Chapter 5 The group index is numerical index property

CHAPTER 5

Soil Classification

SOLUTIONS TO QUESTIONS AND PRACTICE PROBLEMS

5.1 What kinds of soils are contained in loam as the term is used in the USDA classification

system?

Solution

5.2 The USDA classification system pays very little attention to gravel sized portion of a soil.

Why?

Solution

This classification system was developed by the USDA to provide indications of a soils

5.3 What is the purpose of the group index in the AASHTO classification system?

Solution

The group index is numerical index property for the used as an aid in the evaluation

5.4 You have been asked to determine if the soil at a given site is suitable for use as a

pavement sub-base material. What laboratory tests would you recommend performing on

the soil samples taken from the site?

Solution

As a minimum a sieve analysis is required. If more than 10% passes the #200 sieve then

5.5 Is it ever necessary to conduct a hydrometer analysis when classifying soils according to

the USCS, or is a sieve analysis always sufficient?

Solution

No, it is not necessary to perform hydrometer analysis to determine the USCS

5-2 Soil Classification Chap. 5

5.6 Explain the difference between a silty sand and a sandy silt in the Unified Soil

Classification System.

Solution

In the unified soil classification system, the noun represents the primary component, and

5.7 How would you determine if a soil was a silty sand or a sandy silt if:

(a) Laboratory test equipment was available?

(b) Laboratory test equipment was not available?

Solution

If laboratory test equipment is available, we would simply perform a sieve analysis and

Data for Problems 5.8-5.11

As part of a site investigation program for a residential development project you

retrieved four different soil samples. None of the soils appear to be organic. At your

laboratory you performed grain-size analyses and Atterberg limit tests on the samples.

The grain-size distribution data for the four soil samples are shown in Figure 5.2 as

curves D, E, F and G. From your Atterberg limit tests on the -40 portion of theses

samples, you measured the following data

Chap. 5 Soil Classification 5-3

Soil Sample

Identification Liquid

Limit Plastic

Limit

D 60 33

Figure 5.2 Particle size distribution curves.

5.8 Determine the USDA classification for soils D through G

Solution

Soil Percent Passing Percent of total Percent of -2.0 mm

2.0 0.05 0.002

Coarse Sand Silt Clay Sand Silt Clay

mm mm mm

D 100 76 24 0 24 52 24 24 52 24

5-4 Soil Classification Chap. 5

5.9 Determine the AASHTO group classification and group index for soils D through G

using the data given above.

Solution

Soil Percent Passing wL I

P AASHTO Classification

#10 #40 #200

D 100 100 84 60 27 A-7-5 (26)

5.10 Which of the soils D through G would make the best highway subgrade according to the

AASHTO classification system?

Solution

5.11 Determine the USCS group symbol and group name for soils D through G using the data

given above.

Solution

Soil Percent Type Cu C

C w

L I

P USCS Classification

Gravel Sand Fines

D 0 16 84 Fine – – 60 27 MH Elastic silt w/sand

Data for Problems 5.12-5.15

You have collected soil samples as part of the investigation for a dam project.

The grain-size distribution data for the four soil samples are shown in Figure 5.8 as

curves H, I, J and L. Soil H has a strong organic smell but contains no fibrous material.

The other soils are nonorganic. From your Atterberg limit tests on the -40 portion of

theses samples, you have measured the following data

Chap. 5 Soil Classification 5-5

Soil Sample

Identification Liquid

Limit Plastic

Limit

H 60 26

5-6 Soil Classification Chap. 5

5.12 Determine the USDA classification for soils H through L. The gravel portion of soil L

consists predominately of well rounded granitic material.

Solution

Soil Percent Passing Percent of total Percent of -2.0 mm

2.0 0.05 0.002 Coarse Sand Silt Clay Sand Silt Clay

mm mm mm

H 100 75 8 0 17 75 8 17 75 8

5.13 Determine the AASHTO group classification and group index for soils H through L using

the data given above.

Solution

Soil Percent Passing wL I

P AASHTO Classification

#10 #40 #200

5.14 Which of the soils H through L would make the best highway subgrade according to the

AASHTO classification system?

Solution

Chap. 5 Soil Classification 5-7

5.15 Determine the USCS group symbol and group name for soils H through L using the data

given above.

Solution

Soil Percent Type Cu C

C w

L I

P USCS Classification

5.16 You have performed a hydrometer test on a soil and measured the particle size

distribution shown in Figure 4.13 as Soil A. The soil has an organic smell but does not

contain any plant fibers. The Atterberg limits test on the -40 portion of the soil

measured a plastic limit of 21 and a liquid limit of 40. Determine its unified group

symbol and group name.

Solution

Soil Percent Type wL I

P USCS Classification

Gravel Sand Fines

5-8 Soil Classification Chap. 5

5.17 You have performed a sieve analysis on a soil sample and measured the particle size

distribution shown in Figure 4.13 as Soil C. The fine portion of the soil was clearly

nonplastic so you did not measure the Atterberg limits. Determine the unified group

symbol and group name for the soil.

Solution

5.18 A clean well-graded sand would rate very high in the AASHTO soil classification system,

and thus be considered a good soil for use as a highway subgrade. However, it would be

a very poor choice for certain other applications. Give one example of a situation where

this soil would not be a good choice.

Solution

Groundwater can easily flow through clean, well-graded sands, so these soils would be

5.19 A sanitary landfill project needs an import soil for use as an impervious cap over the

refuse. The following soils are available for this purpose:

Soil 1: GC

Soil 2: SC

Soil 3: ML

Soil 4: CL

The design engineer desires a soil with a high clay content because it will help

keep water out of the landfill. Which of the available soils appears to be most promising?

Why?

Solution

The CL soil would be the best choice because it has the lowest hydraulic conductivity.

5.20 Which of the following USCS soils would probably receive the best rating for use as a

highway subgrade per the AASHTO classification: CL, SM, ML, or SW? Why?

Solution

According to Table 5.2, SW would provide the best support for highways. This is

Chap. 5 Soil Classification 5-9

5.21 You are drilling and sampling at a site in an arid region. You encounter a fine grained

soil that is dry and hard. You are able to break chucks of the soil with your fingers but

only with difficulty. You add water to soil until it is in a plastic stated and then form a

ribbon out of the plastic soil by squeezing it between your thumb and forefinger. It is

easy to form a long thin ribbon out of the moistened soil. Is this soil a silt or a clay?

Explain.

Solution

The soil is a clay because of the high dry strength and the ability to form a long ribbon.

5.22 You are examining a soil sample retrieved during drilling. The sample has little or no

plasticity, the particles are very uniform and about the size of the period at the end of this

sentence. What is the most likely USCS classification for this soil?

Solution

No plasticity makes it safe to assume little or no fines. Since the period at the end to