Civil Engineering

• Factors affecting Engineering Projects; ground surface/instability, soil (porosity/compaction), weathered bedrock, jointing in permeable rock, permeable rock layer, faults and strength of rock.
• Rock Strength:

  • Basalt - 250
  • Granite - 200
  • Gneiss - 150
  • Carboniferous Limestone/Marble - 100
  • Slate - 90
  • Palaeozoic Sandstone - 70
  • Schist - 60
  • Mudstone - 40
  • Oolitic Limestone - 25
  • Mesozoic Sandstone/Shale - 20
  • Chalk - 15
  • Clay - 2
• Rock Strength doesn't account for jointing and weathering (field conditions), and values lower in saturated rock; strength of rock determined in lab using hydraulic ram to compress cylindrical sample of rock until it fractures.
• Strongest rocks are crystalline (especially igneous as interlocking crystals) + older rocks are stronger as more cemented/lower porosity:

  • Carboniferous Limestone recrystallised from fossil remains to interlocking calcite crystals + marble recrystallised calcite.
  • Foliated Metamorphic Rocks loses strength as layers not firmly held together; large mica flakes in schist cleave easier than compacted slate.
• Good/Bad Rocks for Engineering:

  • Mudstone loses strength when fissile + thinly bedded shale beds seperated from each others (shale weathers easily).
  • Clay imcompetent/behaves plastically flowing under stress from flexible/flaky clay minerals and high water content + flat clay particles aligned parallel to direction of shear + weak/likely to collaspe (saturated clays may slump).
  • Unconsolidated Deposits (alluvial sand/gravel) weak/permeable so tunnels must be deep enough to pass underneath alluvial deposits.
• Rock Hardness (how easily it can be drilled):

  • competent rocks retain their shape when folded; limb keeps same thickness throughout fold (eg. sandstone/limestone)
  • incompetent rocks change their thickness throughout the fold; rock flows into space available; folding complex and not easy to predict pattern of folding from one outcrop to another (eg. shale, clay and mudstone)
• Factors affecting rock strength:

  • Dip of Strata: sedimentary beds usually laid down in horizontal beds; dipping strata can have water flowing down slopes, inclined planes for bed/material to move down, fractures caused by folding, and movement/slipping between beds during folding.
  • Horizontal Beds (stable): breaks in deposition create bedding planes (planes of weakness/dip towards valley acting as slip surface/water lubricates bedding planes) + massive/thick beds are stable/competent but thin beds are unstable/imcompetent.
• Factors affecting rock strength:

  • Folding: two limbs of fold means two surfaces for sliding to take place in opposite directions + top of anticline stretched over crest forms tension cracks/planes of weakness + folds eroded at surface expose succession of strata (different rocks outcrop).
  • Faulting: rocks on side of fault moved against other side (fault is line of weakness + water percolates down fault plane + different rocks brought together at surface have different properties) + fault breccia may form (percolate water through into porous rocks).
• Factors affecting rock strength:

  • Slaty Cleavage: slate splits easily along closely spaced cleavage planes which weaken slate making it susceptible to weathering.
  • Joints: joint patterns involve fractures (strong/competent rocks weakened by jointing dividing rocks into blocks easily loosened by weathering) + jointing increases fissure porosity/permeability + horizontal beds means vertical joints (vertical lines of weakness which cliff/steep slopes collaspe).
• Groundwater - water occupying pores/other spaces in cracks and sediments derived mostly from rainfall percolating into underlying rock.

  • present in billions of tiny spaces between mineral grains/narrow fractures in bedrock
  • distribution depends on porosity/permeability of rocks, sediment and soil

  Water Table - surface separating unsaturated rock above from saturated rock below.
• • Porosity - volume of pore spaces in a rock/sediment (expressed as a % of total rock volume): % porosity = (volume of pores/total volume of rock/sediment) x 100
  • Permeability - ability of a rock/sediment to transmit fluids/oil/gas; can be expressed as a rate of flow of the fluid through the rock/sediment: permeability = distance fluid travelled/time taken
• Aquifers - a body of porous/permeable rock capable of storing and yielding signicant amounts of water.

  Types: unconfined (open to surface/infiltration) + confined (overlain by less permeable materials) + perched (underlain by low-permeability unit) + artesian (water rises in pipes).
• Properties of Aquifers:

  • porosity, bedding planes, joints, cleavages, faults, vesicles, interstices/gaps between grains and intraparticle pores
  • porosity; soils 30-55%, clay 50-70%, sandstones/limestones 5-30%, chalk 10-45% and fractured igneous 10-40% (size/sorting has impact)
  • permeability (rate which water flows through); not all rocks permeable, eg. clay - Specific Yield; volume of water it yields, when drained naturally/artificially pumped + as grain size increases, specific yield increases
• Properties of Aquifers:

