Apply Footing and Geomechanical Design Principles to Buildings Assessments Up to Three Storeys
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built environment – advanced diploma of building surveying cpc60108.
Apply footing and geomechanical design principles to buildings Assessments up to three storeys - CPCCSV6004A
Assignment 2.
rEV B.
troy dibben.
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2 Outline.
Introduction & Tasks:
In this document, it shall be discussed and understood to apply knowledge of soft soils, ground improvement, embankments, slope stability and retaining walls to a practical example of earthworks construction.
An embankment at the rear of a residence, 2.5m high is to be constructed. The existing soil profile in the area consists of a thick layer of soft clay with a shallow depth to groundwater, over dense sand less than 10 metres in depth. Explain the nature of the problems with embankment construction that maybe expected on this site, and discuss at least three engineering techniques by which the embankment construction and/or performance of the foundation soils can be improved.
For the same site and proposed construction, explain how earth retaining systems can be used to limit the embankment side slopes from encroaching onto neighbouring property. Discuss at least three options for earth retaining systems that may be used, including the relative merits of each.
Illustrate the report with drawings or images of the various ground improvement techniques and earth retaining systems that are discussed.
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Overview and Review.
To begin with, and looking at the behaviour of embankments constructed on soft clay soils, together with the associated problems with the difficulties that geotechnical engineers encounter during design and construction. The costs for remediation of such failures are high.
As noted within, Geotechnical Embankment Construction 2013 Edition, a good and general understanding of soil and its different properties is essential for building a quality embankment. The engineering properties of a soil can vary greatly from gravel to clays. To build a quality embankment, the specific properties of the soil being used must be understood in order to make proper field judgments. (Geotechnical, Embankment Construction1 Revised: 2013, 1)
Soil embankments have been built for many years by engineers for different purposes, such as road networks, motorways, railways, water retention, flood control works and airports, just to list a few. Historically, clay soils were considered to be highly resistant to erosion by flowing water; however, at present it is now recognized that highly erodible clay soils exist in nature. Numerical modelling of embankments on soft soils, Chapter 3, Embankments on soft soils, nd,1).
Embankments can be built on sites with good ground conditions, in order to reduce and avoid stability problems during construction, large settlements and costs associated with technical difficulties. It is often found that regions along the coast and river estuaries are covered with young, shallow to deep deposits of soft clays, muds and compressible silts.
Studies have shown that failures of structures built of dispersive or soft clay soils occurred on first wetting. Failures can be associated with the presence of water and cracking by shrinkage, differential settlement, or construction deficiencies. These failures emphasize the importance of early recognition and identification of dispersive clay soils; otherwise, the problems they cause can result in sudden, irreversible, and major failures.
Lastly, in terms of to the scenario/example to which the review to soft clay soil embankements applies (2.5m high is to be constructed. The existing soil profile in the area consists of a thick layer of soft clay with a shallow depth to groundwater, over dense sand less than 10 metres in depth), this in turn gives a good grounding to move forward to investigate three engineering techniques by which the embankment construction and/or performance of the foundation soils can be improved.
Apply footing and geomechanical design principles to buildings Assessments up to three storeys.
Assignment 2.
Typical Embankment Failures - Sketches.
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Figure 1: Sketch of Rotational Failure.
Taken from; Embankment Construction - Statewide Urban, Chapter 6 - Geotechnical Section 6D-1 - Embankment Construction.
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Figure 2: Sketch of Displacement Failure.
Taken from; Embankment Construction - Statewide Urban, Chapter 6 - Geotechnical Section 6D-1 - Embankment Construction.
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Figure 3: Sketch of Translatory Failure.
Taken from; Embankment Construction - Statewide Urban, Chapter 6 - Geotechnical Section 6D-1 - Embankment Construction.
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Engineering Techniques.
Some of the engineering techniques, through which construction and/or performance of clays or soft soils can be improved are identified and discussed. However, of the methods which are available to stabilize slopes are: re-grading to flatten the slope, construction of stability berms, the use of lightweight fill - geofoam or shredded tires to reduce the load, and structural reinforcing methods such as geosynthetic or geo-mats reinforcements, stone columns, rammed aggregate piers, soil nailing, and lastly concrete piles. (Design Manual, Geotechnical, Embankment Construction, 2013,5) The report shall examine at some of these engineering techniques, in the context of our embankment wall scenario.
