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PE Civil Exam – Geotechnical with
Complete Solutions
Void Ratio, e - ANSWER-or Vv/Vs Porosity, n - ANSWER-or Vv/Vt Saturation, S - ANSWER-or Vw/Vv Dry unit weight (Ɣd) - ANSWER- Wet unit weight (Ɣw) - ANSWER- Specific Gravity (Gs) - ANSWER- dry density (ρd) - ANSWER-Ws/Vt OR ρ / (1+w) Water content, w - ANSWER-Ww/Ws Truss - ANSWER-A truss is an assemblage of straight members connected at their ends by flexible connections (pin-jointed) to form a rigid configuration. Use of Truss - ANSWER-Trusses are widely used for applications such as supporting bridge decks, roofs of buildings, floor slabs, electricity pylons, towers, domes etc. Simple Truss (2D) - ANSWER-2D plain trusses e.g. bicycle frame Planar truss - ANSWER-Consist of joining plane trusses Space Frame Truss - ANSWER-3D framework Pitched Truss - ANSWER-A truss having inclined top chords. Flat truss - ANSWER- Truss Members - ANSWER- Types of connections in timber trusses - ANSWER-Metal plate and bolts
Nail plate Traditional method using Hardwood pegs Types of connections in steel trusses - ANSWER-Bolts and nuts (also rivets, but they are now rarely used) Welded joint Plane truss determinacy calculations - ANSWER-m = 2j - r m = number of members in the truss j = number of joints in the truss r = number of external reactions Assumptions for analysis of Trusses - ANSWER-i. All members are connected only at their ends by frictionless hinges ii. All loads and support reactions are applied only at the joints iii. The centroidal axis of each member coincides with the line connecting the centers of the adjacent joints iv. The dead weight of the truss members is distributed along its length; hence it is ignored Characteristics of engineering soils - ANSWER-Gravel (2mm - 75mm) Sand (0.05mm - 2mm) Silt (0.002mm - 0.05mm) Clay (< 0.002mm) boulders (> 256 mm) cobbles (64 mm - 256 mm) pebbles (4 mm - 64 mm) granules (4 mm - 2 mm) Mechanical Weathering - ANSWER-The process by which rocks are broken into smaller pieces by physical forces e.g. running water, wind, ocean waves, glacier ice, frost, and expansion and contraction caused by the gain and loss of heat. Chemical Weathering - ANSWER-The process of chemical decomposition of the original/parent rock. In the case of mechanical weathering, the rock breaks down into smaller pieces without a change in its chemical composition. However, in chemical weathering, the original material may be changed into something entirely different. Residual soils - ANSWER-These soils stay where they were formed and cover the rock surface from which they were derived The rate of weathering is higher in warm and humid regions compared to cooler and drier regions and, depending on the climatic conditions, the effect of weathering may vary widely Common in the tropics. The nature of a residual soil deposits will generally depend on the parent rock.
Large trees Measuring heave after wetting How to recognize expansive clays - Laboratory evidence - ANSWER-Grading and indicator tests single and double oedometer tests Free swell and swell pressure test Dry density - moisture content X ray diffraction to determine mineralogy Soil Problems - Collapsible Sands - ANSWER-generally low-density, fine-grained combinations of clay and sand left by mudflows that have dried, leaving tiny air pockets. How to recognize collapsible sand - Desktop study - ANSWER-Transported soils Residual soils - chemical decomposition and leaching in annual water surplus areas Granites of basement complex: Randburg, Sandton, Tzaneen, Mbombela How to recognize collapsible sand - Field evidence - ANSWER-Reduction in volume of soil on wetting Tilting, distorting and cracking of buildings around obvious points of water entry How to recognize collapsible sand - Laboratory evidence - ANSWER-Collapse Potential test Microscope examination of soil fabric Dry Densities with depth double oedometer tests Soil Problems - Soft Clays - ANSWER-soils with large fractions of fine particles such as silty and clayey soils, which have high moisture content, peat foundations and loose sand deposits, located near or under the water table Very low strength Large settlements Long time for settlement to occur Often very thick: 20 -50 m Soil Problems - Dolomites - ANSWER-Solution cavities within water soluble Dolomite rock masses result in the formation of sinkholes or subsidence above cavities due to changes in the groundwater regime Reacts with weakly acid rainwater Soil Problems - Dolomite legislation - ANSWER-1. Dolomite stability investigations must be carried out for all new townships and structures underlain by dolomite at depths of less than 100 m below surface.
- SANS standards now apply to all dolomite areas covering the way one investigates and also sets limits on development (type and density).
