CE643_Spring2023_Week6_GS (6)

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Oct 30, 2023

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2/21/2023 1 CE 643 - Advanced Foundation Engineering Spring 2023 Week 6 02/21/2023 Sometimes construction work requires ground excavations with vertical or near- vertical faces. Examples: basements of buildings in developed areas or underground transportation facilities at shallow depths below the ground surface. The vertical faces of the cuts need to be protected by temporary bracing systems to avoid failure that may be accompanied by considerable settlement or by bearing capacity failure of nearby foundations. Introduction 1 2
2/21/2023 2 Brace excavation method (a) profile, (b) plan Installing horizontal struts Infront of retaining walls to resist the earth pressure on the backs of walls is called the braced excavation method. The bracing system of the braced excavation method includes struts, wales, end braces, corner braces, and center posts. Braced excavation method Braced excavation method 3 4
2/21/2023 3 Construction Methods Bottom-up • Walls constructed before excavation • Excavation succeeds slab construction • Temporary supports installed during excavation. • Bottom slab constructed first Top-down • Uppermost slab constructed first • Excavation of each level succeeds the construction of slab above it Construction Method Selection Criteria Bottom-up Top-down Temporary Bracing Needed Not needed Allowed Machinery Size Small-Medium Small-Medium Project Duration Long Short Cost High Moderate Suitable Locations Any Any Constructability Known for contractors Needs experience Construction Risk Medium Medium Suitable Project Size Deep but narrow site Large Design Considerations There are three main geotechnical design considerations: 1. Design the bracing based on the maximum expected loads 2. Stability of the excavation at all stages of construction 3. Magnitudes of deformation Effect on surrounding structures Closely related to the methods of constructions, dewatering, etc. 5 6
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2/21/2023 4 Stress Analysis Struts – the apparent earth pressure method 𝑃 ௦ = 𝜎 ௛ ஺ 𝐵𝑆 𝑃 ௦ = maximum measured strut load 𝜎 ௛ ஺ = the apparent earth pressure 𝐵 = horizontal spacing between supports 𝑆 = vertical spacing between supports Apparent earth pressures method: Sands 7 8
2/21/2023 5 Apparent earth pressures method: Clays Stress Analysis Struts – the apparent earth pressure method Sand Soft to medium soft clay ( ఊு ೐ ௌ ೠ > 4) Stiff clay ( ఊு ೐ ௌ ೠ ≤ 4) 9 10
2/21/2023 6 Source of high earth pressures in soft clays Example Phase 2: • Install strut 1 • Excavate to EL +95ft Phase 1: • Excavate 6ft to elevation +105ft as cantilever wall Phase 3: • Install strut 2 • Excavate to EL +85ft Plot the design lateral earth pressure envelope for each phase and calculate the strut loads 11 12
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2/21/2023 7 Stability analysis Failure of collapse of excavations are disastrous at excavation sites. Their influence range is usually large: much ground settlement may arise and adjacent properties within the influence range of settlement may be damaged significantly. Failure of excavation may arise From the stress on the support system exceeding the strength of its materials. From the shear stress in soil exceeding the shear strength The methods of analyzing whether the soil at the excavation site are able to bear the stress generated by excavation are called the stability analysis. Stability analysis include: Overall shear failure analysis (push-in and basal heave failure analysis) Sand boiling analysis Upheaval analysis Overall shear failure When the shear stress at a point in soil exceeds or equals the shar strength of soil at the point, the point is in the failure state or limiting state. When many failure points connect up into continuities and form a plane, failure surface is thus produced. Once the failure surface is produced, the excavation failure or collapse will occur. This is called the overall shear failure. Two main overall failure modes of excavation: Push-in and basal heave Push-in Basal heave 13 14
