Materials · Study · PE Civil: Water Resources and Environmental · FE → PE Prep
Materials
6% of exam
Soil classification and boring-log interpretation, soil properties and phase relationships, concrete, piping materials, and test-method conformance.
5 concepts
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 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 — D10, D30, D60 (the sizes at which 10%, 30%, 60% are finer) — and form the uniformity coefficient Cu and the coefficient of curvature Cc. A gravel is well graded (GW) when Cu≥4 and 1≤Cc≤3; a sand is well graded (SW) when Cu≥6 and 1≤Cc≤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.
Cu=D10D60,Cc=D10D60(D30)2
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≥50) gives fat clay CH or elastic silt MH. The small hatched wedge where 4≤PI≤7 and the point sits on or above the A-line is the dual CL-ML, silty clay.
A-line:PI=0.73(LL−20),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) or GC/SC (clayey, fines plot as CL/CH). 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. Unlike USCS, AASHTO treats material passing the No. 200 as 'fines' but draws its silt/clay line at PI=10 and uses the No. 10 and No. 40 sieves. 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)[0.2+0.005(LL−40)]+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 N60 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 N60=(Eeff/60)N; the fuller Skempton (1986) correction multiplies in borehole, sampler, and rod factors (Cb, Cs, Cr) but those are NOT in the handbook. From N60 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.
N60=60EeffN(handbook 3.8.1.1; Eeff 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 to N60 first, use sands-only for density, and read the ·stabilized· water table for effective-stress work.
Key equations
Uniformity coefficientCu=D10D60
Spread of grain sizes. D60, D10 = sieve openings (mm) passing 60% and 10%. Cu≥4
Coefficient of curvatureCc=D10D60(D30)2
Plasticity indexPI=LL−PL
Range of water content over which soil is plastic (%). LL = liquid limit, PL = plastic limit.
A-linePI=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 ≈25.5
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 indexLI=PIw−PL
Where in-situ water content w sits between limits. LI≤0
AASHTO group indexGI=(F−35)[0.2+0.005(LL−40)]+0.01(F−15)(PI−10)
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)N60=60EeffN=0.60EmN
Relative density (granular state)Dr=emax−eminemax−e×100%
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 D10=0.17 mm, D30=0.60 mm, D60=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: Cu=1.30/0.17=7.65
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%≥50%, so the soil is fine-grained. PI=LL−PL=42−20=22
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 Em=0.75. Find N60
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 Cc and Cu off the wrong diameters. D10, D30, D60 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 (Cu, Cc) for a dirty sand or gravel. With >12% fines the soil is SM/SC or GM/GC 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)≤40 (i.e. F≤55 in the second term), (F−35)≤40
•Correlating raw SPT N to density or strength. Always correct to N60 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 silt/clay at PI=10 on the No. 10/No. 40 sieves; USCS uses the A-line on the No. 200 fines. 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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(gravel) or
≥6
(sand) is needed for 'well graded.'
Shape of the gradation curve; well graded requires 1≤Cc≤3 together with the Cu test.
, where
0.73(LL−20)=4
), then rises along
0.73(LL−20)
.
brittle/stiff,
0<LI<1
plastic,
LI≥1
behaves as a viscous liquid (sensitive).
F = % passing No. 200. The handbook prints the bare formula, but AASHTO M145 caps the partial products: (F−35)≤40 (F≤75), (LL−40)≤20 in the first term, and (F−15)≤40 (F≤55), (PI−10)≤20 in the second. Report as integer in parentheses; if negative, use 0. Higher = poorer subgrade.
Handbook §3.8.1.1: Eeff = measured hammer efficiency (%) or Em 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 N60=(EmCbCsCr/0.60)N adds borehole Cb, sampler Cs, and rod Cr factors but is NOT in the NCEES handbook.
State of a cohesionless soil between its loosest (emax) and densest (emin) packing; the density class read from N60 on a boring log corresponds to a Dr range (e.g. medium dense ≈ 35-65%).
and
Cc=(0.60)2/(0.17×1.30)=0.36/0.221=1.63
.
Sand criterion:
Cu≥6
✓ and
1≤Cc≤3
✓, so it is well graded.
Group symbol:
SW
(well-graded sand). Sanity check: a wide size range (
D60/D10≈8
) with a smooth curvature is the textbook signature of a dense, well-graded sand.
.
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:
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
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)≤40
, and here
F=68>55
so use
40
:
0.01(40)(12)=4.8
. Thus
GI=6.93+4.8=11.7⇒A-7-6(12)
. (Plugging the uncapped
F−15=53
into the bare handbook formula would give
6.93+6.36=13.3→13
; 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.
by the handbook method and the relative-density class; then note how the fuller Skempton (1986) correction (borehole
Cb=1.05
, sampler
Cs=1.0
, rod
Cr=0.85
) would adjust it.
Solution. Handbook §3.8.1.1 hammer-efficiency form: N60=0.60EmN=0.600.75×22=1.25×22=27.5.
N60≈27.5, i.e. N60 in the 10-30 band ⇒ medium dense sand.
Skempton refinement (NOT in the handbook): N60=0.60EmCbCsCrN=0.600.75×1.05×1.0×0.85×22=0.600.6694×22=1.116×22=24.5 — still medium dense.
Sanity check: an automatic hammer delivers ~75% energy (>60%), so the correction factor exceeds 1 and N60 rises above the raw N=22; the borehole/sampler/rod factors trim it slightly. Both forms land in the medium-dense range.