Materials

6% of exam

Soil classification and boring-log interpretation, soil properties and phase relationships, concrete, piping materials, and test-method conformance.

5 concepts

Exam areas

A. Soil classification and boring log interpretation

Soil Classification and Boring Logs (USCS)

Run a soil through the USCS decision tree from sieve gradation and Atterberg limits, assign group symbol and name, and read SPT, water table, and strata off a boring log.

Almost every geotechnical problem on the PE Civil WRE exam — bearing capacity, settlement, seepage, slope stability, pipe bedding — starts by deciding what the soil ·is·. The Unified Soil Classification System (USCS) is that decision: a two-letter group symbol that compresses a sieve analysis and two Atterberg limits into a label every downstream correlation depends on. Points are lost here not in hard math but in mis-reading the decision tree — calling a soil a clay because it 'looks' fine when the No. 200 sieve says it is coarse, or putting a point on the wrong side of the

A

-line. The classification chart, plasticity chart, and group index live in the NCEES PE Civil Reference Handbook §3.7 Soil Classification and Boring Log Interpretation; this concept builds the fluency to use them under time pressure.

The No. 200 split: coarse vs fine

The very first branch of the USCS is a single number: the percent passing the No. 200 (0.075 mm) sieve, often called the fines content

F

. If more than 50% is retained on the No. 200, the soil is coarse-grained (a gravel or a sand); if 50% or more passes, it is fine-grained (a silt or a clay). For a coarse soil you then split the coarse fraction on the No. 4 (4.75 mm) sieve — more than half the coarse fraction retained on No. 4 makes it a gravel (G), otherwise a sand (S). Get this top branch wrong and every later step is wrong, so always tabulate cumulative percent passing before classifying.

Gradation: uniformity and curvature coefficients

For a clean coarse-grained soil (less than 5% fines), the second letter is decided by gradation. You read three particle diameters off the grain-size curve —

D_{10}

D_{30}

D_{60}

(the sizes at which 10%, 30%, 60% are finer) — and form the uniformity coefficient

C_u

and the coefficient of curvature

C_c

. A gravel is well graded (GW) when

C_u \ge 4

and

1 \le C_c \le 3

; a sand is well graded (SW) when

C_u \ge 6

and

1 \le C_c \le 3

. Fail either test and it is poorly graded (GP or SP). Well graded means a wide, smooth spread of sizes that packs densely; uniform (poorly graded) means one size dominates.

C_u = \dfrac{D_{60}}{D_{10}}, \qquad C_c = \dfrac{(D_{30})^2}{D_{10}\,D_{60}}

The plasticity chart and the A-line

Fine-grained soils — and the fine fraction of dirty coarse soils — are classified on the Casagrande plasticity chart, plotting plasticity index

PI = LL - PL

against liquid limit

LL

. The

A

-line separates clays (on or above) from silts and organics (below); the

U

-line is the empirical upper bound no real soil exceeds. Low liquid limit (

LL < 50

) gives the lean clay CL or the silt ML; high liquid limit (

LL \ge 50

) gives fat clay CH or elastic silt MH. The small hatched wedge where

4 \le PI \le 7

and the point sits on or above the

A

-line is the dual CL-ML, silty clay.

\text{A-line:}\;\; PI = 0.73\,(LL - 20), \qquad \text{U-line:}\;\; PI = 0.9\,(LL - 8)

Group symbol and the decision tree

Coarse soils with 5-12% fines take a dual symbol that pairs the gradation with the fines plasticity (for example SW-SM or SP-SC). Coarse soils with more than 12% fines drop the gradation test entirely and become GM/SM (silty, fines plot as ML/MH), GC/SC (clayey, fines plot as CL/CH), or the dual GC-GM / SC-SM when the fines themselves plot as CL-ML. The full group ·name· then layers on modifiers — 'with sand,' 'with gravel,' 'sandy,' 'gravelly' — driven by the secondary coarse content, but the exam almost always asks for the two-letter symbol. Work the tree in fixed order: No. 200 first, then No. 4, then either gradation (clean) or plasticity (dirty).