2/21/2023 8 Push-in failure The push-in is caused by the earth pressure, reaching the limiting state, on the both sides of the retaining wall, which is thereby moved, a larger distance, towards the excavation zone (especially the pert embedded in soil) until it reaching the full zone failure. Push-in refers to the stability of the retaining wall. Also, causes the soil near the wall to heave Basal heave failure The basal heave arise from the weight of soil outside the excavation zone exceeding the bearing capacity of soil below the excavation bottom, causing the soil move and the excavation bottom to heave so much that the whole excavation collapse. The basal heave refer the stability of the soil below the excavation bottom, and its failure surface may pass through the bottom of the retaining wall or through the soil below the bottom of the retaining wall. Nevertheless, when it occurs to a soft clay ground the earth pressure on both sides of wall may also reach the limiting state cause push-in failure also possible. 15 16
2/21/2023 9 Push-in failure analysis There are two main analysis methods: 1. Free earth support method 2. Fixed earth support method Free support method is not applicable to cantilever wall as there will no force equilibrium. If the free earth support method is applied to a strutted wall, the force acting on wall will be including both the passive and active and the strut load. If the fixed earth support method to a strutted wall, the penetration depth of wall be too large to be economical. Push-in failure analysis For strutted wall, the free earth support method is the commonly used analysis method. Take the retaining wall below the lowest level of strut as a free body and conduct the equilibrium analysis Gross Pressure Method 17 18
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2/21/2023 10 Gross Pressure Method Push-in failure analysis JSA (1988) and TGS (2001) suggested that: 𝐹 ௉ ≥ 1.5 . Nevertheless, when assume, 𝑀 ௦ = 0 , 𝐹 ௉ ≥ 1.2 For cohesive soil with 𝑆 ௨ (undrained shear strength) constant and ଶௌ ೠ ఊு ೐ ≥ 0.7 (i.e., 𝑆 ௨ large and not varying with the increase of depth, or shallow excavations), the results from the gross pressure method come illogical, that is the deeper the penetration depth of the retaining wall, the smaller the FS. Gross pressure method is not applicable to cohesive soil with constant 𝑆 ௨ values and ଶௌ ೠ ఊு ೐ is too large However, this method is good enough for ଶௌ ೠ ఊு ೐ < 0.7 Also, it is applicable when: ௌ ೠ ఙ ೡ ᇲ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 or 𝑆 ௨ increase with increase depth Basal heave failure Analysis Analysis of basal heave failure are only applicable to clayey soils. Since ∅ = 0 for clay, the failure surface of bearing capacity failure in clay are circular arc surfaces. The basal heave failure due to excavation is also a kind of bearing capacity failure and might also have a main circular arc failure surface. The analysis method for basal heave varies with the assumed shapes of failure surface near ground or excavation surface, through the main failure surface is still a circular arc. 19 20
2/21/2023 11 Basal heave failure Analysis The analysis method of basal heave assumes many possible failure surfaces and finds their corresponding factors of safety according to mechanism. The one with the smallest FOS is the most likely potential failure surface. Basal heave failure Analysis Many analysis methods have been proposed for basal heave, the most commonly applied method is Terzaghi’s, Bjerrum and Eide’s method and the slip circle method. These methods will be categorized into the: Bearing capacity method Negative bearing capacity method Slip circle method 21 22
2/21/2023 12 Basal heave failure Analysis - Bearing capacity method B1 keep increase and measure the ratio of ultimate weight of soil (“abd”), until the trail failure surface covers the whole excavation (i.e., B1 = B/√2) The FOS against basal heave (F b ) for the excavation is the smallest one among the FOS corresponding to the trial failure surfaces. Since the width of the B1 wide soil on each side of excavation zone may produce the failures, the schematic diagram to calculate the FOS. Terzaghi (1948) – Basal Heave Failure 23 24
