3-Geotech Design-23s

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1 Geotechnical Engineering (Part 3) Introduction to Geotechnical Design Professor Hadi Khabbaz Email: hadi.khabbaz@uts.edu.au CB11.11.244 Geotechnical Problems ⚫ Variety of geotechnical projects: 1 2

2 ?? Shallow Foundation Geotechnical Problems ?? Deep Foundation 3 4

3 ⚫ Variety of geotechnical projects: Retaining wall ⚫ Variety of geotechnical projects: Slope stability 5 6

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4 Slope stability Retaining Walls 7 9

5 Dams Geotechnical Design 11 11 Offshore Foundations 10 11

6 Light Tower Communication Tower Tower Foundations Circular Foundation for Wind Turbine Square Foundation 12 13

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7 Toronto's CN Tower The World's Tallest Freestanding Structure! (553 m) Ref: http://commons.wikimedia.org/wiki/Image:Toronto's_CN_Tower.jpg Canadian National Tower The World's Tallest Building Burj Khalifa, Dubai (828 m) http://en.wikipedia.org/wiki/File:Burj_Khalifa.jpg 14 15

8 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 ? 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 16 17

9 Nigatta, Japan, 1964 Kissing Buildings ( Buildings Neighbourly Love ) failed due to stress concentration and lack of sufficient bearing capacity of the foundations. 18 19

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10 It has become monuments ! Leaning Tower of Pisa Building Collapsed In Shanghai, 27 June 2009 An unoccupied building still under construction in the Minxing District of Shanghai city toppled over 20 21

11 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 Site Testing 22 23

12 Laboratory Testing Triaxial Cell Shear Box Oedometer 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. 24 25

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13 Design Criteria in Geotechnical Engineering ⚫ 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 … Geotechnical Design Criteria 26 27

14 Geotechnical Design Criteria Deformation Load Failure load Design strength “ serviceable” load “serviceable” load Allowable deformation ⚫ 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. Applied loads < Resisting reactions Strength Criterion 28 29

15 Strength Criterion Value Frequency Load Strength P mean R mean Failure Value Frequency Load P mean Failure Strength R mean Safety Margin ⚫ 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. Strength Strength Criterion 30 31

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16 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 Strength Design Approaches 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; ❖ Different characteristic 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). 32 33

17 Overall Factor of Safety ⚫ Used in 20 th century, still very popular. ⚫ Strength criterion:   i u P F R ⚫ 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. Ultimate strength Factor of safety Applied loading Overall Factor of Safety Average factor of safety for different types of structures Category Typical Structure Load Character Thorough site invest. Limited site invest. A Railway bridges, Warehouses, Silos Max. design load likely to occur often 3 4 B Highway bridges, Industrial buildings, Public buildings Max. design load expected to occur on rare occasions 2.5 3.5 C Residential buildings Max design load does not occur 2 3 Effect of Site Investigations 34 35

18 Overall Factor of Safety 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 Effect of Type of Project The LRFD Approach Strength criterion: F Ru  S a i P i Ultimate strength Load factor Applied loading Strength reduction 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 ………… …………… (Load and Resistance Factor Design) 36 37

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19 The LRFD Approach ⚫ Strength criterion: F Ru  S a i P i Ultimate strength Load factor Applied loading Strength reduction factor ⚫ Values of a i 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. (Load and Resistance Factor Design) F Ru  S a i P i 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 ………… …………… Ultimate strength Load factor Applied loading Strength reduction factor The LRFD Approach ⚫ Strength criterion: (Load and Resistance Factor Design) 38 39

20 The Partial Factors of Safety ⚫ Increasing popularity in Europe. ⚫ Strength criterion: Load factor Applied loading R   S a i P i Design resistance ⚫ 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. Serviceability Criterion Deformation  Allowable deformation Differential deformation  Allowable differential deformation L Cracks d Differential settlement Ratio = d /L 40 41

21 Serviceability Criterion Tolerable deformations depends on the type of structure: ➢ Stiffness of the structure ➢ Building configuration and location of structure within settlement profile Deformation  Allowable deformation Differential deformation ratio  Allowable differential deformation ratio Serviceability Criterion ➢ 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. Examples of serviceability criteria For many structures differential settlements are more important that the absolute value of settlements. 42 43

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22 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 Serviceability Criterion Summary of criteria for settlement & differential settlement of structures Type of structure Type of damage/concern Criterion Limiting value Framed buildings and reinforced load bearing walls Structural damage Differential 1/150 - 1250 Walls & partitions cracking Differential 1/500 (1/1000-1/1400 for end bays) Visual appearance Tilt 1/300 Connection to services settlement 50-75 mm (sand) 75-135 mm (clay ) Tall buildings, structures with unreinforced load bearing walls Operation of lift & elevators Tilt (after lift installation) 1/1200 – 1/2000 Cracking by sagging Deflection ratio 1/2500 (L/H = 1) 1/1250 (L/H = 5) Cracking by hogging Deflection ratio 1/5000 (L/H = 1) 1/2500 (L/H = 5) Bridges – general Ride quality Settlement 100 mm Structural distress Settlement 63 mm Function Horizontal movement 38 mm Bridges – multiple span Structural damage Differential 1/250 Bridges – single span Structural damage Differential 1/200 44 45

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23 Design Methods Category Characteristics Method of parameter estimation 1 Empirical – not based on soil mechanics principles Simple in-situ or laboratory tests with correlations 2 Based on simplified theory or charts- uses soil mechanics principles- uses mostly hand calculations. Routine relevant in-situ or laboratory test – may require some correlations 3 Based on theory using site specific analysis- uses soil mechanics principles- uses advanced numerical or analytical techniques. Careful laboratory and/or in situ tests which follow the appropriate stress paths ⚫ Methods in Categories 1 & 2 are the most commonly used. ⚫ Many of category 2 design charts developed from category 3 analyses. 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 Quiz on Settlement 46 47

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24 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 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. 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 48 49

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