type of
instrument and the client arguing over its cost of installation, cost of operation or even the usefulness of such geotechnical monitoring and instruments for the project. Even worse this argument is sometimes thrown back at the designers through a challenge such as “why should we monitor the wall? Haven’t you designed it to be safe?”
Airlines and government boards do not consider installing black boxes only in planes that are old and with mechanical problems that may have a high risk of falling out of the sky. Imagine if such practices were taking place, would our planes be as safe as they are? Would they have evolved in the way they have?
Why is it so hard to do the same in geotechnical projects? Why is geotechnical monitoring and instrument installation, warranted in critical situations, on critical structures but not on ordinary slopes or embankments etc? How is the profession going to excel in future projects when real behavior of geo-structures is so difficult to find and evaluate?
As a profession, we should try to persuade owners, government officials, policy makers etc of the significance of reliable geotechnical monitoring systems included in the majority of geotechnical works. By doing this future works and infrastructure will become safer and costs will fall far more than we may realize. Don’t put a price tag in current projects without considering future projects…
Every geotechnical engineer knows E. Hoek and his significant contribution to rock mechanics. Here you can find his first on line lecture titled “The Development of Rock Engineering”, you just need to enter your name, e-mail and company and you get the password to view the on line lecture.
This lecture provides interesting historical background regarding the development of rock mechanics and the future trends of the profession.
Another interesting PowerPoint presentation regarding the future direction of Eurocode 7 published by Dr Andrew Bond of Geocentrix is worth reading. In this presentation the Quo Vadis of Eurocode 7, the proposed changes and modifications are presented.
November – December issue of Geo-Strata which is a published forum of the Geo-Institute of the American Society of Civil Engineers (ASCE) featured an article by Patrick C. Lucia, Chairman Emeritus of Geosyntec Consultants, titled “As I See It: Geotechnical Forensic Engineering in Defense of Geotechnical Engineers”.
In the article Patrick shares his over 25 years of experience in forensic geotechnical investigation of failures and the compliance of Geotechnical Engineers to “Standard of Care”. In his opinion the majority of failures occur due to “lack of process in conducting the geotechnical engineering practice”.
Unfortunately it is very difficult to standardize geotechnical engineering practice in a way that other engineering disciplines have. The difficulty of standardizing geotechnical practice is that ground is not standard. This is why geotechnical engineering is so challenging. How can you standardize an investigation in a new project? Is the text book “influence zone” depth an adequate depth to drill? Can a few centimeters thick unfavorable clay seam be found with two 30m borings in a proposed cut? Can an undisturbed or even remolded sample be acquired from that seam? Can we pursue the client to spend additional thousands of dollars when we are unsure of what lies beneath?
Pat is arguing that “when the process of engineering is properly done and properly documented, it will far reduce the number of claims and make the defense of those claims much easier.” This is true but maybe difficult, especially in a world of fast track projects and low bids. Maybe our profession needs to do much more to “standardize” proper engineering process. Firms may need to take action to “educate” potential clients and owners about the importance of a sound geotechnical investigation, peer reviewed process in ground properties evaluation and design and necessary time that is needed.
Time is a fundamental problem in geotechnical engineering profession. It is not easily understood why maybe a month is needed for a simple foundation investigation. How can you argue when you hear “we do not have such time, we need the results in a week!”, as if we control the permeability characteristics of a clay in a consolidation test!!!
These and many other issues make our profession so challenging, difficult but at the same time so rewarding, from a scientific point of view (I don’t know any billionaire geotechnical engineer). We need to practice geotechnical engineering and at the same time educate the rest of involved disciplines in its difficulties. Unfortunately probably we are not doing very well in the second part of educating…
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A brief summary for all the people that could not make it to the 18th International conference on soil mechanics and geotechnical engineering held on Paris between Monday 2 and Friday 6th of September 2013. The conference main theme was “Challenges and Innovations in Geotechnics”.
The conference commenced with the former president J. L. Briaud presentation of “The State of the Society” in which a very interesting point was his 10 rules for success.
The conference continued with the 8th Terzaghi Oration invited lecture from Susan Lacasse of the Norwegian Geotechnical Institute (NGI).
The title of the lecture was “Protecting society from landslides – the role of the geotechnical engineer”. The lecture presented case studies of landslides, their causes and the way they were analyzed and treated. Very interesting was the Kattmarka landslide that occurred on March the 13 2009 (which incidentally was Friday the 13!) and was caused because of the road construction. Main issues that led to the landslide were among others the limited geotechnical investigation and geotechnical design.
