ASTM Certified Soils Technician Examination Practice
Questions & [Verified Answers], Plus Explained Rationales
Question 1: In the Unified Soil Classification System (USCS), which primary soil
type is characterized by more than 50% of the material by weight being
retained on the No. 200 sieve and having a plasticity index greater than 7
when the liquid limit is between 30 and 50?
A. GW
B. SP
C. CL
D. ML
CORRECT ANSWER: C. CL
Rationale: Under USCS criteria, fine-grained soils with more than 50% passing the
No. 200 sieve are classified based on Atterberg limits. A clay of low plasticity (CL)
plots above the A-line with liquid limit between 30 and 50 and plasticity index
greater than 7, distinguishing it from silts (ML) and high-plasticity clays (CH).
Question 2: During a Standard Proctor compaction test (ASTM D698), what is
the correct hammer weight and drop height used to compact soil in a 4-inch
diameter mold?
A. 10 lb hammer dropped from 18 inches
B. 5.5 lb hammer dropped from 12 inches
C. 10 lb hammer dropped from 12 inches
D. 5.5 lb hammer dropped from 18 inches
CORRECT ANSWER: B. 5.5 lb hammer dropped from 12 inches
Rationale: The Standard Proctor test applies lower compactive effort using a 5.5-
pound hammer falling 12 inches for 25 blows per layer in three layers. This produces
approximately 12,400 ft-lb/ft³ of energy, in contrast to the Modified Proctor which
uses a 10-pound hammer dropped 18 inches.
Question 3: When performing a sieve analysis on a soil sample containing both
coarse and fine fractions, at which sieve size is the sample typically split so
that the finer portion can be wet-sieved if necessary?
A. No. 4 sieve
B. No. 10 sieve
C. No. 40 sieve
D. No. 200 sieve
CORRECT ANSWER: A. No. 4 sieve
Rationale: Standard practice separates the sample on the No. 4 sieve (4.75 mm). The
Page 1 of 65
,plus No. 4 material is dry-sieved, while the minus No. 4 material may be washed
over the No. 200 sieve to remove fines before dry sieving the retained portion,
ensuring accurate particle-size distribution.
Question 4: What is the primary purpose of determining the liquid limit of a
soil using the Casagrande cup method?
A. To measure the moisture content at which soil begins to exhibit plastic behavior
B. To determine the moisture content at which soil flows under a standardized
impact
C. To calculate the shrinkage limit of the soil specimen
D. To establish the optimum moisture content for compaction
CORRECT ANSWER: B. To determine the moisture content at which soil flows
under a standardized impact
Rationale: The liquid limit is defined as the water content at which a soil paste
placed in the Casagrande cup and grooved will close a ½-inch groove after 25
blows of the cup. It marks the boundary between liquid and plastic states and is a
fundamental Atterberg limit used in classification and engineering behavior
assessment.
Question 5: In the sand-cone method for determining field density (ASTM
D1556), which material is used to fill the excavated hole after the soil has been
removed?
A. Ottawa sand of known density and volume
B. Water of measured volume
C. Nuclear density gauge calibration sand
D. The same soil that was excavated
CORRECT ANSWER: A. Ottawa sand of known density and volume
Rationale: A calibrated Ottawa sand of known bulk density is poured from the sand
cone apparatus into the excavated hole. The volume of the hole is calculated from
the weight of sand required to fill it, allowing computation of the in-place wet
density of the soil when combined with the mass of the excavated material.
Question 6: Which of the following best describes the plasticity index (PI) of a
soil?
A. The difference between the liquid limit and the plastic limit
B. The average of the liquid limit and plastic limit
C. The moisture content at which the soil becomes non-plastic
D. The ratio of liquid limit to plastic limit
CORRECT ANSWER: A. The difference between the liquid limit and the plastic
limit
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,Rationale: Plasticity index is calculated as PI = LL – PL. It quantifies the range of
water contents over which the soil exhibits plastic behavior and is a key parameter
used in both USCS and AASHTO classification systems as well as for estimating
compressibility and strength characteristics.
Question 7: When conducting a nuclear density gauge test in the direct
transmission mode, what is the typical range of probe depths used for
measuring soil density?
A. 2 to 12 inches
B. 1 to 6 inches
C. 6 to 24 inches
D. Surface only
CORRECT ANSWER: A. 2 to 12 inches
Rationale: In direct transmission mode the source rod is inserted into a pre-driven
hole to depths commonly ranging from 2 to 12 inches. Gamma radiation travels
through the soil to the detector, providing a density measurement averaged over
the depth of insertion, which is preferred for accuracy over backscatter mode.
Question 8: According to ASTM D4318, what is the minimum number of points
required to establish a valid liquid limit flow curve?
