Soil Classification - Memory Anchors
The human brain remembers stories, images, and emotions — NOT raw equations. Every formula, classification rule, and boundary value in Soil Classification has been transformed here into a vivid anchor: a story, an acronym, an analogy, or a rhyme. When you encounter a board exam problem, your brain will not search through notes — it will REPLAY a memorable scene. Use these anchors during your first read, then test yourself with the recall triggers. Spaced repetition of just 5 anchors per day will make you unstoppable on exam day.
Memory anchors
Coefficient of Uniformity formula: Cu = D60 / D10
Memory aid
Think: 'SixTY over TEN gives you UNIformity.' Cu = D60/D10. The subscripts 60 and 10 — remember '6 is BIGGER than 1', so the bigger diameter is always on TOP (numerator). Cu measures how UNIFORM the grain sizes are — a small Cu means everyone is the same size (uniform), a large Cu means a wide range.
Why it works
The phrase anchors the numerator/denominator order, which is the most common mistake students make.
Example usage
Exam asks: D60=1.5 mm, D10=0.15 mm, find Cu. Trigger: '60 on top' → Cu = 1.5/0.15 = 10.
Recall trigger
Think '60 on top, 10 on bottom' — sixty is bigger so it stays on top like the boss.
Coefficient of Gradation (Curvature): Cc = D30² / (D10 × D60)
Memory aid
Cc is the 'MIDDLE CHILD SQUARED.' D30 is the middle value among D10, D30, D60. The middle child always tries TWICE AS HARD (hence squared). The parents (D10 and D60) are in the denominator multiplied together. Formula: Cc = (D30)² / (D10 × D60). Remember: 'Middle child SQUARED, parents MULTIPLIED below.'
Why it works
The family metaphor makes the formula structure intuitive and unforgettable.
Example usage
D30=0.45, D10=0.15, D60=1.5. Trigger: middle child squared → Cc = (0.45)²/(0.15×1.5) = 0.2025/0.225 = 0.90.
Recall trigger
Middle child squared — parents below.
Well-graded criteria for SAND: Cu ≥ 6 AND 1 ≤ Cc ≤ 3
Memory aid
For SAND, use the rule: 'SIX AND SANDWICH.' Cu must be at least SIX. Cc must be sandwiched between 1 and 3 (the bread is 1 and 3, the filling is Cc). Both conditions must be TRUE — it's an AND rule, not OR. If either fails, the sand is poorly graded (SP).
Why it works
The 'sandwich' image makes the bounded range 1≤Cc≤3 tactile and visual.
Example usage
Cu=10 (≥6 ✓), Cc=0.9 (not in sandwich ✗) → fails → SP, not SW.
Recall trigger
'SIX AND SANDWICH' — both must hold for SW.
Well-graded criteria for GRAVEL: Cu ≥ 4 AND 1 ≤ Cc ≤ 3
Memory aid
Gravel is ROUGHER and needs less uniformity — so it only needs Cu ≥ 4 (not 6). Rhyme: 'Gravel at FOUR, Sand at SIX — get both Cu and Cc to fix.' The Cc sandwich (1 to 3) is the SAME for both gravel and sand — only the Cu threshold changes.
Why it works
The rhyme pairs the two thresholds together, preventing mix-ups on the board exam.
Example usage
Gravel with Cu=5 (≥4 ✓) and Cc=2 (in sandwich ✓) → GW (well-graded gravel).
Recall trigger
Gravel=4, Sand=6 — Cc sandwich same for both.
Plasticity Index: PI = LL – PL
Memory aid
Think of plasticity as the WIDTH of a clay's 'plastic zone' — the range of moisture where you can mold it like clay. LL is the upper door (soil flows like liquid above it). PL is the lower door (soil crumbles below it). PI is the WIDTH of the doorway between the two doors. PI = LL − PL. The WIDER the doorway, the MORE plastic the soil.
Why it works
Spatial analogy (doorway width) makes the subtraction formula self-evident.
Example usage
LL=45, PL=22 → PI = 45 − 22 = 23. Wide doorway = highly plastic clay.
Recall trigger
Doorway between LL (top) and PL (bottom) — PI is its width.
