Determine the site class for the subsoil profile shown in Fig. 11.21. Assume the site will be subjected to strong ground shaking during the design earthquake.

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Accepted Answer

The upper layer consists of very soft organic silt, very loose organic silty sand, and very soft peat. The standard penetration tests in this layer indicate uncorrected N values from 1 to 4. Given these low SPT values and the expected strong shaking from the earthquake, it is likely that the silt and silty sand layers will liquefy during the design earthquake. Hence, per Table 14.11, the answer is site class F.

TABLE 14.11 Site Class Definitions
Site class	Soil profile name	Average properties in top 100 ft[latex] {}^{a}[/latex]
Site class	Soil profile name	Standard Shear wave velocity, [latex]v _{s}[/latex], (ft/s)	Soil undrained penetration resistance, [latex]\left(N_{1}\right)_{60}[/latex]	shear strength, [latex]s_{u}[/latex], (psf)
A	Hard rock	[latex]v _{s}[/latex] > 5000	N/A	N/A
B	Rock	2500 < [latex]V _{S}[/latex] ≤5000	N/A	N/A
C	Very dense soil and soft rock	1200 < [latex]v _{s}[/latex] ≤ 2500	[latex]\left(N_{1}\right)_{60}[/latex] > 50	[latex]s_{u}[/latex] > 2000
D	Stiff soil	600 ≤ [latex]v _{s}[/latex] ≤ 1200	15 ≤ [latex]\left(N_{1}\right)_{60}[/latex] ≤ 50	1000 ≤ [latex]s_{u}[/latex] ≤ 2000
E	Soft soil	[latex]v _{s}[/latex] < 600	[latex]\left(N_{1}\right)_{60}[/latex] < 15	[latex]s_{u}[/latex] < 1000
E	Any profile with more than 10 feet of soil having the following characteristics: plasticity index PI > 20, moisture content w ≥ 40%, and undrained shear strength [latex]s_{u}[/latex] < 500 psf
F	Any profile containing soils having one or more of the following characteristics: 1. Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils. 2. Peats and/or highly organic clays (H > 10 feet of peat and/or highly organic clay where H = thickness of soil). 3. Very high plasticity clays (H > 25 feet with plasticity index PI > 75). 4. Very thick soft/medium stiff clays (H > 120 feet) with [latex]s_{u}[/latex] < 1000 psf.
Notes: a – see Chap. 20 of ASCE Standard 7-10 (2010) for further details. For SI: 1 foot = 304.8 mm, 1 square foot = 0.0929 m², 1 pound per square foot = 0.0479 kPa. N/A = Not applicable. ASCE Standard 7-10 recommends use of uncorrected N-values instead of [latex]\left(N_{1}\right)_{60}[/latex]. Sources: Table 20.3-1 and Chap. 20 of ASCE Standard 7-10 Minimum Design Loads for Buildings and Other Structures (2010).

Question 14.7: Determine the site class for the subsoil profile shown in Fi...

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