The NSR-10 classifies the soil of each lot into six profiles (site classes), from A (competent rock) to F (soils that require a site-specific study), based on the top 30 meters of ground (A.2.4). The profile sets the coefficients Fa and Fv that amplify the design earthquake: in Barranquilla, the plateau of the spectrum goes from 0.20 g on type A rock to 0.63 g on a type E soft soil.
How is the soil classified?
In three steps (A.2.4.5). First, profile F is ruled out, since it requires a site-specific seismic study. Next, look for layers of soft clay, with an undrained shear strength below 50 kPa, a water content above 40 % and a plasticity index above 20: if they add up to 3 m or more, the profile is E. Otherwise, the soil is classified by the shear wave velocity vs, which prevails when it is measured; or by the blow count N; or by the undrained shear strength su of the cohesive layers, taking the softer profile.
| Profile | Description | Criterion, in the top 30 m |
|---|---|---|
| A | Competent rock | vs ≥ 1,500 m/s |
| B | Medium-stiffness rock | 760 ≤ vs < 1,500 m/s |
| C | Very dense soil or soft rock | 360 ≤ vs < 760 m/s, or N ≥ 50, or su ≥ 100 kPa |
| D | Stiff soil | 180 ≤ vs < 360 m/s, or 15 ≤ N < 50, or 50 ≤ su < 100 kPa |
| E | Soft soil | vs < 180 m/s, or more than 3 m of soft clay (PI > 20, w ≥ 40 %, su < 50 kPa) |
| F | Require site evaluation | Liquefiable or collapsible soils, peats and organic clays, very high plasticity clays, more than 36 m of soft to medium clay |
How much does the design earthquake change?
| Profile | Fa | Fv | Plateau Sa (g) | TC (s) |
|---|---|---|---|---|
| A | 0.8 | 0.8 | 0.20 | 0.48 |
| B | 1.0 | 1.0 | 0.25 | 0.48 |
| C | 1.2 | 1.7 | 0.30 | 0.68 |
| D | 1.6 | 2.4 | 0.40 | 0.72 |
| E | 2.5 | 3.5 | 0.63 | 0.67 |
With a type D profile in Barranquilla, a 15 m tall concrete moment-frame building has an approximate period of 0.54 s (A.4.2.2) and falls on the plateau of the spectrum. Work out your case with the NSR-10 design spectrum.
What about profile F?
The code gives no coefficients for it. It groups four cases: soils prone to failure or collapse under earthquake loading, such as liquefiable soils; peats and organic clays more than 3 m thick; very high plasticity clays, more than 7.5 m thick with a plasticity index above 75; and more than 36 m of medium to soft clays (table A.2.4-1). In all of them, the geotechnical engineer must carry out a site-specific seismic study (A.2.10).
What should the geotechnical study include?
The geotechnical study submitted for the permit must define the local site effects of section A.2.4, that is, the soil profile, or the site-specific study when one is needed (A.1.5.4). How many borings, and how deep, we explain in geotechnical study.
And seismic microzonation?
Where the municipality has adopted a seismic microzonation study, its regulations replace sections A.2.4 and A.2.6 of the NSR-10 and are mandatory (A.2.9.1). Before using the table, check whether your city has one.
Source: NSR-10, Title A, sections A.2.4, A.2.6, A.2.9 and A.2.10.
Frequently asked questions
What is a type D soil profile?
A stiff soil: shear wave velocity between 180 and 360 m/s, blow count N between 15 and 50, or undrained shear strength between 50 and 100 kPa in the top 30 m (table A.2.4-1).
Who defines the soil profile?
The geotechnical engineer, in the geotechnical study submitted for the permit (A.1.5.4 and A.2.4). For profile F they must also carry out a site-specific seismic study (A.2.10).