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2/21/2023 13 The failure surface is not restricted by the stiff soil The failure surface is restricted by the stiff soil It was found that based on these above equations, the value of the factor of safety against basal heave has nothing to do with the existence of the retaining wall. However, in reality, the retaining wall with high stiffness may be capable of restraining the basal heave failure. Thus, actual FOS should be higher than the values given from above equations. 25 26
2/21/2023 14 Negative bearing capacity method Consider the unloading behavior caused by excavation Using bearing capacity equation, obtain the ultimate unloading pressure FOS = ultimate unloading pressure/unloading pressure 27 28
2/21/2023 15 Slip circle method 29 30
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2/21/2023 16 Slip circle method Hypothetical excavation case 𝑆 ௨ = constant ௌ ೠ ఙ ೡ ᇲ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 31 32
2/21/2023 17 Basal stability –existing methods Basal stability – existing methods 33 34
2/21/2023 18 Basal stability – Existing limit equilibrium method solutions Upheaval Sand or gravel Tendency of lifted by the water pressure The safety against upheave ( 𝐹 𝑢𝑝 ) of the impermeable layer should examined 𝐹 ௨௣ ≥ 1.2 To safeguard the safety of excavation construction, the possibility of upheaval at each stage of excavation should be analyzed. 35 36
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2/21/2023 19 Sand boiling Harza (1935) When the effective stress equal to zero, the soil is unable to bear any load, and this is calling the sad boiling The hydraulic gradient when the effective stress is equal to 0 is called the critical hydraulic gradient 𝑖 ௖௥ 𝑖 ௖௥ = 𝛾′ 𝛾 ௪ = 𝐺 ௦ − 1 1 + 𝑒 𝐺 ௦ is specific gravity and e is void ratio When the exit gradient (point A in figure) is close to the critical hydraulic gradient, the sand boiling occur. Harza (1935) defined the factor of safety against sand boiling: 𝐹 ௦ = 𝑖 ௖௥ 𝑖 ௠௔௫ (𝑒𝑥𝑖𝑡) 𝑖 ௠௔௫ is the maximum hydraulic gradient at the exit of the seepage, and can be obtained with the flow net method Sand boiling Terzaghi (1922) 37 38
2/21/2023 20 Sand boiling Marsland (1953) Following figures are obtained by Marsland (1953) for excavation in sand for piping phenomenon and adopted by NAVFAC DM7.1 Impermeable layer located infinitely deeply Impermeable layer located within finite depth Reasonable factor of safety against piping in excavation be around 1.5-2.0. Required penetration depth Required penetration depth Instrumentation for monitoring of excavations 39 40
2/21/2023 21 Effects on adjacent structures Surface settlements due to installation of diaphragm wall (O’Rourke, 1990) 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑒𝑥𝑐𝑎𝑣𝑎𝑡𝑖𝑜𝑛 𝑀𝑎𝑥. 𝑇𝑟𝑒𝑛𝑐ℎ 𝐷𝑒𝑝𝑡ℎ 𝑆𝑒𝑡𝑡𝑙𝑒𝑚𝑒𝑛𝑡 𝑀𝑎𝑥. 𝑇𝑟𝑒𝑛𝑐ℎ 𝐷𝑒𝑝𝑡ℎ (%) 41 42
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2/21/2023 22 Surface settlements due to installation of diaphragm wall Ou and Yang, 2000 Analysis of ground surface settlement induced by excavation – Empirical methods Peck’s (1969) Method 𝑁 ௕ = Stability number of soil = ఊு ೐ ௌ ೠ್ 𝑁 ௖௕ = Critical stability number for basal heave 43 44
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2/21/2023 23 FOS against basal heave Estimation of wall deflections Estimation of wall deflections Clough et al., 1989; Terzaghi, Peck and Mesri, 1996) 45 46
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2/21/2023 24 Excavation depth The deformation of the retaining wall in soft clay is generally greater than the sand The maximum deformation 𝛿 ௛௠ 𝛿 ௛௠ = 0.2% 𝑡𝑜 0.5% 𝐻 ௘ where 𝐻 ௘ = excavation depth Recommend for soft clay Recommend for sand Prediction of settlement troughs: Semi-empirical from FE analyses Clough et al., 1989 47 48
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2/21/2023 25 Settlements and angular distortion Damage caused by excavation- induced ground movements 49 50
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2/21/2023 26 THANK YOU! 51
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