A-line equation: PI = 0.73(LL – 20) separates CLAY (above) from SILT (below)
Memory aid
STORY: Arthur Casagrande drew a LINE on his plasticity chart in 1948. He called it the A-line — 'A' for Arthur, and also 'A' for ABOVE = CLAY. He noticed clays always plotted ABOVE the line, silts BELOW. The slope is 0.73 — he remembered it as '73 cents on the dollar' (less than 1). The line starts at LL=20 (subtract 20 from LL). So: A-line → PI = 0.73(LL − 20). CLAY is aristocratic — it sits ABOVE the A-line.
Why it works
The story ties the formula to its historical inventor, the slope to money, and position to aristocracy — three independent hooks.
Example usage
At LL=45: A-line PI = 0.73(45−20) = 18.25. If actual PI=23 > 18.25, soil is above A-line → CLAY.
Recall trigger
Arthur's A-line: 73 cents, minus 20, CLAY above.
LL = 50 separates LOW plasticity (L) from HIGH plasticity (H) in USCS fine-grained soils
Memory aid
Visualize a SPEED LIMIT SIGN on the plasticity chart: '50 km/h — LOW SPEED ZONE.' Below LL=50 is the LOW SPEED ZONE (low plasticity, suffix L: CL, ML). Above LL=50 is HIGH SPEED ZONE (high plasticity, suffix H: CH, MH). The sign is painted exactly at LL=50 on the horizontal axis of the plasticity chart.
Why it works
A road sign is a universally familiar image; linking it to the LL=50 boundary makes the threshold automatic.
Example usage
LL=60 → above the 50 sign → HIGH plasticity → suffix H (e.g., CH if above A-line).
Recall trigger
Speed limit 50 — below is L, above is H.
USCS fine-grained symbols: M=Silt, C=Clay, O=Organic
Memory aid
Remember: 'M-C-O = My Clay is Organic!' M = silt (from Swedish 'mo' = fine sand/silt), C = Clay, O = Organic. Or use: 'MaCO' — like the Philippine city Maco, Davao de Oro — a mnemonic city. When you think of Maco, you think M(silt)-C(clay)-O(organic).
Why it works
Using a Filipino place name (Maco) creates a culturally relevant hook unique to Filipino reviewees.
Example usage
Fine-grained soil with LL=45, above A-line → C (clay), LL<50 → L → CL.
Recall trigger
'MACO' — M=silt, C=clay, O=organic.
USCS coarse-grained second-letter codes: W=well-graded, P=poorly-graded, M=silty, C=clayey
Memory aid
'When Policemen Meet Criminals' = W, P, M, C. A coarse soil (G or S) gets a second letter. Well-graded (W) and Poorly-graded (P) are for CLEAN gravels/sands (use Cu and Cc). Silty (M) and Clayey (C) are for DIRTY gravels/sands (fines ≥12%). When 5–12% fines, dual symbols (GW-GM, etc.).
Why it works
Police story creates a narrative sequence for the four coarse-grained second-letter options.
Example usage
Gravel, Cu=20, Cc=2, fines<5% → clean, well-graded → GW.
Recall trigger
'When Policemen Meet Criminals' — W, P, M, C.
USCS: Coarse-grained = >50% retained on #200 sieve; Fine-grained = ≥50% passing #200
Memory aid
The #200 sieve is the BOUNCER at a nightclub. If MORE THAN HALF the soil particles are too big to get past the bouncer (retained on #200), the soil is COARSE (G or S). If HALF or more squeeze through (pass #200), it's FINE (M, C, O). The bouncer's rule: majority decides the crowd's identity.
Why it works
Nightclub bouncer analogy makes the 50% threshold and the sieve action vivid and amusing.
Example usage
Soil: 55% passing #200 → fine-grained → go to plasticity chart for M, C, or O.
Recall trigger
Bouncer at #200 — majority retained = coarse, majority passing = fine.
AASHTO Rating: A-1 (best subgrade) to A-7 (worst mineral soil), A-8 = peat/organic
Memory aid
STORY: Imagine a SCHOOL GRADING SYSTEM — but REVERSED. In AASHTO, A-1 is like a FIRST HONOR student (best subgrade, coarse, gravelly). A-7 is like a failing student (worst, highly plastic clay). A-8 is EXPELLED — that's peat, not even usable. The higher the number, the WORSE the subgrade. So A-1 student builds roads; A-7 student destroys them; A-8 is kicked out of school.
Why it works
Reversing the familiar grading scale creates memorable cognitive dissonance — exactly what makes memories stick.
