3-Geotech Design-23s-1.pdf

Geotechnical Engineering (Part 3) Introduction to Geotechnical Design Professor Hadi Khabbaz Email: [email protected] CB11.11.244 1...

Geotechnical Engineering (Part 3) Introduction to Geotechnical Design Professor Hadi Khabbaz Email: [email protected] CB11.11.244 1 Geotechnical Problems ⚫ Variety of geotechnical projects: 2 1  Geotechnical Problems Shallow Foundation ?? 3 Deep Foundation ?? 4 2  Retaining wall ⚫ Variety of geotechnical projects: Geotechnical Design 5 5 Slope stability ⚫ Variety of geotechnical projects: Geotechnical Design 6 6 3  Slope stability 7 Retaining Walls 9 4  Dams 10 Offshore Foundations Geotechnical Design 11 11 5  Communication Tower Light Tower 12 Tower Foundations Circular Foundation for Square Foundation Wind Turbine 13 6  Toronto's CN Tower The World's Tallest Freestanding Structure! (553 m) Canadian National Tower Ref: http://commons.wikimedia.org/wiki/Image:Toronto's_CN_Tower.jpg 14 The World's Tallest Building Burj Khalifa, Dubai (828 m) http://en.wikipedia.org/wiki/File:Burj_Khalifa.jpg 15 7  Geotechnical Engineering Concerns of geotechnical engineers: Data Accuracy, Safety, Performance, Economy, Convenience, Effectiveness, Environment and Sustainability How can we: ❖ characterise the ground most economically? ❖ estimate the required design parameters? ❖ carry out the most convenient design? ❖ be sure that the design is sufficiently safe? ❖ optimise the cost versus performance? ❖ be sure that the design can be constructed effectively? ❖ consider long-term impacts on environment and natural resources? 16 Geotechnical Engineering ⚫ A good design requires: ❖ An adequate estimation of the engineering properties of the ground strata. ❖ Understanding the behaviour of the geotechnical work. ❖ Understanding the key parameters affecting this behaviour. ❖ Experience. ⚫ Inappropriate interpretation may result in catastrophe 17 8  Nigatta, Japan, 1964 18 Kissing Buildings (Buildings Neighbourly Love) failed due to stress concentration and lack of sufficient bearing capacity of the foundations. 19 9  Leaning Tower of Pisa It has become monuments ! 20 An unoccupied building still under construction in the Minxing District of Shanghai city toppled over Building Collapsed In Shanghai, 27 June 2009 21 10  Approach ⚫ Site Studies Sampling Field testing ⚫ Laboratory Testing Obtain soil properties Assess soil behaviour under loading ⚫ Office Design Use the information obtained Assess safe design ⚫ Construction Observe the soil. Be careful if soil has different characteristics than assumed in design 22 Site Testing 23 11  Laboratory Testing Shear Box Oedometer Triaxial Cell 24 Geotechnical Design ⚫ Early geotechnical designs were based on technical experience and engineering judgment ⚫ Attempts have been made to understand the behaviour of different soils. ⚫ Unlike many other Civil Engineering fields, soil mechanics and geotechnical engineering are fairly new. ⚫ Hence: Always look for new developments. 25 12  Design Criteria in Geotechnical Engineering 26 Geotechnical Design Criteria ⚫ Aims of the criteria: To limit the applied pressure in order to prevent failure; To limit the deformations and prevent serviceability problems. ⚫ Design criteria: Strength criterion: ❖ Applied load < Strength Serviceability criterion: ❖ Deformation < Acceptable Value ⚫ Limitations: Lack of knowledge; Budget: ❖ Nearly all structures can be built and safely supported if there is unlimited budget, but … 27 13  Geotechnical Design Criteria Load Failure load “serviceable” load Design strength “serviceable” load Deformation Allowable deformation 28 Strength Criterion Applied loads < Resisting reactions ⚫ Applied loads: Dead loads, live loads, wind loads, water pressures, earthquake loads, etc. Variable in nature: ❖ Depend on the intensity of the events, density of the material used, and some times on the method of application of the loads. ⚫ Resistances: Based on cohesion and friction of the soil material; Variable in nature: ❖ Soil is not a homogeneous material, strength parameters varies with locations and sometimes with time. 29 14  Strength Criterion Frequency Load Strength Value Pmean Rmean Failure 30 Strength Criterion Frequency Safety Margin Load Strength Strength Value Pmean Rmean Failure ⚫ There are always uncertainties in the determining the actual value of the applied loads and in estimating the true resistance of the system. ⚫ The probability of failure must be limited to a small value. 