- Dolomite Risk Management systems are becoming part of the solution in many state and local government structures. Shallow Foundations - ANSWER-These are placed so that the depth of founding is less than the foundation breadth but no deeper than 3m Deep Foundations - ANSWER-Depth of founding is greater than the foundation breadth, or deeper than 3m requiring specialist excavation plant. Deep foundations are used for heavier structures when great depth is required for supporting the load. Hybrid Foundations - ANSWER-This type comprises elements which are common to both shallow and deep foundations. Retaining Walls - ANSWER-These are constructed to provide permanent lateral support to vertical or near-vertical slopes of soil. They are typically constructed first and then backfilled with soil. Lateral Support - ANSWER-When excavations are required for basements of underground transport systems, lateral support is often required Mechanically Stabilised Retaining Walls - ANSWER-More recently the construction of near-vertical soil walls has been accomplished by reinforcing the mass with metallic strips, geotextiles and geogrids. Gravity transported soils - Lacustrine deposits - ANSWER-Water from rivers and springs flow into lakes. In arid regions, streams carry large amounts of suspended solids. Where the stream enters the lake, granular particles are deposited in the area forming a delta. Some coarser particles and the finer particles; that is, silt and clay, which are carried further into the lake are deposited onto the lake bottom in alternate layers of coarse grained and fine- grained particles. The deltas formed in humid regions usually have finer grained soil deposits compared to those in arid regions. Gravity transported soils - Aeolian deposits - ANSWER-The grain-size distribution of the sand at any particular location is surprisingly uniform. The uniformity can be attributed to the sorting action of the wind.
- The general grain size decreases with distance from the source, because the wind carries the small particles further than the large ones.
- The relative density of sand deposited on the windward side of dunes may be as high as 50 to 60%, decreasing to about 0 to 15% on the leeward side. Geotechnical site investigation - Category 1 - ANSWER-Typically the foundations of relatively small and simple structures (e.g. lightly loaded 1-2 storey buildings) for which the ground conditions (confirmed by simple procedures such as trial holes) are known from experience to be uncomplicated with no loose or compressible strata and no significant groundwater problems.
Soil: What is the soil (or ground) profile and groundwater regime? Structure: size of structure, structural form and layout, the type, magnitude and direction of the applied loads, allowable foundation movements.. Safety: Numerous ground-related hazards may affect the site. Sustainability. This requires consideration of environmental and social factors and the economic implications of different options. Two basic soil types - ANSWER- Cohesive Soils - ANSWER-the load is transferred to the soil and water particles that make it up. However as water cannot take shear stresses the water tends to want to flow away, however in cohesive soils due to the low permeability this cannot take place immediately. Granular Soils - ANSWER-the permeability is much higher, when it is loaded, hence, the pore water will tend to flow away. Consequently the pore water pressure does not increase above the hydrostatic pressure. Soil Problems - Expansive Clays - ANSWER-- Contains Clay minerals
- Increase in volume when wet and shrink when dry >10%
- near surface zone (3m)
- Seasonal influence (Rain - swell, Drought - shrink)
- Largest widespread soil issue in SA Tunnelling - ANSWER-When constructing a tunnel it is necessary to know the stresses that will be exerted on the tunnel so as to design an adequate support system. Soil compaction - ANSWER-increase the consistency of soils thereby improving its strength. Bearing capacity and settlement - Failure - ANSWER-Complete Failure (i.e. Remedial/Repair Work Impossible) Definition: Collapse or loss of safety in terms of day- to-day performance of the structure Alternative Terms: Ultimate limit state Partial Failure (i.e. Remedial/Repair Work Possible) Definition: Failure that limits the day-to-day use of a structure Alternative Terms: Serviceability limit state Depth of foundation - ANSWER-Take into account: ground water rainfall temperature freezing/thawing Shallow foundation Spreading Load - ANSWER- Compaction Vs consolidation - ANSWER-Compaction increses density of an UNSATURATED soil by reducing the volume of air in the voids
Consolidation is a time related process of increasing the density of a SATURATED soil by squeezing water from soil voids Shallow foundation - ANSWER-Depth (measured to bottom of footing) is shallower than width Cementitious materials - ANSWER-Include portland cement, blended hydraulic cements, expansive cement, other cementitious additives, including fly ash pozzolanszl, silica fume, and ground granulated blast furnace slag Portland cement produced by - ANSWER-Burning a mixture of lime and clay in a rotary kiln and grinding the resulting mass Which concrete types available with air entraining? - ANSWER-Types 1, 2, & 3 (1a, 2a, & 3a) D cracking - ANSWER-Concrete disintegration cracking caused by freeze-thaw cycle. Coarse aggregates primary cause of d cracking so reduce the size of Max aggregate. Shrinkage cracking - ANSWER-Higher cement paste content causes shrinkage cracking during setting leading to increased water penetration and corrosion of reinforcing steel. Type k cement - ANSWER-Shrinkage compensating cement Fly ash - ANSWER-Waste product of coal burning power generation stations. Reacts with the calcium hydroxide in setting cement to increase binding. Serves as a microfiller between cement particles, increasing strength and durability, and reducing permeability. Coarse aggregate size - ANSWER-Between 3/8 in and 1 1/2 inches Three main functions of coarse aggregate - ANSWER-1)act as relatively inexpensive filler
- provide mass of particles capable of resisting applied loads
- reduce the volume changes that occur during setting of the cement- water mixture Three functions of water in concrete - ANSWER-1) reacts chemically with cement (hydration)
- wets the aggregate
- water and cement mixture (cement paste) lubricates the concrete mixture and allows it to flow What kind of water can be used to produce concrete - ANSWER-Potable water that conforms to astm c 1602 and that has no pronounced odor or taste Compressive strength of concrete primarily dependent of mixture's - ANSWER-Water - cement ratio
