Notes Week 1 Engineering Geology Problems

Transcription

1 Notes Week 1 Engineering Geology Problems The Big Three of Engineering Geology 1. Will the ground bear the weight? (i.e. Subsurface materials? Rock or soft soil and sediment? Differential settlement?) 2. Will the walls collapse or move? (i.e. soft weathered material or fractured rock?) 3. Will the excavation flood? (i.e. groundwater? Porous sands? Open fractures in rock?) Will the ground bear the weight? Uniform, strong rocks is good. Variable strength rocks with strong on one side and weak on the other is bad. (rarely and not a problem in Sydney) Geotechnical issues: o Differential settlement of the structures foundations. The weight of the structure forces relatively weaker founding materials on one side of the structure to compress more than stronger materials on the other side. One side of the structure will literally sink into the ground at a faster rate than the other. o Structural damage o Floors, walls or founding blocks bend or crack. o Pieces of internal or external wall will break away and fall off. o Windows may shatter explosively as their frames distort beyond the tolerance o Floor slabs can crack and dislocate which creates little ledges or steps Geotechnical solution: o Do a site investigation o Evenly distribute the load of the structure on the foundations by varying the contact area of the foundation block. o Use piles, piers, pads o Use codes to determine footing design suitable for specific soil or rock types present in a particular place. Key examples: St. Francis Dam by William Mullholland, Leaning Tower of Pisa. Will the walls collapse or move? 1 P a g e

2 Will the structures or roads adjacent to an excavation be destabilized? Geotechnical problems: o Soft weathered material or fractured rock present at the site that could slide into the empty space created when the hole that will become the basement levels is dug out? Geotechnical solutions: o Consider joint or bedding system; o Nail the wall with bolts or cable anchors o Place wire mesh over the rock face using faceplates of the rock bolts or cable anchors to catch any smaller rock slabs or stones. o Place concrete over the top of wire mesh to strengthen the excavated rock face. Spray shotcrete to create impermeable barrier and impede drainage. o Make sure concrete layer has weep holes so that water can drain into excavation. o Install sump at bottom of excavation and pump out water to surface drainage system. o Excavate the hole progressively; use sheet piles, solder piles, secant piles for weak materials; anchor the wall and simultaneously install groundwater drainage system for strong materials. Will the excavation flood? Geotechnical problems: o Water making its way through natural fractures, large pores. Geotechnical solutions: o Seal up the wall of excavation and make it watertight using bentonite. o Grouting to fill up fractures. o Install a sump at the very bottom of the basement and install large pumps to pump water out quickly. Integrating all these problems Varying permeability depends on the nature/size of fractures. Varying width of fractures (tends to decrease with increasing depth). Rock types change. Orientation and characteristics of joint systems. Case example: building the law school First question: will the ground bear the weight? 2 P a g e

3 Bearing capacity unlikely to be a problem. Wall stability is a concern. Can put large buildings on this foundation material. Less stable wall. Prone to weathering and disintegration. Uniaxial compression: 15 to 20 MPa Generally a benign construction material. Second question: will the walls collapse or move? Adjacent to Fisher Library. Retaining structure is the soldier pile wall. Cylinders of concrete reinforcing cables closely spaced as we are worried about differential settlement and distortion of the building. The factor of safety is quite high. This is to maintain the stability of the foundation of the building. Adjacent to Victoria Park. Concrete cylinders are more widely spaced, approximately 2m apart and cable anchors are used at the bottom of those concrete cylinders to increase stability. Third question: will the excavation flood? Water gets in through the soil profile. Water flows through the joint system. Is this system sufficiently clamped? Steel reinforced concrete circular sets to provide resistance to the dilation of rock mass. Case Example 2: Highway 1, California, USA Steep mountainous terrain. Geotechnical issue is the desegregation of rock mass and sliding blocks. Potential for large-scale failure is monitored. Geotechnical solution is anchoring system is not preferred due to costs; install steel mesh to prevent loose rocks from falling down. Case Example 3: Leaning Tower of Pisa Geotechnical issue: Tilt continued to worsen throughout the centuries. Columns and arches of the third storey was slightly taller on the sinking side. Angling the eighty storey bell chamber. Differential settlement. Geotechnical solution: 3 P a g e