A. Two points
B. Three points
C. Four points
D. Five points
CORRECT ANSWER: B. Three points
Rationale: A minimum of three determinations at different water contents,
preferably covering the range of 15 to 35 blows, is required to construct the flow
curve. The liquid limit is then interpolated as the water content corresponding to 25
blows on a semi-log plot of water content versus number of blows.
Question 9: In soil compaction control, what does the term “relative
compaction” express?
A. The ratio of field dry density to laboratory maximum dry density multiplied by
100
B. The ratio of field wet density to laboratory maximum wet density
C. The difference between optimum moisture content and field moisture content
D. The percentage of voids remaining after compaction
CORRECT ANSWER: A. The ratio of field dry density to laboratory maximum
dry density multiplied by 100
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, Rationale: Relative compaction is defined as (field dry density / laboratory maximum
dry density) × 100. It is the primary acceptance criterion specified in construction
specifications to ensure the compacted fill achieves the required density relative to
the Proctor maximum.
Question 10: Which soil classification system is most commonly specified for
highway and transportation projects in the United States?
A. Unified Soil Classification System (USCS)
B. AASHTO Soil Classification System
C. USDA Textural Classification
D. British Standard Classification
CORRECT ANSWER: B. AASHTO Soil Classification System
Rationale: The AASHTO system (M 145) is specifically developed for highway
subgrade evaluation and is widely referenced in state DOT specifications. It classifies
soils into groups A-1 through A-7 based on particle size and plasticity
characteristics, with group index values indicating relative quality as a subgrade
material.
Question 11: When performing the plastic limit test, a soil thread is rolled to a
diameter of approximately 3 mm. At what condition is the plastic limit
considered reached?
A. When the thread can be rolled without cracking
B. When the thread crumbles and can no longer be re-rolled
C. When the thread reaches a length of 100 mm
D. When the thread becomes shiny and sticky
CORRECT ANSWER: B. When the thread crumbles and can no longer be re-
rolled
Rationale: The plastic limit is the water content at which a soil thread approximately
3 mm in diameter begins to crumble and can no longer be reworked into a
continuous thread. This defines the lower boundary of the plastic state.
Question 12: What is the primary advantage of the Modified Proctor test over
the Standard Proctor test?
A. It uses less energy and is faster to perform
B. It better simulates the higher compaction energies of modern heavy equipment
C. It requires a smaller sample size
D. It is only applicable to coarse-grained soils
CORRECT ANSWER: B. It better simulates the higher compaction energies of
modern heavy equipment
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Questions & [Verified Answers], Plus Explained Rationales
Question 1: In the Unified Soil Classification System (USCS), which primary soil
type is characterized by more than 50% of the material by weight being
retained on the No. 200 sieve and having a plasticity index greater than 7
when the liquid limit is between 30 and 50?
A. GW
B. SP
C. CL
D. ML
CORRECT ANSWER: C. CL
Rationale: Under USCS criteria, fine-grained soils with more than 50% passing the
No. 200 sieve are classified based on Atterberg limits. A clay of low plasticity (CL)
plots above the A-line with liquid limit between 30 and 50 and plasticity index
greater than 7, distinguishing it from silts (ML) and high-plasticity clays (CH).
Question 2: During a Standard Proctor compaction test (ASTM D698), what is
the correct hammer weight and drop height used to compact soil in a 4-inch
diameter mold?
A. 10 lb hammer dropped from 18 inches
B. 5.5 lb hammer dropped from 12 inches
C. 10 lb hammer dropped from 12 inches
D. 5.5 lb hammer dropped from 18 inches
CORRECT ANSWER: B. 5.5 lb hammer dropped from 12 inches
Rationale: The Standard Proctor test applies lower compactive effort using a 5.5-
pound hammer falling 12 inches for 25 blows per layer in three layers. This produces
approximately 12,400 ft-lb/ft³ of energy, in contrast to the Modified Proctor which
uses a 10-pound hammer dropped 18 inches.
Question 3: When performing a sieve analysis on a soil sample containing both
coarse and fine fractions, at which sieve size is the sample typically split so
that the finer portion can be wet-sieved if necessary?
A. No. 4 sieve
B. No. 10 sieve
C. No. 40 sieve
D. No. 200 sieve
CORRECT ANSWER: A. No. 4 sieve
Rationale: Standard practice separates the sample on the No. 4 sieve (4.75 mm). The
Page 1 of 65
,plus No. 4 material is dry-sieved, while the minus No. 4 material may be washed
over the No. 200 sieve to remove fines before dry sieving the retained portion,
ensuring accurate particle-size distribution.
Question 4: What is the primary purpose of determining the liquid limit of a
soil using the Casagrande cup method?