Example usage
AASHTO classification A-6 → poor subgrade, high plasticity fines → needs treatment before use as subbase.
Recall trigger
Reversed school grades — A-1 is honor, A-7 fails, A-8 expelled.
Group Index formula: GI = (F–35)[0.2 + 0.005(LL–40)] + 0.01(F–15)(PI–10)
Memory aid
Break the GI formula into TWO CHUNKS: CHUNK 1 (LL part) = (F–35)[0.2 + 0.005(LL–40)]. CHUNK 2 (PI part) = 0.01(F–15)(PI–10). Each chunk has its own 'starting F': Chunk 1 uses (F–35), Chunk 2 uses (F–15). Chunk 1 uses LL–40, Chunk 2 uses PI–10. Memory: '35 and 15 are the F-anchors; 40 and 10 are the limit-anchors.' Higher GI = WORSE soil. Minimum GI = 0 (never negative).
Why it works
Chunking the complex 4-variable formula into two independent parts reduces cognitive load by 50%.
Example usage
F=60, LL=45, PI=23: Chunk1=(60-35)[0.2+0.005(45-40)]=25(0.225)=5.625; Chunk2=0.01(60-15)(23-10)=0.01(45)(13)=5.85; GI=5.625+5.85≈11.
Recall trigger
Two chunks: LL-chunk (F-35, LL-40) and PI-chunk (F-15, PI-10).
GI caps: (F–35) max 40, (F–15) max 40, (LL–40) max 20, (PI–10) max 20
Memory aid
Visualize FOUR CAPPED BOTTLES arranged in a row. Each bottle's cap limits how much liquid comes out. Bottle 1: (F–35) capped at 40. Bottle 2: (F–15) capped at 40. Bottle 3: (LL–40) capped at 20. Bottle 4: (PI–10) capped at 20. All four bottles have caps! If your calculation exceeds the cap, USE THE CAP. Two bottles cap at 40, two cap at 20.
Why it works
Visual symmetry (two 40s and two 20s) plus the physical cap image prevents forgetting the upper limits.
Example usage
If F=90, then F-35=55 → exceeds cap of 40 → use 40. F-15=75 → exceeds 40 → use 40.
Recall trigger
Four capped bottles — two at 40, two at 20.
D10 is called the Effective Size
Memory aid
D10 is the SMALLEST 10% of particles — they control FLOW through the soil. Think of a pipe: the NARROWEST POINT controls how much water flows through, not the average pipe diameter. D10 is that narrowest bottleneck. That's why it's 'effective' — it effectively controls permeability. Hazen's permeability equation (k ≈ C × D10²) is literally built around D10.
Why it works
The pipe bottleneck image makes the concept of 'effective' size physically intuitive.
Example usage
Board exam: 'What does D10 represent?' → Effective size — the diameter controlling permeability.
Recall trigger
Bottleneck pipe = D10 controls flow = Effective Size.
The four Atterberg limits in order of increasing moisture: SL < PL < LL
Memory aid
STORY: A clay sample wakes up in the morning DRY and SOLID (below Shrinkage Limit, SL). As it rains, it absorbs water and enters the SEMI-SOLID state. More rain — it crosses the PLASTIC LIMIT (PL) and becomes moldable like clay pottery. Heavy monsoon — it crosses the LIQUID LIMIT (LL) and FLOWS like a landslide. Sequence: SL → PL → LL (Solid → Semi-solid → Plastic → Liquid). 'SPL-L' or think: 'Some Poor Levees Leak' = Solid, Plastic, Liquid, Limits.
Why it works
The monsoon story is culturally Filipino and maps directly to increasing moisture content states.
Example usage
Board: 'What is the order of Atterberg limits?' → SL < PL < LL with increasing moisture.
Recall trigger
Monsoon story: dry → SL → PL → LL → flows.
Poorly graded: uniform particle size (all same) vs Gap-graded: missing middle sizes
Memory aid
POORLY GRADED = a class where ALL students are the same height (uniform — like monoculture rice). GAP-GRADED = a class with tall and short students but NO medium-height students (the middle is missing — like a bimodal size distribution). Both are NOT well-graded. A WELL-GRADED soil has a full range like a diverse class of many different heights.
Why it works
Classroom height analogy is universally relatable and clearly distinguishes three gradation types.
Example usage
Two-humped gradation curve → gap-graded (missing middle sizes) → NOT well-graded.
