Reflective Entries

Start of Semester
Geotechnical Engineering N is the continuous from the Rock and Soil Mechanics which provides further knowledge, including embedded retaining walls, analysis of the slope stability of an earth dams, design of slabs on expansive soils and pavement designs for the given traffic conditions. Nowadays, Geotechnical Engineering plays an important role in the engineering field which concerned the behaviour of earth materials. Start of the couple of weeks for the semester were quite interesting because the 1st assignment was dealing with the GALENA software which used to design the dam stability and this assignment was much challenging than the previous assignments on Rock and Soils and it also provides deeper knowledge on designing. From the dam stability assignment, it was quite interesting which I learnt 3 cases in the design in order to anticipate conditions over the life of the dam. This assignment provides good fundamental knowledge of designing dam stability which can be used in my workplace in the future. Reactivity practical was good which we needed to work out the soil suction value, shrinkage swell and etc. For the following weeks the semester was getting more and more challenging where we introduced some new programs such as Cord and Slog for Slab design and CIRCLY for Pavement design which are very useful in the geotechnical engineering field.

End of Semester
Apart from that, Expansive soil Technology, Design of Slabs for Expansive Soils, Road Pavements, Elements of Pavement Design, Pavement material specification and design and finally management of pavement lectures were quite interesting. Cord and Slog programs are very useful in designing the slab of a structural which help us to design the minimum of steel requirement for slab and beam depth. The latter part of the course was focuses on pavement engineering which use program CIRCLY to make economic pavement designs for the given traffic conditions. We also have Mr. Hugo van Loon as our guest lecturer on few topics of the pavement designs. Mr. Hugo also expert in pavement industry and he has been working for DTEI several years. The lectures from him were good. From the CIRCLY practical, I found that it was interesting because we need to design the thickness of the pavement layer in order to achieve the required DESA ratio. Pavements are designed and tested whether it can withstand the stress from the different kind of traffic. In week 13, we went for a field trip to DTEI at Walkleys Height on Melbourne Cup Day. From the trip, we were shown how does the asphalt are mixed and tests done in the lab. Dr Don has done a good job for the entire semester and I was very impressive and satisfy with the knowledge I learnt from this course.

Revetment Walls of Unsaturated Clay(AGS Meeting)

AGS meeting on revetment walls for unsaturated clay. Clay pit at Thebarton : sandy clay which the design using saturated clay and the friction angle from CIU Triaxial Testing. Installing soil nails for the Bakewell Underpass. Underpass Millswood consists of 100mm thick which it is cheaper. Project aims when a design approach need to consider the construction method need to be thick of safety. Field investigation method and lab testing needed to provide input to design.

Mr. John Woodburn who is an expert in unsaturated soil presented the basic stability analysis, background information about soil suction and design suction profiles. Typical underpass like North South Corridor consists of 34m wide and 4m depth of underpass. Australia Standard 4678, 2002 recognizes a class of structure known as revetment wall which utilizes the inherent strength of the soil mass with vertical 70 degrees. Revetment walls relatively shallow sloping cuts in unsaturated soil are stable and not required retaining wall. It can protect the soil surface from wetting and drying. Maximum height of vertical cuts is 4c/γ for cohesive soil. Increase in height helps to stable slope with deceasing face angle. Design consideration row needed to include investigation range of soil and pressure. Large trees will cause shrinkage and foundation problems.

Soil suction at in situ moisture content are normal missing. To argument field test can increase confidence. To characterize the soil can use the Sieve analysis and Atterberg Limits Test. Saturation or equilibration can take weeks, not overnight otherwise results are unreliable in CIU test. Soil strength characterization involves measurement of shear strength and total suction. Soil is wetted by applying hydraulics gradient across and is confined state.

Two contributing factor of soil suction – Governing Freland’s for US which assume the pore water pressure is zero. For dry Hindmarsh clay undrained values Cu = 0 kPa and ϕu 38 degrees. Two broad methods of revetment analysis are limit equilibrium method to seach for lowest factor of safety(FOS) and Finite Element method used for modeling suction profiles. It uses matric suction in kPa and need to convert pF in to kPa.

For excavation or construction need to know likehood of perched watertables, pockets of loose dry sands, saturated sands in old stream channels and blocky clays. Feasibility as for long deep excavation consists of range of soil present. Stage excavation need to be carried out quickly and safely without the need of ground support. Verification of ground condition and implementation of drainage consideration design of revetment wall should be flexible with articulation, drainage required behind the wall to ensure that saturation does not occur. Advantages of precast panel construction over the 1st and 2nd methods as work can proceed rapidly. Final trimming can be made before. Precast concrete for revetment wall would allow rapid and much cheaper construction of underpass.