31 15  Strength Design Approaches The margin of safety can be applied using: 1. The Overall Factor of Safety approach 2. The Load and Resistance Factor Design approach (LRFD): 3. The Partial Safety Factors 4. Using Probabilistic Methods 32 Strength Design Approaches 1. The overall factor of safety approach: ❖ The resisting forces (strength) reduced by a single factor. 2. The load and resistance factor design approach (LRFD): ❖ The resisting forces reduced by a factor and the applied loads increased by another factor. 3. The partial safety factors; ❖ Differentcharacteristic strength values reduced by different factors and various applied loads increased by different factors. 4. Using probabilistic methods; ❖ The calculated probability of failure must be less than the acceptable probability (mostly in research). 33 16  Overall Factor of Safety ⚫ Used in 20th century, still very popular. ⚫ Strength criterion: Ru Applied loading Ultimate strength F  P i Factor of safety ⚫ Choice of F depends on several factors: Type of structure: ❖ Important structures where the consequences of failure are severe require a larger safety factor. Extent of site investigation: ❖ A limited site investigation increases the uncertainties in estimation of strength and requires a higher safety factor. Type of earthwork: ❖ Works with better understanding of their behaviour require a lower factor of safety. 34 Overall Factor of Safety Effect of Site Investigations Average factor of safety for different types of structures Category Typical Structure Load Character Thorough Limited site invest. site invest. A Railway bridges, Max. design load likely 3 4 Warehouses, Silos to occur often B Highway bridges, Max. design load 2.5 3.5 Industrial buildings, expected to occur on Public buildings rare occasions C Residential Max design load does 2 3 buildings not occur 35 17  Overall Factor of Safety Effect of Type of Project Factor of safety for geotechnical projects Work Range of F Earthworks – dams, slopes, fills 1.2 - 1.6 Retaining walls 1.5 - 2.0 Braced excavations 1.2 - 1.5 Shallow foundations 2.5 - 4.0 Piled footings 2.0 - 5.0 Seepage forces – uplift, heave, piping 1.5 - 2.0 36 The LRFD Approach (Load and Resistance Factor Design) Strength criterion: Applied loading Strength reduction factor F Ru  S ai Pi Ultimate strength Load factor Typical load factors for load combinations (AS1170-1993) Case Combinations Dead + Live 1.25D+1.5L or 0.8D + 1.5L Dead + Live + Wind 1.25D + W + 0.4L or 0.8D + W Dead + Live + Earthquake 1.25D +1.6E +0.4L or 0.8D +1.6E ………… …………… 37 18  The LRFD Approach (Load and Resistance Factor Design) ⚫ Strength criterion: Applied loading Strength reduction factor F Ru  S ai Pi Ultimate strength Load factor ⚫ Values of ai are specified in codes or standards. ⚫ Values of F may be specified in standards, otherwise selected based on: Type of earthwork: ❖ A good understanding of the behaviour of the foundation requires a higher strength reduction factor, F. Extent of site investigation: ❖ A limited site investigation increases the uncertainties in estimation of strength and requires a lower strength reduction factor. 38 The LRFD Approach (Load and Resistance Factor Design) ⚫ Strength criterion: Applied loading Strength reduction factor F Ru  S ai Pi Ultimate strength Load factor Typical strength reduction factors for piles (AS2159-1995) Methods F Load test 0.5 – 0.9 Dynamic formulae 0.45 – 0.65 Penetration test 0.4 – 0.65 ………… …………… 39 19  The Partial Factors of Safety ⚫ Increasing popularity in Europe. ⚫ Strength criterion: Applied loading Design resistance R  S ai Pi Load factor ⚫ Design resistance is calculated using reduced characteristic strength parameters. Reduction factors varies depend upon the uncertainty associated with soil parameters, ❖ Different for c and f. ❖ Different for drained and undrained conditions. 