Category Archives: Landslide

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Geosysta at 12th International Symposium on Landslides (ISL)

Training and professional development have always been integral to operations at Geosysta. It is our priority to keep our team learning new skills and maintain our strong connections with academia and professionals by always trying to share our knowledge in the fields of geotechnical and mining engineering.

As part of our constant evolvement, Geosysta participated in the 2016 12th International Symposium on Landslides (ISL) held 12-19 June in Napoli, Italy, where well documented case histories of landslides were presented providing a better understanding of related mechanics, accounting for relevant soil and rock properties and their behaviour.

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As per the conference committee: “The landslides risk has strongly increased over the last decade, mainly because of ever growing population and relevant bigger exposure. In many countries, this is also due to expanded civil and industrial settlements, as well as widened infrastructures and lifelines. For this reason, in order to perform risk analysis and management, it is necessary a better understanding of landslides’ mechanics, accounting for relevant soil and rock properties and their behaviour in well documented case histories.”

Sessions that focused on landslides development mechanisms and the findings of post geotechnical investigation, moving from experience to practice through theory, were presented during the conference. The conference’s presentations covered a wide range of both practical and theoretical approaches when investigating the reasons resulting to landslides and how their findings could be used in order to predict similar future events.

Georgia Papavgeri, Civil and Geotechnical Engineer at Geosysta, said: “Everything came together for a successful conference. It was exciting to be among so many experienced professionals being able to share Geosysta’s unique story and expertise on landslides events”.

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Georgia Papavgeri at ISL 2016

Georgia presented a 10 minutes session on “Large moving landslide inside a lignite mine in Northern Greece”. The presentation discussed an actual case inside an 170-180-meter deep lignite mine in Northern Greece, where the combination of thin clay layers, overlaying deep sandy layers with an unfavorable inclination and complex tectonic formation have resulted in a slow moving landslide. Since this is an active mine, with large earthwork moving operations, it was important to accurately evaluate the mechanisms that led to the formation of the landslide, and the effects of the moving landslide body to the operations of the mine. During the investigation of this large landslide area, different data were utilized, starting from borehole data of more than 50 borings ranging from 150-300m deep, 6 borehole in which High Resolution Acoustic Televiewer loggings where performed, and three deep inclinometers. All data were combined and critically evaluated to generate the possible landslide mechanisms and to evaluate different analysis methodologies.received_881193118652763

Another important component of ISL 2016 was the trade show and the participation of many sponsors from across Italy, Europe and around the world. During the Symposium there was the opportunity for Exhibitors, Producers and Distributors, Public and Private Societies, Contractors and Consultants to promote the name and the image of their companies to all registered participants.

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Yeager airport landslide

Yeager Airport Expansion slide additional comments

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).

Mine landslide Geotechpedia
Mine landslide

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.

Slope model
Slope model

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 notedstability 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.

References:

R. Peck, (1998). “The Place of Stability Calculations in Evaluating the Safety of Existing Embankmnet Dams”, Civil Engineering Practice, Fall 1998.

Yeager failed slope

What could have gone wrong in Yeager Airport Expansion slide?

Yeager airport landslide
Yeager airport landslide

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.

2010 STGEC - Yeager Airport - Tallest Reinforced Slope in N America
Figure 1: Reinforced 1:1 earth slope extension of runway 5

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.

Yeager SW slope copy
Figure 2: Google earth image of excavation at Southwest slope of runway 5

A newer picture taken on 4/2006 presents the progress of works in which a part of the reinforced earth slope has been constructed.

Yeager airport partially constructed slope
Figure 3: Google earth image with partially constructed slope

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.

Yeager airport reinforcement placement
Figure 4: Site photograph shown the placement of reinforcement near the base of the slope, rock formation is clearly visible in the back (Lostumbo 2010).

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, 2014the 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).

Yeager airport slide model copy
Figure 5: Very simple model evaluating the possibility of failure due to weathered shale intercalation

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.

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Figure 6: Google earth image before the failure, note the rocks between slope and road
Yeager airport toe slope possible failure surface
Figure 7: Google street view in the road just where the rocks are seen. Observe a possible shear surface.

References:

Duncan J. M. (2013). “Impact of time on the performance of reinforced slopes” Geo-Congress 2013.

Huang Z., Al-Saad Q., Nasrazadani S., Wu Felix H. (2014). “Understanding and optimizing the geosynthetic-reinforced steep slopes“, EJGE Vol. 19, 2014

Mohr – Coulomb failure criterion continued

Things to remember when using the Mohr – Coulomb failure criterion:

  • The linear failure envelope is just an approximation to simplify calculations
  • The failure envelope is stress dependent and will produce some kind of curvature if shear strength tests are executed in much different confining stresses (fig 1, from Duncan and Write, 2005).