AASHTO and the group index

The AASHTO system (M 145) rates soils as highway subgrade, A-1 through A-7, and is graded by a group index

GI

written in parentheses, like A-7-6(13). Higher

GI

means a poorer subgrade; granular A-1/A-3 soils carry

GI = 0

. AASHTO separates granular from silt-clay materials at 35% passing the No. 200 (USCS splits coarse from fine at 50%), draws its silty/clayey line at

PI = 10

on the fraction passing the No. 40 — the same fraction both systems run the Atterberg limits on — and reports the No. 10 and No. 40 percentages to place the A-1/A-3 granular groups. The two systems disagree on purpose: USCS optimizes for engineering behavior (an A-line clay), AASHTO for pavement support, so a USCS 'CL' can map to several AASHTO groups.

GI = (F-35)\big[\,0.2 + 0.005(LL-40)\,\big] + 0.01\,(F-15)(PI-10)

Reading a boring log: SPT N, water table, strata

A boring log is the field counterpart to the lab classification: it records the strata, the depth to the water table (often two readings — at completion and after 24 h stabilization), and the Standard Penetration Test blow count

N

(blows to drive the split spoon the second and third 6 in increments). Raw

N

must be energy-corrected to

N_{60}

before any correlation, because hammers deliver different fractions of the theoretical 60% energy. The NCEES handbook (§3.8.1.1 Hammer Efficiency) gives only the hammer-efficiency form

N_{60} = (E_{eff}/60)\,N

; the fuller Skempton (1986) correction multiplies in borehole, sampler, and rod factors (

C_b

C_s

C_r

) but those are NOT in the handbook. From

N_{60}

you read relative density of sands or consistency of clays — but SPT in clays is unreliable, so never set a design clay strength from

N

alone.

N_{60} = \dfrac{E_{eff}}{60}\,N \qquad\text{(handbook 3.8.1.1; } E_{eff}\text{ in \%)}

Exam strategy

Build a tiny table of cumulative percent passing the moment a gradation problem appears, then walk the tree top-down: No. 200 (coarse vs fine), No. 4 (gravel vs sand), then gradation or plasticity. Compute

PI = LL - PL

and test it against

0.73(LL-20)

— at or above is a clay (C), below is a silt (M). For 'classify and give the group index,' remember

GI

is an AASHTO quantity, round it to the nearest integer, and never let it go negative (report

0

). On boring-log questions, correct

N

N_{60}

first, use sands-only for density, and read the ·stabilized· water table for effective-stress work.

Key equations

Uniformity coefficient

C_u = \dfrac{D_{60}}{D_{10}}

Spread of grain sizes. $D_{60}$ , $D_{10}$ = sieve openings (mm) passing 60% and 10%. $C_u \ge 4$ (gravel) or $\ge 6$ (sand) is needed for 'well graded.'

Coefficient of curvature

C_c = \dfrac{(D_{30})^2}{D_{10}\,D_{60}}

Shape of the gradation curve; well graded requires $1 \le C_c \le 3$ together with the $C_u$ test.

Plasticity index

PI = LL - PL

Range of water content over which soil is plastic (%). $LL$ = liquid limit, $PL$ = plastic limit.

A-line

PI = 0.73\,(LL - 20)

Casagrande chart divider. On or above = clay (CL/CH); below = silt or organic (ML/MH/OL/OH). The A-line runs horizontal at $PI=4$ for low $LL$ (below $\approx 25.5$ , where $0.73(LL-20)=4$ ), then rises along $0.73(LL-20)$ .

U-line (upper bound)

PI = 0.9\,(LL - 8)

Empirical limit no natural soil exceeds; a plotted point above it signals a test or transcription error.

Liquidity index

LI = \dfrac{w - PL}{PI}

Where in-situ water content $w$ sits between limits. $LI \le 0$ brittle/stiff, $0<LI<1$ plastic, $LI \ge 1$ behaves as a viscous liquid (sensitive).

AASHTO group index

GI = (F-35)\big[0.2 + 0.005(LL-40)\big] + 0.01(F-15)(PI-10)

$F$ = % passing No. 200. The handbook prints the bare formula, but AASHTO M145 caps the partial products: $(F-35)\le 40$ ( $F\le75$ ), $(LL-40)\le 20$ in the first term, and $(F-15)\le 40$ ( $F\le55$ ), $(PI-10)\le 20$ in the second. Report as integer in parentheses; if negative, use 0. Higher = poorer subgrade.

