ENCE 4610 Foundation Analysis and Design

Transcription

1 This image cannot currently be displayed. ENCE 4610 Foundation Analysis and Design Shallow Foundations Total and Differential Settlement Schmertmann s Method

2 This image cannot currently be displayed. Strength Requirements Geotechnical Strength Requirements Design to prevent failure by soil shear failure Geotechnical strength for shear failure is referred to as the bearing capacity of the soil Analysis usually performed by ASD analysis; LRFD becoming more common Structural Strength Requirements Design to avoid structural failure of foundation components Similar to other structural analyses Most common strength requirement: avoid bearing capacity failure

3 This image cannot currently be displayed. This image cannot currently be displayed. Serviceability Considerations Most common issue in serviceability: settlement

4 Types of Settlement Definitions of Settlement o Absolute settlement, usually associated with uniform/total settlement o Angular distortion settlement, usually associated with differential settlement (ratio of settlement to distance between foundations and structures)

5 Factors to Determine Acceptable Settlement Connections with existing structures Utility Lines Total settlement of permanent facilities can harm or sever connections to outside utilities such as water, natural gas, and sewer lines. Water and sewer lines may leak contributing to localised wetting of the soil profile and aggravating differential displacement. Leaking gas from breaks caused by settlement can lead to explosions. Surface Drainage Access Aesthetics Material of structure (steel, concrete) Usage Requirements Settlement of bridges/overpasses vs. settlement of embankments, the bump in the bridge

6 This image cannot currently be displayed. This image cannot currently be displayed. This image cannot currently be displayed. Typical Values of Acceptable Settlement

7 Example of Settlement Calculations Given Steel framed office building, 20' column spacing Supported on spread footings founded on clayey soil Find Allowable total settlement Allowable differential settlement Solution Typical total settlement specification = 4 (Frames structure) Use δ = 1/500 (Steel and concrete frame); δ du = (1/500)(20') = 0.04' = 0.5

8 Schmertmann s Method: Procedure and Example Find o Settlement in inches at the end of construction o Settlement in inches one (1) year after the end of construction Given o o o 6 x 24 footing, shown below 2 ksf applied bearing pressure Soil Profile and foundation depth as shown below Note that N1 60 are corrected for both overburden and hammer efficiency

9 Schmertmann s Method Step 1: Draw the Strain Influence Diagram, Compute I zb at Surface Strain influence diagrams for square and continuous foundations are shown at the right Compute L f /B f (Equivalent Footing) o Uniform loading, so L f /B f = L/B = 24/6 = 4 o For L/B = 1, I z z= 0 = 0.1 o For L/B = 10, I z z= 0 = 0.2 o By linear interpolation, for L/B = 4, I z z= 0 = 0.133

10 Schmertmann s Method Step 2: Draw the Strain Influence Diagram, Compute Maximum Depth of Influence Compute D I o Uniform loading, so L f /B f = L/B = 24/6 = 4 o For L/B = 1, D I = 2B f o For L/B = 10, D I = 4B f o By linear interpolation, for L/B = 4, D I = 8B f /3 o For B = 6, D I = (8)(6)/(3) = 16

11 Schmertmann s Method Step 3: Draw the Strain Influence Diagram, Determine Depth of Peak Strain Influence Factor Compute D IP o Uniform loading, so L f /B f = L/B = 24/6 = 4 o For L/B = 1, D IP = B f /2 o For L/B = 10, D IP = B f o By linear interpolation, for L/B = 4, D IP = 2B f /3 o For B = 6, D IP = (2)(6)/(3) = 4 o Alternate: D IP = D I /4

12 Schmertmann s Method Step 4: Draw the Strain Influence Diagram, Determine Peak Strain Influence Factor Compute I ZP o D IP = (2)(6)/(3) = 4 o This is 4 below the foundation; since the foundation is 3 below the surface, the depth of the peak strain influence factor is = 7 below the soil surface (important for effective stress computations) o I ZP = (Δp/p op ) 0.5 o Increase in stress at depth of footing Δp = 2 ksf (3 )(0.115 kcf) = ksf o p op = (3)(0.115) + (3)(0.125) + (1)(120) = ksf o I ZP = (1.665/0.840) 0.5 = 0.64

13 Schmertman s Method Step 5: Draw the Strain Influence Diagram Layer Boundaries are SOLID Helpful Guidelines: o o o o The depth of the peak value of the strain influence is fixed. To aid in the computation, develop the layering such that one of the layer boundaries occurs at this depth even though it requires that an actual soil layer be sub-divided. Limit the top layer as well as the layer immediately below the peak value of influence factor, I zp, to 2/3B f or less to adequately represent the variation of the influence factor within D IP. Limit maximum layer thickness to 10 ft (3 m) or less. Match the layer boundary with the subsurface profile layering. Layer Mid-Points are DASHED