Landfill

b) Report on the use of geotextiles and geosynthetic clay liners in the construction of modern landfills.


In modern landfills, the waste is contained by a liner system. The primary purpose of the liner system is to isolate the landfill contents from the environment and, therefore, to protect the soil and groundwater from pollution originating in the landfill. The greatest threat to groundwater posed by modern landfills is leachate. Leachate consists of water and water-soluble compounds in the refuse that accumulate as water moves through the landfill.


Landfill liners are designed and constructed to create a barrier between the waste and the environment and to drain the leachate to collection and treatment facilities. This is done to prevent the uncontrolled release of leachate into the environment. The liner system of a landfill performs the vital task of retaining the leachate produced by the waste. The overall stability of a landfill may also be determined by the liner system.


Geotextiles: In landfill liners, geotextiles used for filtration, separation and protection which prevent the movement of small soil and refuse particles into the leachate collection layers and to protect geomembranes from punctures. These materials allow the movement of water but trap particles to reduce clogging in the leachate collection system. Usually geotextiles are placed at the tension surface to strengthen the soil. Geotextile also prevent damage to synthetic liners due to the rock aggregates hardness and angularity– comparative to the softness of some synthetic membrane liners.


Geosynthetic Clay Liner (GCL): Geosynthetics have revolutionalized many aspect of geotechnical design and construction. Geosynthetic clay liners are becoming more common in landfill liner designs. These liners consist of a thin clay layer (four to six millimeters) between two layers of a geotextile. These liners can be installed more quickly than traditional compacted clay liners, and the efficiency of these liners is impacted less by freeze-thaw cycles. GCL represent a potential alternative to a conventional compacted clay liner or as a means of augmenting the performance of a compacted clay liner as part of a composite liner system in the base of municipal solid waste landfills.


Geosynthetic materials also offer the following advantages:
• ease of use, and speed of deployment.
• a high level of manufacturing quality assurance – consistency and homogeneity.
• increase airspace in constructed cells.
• reduced requirement to quarry virgin gravels, clays and sands.
• reduced requirement to transport large volumes of gravels, clays and sands over urban, rural, or regional road systems.
• technically superior performance over many natural barrier and drainage materials when used alone.
• utilisation of recycled plastics that would otherwise occupy ‘dead’ space in a landfill cell.


A geosynthetic clay liner can be incorporated into different kinds of liner systems which are single single-liner systems, double-liner systems or composite liner systems. Composite-liner systems are more effective at limiting leachate migration into the subsoil than either a clay liner or a single geomembrane layer. A composite liner consists of a geosynthetic or geomembrane in combination with a clay line.

Figure 1: Double composite liner system


CASE HISTORY – GORDONVALE LANDFILL CAP, CAIRNS.
Cairns Water, a business unit of Cairns City Council, were given the responsibility of controlling and managing all aspects of capping the Gordonvale landfill - approximately 20km south of Cairns. After consulting with landfill design specialists (Golder Associates) and reviewing the performance of the options available, they selected a Bentofix® Geosynthetic Clay Liner over a 400mm thick compacted clay liner (CCL). Along with the many advantages of using a GCL over a CCL included the recent successful installation of a Bentofix® GCL at Babinda Landfill nearby. Bentofix® X2000 (a specialised capping grade of GCL) supplied by Geofabrics Australasia was used to cap the entire aerial extent of the landfill, with Geofabrics staff offering initial briefing andtechnical support to Cairns City Council employees who carried out the work in-house.

The GCL was laid directly onto a suitably prepared subgrade using an excavator and spreader bar supplied by Geofabrics. Rolls were run longitudinally from the base up the slope to the apex of the landfill. Adjacent roll had the standard 300mm overlap - no physical or mechanical joining is necessary. Transverse joints at the end of each roll also required a 300mm overlap, and were sealed with bentonite paste and protected from soil and foreign matter with geotextile cover strip (XROLL). Above the GCL barrier, a minimum of 400mm cover, a blended material comprised of approximately 50% shredded green waste and 50% sandy soil, was used as the confining cover soil layer.

The total project time required for complete installation of the 23,890m² of GCL was 6 weeks. Since the completion of capping works, Cairns Water staff have made comment on the significant reduction in leachate volume generated by rainfall infiltration into the waste body.