In terms of our embankment and establishing context for this report, some of the causes of slope instability are characterized by a balance between the gravitational forces tending to pull soils downslope and the resisting forces comprised of soil shear strength. It is stated that, the state of temporary equilibrium may be compromised when the slope is subject to de-stabilizing forces. Further, the factors affecting slope stability may include those that increase the gravitational force (e.g. slope geometry, undercutting, surcharging) or those that reduce soil shear strength (e.g. weathering, pore water pressure, vegetation removal) (Design Manual, Geotechnical, Embankment Construction, 2013,5)
Slope instability poses problems generally, some of these failures occur on both new embankments and cut slopes. These occur because identifying factors that affect stability at a particular location, such as soil shear strength parameter values, ground water surface elevations, and negative influences from construction activities, are often difficult to measure. Identification of the hazards is key of landslide mitigation. Once a failure occurs or a potential failure is identified (i.e. low factor of safety), knowledge of which methods of remediation will be most effective to stabilize the slope. Ideally, these stability problems can be discovered and addressed before a slope failure occurs. (Design Manual, Geotechnical, Embankment Construction, 2013,5)
1. Mechanically Stabilized Earth Walls.
Mechanically stabilized earth walls can be seen as a type of embankment wall(s) constructed in fill situations and consist of horizontal soil reinforcing elements to prevent erosion. Mechanically stabilized earth, also called “MSE”, is soil constructed with artificial reinforcing via layered horizontal mats (geosynthetics or geomats) fixed at their ends. The benefits of these mats, provide added internal shear resistance beyond that of simple gravity wall structures.
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Figure 4: Imagery of a typical “Geomat Product” performed on a sloping embankment. Presto, Geo Systems, http://www.prestogeo.com/slope_protection, Presto's Geoweb® Slope Protection System. (accessed 20th July 2014)
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
1. Mechanically Stabilized Earth Walls.
Section Sketch – (as requested for this example).
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
2. Lightweight Fill or Geo-Foam.
Another option to by which the performance of the foundation soils can be improved is introducing geofoam or a lightweight substance or product. This is a lightweight, rigid engineered fill material made from expanded polystyrene used in widely across the world for geotechnical construction, residential and commercial applications, providing a highly controllable and cost effective fill product that will significantly provide superior earth stabilization.
It is noted with traditional earth materials can be heavy and may cause settlement, instability and/or lateral pressures to the earth wall. Some other lightweight fill materials such as foamed concrete, waste tires, soil, woodchips and wood fiber have higher densities, are variable in their makeup and are not engineered, due to field execution variables.
Lastly, the relevance & some of the benefits for our “scenario” are; this option will provide an easy build, lightweight, cost effective, its stable, has water absorption and is sustainable which for the most part, is one of the important factors for consideration in terms of any construction.
Features and Benefits.
• Lightweight and Rigid. • Safe and easy to handle. • Eliminates staged construction & preloading. • Engineered, Controllable and Predictable. • Inert over long periods of time. • Approx. 100 times lighter than soil. • 20 to 30 times lighter than other lightweight fill. • Perfect for earth stabilization, relieved load & civil engineering projects.
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Figure 5: Imagery of a typical geofoam can be used to create topography without adding significant load to underlying structures and services. Imagery Taken from; Expanded Polystyrene (EPS) Geofoam Application & Technical Data – see reference page). (accessed 20th July 2014).
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
3. Soil Nailing.
One of the mast cost effective methodologies to by which to assist with the performance of the foundation soils is soil nailing. This is a construction technique that can be used as a remedial measure to treat unstable natural soil slopes or as a construction technique that allows the safe over-steepening of new or existing soil slopes.
This involves the insertion of relatively slender reinforcing elements into the slope – often general purpose reinforcing bars (rebar) although proprietary solid or ‘hollow-system bars’ are also available. The solid bars are usually installed into pre-drilled holes and then grouted into place using a separate grout line, however hollow bars may be drilled and grouted simultaneously by the use of a sacrificial drill bit and by pumping grout down the hollow bar as drilling progresses.
Methods of firing relatively short bars into soil slopes have also been developed. These bars are installed using drilling techniques are usually fully grouted and installed at a slight downward inclination with bars installed at regularly spaced points across the slope face. A rigid facing (often pneumatically applied concrete, otherwise known as shotcrete) or isolated soil nail head plates may be used at the surface. As another option - a flexible reinforcing mesh may be held against the soil face beneath the head plates. Erosion control fabrics and may be used in conjunction with flexible mesh facing where environmental conditions dictate.
Lastly, the relevance & some of the benefits for our “scenario” in terms of improving the performance of the foundation soils are; highly efficient method of constructing near vertical slopes plus allows the build/developer/home handyperson the ability to maximise the potential development footprint of a site. This option in my view be one of the most cost effective methods to “improving the performance of the foundation soils”.
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Figure 6: Imagery of a Soil Nailing - to embankment walls. A cost effective use to treat the performance of soft or steep foundation soils.
M&J Drilling Services, Soil Nailing and Rock Anchors, http://www.mandjdrilling.com/?ID=30&SubID=50 (accessed 20th July 2014)
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Earth Retaining Systems – Overview and Options.
A retaining wall is a structure designed and constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.
The understanding is, the walls must resist the lateral pressures generated by loose soils and in some cases, water pressures.
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Figure 7: Imagery of various types of retaining walls.
Every retaining wall supports a “wedge” of soil. The wedge is defined as the soil which extends beyond the failure plane of the soil type present at the wall site, and can be calculated once the soil friction angle is known. As the setback of the wall increases, the size of the sliding wedge is reduced. This reduction lowers the pressure on the retaining wall.
The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained material to move down slope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction and the cohesive strength of the retained material, as well as the direction and magnitude of movement the retaining structure.