40 Serviceability Criterion Deformation  Allowable deformation Differential deformation  Allowable differential deformation L Cracks d Differential settlement Ratio = d/L 41 20  Serviceability Criterion Deformation  Allowable deformation Differential deformation ratio  Allowable differential deformation ratio Tolerable deformations depends on the type of structure: Stiffness of the structure Building configuration and location of structure within settlement profile 42 Serviceability Criterion Examples of serviceability criteria Settlement of a normal foundation must be less than that causes damage to the super structure. Settlement of the foundation of machineries must be limited to prevent damage to sensitive equipment. Settlement of foundations of a nuclear power plants must be limited to prevent any cracking in the walls. Settlement of a dam must be limited to prevent fissures in the core of the dam. For many structures differential settlements are more important that the absolute value of settlements. 43 21  Serviceability Criterion ⚫ AS 2870 (Residential Slab and Footings) Settlement Limits: Type of building max. (mm) Differential (d/L) Clad frame 40 1/300 Articulated brick veneer 30 1/400 Brick veneer 20 1/600 Articulated full brick 15 1/800 Full brick 10 1/2000 44 Serviceability Criterion Summary of criteria for settlement & differential settlement of structures Type of structure Type of damage/concern Criterion Limiting value Framed buildings and Structural damage Differential 1/150 - 1250 reinforced load bearing Walls & partitions cracking Differential 1/500 (1/1000-1/1400 walls for end bays) Visual appearance Tilt 1/300 50-75 mm (sand) Connection to services settlement 75-135 mm (clay) Tall buildings, Operation of lift & elevators Tilt (after lift 1/1200 – 1/2000 installation) structures with Cracking by sagging Deflection 1/2500 (L/H = 1) unreinforced load ratio 1/1250 (L/H = 5) bearing walls Cracking by hogging Deflection 1/5000 (L/H = 1) ratio 1/2500 (L/H = 5) Bridges – general Ride quality Settlement 100 mm Structural distress Settlement 63 mm Function Horizontal 38 mm movement Bridges–multiple span Structural damage Differential 1/250 Bridges–single span Structural damage Differential 1/200 45 22  Design Methods Category Characteristics Method of parameter estimation 1 Empirical – not based on soil Simple in-situ or laboratory tests mechanics principles with correlations 2 Based on simplified theory or charts- Routine relevant in-situ or uses soil mechanics principles- uses laboratory test – may require mostly hand calculations. some correlations 3 Based on theory using site specific Careful laboratory and/or in situ analysis- uses soil mechanics tests which follow the appropriate principles- uses advanced numerical stress paths or analytical techniques. ⚫ Methods in Categories 1 & 2 are the most commonly used. ⚫ Many of category 2 design charts developed from category 3 analyses. 46 Quiz on Settlement The bearing pressure versus settlement curve of a strip footing constructed on the top of a compacted sand layer is given in the following figure. The width of the footing is 2 m. A factor of safety of 3 is specified to determine the allowable bearing capacity. The total settlement of this footing should not exceed settlement limits stated in AS2870 after application of the load. Accordingly, what would be the allowable load to be applied on this footing? What would be the expected total settlement of the footing under this allowable load? a. Type of building is brick veneer b. Type of building is full Bick 47 23  Assume the Factor of safety is 3 and width of the strip footing is 2 m. a. Find the total settlement if the type of building is brick veneer. b. Find the total settlement if the type of building is full Bick 48 Serviceability Criterion AS 2870 (Residential Slab and Footings) Settlement Limits: Type of building max. (mm) Differential (d/L) Clad frame 40 1/300 Articulated brick veneer 30 1/400 Brick veneer 20 1/600 Articulated full brick 15 1/800 Full brick 10 1/2000 Brick veneer is a method of construction where a property of either a wooden or steel frame is concealed with a single layer of bricks as the exterior layer. 49 24  Thank You Please Be Ready for the Next Part Landslide covers National Highway in Taiwan 50 25