14 Schmertmann s Method Step 6: Determine the Values of Elastic Modulus Estimate from SPT Value o Layer 1: Sandy Silt, E s = 4(N1 60 ) = (4)(25) = 100 tsf = 200 ksf o Layer 2: Coarse Sand, E s = 10(N1 60 ) = (10)(30) = 300 tsf = 600 ksf o Layer 3: Coarse Sand, E s = 10(N1 60 ) = (10)(30) = 300 tsf = 600 ksf o Layer 4: Sandy Gravel, E s = 12(N1 60 ) = (12)(68) = 816 tsf = 1632 ksf Values computed in this fashion must be corrected by a factor X

15 Schmertmann s Method Step 6: Determine the Values of Elastic Modulus Modulus of Elasticity Correction Factor X o X = 1.25 for L f /B f = 1 o X = 1.75 for L f /B f >10 o By linear interpolation, for L f /B f = 4, X = 1.42 Corrected Values of E s o 1: (100)(1.42) = 142 tsf o 2: (300)(1.42) = 426 tsf o 3: (300)(1.42) = 426 tsf o 4: (816)(1.42) = 1159 tsf

16 Schmertmann s Method Step 7: Compute Basic Total Settlement Basic Formula for Schmertmann s Method o We first concentrate on computing the summation, which will represent the settlement divided by the applied bearing pressure S i ΔH = C C Δp 1 2 i = H c n i= 1 I z XE s ΔH i

17 Schmertmann s Method Step 8: Determine Embedment and Creep Factors Embedment Factor Creep Factor C 1 C 1 po = Δp 3' 115 pcf = pcf = t years C2 = log For end of construction, C 2 At end of = 1 one year, C 2 = log = 1.2

18 Schmertmann s Method Step 9: Determine Settlement at End of Construction Step 10: Determine Settlement at End of One Year End of Construction S i End of One Year S i S i n = C1C 2Δp i= 1 = (0.896)(1)(1.655 ksf )( in/tsf )(1 tsf/ 2 ksf ) S i S i = 0.130" n = C1C 2Δp i= 1 = 0.156" ΔH = (0.896)(1.2)(1.655 ksf )( in/tsf )(1 tsf/ 2 ksf ) S i i ΔH i

19 Chart for Interpolated Values

20 Settlement vs. Bearing Capacity (Shear Failure)

21 Bearing Capacity Charts Example

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23 Comments on Bearing Capacity Chart Example

24 Lightly Loaded Footings and Presumptive Bearing Pressures The use of presumptive bearing capacities for shallow foundations bearing in soils is not recommended for final design of shallow foundations for transportation structures, especially bridges. Guesses about the geology and nature of a site and the application of a presumptive value from generalizations in codes or in the technical literature are not a substitute for an adequate site-specific subsurface investigation and laboratory testing program. As an exception, presumptive bearing values are sometimes used for the preliminary evaluation of shallow foundation feasibility and estimation of footing dimensions for preliminary constructability or cost evaluations. Lightly loaded footings are those which meet the following criteria: o o Square, circular, or rectangular footings subjected to vertical loads less than 200 kn (45 kips) Continuous footings subjected to vertical loads less than 60 kn/m (4 kips/ft) Include typical one and two-story wood frame buildings and other similar structures A conservative approach; normally easier in these cases to design a conservative structure than to perform the analysis

25 Presumptive Bearing Pressures Sands Allowable Bearing Pressure Tons Per sq ft Type of Bearing Material Consistency In Place Range Recommended Value for Use Well graded mixture of fine and coarsegrained soil: glacial till, hardpan, boulder clayvery compact 8 to (GW-GC, GC, SC) Gravel, gravel-sand mixtures, boulder gravel mixtures (SW, SP, SW, SP) Coarse to medium sand, sand with little gravel (SW, SP) Fine to medium sand, silty or clayey medium to coarse sand (SW, SM, SC) Very compact 6 to Medium to compact 4 to Loose 2 to Very compact 4 to Medium to compact 2 to Loose 1 to Very compact 3 to Medium to compact 2 to Loose 1 to 2 1.5

26 Presumptive Bearing Pressures Clays and Silts Type of Bearing Material Homogeneous inorganic clay, sandy or silty clay (CL, CH) Inorganic silt, sandy or clayey silt, varved silt-clay-fine Sand Consistency In Place Allowable Bearing Pressure Tons Per sq ft Range Recommended Value for Use Very stiff to hard 3 to Medium to stiff 1 to Soft.5 to Very stiff to hard 2 to Medium to stiff 1 to Soft.5 to 1 0.5

27 Presumptive Bearing Pressures Notes o o o o Compacted fill, placed with control of moisture, density, and lift thickness, has allowable bearing pressure of equivalent natural soil. Allowable bearing pressure on compressible fine grained soils is generally limited by considerations of overall settlement of structure. Allowable bearing pressure on organic soils or uncompacted fills is determined by investigation of individual case. If tabulated recommended value for rock exceeds unconfined compressive strength of intact specimen, allowable pressures equals unconfined compressive strength.

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