CAPITAL HUMANO
A) El capital humano innato
6. Análisis espacial y temporal de las dotaciones de capital humano
6.1 Análisis espacial de las dotaciones de capital humano
Interactive Concept Learning and Self-Assessment
Access Chapter 3, Section 3.7 on the CD to interactively learn about the unified classification system and then access Problem Solver on the left sidebar to interactively classify soils. Take Quiz 3.7 to test your understanding of the concepts.
3.7 SOIL CLASSIFICATION SCHEMES 55
A classification scheme provides a method of identifying soils in a particular group that would likely exhibit similar characteristics. Soil classification is used to specify a certain soil type that is best suitable for a given application. There are several classification schemes available. Each was devised for a specific use. For example, the American Association of State Highway and Transportation Officials (AASHTO) developed one scheme that classifies soils according to their usefulness in roads and highways while the Unified Soil Classification System (USCS) was originally developed for use in airfield construction but was later modified for general use.
Unified Soil Classification System The USCS is neither too elaborate nor too simplistic. The USCS uses symbols for the particle size groups. These symbols and their representations are: G—gravel, S—sand, M—silt, C—clay. These are combined with other symbols expressing gradation characteristics—
W for well graded and P for poorly graded—and plasticity characteristics—H for high and L for low, and a symbol, O, indicating the presence of organic material. A typical classification of CL means a clay soil with low plasticity, while SP means a poorly graded sand. The flowcharts shown in Figs. 3.11a,b provide systematic means of classifying a soil according to the USCS. The American Society for Testing and Materials (ASTM) flowcharts linking the group symbols with group names are shown in Figs. 3.12a,b.
Experimental results from soils tested from different parts of the world were plotted on a graph of plasticity index (ordinate) versus liquid limit (abscissa). It was found that clays, silts, and organic soils lie in distinct regions of the graph. A line defined by the equation
PI¼ 0:73ðLL 20Þ% (3.27)
Are 50% of particles > 0.075 mm?
No
Proceed to flowchart of Fig. 2.15b Soil is coarse–grained
Soil is gravel First letter is G
Soil is sand First letter is S Is gravel fraction > sand fraction?
Is clay + silt fraction
> 12%? < 5%? Between 5% and 12%?
Is clay fraction > silt fraction?
Plastic clayey fines Second letter C
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3
If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 second letter is W, otherwise second letter is P
second letter is W, otherwise second letter is P Non–plastic silty fines
Second letter M
Is clay fraction > silt fraction?
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3 Classification is: GW–GC Otherwise,
Classification is: GP–GC If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 Classification is: SW–SC Otherwise,
Classification is: SP–SC
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3 Classification is: GW–GM Otherwise,
Classification is: GP–GM If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 Classification is: SW–SM Otherwise,
Classification is: SP–SM Yes
FIGURE 3.11a Unified Soil Classification flowchart for coarse-grained soils.
called the ‘‘A-line,’’ delineates the boundaries between clays (above the line) and silts and organic soils (below the line) as shown in Fig. 3.13. A second line, the U-line expressed as PI¼ 0:9ðLL 8Þ, defines the upper limit of the correlation between plasticity index and liquid limit. If the results of your soil tests fall above the U-line, you should be suspicious of your results and repeat your tests.
Are 50% of particles < 0.075 mm?
Soil is coarse-grained Proceed to flowchart of Fig. 2.15a Soil is fine–grained
Is___________________LL (oven-dried) < 0.75%
LL (not dried)
Yes No
Yes No
Yes No
Yes No
Is clay fraction > silt fraction?
Is liquid limit > 50%?
Soil is organic Soil is fat clay or elastic silt.
Plasticity is low Second letter is L Soil is lean clay or silt.
Soil is silt First letter is M
FIGURE 3.11b Unified Soil Classification flowchart for fine-grained soils.
GRAVEL
fines=CL or CH GP-GC Poorly graded gravel with clay Poorly graded gravel with clay and sand
<15% sand
>_15% sand
fines=ML or MH GP-GM Poorly graded gravel with silt Poorly graded gravel with silt and sand
<15% sand
>_15% sand
fines=CL or CH GW-GC Well-graded gravel with clay Well-graded gravel with clay and sand
<15% sand Well-graded gravel with silt and sand
<15% sand
>_15% sand Poorly graded gravel Poorly graded gravel with sand
<15% sand
>_15% sand Well-graded gravel
GROUP SYMBOL GROUP NAME
Note—Percentages are based on estimating amounts of fines, sand, and gravel to the nearest 5%.
Well-graded
fines=CL or CH SP-SC Poorly graded sand with clay Poorly graded sand with clay and gravel
<15% gravel
>_15% gravel
fines=ML or MH SP-SM Poorly graded sand with silt Poorly graded sand with silt and gravel
<15% gravel
>_15% gravel
fines=CL or CH SW-SC Well-graded sand with clay
Well-graded sand with clay and gravel
<15% gravel Well-graded sand with silt and gravel
<15% gravel
>_15% gravel Poorly graded sand Poorly graded sand with gravel
<15% gravel
>_15% gravel Well-graded sand Well-graded sand with gravel
Clayey sand Clayey sand with gravel
FIGURE 3.12a Flowchart for identifying coarse-grained soils (less than 50% fines). (Source: ASTM standards.)
