Report - Oregon State Library: State Employee Information Center ...

Table 3.5: Proposed Simplified Site Classification System
Site Site
General Description Site Characteristics 1,2
Class Condition
(A 0 ) A 0 Very Hard Rock V s (avg.) > 5,000 ft/sec in top 50 ft.
2,500 ft/sec ≤ V
Competent Rock with Little or No Soil and/or
s (rock) ≤ 5,000 ft/sec, and
A A 1
H
Weathered Rock Veneer.
soil + weathered rock < 40 ft with V s > 800 ft/sec
(in all but the top few feet 3 )
AB
B
C
AB 1
AB 2
D D 1
Soft, Fractured and/or Weathered Rock
Stiff, Very Shallow Soil over Rock and/or Weathered
Rock
For both AB 1 and AB 2 :
40 ft < H soil + weathered rock ≤ 150 ft, and
V s ≥ 800 ft/sec (in all but the top few feet 3 )
No "Soft Clay" (see Note 5), and
B 1 Deep, Primarily Cohesionless 4 Soils. (H soil < 300 ft)
H cohesive soil < 0.2 H cohesionless soil
B 2
Medium Depth, Stiff Cohesive Soils and/or Mix of H all soils ≤ 200 ft, and
Cohesionless with Stiff Cohesive Soils; No "Soft Clay" V s (cohesive soils) > 500 ft/sec (see Note 5.)
C 1
Medium Depth, Stiff Cohesive Soils and/or Mix of
Same as B
Cohesionless with Stiff Cohesive Soils; Thin Layer(s) of
2 above, except
0 ft < H
Soft Clay.
soft clay ≤ 10 ft (see Note 5.)
C 2
Deep, Stiff Cohesive Soils and/or Mix of Cohesionless H soil > 200 ft. and
with Stiff Cohesive Soils; No "Soft Clay"
V s (cohesive soils) > 500 ft/sec.
C 3 Very Deep, Primarily Cohesionless Soils Same as B 1 above except H soil > 300 ft
C 4
Soft, Cohesive Soil at Small to Moderate Levels of 10 ft ≤ H soft clay ≤ 100 ft, and
Shaking
A max rock < 0.25g
10 ft < H
Soft, Cohesive Soil at Medium to Strong Levels of
soft clay < 100 ft, and
0.25 g < A
Shaking.
max rock ≤ 0.45 g, or
[0.25 g < A max rock ≤ 0.55 g and M ≤ 7¼]
(E) 6 E 1 Very Deep, Soft Cohesive Soil. H soft clay > 100 ft (See Note 5.)
E 2 Soft, Cohesive Soil and Very Strong Shaking.
H soft clay > 10 ft and either: A max rock > 0.55 g,
or A max rock > 0.45 g and M > 7¼]
H
E 3 Very High Plasticity Clays.
clay > 30 ft with PI > 75% and V s < 800
ft/sec
F 1 Highly Organic and/or Peaty Soils. H > 20 ft of peat and/or highly organic soils.
(F) 7 F 2
Sites likely to suffer ground failure due to significant
liquefaction/ other potential modes of ground instability
Liquefaction and/or other types of ground
failure analysis required.
1. H = total (vertical) depth of soils of the type or types referred to.
2. V s = seismic shear wave velocity (ft/sec) at small (shear strain 10 -4 %).
3. If surface soils are cohesionless, V s may be less than 800 ft/sec in top 10 feet.
4. "Cohesionless soils" = soils with less than 30% "fines" by dry weight; “Cohesive soils” = soils with more than 30% “fines” by
dry weight, and 15% ≤ PI (fines) ≤ 90%. Soils with more than 30% fines, and PI (fines) < 15% are considered “silty” soils, and
these should be (conservatively) treated as “cohesive” soils for site classification purposes in this Table. (Evaluation of approximate
V s for these “silty” soils should be based either on penetration resistance or direct field V s measurement; see Note 8 below.)
5. "Soft Clay" is defined as cohesive soil with: (a) Fines content ≥ 30%, (b) PI (fines) ≥ 20%, and (c) V s ≤ 500 ft/sec.
6. Site-specific geotechnical investigations and dynamic site response analyses are strongly recommended for these conditions.
Variability of response characteristics within this Class (E) of sites tend to be more highly variable than for Classes A 0 through D,
and the very approximate response projections should be applied conservatively in the absence of (strongly recommended) sitespecific
studies.
7. Site-specific geotechnical investigation and dynamic site response analyses are required for these conditions. Potentially
significant ground failure must be mitigated, and/or it must be demonstrated that the proposed structure/facility can be engineered
to satisfactorily withstand such ground failure.
8. The following approaches are recommended for evaluation of V s :
(a) For all site conditions, direct (in situ) measurement of V s is recommended.
(b) In lieu of direct measurement, the following empirical approaches can be used:
(i) For sandy cohesionless soils: either SPT-based or CPT-based empirical correlations may be used.
(ii) For clayey soils: empirical correlations based on undrained shear strength and/or some combination of one or more of the
following can be used (void ratio, water content, plasticity index, etc.). Such correlations tend to be somewhat approximate,
and should be interpreted accordingly.
(iii) Silty soils of low plasticity (PI < 15%) should be treated as "largely cohesionless" soils here; SPT-based on CPT-based
empirical correlations may be used (ideally with some "fines" correction relative to "clean sand" correlations.) Silty soils of
medium to high plasticity should be treated more like "clayey" soils as in (iii) above.
(iv) "Other" soil types (e.g. gravelly soils, rockfill, peaty, and organic soils, etc.) require considerable judgment, and must be
evaluated on an individual basis; no simplified "guidance" can appropriately be offered herein.
52