A. To measure the moisture content at which soil begins to exhibit plastic behavior
B. To determine the moisture content at which soil flows under a standardized
impact
C. To calculate the shrinkage limit of the soil specimen
D. To establish the optimum moisture content for compaction
CORRECT ANSWER: B. To determine the moisture content at which soil flows
under a standardized impact
Rationale: The liquid limit is defined as the water content at which a soil paste
placed in the Casagrande cup and grooved will close a ½-inch groove after 25
blows of the cup. It marks the boundary between liquid and plastic states and is a
fundamental Atterberg limit used in classification and engineering behavior
assessment.
Question 5: In the sand-cone method for determining field density (ASTM
D1556), which material is used to fill the excavated hole after the soil has been
removed?
A. Ottawa sand of known density and volume
B. Water of measured volume
C. Nuclear density gauge calibration sand
D. The same soil that was excavated
CORRECT ANSWER: A. Ottawa sand of known density and volume
Rationale: A calibrated Ottawa sand of known bulk density is poured from the sand
cone apparatus into the excavated hole. The volume of the hole is calculated from
the weight of sand required to fill it, allowing computation of the in-place wet
density of the soil when combined with the mass of the excavated material.
Question 6: Which of the following best describes the plasticity index (PI) of a
soil?
A. The difference between the liquid limit and the plastic limit
B. The average of the liquid limit and plastic limit
C. The moisture content at which the soil becomes non-plastic
D. The ratio of liquid limit to plastic limit
CORRECT ANSWER: A. The difference between the liquid limit and the plastic
limit
Page 2 of 65
,Rationale: Plasticity index is calculated as PI = LL – PL. It quantifies the range of
water contents over which the soil exhibits plastic behavior and is a key parameter
used in both USCS and AASHTO classification systems as well as for estimating
compressibility and strength characteristics.
Question 7: When conducting a nuclear density gauge test in the direct
transmission mode, what is the typical range of probe depths used for
measuring soil density?
A. 2 to 12 inches
B. 1 to 6 inches
C. 6 to 24 inches
D. Surface only
CORRECT ANSWER: A. 2 to 12 inches
Rationale: In direct transmission mode the source rod is inserted into a pre-driven
hole to depths commonly ranging from 2 to 12 inches. Gamma radiation travels
through the soil to the detector, providing a density measurement averaged over
the depth of insertion, which is preferred for accuracy over backscatter mode.
Question 8: According to ASTM D4318, what is the minimum number of points
required to establish a valid liquid limit flow curve?
A. Two points
B. Three points
C. Four points
D. Five points
CORRECT ANSWER: B. Three points
Rationale: A minimum of three determinations at different water contents,
preferably covering the range of 15 to 35 blows, is required to construct the flow
curve. The liquid limit is then interpolated as the water content corresponding to 25
blows on a semi-log plot of water content versus number of blows.
Question 9: In soil compaction control, what does the term “relative
compaction” express?
A. The ratio of field dry density to laboratory maximum dry density multiplied by
100
B. The ratio of field wet density to laboratory maximum wet density
C. The difference between optimum moisture content and field moisture content
D. The percentage of voids remaining after compaction
CORRECT ANSWER: A. The ratio of field dry density to laboratory maximum
dry density multiplied by 100
Page 3 of 65
, Rationale: Relative compaction is defined as (field dry density / laboratory maximum
dry density) × 100. It is the primary acceptance criterion specified in construction
specifications to ensure the compacted fill achieves the required density relative to
the Proctor maximum.
Question 10: Which soil classification system is most commonly specified for
highway and transportation projects in the United States?
A. Unified Soil Classification System (USCS)
B. AASHTO Soil Classification System
C. USDA Textural Classification
D. British Standard Classification
CORRECT ANSWER: B. AASHTO Soil Classification System
Rationale: The AASHTO system (M 145) is specifically developed for highway
subgrade evaluation and is widely referenced in state DOT specifications. It classifies
soils into groups A-1 through A-7 based on particle size and plasticity
characteristics, with group index values indicating relative quality as a subgrade
material.
Question 11: When performing the plastic limit test, a soil thread is rolled to a
diameter of approximately 3 mm. At what condition is the plastic limit
considered reached?
A. When the thread can be rolled without cracking
B. When the thread crumbles and can no longer be re-rolled
C. When the thread reaches a length of 100 mm
D. When the thread becomes shiny and sticky
CORRECT ANSWER: B. When the thread crumbles and can no longer be re-
rolled
Rationale: The plastic limit is the water content at which a soil thread approximately
3 mm in diameter begins to crumble and can no longer be reworked into a
continuous thread. This defines the lower boundary of the plastic state.
Question 12: What is the primary advantage of the Modified Proctor test over
the Standard Proctor test?
A. It uses less energy and is faster to perform
B. It better simulates the higher compaction energies of modern heavy equipment
C. It requires a smaller sample size
D. It is only applicable to coarse-grained soils
CORRECT ANSWER: B. It better simulates the higher compaction energies of
modern heavy equipment
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