H max = V s / (4 * f c )

Transcription

1 CVEEN 5330 Homework Assignment 7 1. Use DEEPSOIL to perform a soil response analysis the 600 South at I-15 soil profile which is attached to this assignment. The design earthquake is a M w = 7.0 earthquake with r rup equal to 5 km. Develop a design target response spectrum for Site CLASS B (soft rock) using the Abrahamson and Silva Next Generation attenuation relation. To determine the Z 1.0 and Z 2.5 depths use Deep Profile 1 in the attached figures. Assume that these depths do not vary as a function of distance from the fault. Also assume that the average dip angle of the fault (i.e., rupture plane) is 45 degrees. Make a plot of the design target response spectrum for M = 7.0 and r rup = 5 km and label this as the design target spectrum. 2. DEEPSOIL has several possible time histories that can be used for analysis that are included with the program. The Kobe record provides a reasonable candidate for a M = 7.0 earthquake. Use DEEPSOIL to scale the pga (peak ground acceleration) of the Kobe record to match the pga value of the design target spectrum. Plot the scaled response spectrum of the Kobe record and overlay it on the design target spectrum determined in problem 1. The overlay plot of the two spectra will most likely need to be done in Excel, or some other plotting program. Using this overlay plot, comment regarding how well the scaled Kobe record matches the design target spectrum as a function of period. 3. Input the site-specifi soil profile for the 600 South site using the attached borehole information. a. Enter the V s and total unit weight data into EduShake. b. Assume the water-table to be at the ground surface. c. Calculate the maximum sublayer thickness using the following instructions: The accuracy of the high frequency response of EduShake is affected by thickness of the V s layers that you use in the model. Use the following formula to calculate the maximum thickness of any sublayer that should be used: H max = V s / (4 * f c ) where: H max is the maximum layer thickness, V s is the shear wave velocity for the layer and f c is the cutoff frequency. (The cutoff frequency is that frequency above which we do not need to accurately predict the surface response. For most geotechnical analysis, a cutoff frequency of about 20 Hertz is appropriate.) If a layer is too thick, using the above equation, then subdivide it into sublayers, each with the same material and V s properties. Make an EXCEL spreadsheet that does this calculation, so that I can check the sublayer thicknesses that you have used in the model. Include the sublayer number, thickness, unit weight and shear wave velocity. Also use the print screen command to make a plot of the soil profile as shown in step 2 of the DEEPSOIL user interface.

2 4. For each soil sublayer developed in 3, select the appropriate modulus and damping curve to represent the soil dynamic properties. It is recommended that you use Sand (Seed and Idriss) average value for sandy soils and the Vucetic and Dobry curves for clayey soils. Note that to use the Vucetic and Dobry curves, you must also provide the corresponding plastic index (PI) value in the input screen that is given on the soil log. Verify that the shear modulus and damping reduction curves you have selected are consistent for each sublayer. For nonplastic silts (i.e., PI = 0), you may use the Vucetic and Dobry curves and assign a PI equal to zero. Include this information in the EXCEL spreadsheet developed in 3. In this spreadsheet, use S&I to indicate sublayers that use the Seed and Idriss curves and V&D for sublayers that use the Vucetic and Dobry curves. For the latter curves include a column in the spreadsheet that includes the value of the PI assigned to each clay layer. 5. Assume that the bottom of soil profile is at a depth = 200 feet. Use STEP 2b in DEEPSOIL to assign the rock properties below this depth. you must assign an elastic rock halfspace. You can create this by doing the following a. Vs = 1200 ft/s b. Unit weight = 140 pcf c. Damping ratio = 2% (Note that assigning the infinite half space at a depth of 200 feet and treating this layer as a rock layer is not actually correct for this site. The actual depth to bedrock may be several hundreds of feet in the middle of the Salt Lake Valley. We have ended the soil profile at a depth of 200 feet so that you can complete this assignment and not spend several hours inputting shear wave velocity values to great depths.) 6. From the DEEP analysis, plot the calculated response spectrum for layer 1 (i.e., surface soil layer) against the input response spectrum (i.e., input response spectrum at the base of the model). Comment about how the site-specific soil column has changed the input rock motion as a function of period using the response spectrum plots. Make sure you comment how the spectral acceleration has been changed for both the long and short period motion.

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5 TOP PORTION OF DEEP PROFILE: m Vs (m/s) End, Wong et al. 2002, published (152m) depth (m) Unconsolidated Semi-consolidated SLC Airport East, Wong & Silva (1993) Lacustrine-alluvial silt and clay (Northern CA Bay Mud), Wong et al. (2002, published) Interpreted cross section, distance = 15.5km, Hill et al. (1990) Generic U.S. Rock, Boore & Joyner (1997) Deep Profile I, this study Deep Profile II, Wong et al. (2002, unpublished)

6 1000 BOTTOM PORTION OF DEEP PROFILE: m Vs (m/s) Consolidated 2500 Bedrock End, Wong and Silva 1993 (>2600m) depth (m) SLC Airport East, Wong & Silva (1993) Lacustrine-alluvial silt and clay (Northern CA Bay Mud), Wong et al. (2002, published) Interpreted cross section, distance = 15.5km, Hill et al. (1990) Generic U.S. Rock, Boore & Joyner (1997) Deep Profile I, this study Deep Profile II, Wong et al. (2002, unpublished) Matchpoint (6000m) Infinite Half-Space