End of Study Period 5 Post

The official teaching period is now over, with preparation for the final exam beginning. Since my last post, Geotechnical Engineering N has studied the design of slabs, expansive soils, unsaturated soil mechanics and pavement design and management.

I particularly enjoyed the lectures on pavement design as it related well to the second-year course Civil Engineering Practice, and now I feel confident that, with a little more practical experience, I can design a road. It was these practical/design aspects of the subject that were particularly enjoyable as I learnt how to use software such as CIRCLY, CORD, and (GALENA). I can imagine that this software, or similar, would be used in the engineering firms I hope to gain future employment with.

As mentioned previously, I have a desire to become a geotechnical engineer and this provided great motivation throughout the study period. Unfortunately, I have been taking a study overload (5 subjects), as well as working full time this semester, and believe my grades have reflected these outside commitments. Although my results have still been strong (avg ~90%), I haven't been able to spend the time researching topics as deeply as I would like and have rushed a few reports.

Having said that however, I have made an effort to attend and research as many topics relating to geotechnical engineering as possible by attending almost all AGS meetings and a special Earthquake Engineering seminar in the study break. I found these seminars particularly interesting.

Overall, the lectures delivered and course content discussed has been insightful, but I did feel as though the last 5 weeks or so (after study break) could've been condensed into 3 or 4 weeks.

Revetment Walls in Unsaturated Clays

Report on an Australian Geomechanics Society or Footings Group Lecture


On the 17th August 2009, I attended a lecture titled 'Revetment Walls in Unsaturated Clays' presented by a team of geotechnical professionals to the SA Chapter of the Australian Geomechanics Society. As a part of the proposed South Road upgrade and future North-South Corridor, Richard Herraman (Geotechnical Engineering Group Manager, DTEI) and his department have received funding to lead a study into more cost-effective ways of stabilising excavated slopes in stiff clays. This study has involved numerous parties including; John Woodburn (Soil Mechanics Instrumentation) who discussed a feasibility study and possible excavation methods; Dr William Kaggwa (Adelaide University) who discussed the geotechnical testing of unsaturated clay soils; and Chris Ward (Parsons Brinckerhoff) who discussed his modelling of the design suction profile.

Often the construction of a retaining wall in stiff clay involves an excavation (often at a steep slope), building the wall and then backfilling. However many stiff clay excavations, such as that shown in figure 1, will stand for decades without any support (unless it gets wet and softens). This leads to the question of whether there is any need to retain and if it is possible to simply protect the slope.

Figure 1: Steep slope excavations in stiff clay often stand up without any support [Source: SA Chapter Australian Geomechanics Society, URL: http://www.australiangeomechanics.org/common/files/sa/20090817-RevetmentWalls.pdf]

A revetment wall utilises the inherent strength of the soil mass and comprises a protective covering on an embankment of earth which is designed to maintain the slope at a steeper angle than the material would naturally assume. According to AS4678, a structure is classed as a revetment wall if the angle of inclination is less than 70 degrees from the horizontal. Richard Herraman highlighted the 60 degree inclination revetment wall of the Millswood underpass on Goodwood Road as an example of a cost-effective method of retaining slopes (figure 2). This wall comprises 100mm of shotcrete with no soil nailing and has stood since 1915 with only relatively minor maintenance work performed (installation of weep holes and individual concrete panel replacement).

Figure 2: The Millswood underpass was used as a case study for the potential future use of revetment walls for underpasses [Source: Google Maps]

John Woodburn then discussed how saturated soil mechanics predicts the maximum height of a vertical cut to be equal to the depth of cracking, but much greater heights are predicted by unsaturated soil mechanics. Luckily I was able to follow this discussion as we had only recently discussed unsaturated soil mechanics and the influence of total suction.

It was then outlined that John's study involved investigating the range of soils along Adelaide's north-south corridor and the depth to groundwater, as well as modelling the design suction profile at the wall and away from the wall of the Millswood underpass case study (modelling performed by Chris Ward).

This modelling was performed using SEEP/W for the suction profile and SLOPE/W for the slope stability investigation and considered 3 cases; a deep watertable, shallow watertable, and a leaking service. My notes from the meeting are a little sketchy on the results of this modelling (my handwriting couldn't keep up with the presentation), but the final message was that revetment stability in unsaturated clays is realisable, given a deep watertable.