Recall trigger
Same height class = poorly graded; missing middle heights = gap-graded.
U-line on plasticity chart: PI = 0.9(LL – 8) — upper bound for natural soils
Memory aid
The U-line is the UPPER FENCE of the plasticity chart. No real soil should plot ABOVE the U-line — if yours does, CHECK YOUR DATA for errors. U = Upper = 0.9(LL–8). Remember: U-line uses 0.9 (bigger slope than A-line's 0.73) and subtracts 8 (not 20). If A-line is the 'road,' U-line is the 'guardrail above the road.' U-line for U-pper bound.
Why it works
Road vs guardrail metaphor creates spatial hierarchy between A-line and U-line on the chart.
Example usage
Plotted point above U-line → suspect lab error in LL or PL measurement.
Recall trigger
U-line = upper guardrail = 0.9(LL–8). Error check if above it.
PT = Peat in USCS — highly organic, not classified by gradation or plasticity
Memory aid
STORY: PT stands for 'Problema Talaga' (Real Problem). Peat is the rebel of soil classification — it refuses to follow normal USCS rules. It is HIGHLY ORGANIC, dark brown/black, smells like a swamp, has very HIGH compressibility and LOW strength. Engineers call it PT because it IS a problem — it cannot be used as structural fill or subgrade without total replacement or deep foundation bypass.
Why it works
Filipino expression 'Problema Talaga' is humorous and relatable, making PT instantly memorable.
Example usage
Site investigation reveals PT — recommend pile foundation or soil replacement. Never use PT as subgrade.
Recall trigger
'Problema Talaga' = PT = Peat = skip or replace.
AASHTO A-1 subgroups: A-1-a (gravel/stone) and A-1-b (coarse sand); A-3 = fine sand
Memory aid
A-1-a: 'A-1-a = Aggregate' → stone fragments and gravel. A-1-b: 'A-1-b = Bigger sand' → coarse sand. A-3: 'A-3 = Alone (clean fine sand) = Arena sand' → fine clean beach sand. Remember the order A-1 > A-3 in quality: gravels before fine sands. A-2 is the borderline silty/clayey gravel group.
Why it works
Connecting A-1-a to Aggregate and A-3 to Arena (beach/fine sand) creates two strong Filipino-relevant word hooks.
Example usage
AASHTO question: fine clean sand, non-plastic → A-3 classification.
Recall trigger
A-1-a=Aggregate (gravel), A-1-b=Bigger sand (coarse), A-3=Arena (fine sand).
GI = 0 for all A-1, A-2-4, A-2-5, and A-3 soils
Memory aid
Rhyme: 'The BEST soils score ZERO — heroes don't need GI, GI is for zero heroes!' The best AASHTO subgrades (A-1 and A-3) report GI=0 always — no formula needed. Only A-4 through A-7 (and A-2-6, A-2-7) need the GI formula. So: if your AASHTO group is in the top tier, just write GI=0 and move on.
Why it works
The rhyme creates a humor link ('zero heroes') that makes the special case rule memorable.
Example usage
A-3 soil: GI = 0 (always). No formula calculation needed.
Recall trigger
'Zero heroes' — top AASHTO groups always GI=0.
Revision game
I am a ratio that tells you how wide the grain-size distribution is. Mathematically, I am D60 divided by D10. For a well-graded sand, I must be at least 6. What am I?
Answer
Coefficient of Uniformity (Cu)
Memory link
A1 — '60 on top, 10 on bottom, boss stays on top.'
I am the middle child of the gradation family. I appear SQUARED in the numerator of my formula and my two siblings are multiplied in the denominator. For well-graded soil, I must be between 1 and 3. What am I?
Answer
Coefficient of Gradation / Curvature (Cc)
Memory link
A2 — 'Middle child squared, parents multiplied below.'
I am the LINE on the plasticity chart that separates clays from silts. My slope is 0.73, and I subtract 20 from the liquid limit. If your data point is ABOVE me, you have clay. Below me, silt. Who drew me?
Answer
A-line, drawn by Arthur Casagrande; equation: PI = 0.73(LL − 20)
Memory link
A6 — 'Arthur's 73-cent line starting at 20.'
In AASHTO, I am the NUMBER that summarizes subgrade quality. The higher my value, the WORSE the soil. I am always an integer, and I am never negative. If you compute me as −2, you report me as this value instead.