Partial group index (A-2-6, A-2-7)

GI = 0.01\,(F-15)(PI-10)

Only the plasticity term is used for the A-2-6 and A-2-7 subgroups.

SPT energy correction (handbook form)

N_{60} = \dfrac{E_{eff}}{60}\,N = \dfrac{E_m}{0.60}\,N

Handbook §3.8.1.1: $E_{eff}$ = measured hammer efficiency (%) or $E_m$ as a fraction (~0.45-0.55 donut, ~0.6 safety, ~0.8-0.9 automatic/trip). Correct before any correlation. The Skempton (1986) refinement $N_{60} = (E_m C_b C_s C_r/0.60)N$ adds borehole $C_b$ , sampler $C_s$ , and rod $C_r$ factors but is NOT in the NCEES handbook.

Relative density (granular state)

D_r = \dfrac{e_{max}-e}{e_{max}-e_{min}}\times 100\%

State of a cohesionless soil between its loosest ( $e_{max}$ ) and densest ( $e_{min}$ ) packing; the density class read from $N_{60}$ on a boring log corresponds to a $D_r$ range (e.g. medium dense $\approx$ 35-65%).

Worked examples

Classify a clean coarse-grained soil

Problem. A sieve analysis gives 3% passing the No. 200 sieve; of the coarse fraction, 60% passes the No. 4. The grain-size curve gives $D_{10}=0.17$ mm, $D_{30}=0.60$ mm, $D_{60}=1.30$ mm. Give the USCS group symbol.

Solution. Step 1 — No. 200 split: only 3% passes (

< 50\%

), so the soil is coarse-grained, and with

< 5\%

fines it is 'clean.' Step 2 — No. 4 split: 60% of the coarse fraction passes No. 4 (

> 50\%

), so it is a sand (S). Step 3 — gradation:

C_u = 1.30/0.17 = 7.65

and

C_c = (0.60)^2/(0.17\times 1.30) = 0.36/0.221 = 1.63

. Sand criterion:

C_u \ge 6

✓ and

1 \le C_c \le 3

✓, so it is well graded. Group symbol:

\textbf{SW}

(well-graded sand). Sanity check: a wide size range (

D_{60}/D_{10}\approx 8

) with a smooth curvature is the textbook signature of a dense, well-graded sand.

Classify a fine-grained soil — USCS and AASHTO

Problem. A soil has 68% passing the No. 200 sieve, $LL = 42$ , $PL = 20$ . Give the USCS symbol, the AASHTO group, and the group index.

Solution. Fines:

68\% \ge 50\%

, so the soil is fine-grained.

PI = LL - PL = 42 - 20 = 22

. USCS:

LL = 42 < 50

(low plasticity). A-line value

= 0.73(42-20) = 0.73(22) = 16.1

. Since

PI = 22 > 16.1

, the point plots above the A-line, so it is a clay:

\textbf{CL}

(lean clay). AASHTO: with

> 35\%

fines it is a silt-clay (A-4 to A-7).

PI = 22 > 11

and

LL = 42 > 41

, and

PI > LL - 30

(

22 > 12

), so it is

\textbf{A-7-6}

. Group index:

GI = (F-35)[0.2+0.005(LL-40)] + 0.01(F-15)(PI-10)

. First term

= (68-35)[0.2+0.005(2)] = 33(0.21) = 6.93

. Second term: AASHTO M145 caps the fines factor at

(F-15)\le 40

, and here

F=68>55

so use

40

0.01(40)(12) = 4.8

. Thus

GI = 6.93 + 4.8 = 11.7 \Rightarrow \textbf{A-7-6(12)}

. (Plugging the uncapped

F-15=53

into the bare handbook formula would give

6.93+6.36=13.3\to13

; applying the M145 cap is correct.) Sanity check: an A-7-6(12) lean clay is a poor, frost-and-moisture-sensitive subgrade — consistent with a plastic CL.

Energy-correct an SPT blow count

Problem. A boring log lists $N = 22$ in a clean sand. The rig uses an automatic hammer with measured energy ratio $E_m = 0.75$ . Find $N_{60}$ by the handbook method and the relative-density class; then note how the fuller Skempton (1986) correction (borehole $C_b = 1.05$ , sampler $C_s = 1.0$ , rod $C_r = 0.85$ ) would adjust it.