Another component of the presentation, which I found interesting, related to the field investigations and detailed testing required for the design of a revetment wall. Dr William Kaggwa discussed the use of SPT/CPT/Dilatometer data, but believes that many companies do not test soil suctions adequately, even though we deal primarily with unsaturated soils.

The possible construction technique of a revetment wall was also discussed and John Woodburn outlined the need to monitor for the presence of perched watertables, blocky clays, pockets of loose dry sands and saturated sands from old stream channels. He suggested that a staged excavation take place with an initial cut of 1:1, which can then be cut further after investigation of the factors outlined previously. It was also made clear that the wall should be flexible with articulation and adequately drained to ensure saturation does not occur. The importance of proper drainage appears to be a recurring theme throughout my study of geotechnical engineering.

John also warned that potential shrinkage behind the wall may lead to a loss of facing support and that although vertical movement (i.e. settlement) will be low, it should be accounted for in the design. He also advised that using pre-cast panels allowed for rapid construction of revetment walls in a cost effective manner. Somewhat unfortunately, the major theme of the presentation was on saving money and questions were raised about whether we (geotechnical engineers) should be sacrificing safety, particularly given the cost of professional indemnity insurance.

Dr. Peter Mitchell also raised the question of slickensides in fissured clay soil, cracks and slippage planes and how these can possibly go undiscovered during the geotechnical testing phase and lead to a potential failure of a revetment wall. John countered the question by arguing that these would be discovered during the staged excavation (i.e. cut 1:1 then dig further).

Overall, I found this presentation useful, interesting and ultimately rewarding as it directly related to what was being taught during Geotechnical Engineering N and my potential future career. I believe the use of revetment walls in stiff clay has merit, but is in need of further investigation before it can be fully adopted as a method of stabilising excavations for road construction.

The Use of Geotextiles and Geosynthetic Clay Liners in Landfills

Report on the Use of Geotextiles and Geosynthetic Clay Liners in the Construction of Modern Landfills. Discuss a recent example.

Modern landfills are highly engineered containment systems that are designed to isolate surrounding soil and groundwater from the potentially harmful impacts of solid waste. Of particular concern is the segregation, collection and treatment of leachate, which consists of water and water-soluble compounds that accumulate as water percolates through solid waste.

In order to contain leachate, many modern landfills employ a composite liner system consisting of a geomembrane (or natural low permeability clay) overlying a compacted clay liner or geosynthetic clay liner (GCL). Geomembranes are relatively impervious polymer sheets (such as high-density polypropylene (HDPE) or polyvinyl chloride (PVC)) that slow the movement of leachate and permit its collection.

However, due to the angular nature of drainage gravel placed above the geomembrane and the large vertical stresses experienced due to dead weight and dynamic loads from earthmoving equipment, it is important to protect the geomembrane from puncture. This is achieved through the use of heavyweight geotextiles. Geotextiles are also used in modern landfills as a filter, preventing the movement of small soil and refuse particles into the leachate collection layer.

Underneath the geomembrane in many landfill systems is a secondary liner, consisting of either a geosynthetic clay liner (GCL) or compacted clay liner (CCL). Consisting of a thin layer of clay, typically sodium bentonite, sandwiched between two layers of a geotextile, GCL's are used as a hydraulic barrier to leachate movement. GCL's also have the advantage of being able to be used with a steeper slope than a CCL, allowing a greater waste storage volume.

A recent, local example of a modern landfill that employs a composite liner system is the Uleybury Landfill in Adelaide, South Australia (figure 1).

Figure 1: The Uleybury Landfill in South Australia is a modern landfill, situated in a disused quarry, that uses a composite liner system to protect the surrounding environment from leachate contamination [Source: personal photograph]

The Uleybury landfill uses a double composite liner system consisting of a HDPE geomembrane, protected by a geotextile layer, overlaying a geocomposite clay liner. Above the geomembrane is a drainage layer to ensure leachate is collected and treated (see figure 2).

Figure 2: The Uleybury Landfill utilises a double composite liner system to prevent the movement of leachate into the local groundwater [Source: Interpretative Sign photographed on site visit to Uleybury Landfill]

It is this liner system, combined with the baled waste operation, landfill gas management and future monitoring program that has lead to the Uleybury landfill being awarded the 2006 Case Earth Award.

Early Study Period 5 Post

So far this semester, Geotechnical Engineering N (can't explain the 'N') has investigated embedded retaining walls, earth dams and tailings dams, as well as landfill technologies.