  • most groundwater in aquifers slowly circulates in upper 100-200m of saturated zone + fresh water penetrates over 2km depth; groundwater deeper than 2km mineralised with sodium/chloride (too saline for potable water)
• • Aquifers of Younger Cover; provide most of groundwater abstracted for public/industrial use + other limestones/sandstones with less favourable water-bearing properties provide supplies locally.
  • Aquifers of Older Cover; provide many local supplies + basement rocks not entirely impermeable (at surface, small yields of groundwater from fractures in 50-100m of saturated zone).
• Groundwater Quality/Chemical Composition:

  • hard calcium-bicarbonate water at outcrop passes into soft sodium-biocarbonate water which passes into saline sodium chloride water
  • groundwater good quality as rocks filter away bacteria (from surface/soil) + only need to percolate 3-30m to get high quality
  • as water percolates deeper, concentrated of dissolved minerals increases
• Increasing River Flow with Groundwater: can be pumped from boreholes into rivers in dry periods to maintain river flow + boreholes located some distance from rivers/water take from groundwater storage so doesn't affect river flow in short term + boreholes away from river mean cones of depression don't interfere with natural groundwater flow to river.
• Effects of Global Warming on Groundwater:

  • 2050, average temperature likely 1.6$^oC$ higher than 1961-90 average + total annual rainfall predicted 10% increase (but decline in UK summer rainfall)
  • prolongued summers (reduces winter recharge season) + droughts more common/dry winter more significant - groundwater storage more important as surface waters at risk
  • drier summers mean seasonal defecits in soil moisture content + aquifers recharged most effectively by prolongued steady rain (winter rainfall intense/overland flow quickly into rivers)
• Seasonal Changes in Groundwater Levels:

  • rivers draining areas of permeable rock obtain most water from aquifers; river flows highest at end of winter/early spring and progressively declines
  • as water table falls, streams dry up and point of groundwater discharge moves downstream (these bournes may remain dry for extended periods during droughts)
• Groundwater around Structures:

  • water seeping below dams can erode soil particles away (piping) + hydrostatic force acting againt a dam can cause it to slide downstream or overturn
  • if structure base below water table, groundwater applies pressure from bottom counteracting its weight (uplift pressure)
  • shorter flow paths = greater hydraulic gradient (+higher volume/velocity of seepage); increases risk of sinkholes
• Groundwater around Structures (Solutions):

  • cut-off wall beneath dam stops flows of groundwater reducing volume of flow (therefore pressure); works as length the flow has to travel is increased, so hydraulic gradient also decreases
  • use drains to deal with seepage/uplift; filter seepage with sand/gravel so soil can't be piped out from foundation + relieve uplift pressure by removing water + pumps can depress groundwater further (minimises uplift/increase structure stability)
• Changing Groundwater Levels under Cities:

  • Levels rising as industrial activity declined/private borehole users switched to public supply + groundwater quality declined (contamination by saline and, industrial/agricultural pollution) + as levels rise, danger of flooding in basement/tunnels.
  • Solution: abstract groundwater from boreholes to control rise of water table (done locally to create cones of depressions around at risk buildings or regionally by water companies for supplies).
• Radon (Rn) Gas:

  • noble gas (+radioactive, colourless, odourless, and tasteless) + radon a decay product of Uranium/Thorium with half-life of 3.825 days
  • one of densest substances that remains a gas under normal conditions (8x denser than air at sea level) + formation; uranium minerals (pitchblende/uraninite) into radium then radon
  • found in igneous (felsic; granite/rhyolite), metamorphic (schist/gneiss) and sedimentary rocks (dark, organic rich shales and limestones/iron-rich sandstones)
• • Radon Sources; soil 69.3%, well water 18.5%, air 9.2%, building materials 2.5% and public water supply 0.5% - main sources entering buildings from groundwater/soil through cracks, joints and gaps in foundations/service utilities.
  • Measuring Radon Gas; Becquerel (Bq) unit of radioactivity (Bq/m$^3$) + average concentration in UK homes 20Bq/m$^3$ and maximun exposure is 200Bq/m$^3$.
• Radon Gas Levels in England/Wales; 100,000 have higher than average concentrations in their homes + high concentrations in Cornwall, Derbyshire, and Northamptonshire (+ parts of Devon/Somerset significantly higher levels of radon).

  • Goverment Policy; target areas of country where 5%+ homes above action level + free radon detector kits made available + fix at own expense if above 200Bq/m$^3$ action level.
• Radon enters homes through cracks in solid floors/walls (below ground level), gaps in suspended floors or around service pipes/construction joints.

  • Health Risks; increases risk of cancer (higher radon/longer exposure = greater risk); causes over 1,000 deaths from lung cancer each year in UK + radon produces radioactive dust in air; gets trapped in airways emitting radiation damaging lungs (increases lung cancer risk).
• Reducing Radon Gas Levels:

  • Ventilate - open windows + install air bricks in walls just above ground level + install an air pump that pumps gas out.
  • Reduce Access - seal cracks/fissures in floors and foundations + seal gaps where utilities enter the house + pressurise buildings by blowing air into house to exclude soil air.
  • Divert - install a sump beneath the building to collect gases which can then be pumped away.
• Radon Gas Analysis:

  • increase levels in wells precursor of earthquake; short half life (98hrs) so unlikely to seep to surface through rocks at depth + soluble in water so monitored in deep springs/wells
  • stress released causing micro-fractures to open in rocks at depth; groundwater flows in and levels fall + radon trapped with rocks escapes through micro-fractures/goes into groundwater solution
  • carbon dioxide acts as carrier for radon/taken into solution in groundwater
• • Radon formed by radioactive decay of small amounts of uranium that occurs naturally in rocks/soil; radioactive elements from decay of radon can be inhaled/enter our lungs.
  • Cornwall/Southwest designated radon affected area (1% domestic households have radon levels above 200Bq/m$^3$) as granite in landscape produces radon more rapidly than other rock types.
• Reasons for Dam Contruction; industrial/domestic water supply, irrigation water for agriculture, generation of hydroelectric power, river regulation/flood control, improved navigation/recreational use, and tailing dams to store waste from mining.
• Arch Dams: eg. Monar Dam, Scotland

  • curved shape + constructed from concrete and reinforced with steel rods/cable + built in narrow, steep valleys but need solid/strong rock for foundations/valley sides (suitable for remote areas)
  • curved upstream into water so hydrostatic pressure from water compressed the structure against the sides of the valley strengthening it + thinner dam so uses less construction material
• Gravity Dams: eg. Itaipu Dam, Brazil

  • triangular cross section + heavy dam holds back water + toe of dam sunk to prevent sliding + can be solid/hollow or overflow/non-overflow + made of concrete/masonry + expensive but most stable
  • held in place by gravity due to immense mass of concrete/masonry + built on solid/impermeable/high-load bearing rock foundation + supported on downstream side by inflexible butresses
• Embankment Dams: eg. Quoitch Dam, Scotland

  • impermeable clay/concrete core held in place by piles of rock/earth, sand and clay with impervious covering (require large quantities of fill material) + built on hard rock + reinforced concrete cover common
  • material binds itself together by friction between particles so cement unneccessary + dams built on broad, shallow, and flat valleys so mass spread over wider area (foundation don't need to be as strong)
• • Arch-Gravity Dam: combines strenght of arch with force of gravity + useful in areas of high water flow but limited building material.
  • Butress Dams: solid/watertight upstream side/downstream side supported by inflexible butresses + flat/curved wall made of reinforced concrete + used in wide valleys in absence of solid rock, eg. Roseland Dam, France.
• Factors Affecting Dam Construction:

  • geological stability; earthquakes/faults can cause dam collaspe or failure + volcanic eruptions may fill reservoirs with pyroclastic/lava flows/lahars (lack of seismic activity best)
  • river catchment needs sufficient rainfall (for reservoirs to be filled in autumn/winter) + underlain by impermeable bedrock promoting surface runoff/storage in reservoirs + low sediment load so doesn't silt up quickly
• Factors Affecting Dam Construction:

  • stable valley sides (reduce risk of mass movement); beds dipping into valley prone to landslips (risk increased if interbedded competent/permable and incompetent/impermeable beds)
  • absence of mineral veins in catchment containing toxic minerals (lead, arsenic or uranium)
  • narrow/deep valley; length of dam/area flooded minimised) + upland sites preferable (high rainfall/low evaporation)
• Underlying Dam Rock Types:

  • strong/competent rock with high-load bearing strength support mass of dam/water (eg. crystalline igneous/metamorphic or well-cemented sedimentary rocks)
  • impermeable rocks prevent leakage of water from reservoir + competent rocks with joints cause leakage problems (eg. limestone is strong, but permeable with caves/solution cavities so unsuitable)
  • uniform rock type; if across 2 rock types, differential subsidence may occur making dam unstable + depth of weathering weakens rock/increases permeability
• Factors Affecting Dam Construction:

  • Dip of strata beneath dam; horizontal beds/beds dipping upstream make stable foundations + beds dipping downstream are unstable (leakage/slippage along bedding planes possible)
  • faults/joints (zones of permeability/weakness) + weight may reactivate old faults/increase seismic activity + joints increase leakage/permeability
  • anticlines unstable foundation (slippage occurs on bedding planes of fold limbs) + tension joints on anticlines allow leakage - synclines more stable, but water bypasses dam through permeable beds
• Ground Improvement Measures Prevent Leakage:

  • Grouting - holes drilled into rock and liquid cement pumped in filling pore spaces, joints and fissures reducing permeability/increases rock strength.
  • Clay/Plastic Geomembrane - prior to filling, reservoir lined with impermeable material to prevent leakage of water into underlying rock (clay is cheap if in local supply).
  • Cut-off Curtain - impermeable (concrete) barrier constructed as extension below dam preventing leakage + strengthens foundations and prevents slippage of beds dipping downstream.
• Problems with Dam Construction:

  • land behind resevoir floods; loss of quality farmland + decaying vegetation produces CO2/CH4 + settlements need relocating (splits up communities)/loss of archaeological sites
  • damage to aquatic ecosystems (changes water depth, temperature, and dissolved oxygen downstream of dams) + prevent fish swimming upstream to breeding grounds + reduction in nutrients transported to sea (affects coastal ecosystems)