4 o Cables first attached around third storey and tensioned to stabilize tower during drilling. o Drill inserted at shallow angle to extract small volumes of soil on the north side of the tower. o Cavity fills under pressure of tower, slowly reversing its southward tilt. o Drainage system to allow consolidation of sediment. Moral of the story: site investigation is important! Case Example 4: Nelson, New Zealand South Island Geotechnical problem: Potential sliding of loose rocks on steep slope Geotechnical solution: o Drain the site. o Install steel mesh which prevents loose rock. o Cable anchors keeps steel mesh in place. 4 P a g e

5 Notes Week 3 Igneous Rocks Definition Solidified molten rock (freezes at high temperature) 1100 C to 650 C. Depends on composition. Earth is mostly igneous rock. Magma = subsurface melt (erupts via volcanoes), lava = melts at surface. Extrusive vs intrusive Melted rock can cool above or below ground. Intrusive igneous rocks = cool slowly underground. Extrusive igneous rocks = cool quickly at Earth s surface. o Lava cooled liquid. o Pyroclastic debris e.g. ash, fragmented lava. Magma formation Partial melting in crust / upper mantle. Melting is from: o Pressure release o Volatile addition o Heat transfer. Upon heating, silica-rich minerals melt first. Partial melting, then yields a silica-rich magma. Removing a partial melt from its source creates: o Felsic magma o Mafic residue Fractional crystallization As magma cools, early crystals settle by gravity. Melt composition changes as a result. o Fe, Mg and Ca is removed in early settled solids. o Si, Al, Na and K remain in melt and increase. Felsic magma can evolve from mafic magma. Progressive removal of mafic minerals. 5 P a g e

6 Cooling rates Cooling rate how fast is heat lost? o Depth: deep is hot, shallow is cool. Deep plutons cool slowly (centuries to a million years) Shallow flows cool rapidly. o Shape: surface to volume ratio Spherical bodies cool slowly Tabular bodies cool faster o Groundwater removes heat. Igneous textures Texture directly reflects magma history. o In response to composition and cooling of magma. The size, shape and arrangement of the minerals. o Glassy made of solid glass or glass shards, e.g. obsidian. o Interlocking crystals minerals that fit like jigsaw pieces, e.g. granite. o Fragmental pieces of pre-existing rocks, e.g. pyroclastic. Intrusive = cool slowly below ground. Individual mineral grains grow larger. Phaneritic texture = coarse. Extrusive = cool rapidly above ground. Mineral grains small, only have a short time to grow. Aphanitic texture = fine. Appears massive, structureless. Igneous classification Composition felsic, intermediate, mafic, ultramafic. Texture fine (aphanitic), coarse (phaneritic). Aphanitic (felsic to mafic): rhyolite, andesite, basalt. Phaneritic (coarse): granite, diorite, gabbro. Pyroclastic classification Pyroclastic fragments of violent eruptions. o Tuff volcanic ash that has fallen on land. o Volcanic breccia made of larger volcanic fragments. o Hyaloclasite fragments of lava exploded into water. Igneous activity distribution Igneous activity tracks tectonic plate boundaries. 6 P a g e

7 o Hot spots mafic volcanic activity. o Convergent boundaries felsic igneous activity. o Divergent boundaries mafic igneous activity. Igneous rocks and processes There are relationships between chemical composition and the mineralogy and magma viscosity. Viscosity determines the explosivity of an eruption. Viscosity as a function of silica content: basalt (mafic), low viscosity; rhyolite (felsic), high viscosity. Note: liquid silica is a highly structured liquid. 7 P a g e