The lecture on embedded retaining walls relied upon some concepts from Rock and Soil Mechanics (effective stresses, active/passive pressure, moment equilibrium) which was beneficial as it felt as though we simply 'picked up where we left off'. The examples covered in this lecture were particularly useful and I found the DVD of the basement excavation in Melbourne interesting. Following the largely theoretical nature of embedded retaining walls, the lectures moved into a very descriptive examination of earth dams, tailings dams and landfill technologies. Although I am a fan of equations, theories and numbers, I found these lectures very interesting as I want to be designing such structures in the future.

In regards to semester 1, I was very happy with how Rock and Soil Mechanics went and am very much enjoying and looking forward to the rest of Geotechnical Engineering N. In fact, I would be surprised if I missed out on more than 4 marks in the end of semester examination as I felt really confident with the previous material.

The content to be covered this semester will be particularly beneficial to me as I have been successful in obtaining vacation work in geotechnical engineering at Golders Associates. This represents a fantastic opportunity and every application discussed so far (sheet piles, contiguous pile walls, landfills, tailing dams) gets me thinking about what I may be designing in the future. This is a great source of motivation which has resulted in me spending many hours this past week in the computer pools designing an earth embankment.

In regards to assessment, I was pleased to discover that the practicals will be a greater focus this semester (50%) as it these that I learn most from, and put the most effort into throughout the semester (not just a cram at the end).

I apologise for the somewhat late entry for this early study period post (I inferred it meant anytime in the first half of semester until I talked to a few other students).

Potential Consequences of Failing to Control Seepage and Decant Water Level in a Tailings Dam

Tailings dams are used to contain the waste material of mining operations and represent the greatest environmental footprint of a mine. Although the size of tailings dams vary they can be as large as 4 square kilometres and, on flat terrain, are built in the form of a ring dyke. Tailings dams are used to dry out the slurried waste material of a mine and subsequently capture excess water so it can be reused. A ring dyke tailings dam spigots out the thickened slurry from the dam perimeter, allowing it to drain along a sloping beach towards a low point.

Although the preferred engineering option for such dams are similar to a water retaining earth dam, tailing dam embankments are progressively built using readily available materials in either an upstream, downstream, or centreline orientation as the mining operation progresses (figure 1).

Figure 1: Conventional embankment design techniques for a tailings dam [Source: www.tailings.info/conventional]

From a geotechnical engineering perspective, the downstream embankment design is preferred as it does not rely on the tailings material for its strength. Often a combination of high moisture content, low density and low stresses combine to produce a low strength tailings material. However, the downstream option is rarely used by mine managers as more and more material (and land) is required as the embankment is progressively built.

In all dams the designer aims to control the amount of seepage, seepage induced pore pressures and internal erosion/piping to ensure dam stability. This is often achieved through the application of drains (and filters) within the embankment to ensure the phreatic surface does not daylight on the downstream face of the embankment. If daylighting of the water table does occur then the effect is two fold; a reduction in shear strength and increased erosion/internal piping which can lead to a catastrophic failure of the embankment. Such a failure can have dire consequences to life and the environment, such as a large mudflow and the release of toxic/acidic tailings. One such failure occured in an Italian fluorite mine which sent 180000 cubic metres of material flowing towards the town of Stava, killing 268 people and causing 155 million Euros of damage.

During the design of a tailings dam embankment it is also important to cater for larger than expected inflows by designing spillways and bypass channels. Flood erosion is a problem common to all surface water impoundments, but overtopping is particularly important for tailings dams. This is because overflow across unconsolidated tailings can cause rapid erosion, leading to drainage from the tailings impoundment.

Overtopping occured in the Aurul S.A. tailings dam located in Baia Mare, Romania sending 100000 cubic metres of cyanide contaminated liquid into the Lapus stream (Figure 2). Described as the greatest European environmental disaster since Chernobyl the cyanide waters killed tonnes of fish and poisoned Hungarian drinking water supplies.

Figure 2: The failure of the Aurul Tailings dam to account for overtopping caused a great environmental disaster [Source: http://www.bristolbayalliance.com/mines_and_fish.htm]

The operation in Baia Mare revolved around the reprocessing of gold tailings from an old, unlined mine using sodium cyanide to recover the residual gold. The tailings stored in the Aurul impoundment had a cyanide concentration of 400 mg/L, but the designers constructed an embankment of inadequate height to account for melting snow and a heavy rain event. The rise in water level caused the dam crest to wash away, releasing the poisionous tailings. Except for the crest, the embankment remained structurally intact.