 

  • According to Lade, 2010 the failure envelope is curved and at low effective stresses which can be found in superficial failures on slopes, the use of linear Mohr – Coulomb may be in the unsafe side. Soils without cementation do not provide any effective cohesion in very low effective stresses (fig 2, from Lade, 2010).

 

  • When the linear Mohr – Coulomb criterion is used it must be evaluated for the expected stress range in the field.
  • Small cohesion values will not produce significant errors when high effective stresses are anticipated in the calculation.
  • In low effective stress even minimum values of effective cohesion (in cohesionless soils) can produce significant errors in factor of Safety (FS) calculations.

References:

Duncan J. M.,  Wright S. G., (2005). “Soil Strength and Slope Stability”. Wiley, New York.

Lade P. V. (2010). “The mechanics of surficial failure in soil slopes”. Engineering Geology 114, pp 57-64.

Slope stability and scale effects

In previous entries the issue of stiff fissured clays and the time to failure was briefly touched. The design of such slopes is not a trivial matter and requires significant knowledge of soil mechanics, geology, hydrogeology etc. One additional issue mentioned (one that sometimes is neglected) is the scale effect. This was presented in the previous entry for a very deep mine in rock. This issue of scale effect in relation to stress field will be briefly presented for the case of stiff fissured clays and hard soils.

In the following picture a large highway cut of about 30m is shown. For a civil engineering project this is a significantly high cut. The effective stress filed in this cut can range from of 50 – 500kPa which is the normal range for laboratory testing.

Highway cut

In the second picture a large excavation for a lignite mine is presented. The depth of excavation of this multi bench cut is around 135m. The excavation of this type needs to consider bench stability of slopes with heights of around 18m and also overall slope stability for highs above 135m. In the second case a large part of a possible failure surface could be in a stress field of around 1500-2000kPa or even more.

 Coal mine slopes

In the following figure the two types of cuts are compared and one can easily understand the significance of scale effects in the design of the different cuts.

Scale difference of coal mine and highway slopes

The scale of the mine excavation is such that even in one cross section, one has to consider besides the stress field, differing geology (pic 4), presence of faults, ground water locations and pore pressures etc. We will focus on the stress dependency at this point.

mine slopes

According to Stark et al, (2005) both fully softened and residual failure envelopes are stress dependent. In this work Stark et al provides an empirical graph regarding the stress dependency until 700kPa of normal stress for residual friction angle and 400kPa for fully softened friction angle.

Shear strength information for higher effective stresses >1MPa are not readily available. Furthermore execution of such tests in very high effective loads is not easy for most commercial laboratories. It may even be very difficult to execute ring shear tests in very high loads due to sample thickness and squeezing out from the sides.

In such high slopes the failure surface can pass from a number of soil layers with different shear strength properties. It is not easy to evaluate the “average” shear strength of layers involved in a possible failure surface. Unfortunately a rule of thumb for selecting shear strength parameters for such slopes cannot be provided. Engineering judgment is required in selecting such parameters and the stress conditions must not be ignored. Shear strength tests should be evaluated in relation to the expected stress field.

Slope failures, landslides and mines

On 11 of April 2013, around 9:30 p.m. a large slide (maybe the largest) in the northeast section of the Kennecott mine occured (fig 1). The slide was preceded by slope movements that reached ~50mm per day. Two major questions could be raised, why this slide occurred and could it have been predicted before hand and remediated?

Kennecott mine

These are very difficult questions and require significant knowledge of the geology, geotechnical conditions of the area, operational practices, climatic conditions etc. In the following paragraphs some initial ideas regarding the stability of high mine slopes and some interesting references will be provided for interested individuals. The incident in Kennecott is an important lesson of how important continuous monitoring of slopes is in such mine operations.

I would like to start with a very interesting graph published by Hoek et al (2000), “Large-scale slope Design – A Review of the State of the Art”.

This chart presents slope height versus overall angle with solid markers representing unstable slopes and open markers represent stable slopes. This chart is for copper porphyry open pits. In this graph the Kennocott mine (Bingham Canyon) is also shown but not the April 2013 one.

It is very interesting to note that most of the unstable markers are located in a range between 35 and 45 degrees of slope angle. Although much information is required for detail evaluation of each point and why instability occurred, a trend can be seen. Can we assume that slopes designed bellow 32-35o would not provide stability problems?

In the next figure I would like to focus on scale effects when dealing with mine slopes in rock or even hard rock materials. In the down left side of the figure 2 a slope with 30 meters height is depicted. In the upper left one of 90m with the same spacing of joints and finaly on the right a slope of 500m again with the same spacing and trance length of discontinuities (figures adopted from Sjoberg, 1996).