Question 14.4: The table below summarizes the results of subsurface explora...
The table below summarizes the results of subsurface exploration. Determine the site class.
| Layer number | Depth below ground surface (m) | Description | Undrained Shear Strength | USCS Group Symbol & Plasticity index |
| 1 | 0–5 | Desiccated clay, highly over-consolidated | 150 kPa | CH PI = 60 |
| 2 | 5–15 | Slightly over-consolidated clay | 100 kPa | CH PI = 65 |
| 3 | 15–20 | Normally consolidated clay | 60 kPa | CH PI = 35 |
| 4 | 20–30 | Till and rock | N/A | N/A |
The procedure is as follows:
Step 1: Check site class F, which must have one or more of the following characteristics:
a. Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils.
b. Peats and/or highly organic clays (H > 10 feet of peat and/or highly organic clay where H = thickness of soil).
c. Very high plasticity clays (H > 25 feet with plasticity index PI > 75).
d. Very thick soft/medium stiff clays (H > 120 feet) with s_{u} < 1000 psf.
None of these requirements are met; hence, it is not a site class F.
Step 2: Check site class E, which must have more than 10 feet of soil having the following characteristics: plasticity index PI > 20, moisture content w ≥ 40%, and undrained shear strength s_{u} < 500 psf. All three layers exceed s_{u} of 500 psf (24 kPa), so this condition is not met.
Step 3: Site class will be C, D, or E, depending on the undrained shear strength. Using Eq. (14.1):
From Table 14.11 the answer is site class D—stiff soil profile.
| TABLE 14.11 Site Class Definitions | ||||
| Site class | Soil profile name | Average properties in top 100 ft {}^{a} | ||
| Standard Shear wave velocity, v _{s}, (ft/s) | Soil undrained penetration resistance, \left(N_{1}\right)_{60} | shear strength, s_{u}, (psf) | ||
| A | Hard rock | v _{s} > 5000 | N/A | N/A |
| B | Rock | 2500 < v _{s} ≤5000 | N/A | N/A |
| C | Very dense soil and soft rock | 1200 < v _{s} ≤ 2500 | \left(N_{1}\right)_{60} > 50 | s_{u} > 2000 |
| D | Stiff soil | 600 ≤ v _{s} ≤ 1200 | 15 ≤ \left(N_{1}\right)_{60} ≤ 50 | 1000 ≤ s_{u} ≤ 2000 |
| E | Soft soil | v _{s} < 600 | \left(N_{1}\right)_{60} < 15 | s_{u} < 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 s_{u} < 500 psf |
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| 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 s_{u} < 1000 psf. |
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| 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 \left(N_{1}\right)_{60}. Sources: Table 20.3-1 and Chap. 20 of ASCE Standard 7-10 Minimum Design Loads for Buildings and Other Structures (2010). |
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