Solution. Handbook §3.8.1.1 hammer-efficiency form:

N_{60} = \dfrac{E_m}{0.60}\,N = \dfrac{0.75}{0.60}\times 22 = 1.25\times 22 = 27.5

N_{60} \approx \textbf{27.5}

, i.e.

N_{60}

in the 10-30 band

\Rightarrow

medium dense sand. Skempton refinement (NOT in the handbook):

N_{60} = \dfrac{E_m C_b C_s C_r}{0.60}\,N = \dfrac{0.75\times1.05\times1.0\times0.85}{0.60}\times 22 = \dfrac{0.6694}{0.60}\times 22 = 1.116\times 22 = 24.5

— still medium dense. Sanity check: an automatic hammer delivers ~75% energy (

>60\%

), so the correction factor exceeds 1 and

N_{60}

rises above the raw

N=22

; the borehole/sampler/rod factors trim it slightly. Both forms land in the medium-dense range.

Common pitfalls

•Calling a soil 'clay' because it feels fine without checking the No. 200 sieve. USCS coarse/fine is decided strictly at 50% passing No. 200 — a soil with 40% fines is still coarse-grained.
•Reading $C_c$ and $C_u$ off the wrong diameters. $D_{10}$ , $D_{30}$ , $D_{60}$ are the sizes at which 10/30/60% are FINER (passing), not retained — and they are in mm.
•Plotting on the wrong side of the A-line. Compute $0.73(LL-20)$ and compare to $PI$ ; 'on or above' is clay (C), 'below' is silt (M). The 4-7 PI band on/above the line is the dual CL-ML.
•Using gradation ( $C_u$ , $C_c$ ) for a dirty sand or gravel. With $>12\%$ fines the soil is SM/SC or GM/GC (SC-SM / GC-GM if the fines are CL-ML) and gradation is irrelevant; with 5-12% fines use a dual symbol.
•Reporting a negative group index. If the $GI$ formula returns a negative number, the soil is good subgrade and $GI = 0$ — never write a negative value.
•Forgetting the AASHTO M145 caps on the group index. The handbook prints the bare formula, but the partial products are limited: $(F-15)\le40$ (i.e. $F\le55$ in the second term), $(F-35)\le40$ , $(LL-40)\le20$ , $(PI-10)\le20$ . Plugging an uncapped $F$ overstates GI by a point or more.
•Correlating raw SPT $N$ to density or strength. Always correct to $N_{60}$ first, and never use SPT to set a design clay strength — the handbook flags fine-grained $N$ as unreliable.
•Mixing AASHTO and USCS sieves and limits. AASHTO splits silty/clayey at $PI = 10$ and granular/silt-clay at 35% passing the No. 200; USCS uses the A-line and a 50% No. 200 split. Both run the Atterberg limits on the minus-No. 40 fraction. They are not interchangeable.

References

NCEES PE Civil Reference Handbook — §3.7 Soil Classification and Boring Log Interpretation
NCEES PE Civil Reference Handbook — §3.8 Material Test Methods (Atterberg Limits, Index Testing)
ASTM D2487 — Unified Soil Classification System (USCS) — Source of the group-symbol chart and plasticity-chart A/U lines.
AASHTO M 145 — Classification of Soils and Soil-Aggregate Mixtures — Defines the A-1…A-7 groups and the group-index formula.

B. Soil properties

Soil Phase Relationships (Weight-Volume)

Use the three-phase block to convert among void ratio, porosity, water content, saturation, specific gravity, and the family of unit weights without memorizing every formula.

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C. Concrete

Concrete: Mix Proportioning and Properties

Proportion a concrete mix by absolute volume, set strength through the water-cement ratio, correct batch weights for aggregate moisture, and read the fresh and hardened property targets.

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D. Piping materials

Piping Materials and Pressure Ratings

Select water and wastewater pipe by material, translate DR/SDR and pressure class into allowable working pressure, add a surge allowance, and respect corrosion and bedding limits.

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E. Material test methods and specification conformance

Material Test Methods and Specification Conformance

Match the standard test to the property it measures — gradation, plasticity, density, strength — compute the result, and judge it against a specification band.

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