The first day continued with the Ishihara lecture presented by George Gazetas from the National Technical University of Athens (NTUA). The presentation title was “Soil-Foundation-Structure systems beyond conventional seismic failure thresholds”.
He presented a novel approach of designing shallow foundations that are not designed to behave elastic in earthquake loading but to be able to work in extreme conditions and allow for uplift and bearing capacity slippage with acceptable limits of temporary and permanent deformations (settlements). This approach is contrary to current codes but it was shown that it could avoid structural damage and collapse.
The conference continued with the Manard Lecture presented by J. L. Briaud with title “The pressuremeter test: Expanding its use” in which he explained how to correctly utilize the PMT, how to execute the drillings and what the advantages of the pressuremeter test are. Furthermore he gave some reference values for preliminary design and some further extend of the test in liquefaction.
A.Sim of Soletanche-Bachy provided an excellent presentation regarding the construction challenges and difficulties for the new Bugis Station and associated tunnels for the Mass Rapid Transit in Singapore. Especially interesting were the methods used to overcome the passage of the tunnels and the station under or very near buildings.
Professor R. Jardine of Imperial Collage presented the Bishop Lecture in which he presented a state of art of laboratory testing and the use in research and practice. The lecture covered driven piles in sand and the detailed laboratory evaluation of these sands in order to predict pile behavior in static and cyclic loading.
The conference continued the next day with very interesting invited lectures that will be presented in a following entry.
I am sure that many geotechnical designers have either been asked this question or have had to answer it internally in order to price a project. After the offer has been prepared, comes the negotiation phase, where the owner of the project starts asking questions about the “high” price (in his opinion) or about a different offer he has had which was half that price!
I would like to point out some aspects that come into play in this negotiating tango between the Consultant and the Owner and some pitfalls that can come about with relation to this issue.
In geotechnical engineering a design is never “easy” or “simple” and this is because the ground is inherently variable, anisotropic and with minor details that cannot be easily assessed but nevertheless can have a detrimental effect during construction. Do we forget this, many times in our practice?
So how do you go about performing a geotechnical design? A geotechnical investigation is executed initially with a predefined number of boreholes, usually less than we would like and a selective number of field and laboratory tests are executed. This investigation may be based on prior experience of the area but often it is not. The depth and location are governed with minimum information and mostly based on the structure to be constructed. Then with the geotechnical information gathered and evaluated the subsurface is formulated and the geotechnical design is executed, based on some form of standard (Eurocode, LRFD etc).
So the question now becomes “how many man hours will your engineers work determining the price you will ask for?” So in an effort to reduce the cost of design, the limited geotechnical investigation parameters are used with some partial factors of safety and the calculations are executed with nice software for bearing capacity or slope stability etc and the design is completed, on time, satisfying the standards and everybody is comfortable over the outcome. So how many man hours does such a procedure require? Don’t you think you should reduce your offer?
This is a recipe for disaster. In order to cut the cost of design, many things that should have been evaluated are not, inexperienced engineers work in the office with the software that they know so well but at the same time they may completely lose touch with the actual conditions or the geotechnical details that will actually control the performance of the project.
The cost of performing a geotechnical design is not merely the man hours spent doing some mainstream calculations but the time and experience that has been devoted to evaluate the most probable conditions and the most unfavorable conceivable deviations from these conditions and how they will affect the proposed project. This is not an easy task; it needs great experience (shouldn’t this be paid?) and many hours of thinking, sketching, performing simple hand or computer calculations, revisiting the site and the site investigation information etc. But this cannot be easily measured or quantified and produced as a cost estimate. So how can two Consultants compete when one routinely executes such practices and the other doesn’t? Sometimes luck favors the bold so the second consultant could have the same track record as the first one. And if a failure or excessive deformation etc happens then it is easy to blame it on “the unforeseen geological conditions”. No harm done! Just the budget and time of the project may significantly increase, maybe increase orders of magnitude in relation to the reduction that was achieved with the negotiation of the geotechnical design fees or with the selection of the geotechnical consultant with the lowest bid.
So Geotechnical Designers should advertise in more detail what they actually do, advertise the experience and expertise they pose in house and the way they tackle a geotechnical design. They may need to make the owner aware of what is at stake with an improper geotechnical design even if it meets all available standards.
Owners should take a step back and think; is the lower bid the best way to go? Is the reduced price that was achieved after hours of negotiations worth the risk of an improper geotechnical design? What is the gain of a reduced cost of design in relation to the actual cost of construction? Never forget that you get what you pay for and this in geotechnical design can really have a significant cost!