Figure 2 Figure 3

Figure 2 and 3:Deployment of the Geosynthetic Clay Liner (GCL) cap, and sealing around aleachate collection manhole.
Figure 4: The finished Cap profile.


References
i) http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_8.pdf
ii) http://www.geosyntheticssociety.org/source_documents/Leaflets/Landfills/English.pdf
iii) https://static.wmdisposal.com/files/brochures/modern_landfill.pdf
iv) http://books.google.com.au/books?id=pWkxjs1vAqMC&pg=PA325&lpg=PA325&dq=use+of+geotextiles+in+the+construction+of+modern+landfills&source=bl&ots=jqBdm1Pkmm&sig=1wIBQ3B623MdbOTm6jrO-UgCr8E&hl=en&ei=eR6tSuDzFp6G6APcsrzoBw&sa=X&oi=book_result&ct=result&resnum=3#v=onepage&q=&f=false

Question (C) Report on the use of grout curtains in water retaining dams

A dam is a barrier across flowing water that obstructs, direct or slows down the flow, often create a reservoir, lake or impoundment. The foundation for most dams more than 15m high, and for some which are smaller, are treated by grouting. Grouting consists of a drilling a line or lines of drill holes from the cutoff level of the dam into the dam foundation, and forcing cement slurry, or chemicals into the foundation under pressure. Grouting is often done before constructing the dam.

Curtain grouting is design to create a thin barrier (or curtain) though an area of high permeability. It usually consists of a single row of grout holes which are drilled and grouted to the base of the permeable rock, or to such depth that acceptable hydraulic gradients are achieved. For large dams on rock foundations, or dams on very permeable rock or soil foundations, 3 or 5 or even more lines of grout holes maybe adopted.

The holes are drilled and grouted in sequence to allow testing of the permeability of the foundation (by parker testing) before grouting and allow to a later check on the effectiveness of grouting from the amount of the grout accepted by the foundation.

Figure 1

The purpose of the grout curtains are used under dams where the foundation would otherwise pass too much seepage, reduce the seepage erosion potential and reduce leakage through the dam foundation. Grout curtain is restrict seepage to such an amount that it does not cause too much loss of storage, and does not dislodge the foundation downstream or erode the base of the dam.

These curtains are never completely water-tight; the reduced amount of seepage passing through them should be picked up in drainage holes downstream of the curtain. A grout curtain in the rock are provided for under seepage control. These discharges to a low pressure region .They provide a 'safety valve' for the dam. Existing of curtain grouting could reduce the permeability of the bedrock thus minimize the infiltration of groundwater and the exfiltration. Besides that, it can reduce uplift pressures (under concrete gravity dams when used in conjunction with drain holes) and strengthen the dam foundation and reduce settlements in the foundation.

Figure 2

Wide cracks in a foundation emptying a dam where there is no grout curtain. The seepage passes underneath and emerges downstream.

Warragamba dam is one for the Australian Dams which constructed before 1962. The dam is on Triassic Sandstone with intervening shales in a gorge with a chord/height ratio of 3 1/4 .

The grouting of the Triassic sandstones for the dam is extensive:
a) Intermediate pressure curtain – When the concrete of the dam had reached a height of about 35ft across the valley holes 10ft apart, were drilled diagonally under the heel of the dam to reach a depth of 75ft below foundation level.

b) High pressure curtain – This curtain is to provide a high permeable zone to reduce leakage and uplift under the dam.

Reference :

a) Dam Geology by R.C.S. Walters ( 2nd Edition )

b) http://www.geosystemsbruce.com/v20/html/pr_GroutCurtain.html

c)http://books.google.com.au/books? id=XH5o_KpmU0QC&pg=PA243&lpg=PA243&dq=grout+curtains+dam&source=bl&ots=lAnUtzzoY1&sig=hngm_o32yRQY9GQuXx0SWc_YX8k&hl=en&ei=xk-JSsvGF8n-kAX8v8mpCA&sa=X&oi=book_result&ct=result&resnum=9#v=onepage&q=grout%20curtains%20dam&f=false

d)http://books.google.com.au/books?id=jHJbOtLxmBoC&pg=PA148&lpg=PA148&dq=grout+curtains+dam&source=bl&ots=-EAPz_4BiW&sig=4iu8tU45Dn4UaOez07v8DZsQOdo&hl=en&ei=xk-JSsvGF8n-kAX8v8mpCA&sa=X&oi=book_result&ct=result&resnum=6#v=onepage&q=grout%20curtains%20dam&f=false