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Geotech
| Question | Answer |
|---|---|
| test method for water content | dry oven test |
| boyont unit weight y' | ysat - yw |
| 4 densities relationship | ysat > y .> yd > y' |
| three limits | shrinkage limit, plastic limit, liquid limit |
| test method for PL | thread rolling |
| test method for LL | casagrande cup |
| vert total stress | sum yiHi |
| pore pressure | u = ywhw |
| hydraulic head | hz+hp = z +p/yw |
| what soil is used for constant head test | sandy soils (high permeability) |
| what does constant head test measure | volume over time |
| what soil is used for falling head test | clayey soils (lower permeability) |
| what does constant head test measure | head loss over time |
| in-situ method advantages and disadvnatages | ad- relatively reliable average permeability dis - high cost and long time |
| when to use hazens equation | cohesionless sandy soils |
| factors affecting permeability | particle size, grading void ratio mineralogy structure saturation water property |
| horizontal flow characteristics | same head loss in each layer but different velocities. the most pervious layer dominates the permeability |
| vertical flow characteristics | different head loss in each layer but same velocity in each. the least pervious layer dominates the permeability |
| flow line | path of a particle of water |
| equipotential lines | constant total head |
| potential function features | lines dont intersect equal total head the difference between two is prportional to the total head difference |
| stream function features | do not intersect tangent is flow direction |
| flow net drawing features | flow line on bottom and on wall equipotential line on both upstream and downstream water surface curvilnerar squares |
| total head at any point in flow net eg a | ha = h1-nxh/Nd |
| pore pressure at any point in flow net eg a | hap=ha-z p = ywhap |
| stream function features | do not intersect tangent is flow direction |
| flow net drawing features | flow line on bottom and on wall equipotential line on both upstream and downstream water surface curvilnerar squares |
| total head at any point in flow net eg a | ha = h1-nxh/Nd |
| pore pressure at any point in flow net eg a | hap=ha-z p = ywhap |
| total seepage force | J |
| seepage force per unit volume | j |
| seepage hazards | heaving boiling piping |
| heaving | pushing of the bottom of the foundation upwards |
| boiling | when the upward seepage force exceeds the effective weight of the soil. occurs at icritical |
| piping | fine particles in cohensionless soils translates through the pores of large particles and pipe forms due to large voids forming and the larger particles being washed away |
| mitigation measures to seepage | reduce head differnce make seepage path longer reduce permeability |
| typical infrasturctures | sheet piles coffer dam dam |
| cofferdam | temporary structure used in submerged areas to create a a dry area which can withstand very high pressure. usually constructred with sheet piles drived into ground |
| sheet piles | driven into the ground to act as an impermeable barrier |
| dam | barrier that stops or restricts flow of water. retains water |
| drains for dams | reduces the hyrdaulic gradient and avoid seepage exit on downstream |
| filters for dam | retains soil particles in places and prevents there migration (piping) through the filter |
| objective of site investigation | to establish the ground and groundwater conditions ideally as a 3D model. what soil and rock types -soil type -strength -stiffness -porosity -hyrdaulic conductivity the depth and thickness of each layer ground water levels costs about 1% of proje |
| stages of site investigation | preliminary site investigation main site investigation |
| prelimainary site investigation | documentary evidence (google maps) field reconnaissance (visit site, take photos) collect local experience (previous investimgations, local records) |
| main site investigation | drilling and sampling laboratory testing in sity testing (alternative which is useful for soils difficult to sample such as sand) |
| drilling methods | coring wet rotary solid stem or hollow stem continous flight auger |
| AR | drilling measurement whioch if less then 10% is considered undisturbed |
| borehole number | 1 for every 250m^2 |
| ground models | often use geological cross section |
| geophysical vs geotechnical investigation | geotechnical is accurate but normall destructive whilst geophysical is less accurate but can be non destructive |
| geophysical advantages | non destrucitve fast and economical applicable to soils and rocks |
| geophysical disadvnatges | no samples models assumed for interpretation affected by cememted layers results influenced by water clay and depth |
| geophysical survey | electrical measurements of resistivity, conductivity, permittivity and was the magnetometer and gravimetric data |
| direct shear test advantages | simpliest and most economical for dry/saturated sandy soils |
| direct shear test disadvantages | does not always fail along the weakest plane, pore water pressure cannot be monitored and controlling drainage is difficult |
| cohesion and friction | cohesion is y int and is 0 for cohesionless soils friction is the angle and depends on vertical stress |
| landslides triggers | intense rainfall water level change ground water flow rapid snow melt earthquake human activity |
| landslipe types | sliding flow fall topple |
| sliding | moving material largely in contact with parent material during movement. has distint slip surface |
| flow | movement of material down a slope in the form of a fluid where the slip surface is ill difines with multiuple slip surfaces which change continously |
| fall | sudden collapse of material from steep slope causing free fall and rock collecting near the base |
| topple | foward rotation and movement of rock out of a slope |
| slope stability analysis steps | evaluate the possibility of failure of slopes adopt stabilsation methods retrofit failed slopes |
| causes of slope failure | external force distrubing the oringal equilbirium of the soil -eg earhquake or excavation on top of slope soil strength reduction caused by external impacts -rain, earth quake |
| FOS | resistance capacity/ distrubance |
| FOS for static | tan / tan |
| FOS for seepage | |
| direct shear test advantages | simpliest and most economical for dry/saturated sandy soils |
| direct shear test disadvantages | does not always fail along the weakest plane, pore water pressure cannot be monitored and controlling drainage is difficult |
| cohesion and friction | cohesion is y int and is 0 for cohesionless soils friction is the angle and depends on vertical stress |
| landslides triggers | intense rainfall water level change ground water flow rapid snow melt earthquake human activity |
| landslipe types | sliding flow fall topple |
| sliding | moving material largely in contact with parent material during movement. has distint slip surface |
| flow | movement of material down a slope in the form of a fluid where the slip surface is ill difines with multiuple slip surfaces which change continously |
| fall | sudden collapse of material from steep slope causing free fall and rock collecting near the base |
| topple | foward rotation and movement of rock out of a slope |
| slope stability analysis steps | evaluate the possibility of failure of slopes adopt stabilsation methods retrofit failed slopes |
| causes of slope failure | external force distrubing the oringal equilbirium of the soil -eg earhquake or excavation on top of slope soil strength reduction caused by external impacts -rain, earth quake |
| FOS | resistance capacity/ distrubance |
| FOS for static | tan / tan |
| FOS for seepage | ysat-yw/yw tan / tan |
| FOS for pure cohesive (no friction) | F=cLr/Wx -length of slip surface = r x degrees/360 *2pi r- radius c - sheer strength x- distance from W to origin gorizontally W - area between slip surface and surface x y |
| steepness | Run : Rise H:V |
| soil nail effective load is considered as minimum of | bond strength of cement grout and nail - strongest usually bond strength of cement grout and soil tensile strength of nail |
| effect of water table change | decreasing depth increases resisting forces and stability whilst oposite for rising water table |
| rapid drawdown | can cause instability as the lateral force provided by water is removed so the pore pressure doesnt have enough time to disipate which can provoke failure. |
| mitigation to rapid drawdown effect | provide drainage systems which can help disipate pore pressure faster and thus increase stability |
Created by:
Scalzo04