Some of the Earth Retaining systems to be discussed which can be used to limit the embankment side slopes from encroaching onto neighbouring properties are much and varied. However to outline a few some of the methods which are available to are: Cut Walls, Tiedback walls, Sheet piles walls, Brick/ Pre cast Masonry block walls, Steepened Reinforced Soil Slopes, Gabion & Rock Mattress Structures, Concrete/Stone/Timber Retaining Walls - In-situ/Pre Cast reinforced concrete walls , Drilled shaft walls and Crib Walls (Pre Cast Conc.).
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Earth Retaining Systems – Options.
1. Crib Walls (Pre Cast Conc.)
Pre Cast “Crib Walls” are low cost, of open web construction and can be quickly and inexpensively erected. The benefits and merits of these are they can be used almost anywhere a retaining wall is needed and walls do not require skilled labour and are easily and quickly erected. The components can be handled by two men and there are no costly foundations involved. The open web construction and use of free draining material eliminates two common causes of failure in retaining walls, namely build up of hydrostatic pressure and the destructive pressure. Some of the applications are; driveways, building sites, steep embankments and the like. (Concrib. Concrete Crib Walls, 2004,10)
Pre Cast “Crib Walls” are gravity retaining walls constructed from interlocking precast concrete components, filled with free draining material and earth backfill, eliminating the hazards of hydrostatic pressure building up behind the wall. (Concrib. Concrete Crib Walls, 2004,10)
This example would be an easy and quick build time and being a cost effective methodology, in terms limiting the embankment side slope from encroaching onto a neighbouring property.
[pic]
Figure 8: Imagery of “Gravity Retaining” Pre Cast Concrete Crib Wall.
(Concrib. Concrete Crib Walls, 2004)
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Earth Retaining Systems – Options.
2. Sheet (Metal) Piled Walls.
Another alternate for this application, which is also very cost effective is “Steel Sheet Piling” is a hot-rolled structural shape with interlocks on the flange tips. The interlocks permit individual sections to be connected to form a continuous steel wall which is earth-tight and water resistant. Because it is readily available and transportable, steel sheet piling is in many cases a fast and economical solution to a durable, long lasting wall system.
The applications are vast, and include, for permanent construction, retaining walls, bulkheads, bridge abutments, graving docks, cut-off walls, mooring dolphins and pier protection cells. Common uses also include temporary structures, such as trenches, cofferdams for building excavations, bridge piers, and lock and dams on the inland river system. One of the overall advantages to this ‘system’ is at the end of the project, the steel sheet piles can be extracted, and the steel reused, if required. (Burt G. Look, 2007,276-287)
One method (not the only), in which this system can be performed or installed by vibrating hammers operated off leaders mounted on tracked base machines or suspended from crawler cranes. Diesel impact hammers and hydraulic press in machines can also be used to drive or push the piles into place. Sometimes water jetting or preboring is used to assist penetration through stiff or hard layers. The chosen pile section also needs to be strong enough to be driven through the various soil strata to the required penetration depth. The drivability of a piling section is a function of its cross sectional properties, length, the steel grade used and, in some cases, the installation method used. The cross-sectional properties are available from the producers of the product, and vary with the section’s thickness, depth, width, and shape.
Figure 10: Sectional View of “Sheet Piling” retaining Wall.
(Burt G. Look, 2007,277)
Apply footing and geomechanical design principles to buildings Assessments up to three storeys
Assignment 2.
Earth Retaining Systems – Options.
3. Cantilevered walls.
One last option for this exercise - (of many), cantilevered walls are/ were the most common type of taller retaining wall. Cantilevered walls are made from a relatively thin stem of steel-reinforced, cast-in-place concrete or mortared masonry (essentially in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing; converting horizontal pressures from behind the wall to vertical pressures on the ground below. It is noted that, cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their stability against high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below and the cost considerations are that this type of wall uses much less material than a traditional gravity wall. (Burt G. Look, 2007)
Advantages of cantilever walls:
• Cantilever walls offer an unobstructed open excavation. • Cantilever walls do not require installation of tiebacks below adjacent properties. • Cantilever walls offer a simpler construction procedure as the construction staging is much simpler.
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Figure 11: Sectional View of “Cantilever Retaining Wall”.
(civil engineering, retaining wall, types and uses of retaining walls, np)
References:
1. Numerical modeling of Embankments on soft soils - Noppa https://noppa.aalto.fi/.../Rak 50_3149_embankments_on_soft_soils.pdf (accessed July 20 2014).
5. Geofoam - National Polystyrene Systems http://www.nationalpolystyrene.com.au/nps/products/road_building_geofoam.html (accessed July 20 2014).
6. Pilingcontractors.com.au http://www.pilingcontractors.com.au/processes/driven-sheet-piles (accessed July 20 2014).
7. Retaining Wall Solutions. http://www.retainingsolutions.com.au/retaining-walls/concrete-crib-walls (accessed July 20 2014).
8. Chararcteristics of Dispersive and Problem Clay Soils. Paul C. Knobel, 1990,1-24 http://www.usbr.gov/pmts/hydraulics_lab/pubs/R/R-91-09.pdf (accessed July 20 2014).