3.7 SOIL CLASSIFICATION SCHEMES 57
AASHTO Soil Classification System The AASHTO soil classification is used to determine the suitability of soils for earthworks, embankments, and road bed materials (subgrade—natural material below a constructed pavement, subbase—a layer of soil above the subgrade, and base—a layer of soil above the subbase that offers high stability to distribute wheel loads). According to AASHTO, granular soils are soils in which 35% or less are finer than the No. 200 sieve (0.075 mm). Silt-clay soils are soils in which more than 35% are finer than the No. 200 sieve (Table 3.6).
The AASHTO system classifies soils into seven major groups, A-1 through A-7. The first three groups, A-1 through A-3, are granular (coarse-grained) soils while the last four groups, A-4 through A-7, are silt-clay (fine-grained) soils (Table 3.7).
CL
<30% plus No. 200 <15% plus No. 200
15-25% plus No. 200 % sand >_% gravel Lean clay with sandLean clay
≥30% plus No. 200 % sand >_% of gravel
Sandy lean clay with gravel
>_15% sand Gravelly lean clay with sand
<15% sand
ML
<30% plus No. 200 <15% plus No. 200
15-25% plus No. 200 % sand >_% gravel Silt with sandSilt
% sand <% gravel Silt with gravel
>_30% plus No. 200
>_15% sand Gravelly silt with sand
<15% sand
CH
<30% plus No. 200 <15% plus No. 200
15-25% plus No. 200 % sand >_% gravel Fat clay with sandFat clay
% sand <% gravel Fat clay with gravel
>_30% plus No. 200
Sandy fat clay with gravel
>_15% sand Gravelly fat clay with sand
<15% sand
MH
Note—Percentages are based on estimating amounts of fines, sand, and gravel to the nearest 5%.
<30% plus No. 200 <15% plus No. 200
15-25% plus No. 200 % sand >_% gravel Elastic silt with sandElastic silt
% sand <% gravel Elastic silt with gravel
>_30% plus No. 200
Sandy elastic silt with gravel
>_15% sand Gravelly elastic silt with sand
<15% sand
GROUP SYMBOL GROUP NAME
FIGURE 3.12b Flowchart for indentifying inorganic fine-grained soil (50% or more fines). (Source: ASTM standards.)
U–line
Silt and clay soils are located within the plasticity chart as shown in Fig. 3.14.
A group index (GI) value is appended in parentheses to the main group to provide a measure of quality of a soil as highway subgrade material. The group index is given as
Group Index: GI ¼ ðF 35Þ½0:2 þ 0:005ðLL 40Þ þ 0:01ðF 15ÞðPI 10Þ (3.28) where F is percent passing No. 200 sieve and the other terms have been defined before. The GI index is reported to the nearest whole number (2.4 reported as 2, 2.5 reported as 3) and if GI< 0, it is set to 0.
TABLE 3.6 Soil Types, Average Grain Size, and Description According to AASHTO
Gravel 75 mm to 2 mm (No.10 sieve)
Sand 2 mm (No.10 sieve) to 0.075 mm (No. 200 sieve) Silt & Clay <0.075 mm (No. 200 sieve)
Silty: PI<10 % Clayey: PI>11 %
TABLE 3.7A AASHTO Classification of Soils and Soil-Aggregate Mixtures General Classification
General rating as subgrade
AThe placing of A–3 before A–2 is necessary in the “left to right elimination process” and does not indicate superiority of A–3 over A–2.
BSee Table 3.7B for values.
Reprinted with permission of American Association of State Highway and Transportation Officials.
Excellent to Good Fair to Poor
Granular Materials
(35% or less passing No. 200) Silt-Clay Materials (More than 35% passing No. 200)
Group Classification A–1 A–2 A–4 A–5 A–6 A–7
Sieve analysis, % passing No. 10 (2.00 mm) No. 40 (425 µm) No. 200 (75 µm) Characteristics of fraction passing No. 40 (425 µm) Liquid limit
TABLE 3.7B AASHTO Classification of Soils and Soil-Aggregate Mixtures General Classification
General rating as subgrade Excellent to Good Fair to Poor
Granular Materials (35% or less passing No. 200)
Silt-Clay Materials (More than 35% passing No. 200) Group Classification
A–1–a A–1–b A–2–4 A–2–5 A–2–6 A–2–7
A–2
Silty or Clayey Gravel and Sand Silty Soils Clayey Soils
A–4 A–5 A–6 A–7–5, passing No. 40 (425 µm) Liquid limit
Reprinted with permission of American Association of State Highway and Transportation Officials.
3.7 SOIL CLASSIFICATION SCHEMES 59
GI for groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is zero. For groups A-2-6 and A-2-7, the partial group index equation
GI¼ 0:01ðF 15ÞðPI 10Þ (3.29)
is used. The higher the group index the lower the quality of the soil as a subgrade material. The GI should not exceed 20 for any of groups A-4 through A-7.