Answer
Group Index (GI); report as 0 if negative
Memory link
A12 and A20 — 'Two chunks, floor at zero.'
I am a USCS soil symbol. My first letter tells you I am predominantly gravel (retained on No. 4 sieve). My second letter W tells you I satisfy BOTH the Cu and Cc criteria. What am I and what are those criteria?
Answer
GW (Well-graded Gravel); Cu ≥ 4 AND 1 ≤ Cc ≤ 3
Memory link
A4 — 'Gravel at FOUR, Sand at SIX — Cc sandwich same for both.'
I am the Filipino-language memory aid for remembering USCS peat. My USCS symbol is PT. My English name starts with P. In Filipino I am 'Problema Talaga' because I cause foundation nightmares. What soil am I?
Answer
PT = Peat (highly organic soil)
Memory link
A18 — 'Problema Talaga' = PT = Peat.
I have four brothers in the Group Index formula. Two of us are capped at 40, and two of us are capped at 20. We are the DIFFERENCES inside the GI equation: (F-35), (F-15), (LL-40), and (PI-10). Which two of us are capped at 40?
Answer
(F − 35) and (F − 15) are both capped at 40
Memory link
A13 — 'Four capped bottles — two at 40 (the F-terms), two at 20 (the LL and PI terms).'
I am a moisture content. At exactly my value, a fine-grained soil transitions from the plastic state to the liquid state. I am measured using the Casagrande cup or the fall-cone test. My symbol is two capital letters. What am I?
Answer
Liquid Limit (LL)
Memory link
A15 — Monsoon story: soil crosses LL and flows like a landslide.
Formula mnemonics
Cu = D60 / D10
Mnemonic
'SIXTY over TEN = uniformiTY' — 60 is bigger, it sits on top. Cu measures the SPREAD of grain sizes.
When to use
Always when evaluating gradation of coarse-grained soils. Cu ≥ 6 for well-graded sand, Cu ≥ 4 for well-graded gravel.
What each part means
D60 = diameter at which 60% of sample passes (by mass); D10 = effective size, diameter at which 10% passes. Cu = ratio showing breadth of grain-size distribution.
Cc = D30² / (D10 × D60)
Mnemonic
'Middle child SQUARED, parents multiplied below' — D30 is the middle value, squared in the numerator; D10 and D60 are the extremes, multiplied in the denominator.
When to use
Second check for well-graded classification. 1 ≤ Cc ≤ 3 required (same for gravel and sand). Must satisfy BOTH Cu AND Cc for well-graded.
What each part means
D30 = diameter at which 30% of sample passes; D10 and D60 = lower and upper reference diameters. Cc = shape of the gradation curve in the middle range.
PI = LL − PL
Mnemonic
'Plasticity Index = width of the plastic DOORWAY between LL and PL.' Subtract the lower door from the upper door.
When to use
For all fine-grained soil classification (USCS and AASHTO). PI used to plot on plasticity chart. PI < 4 = non-plastic.
What each part means
LL = liquid limit (upper boundary of plastic state); PL = plastic limit (lower boundary of plastic state); PI = range of moisture over which soil behaves plastically.
A-line: PI = 0.73(LL − 20)
Mnemonic
'Arthur's 73-cent line starting at 20' — slope 0.73 (like 73 cents per dollar of LL above 20). Points ABOVE = clay (C), points BELOW = silt (M).
When to use
Plotting fine-grained USCS classification. Evaluate computed PI vs. A-line PI at same LL to determine C vs. M classification.
What each part means
0.73 = slope of the A-line on the plasticity chart; 20 = horizontal offset (the line crosses LL=20 at PI=0); PI and LL are from lab tests.
GI = (F−35)[0.2 + 0.005(LL−40)] + 0.01(F−15)(PI−10)
Mnemonic
'Two chunks: LL-chunk uses F-35 and LL-40; PI-chunk uses F-15 and PI-10. Four caps: two at 40, two at 20. Floor at zero, round to whole number.'
When to use
For AASHTO A-4 through A-7 and A-2-6, A-2-7 soils only. Always report as integer ≥ 0. Written in parentheses after AASHTO designation, e.g. A-6(11).
What each part means
F = percent passing No. 200 sieve (as a number, e.g. 60 not 0.60); LL = liquid limit; PI = plasticity index; GI = group index (higher = poorer subgrade quality).
Quick recall chains
USCS Classification Steps for Fine-Grained Soils
Recall test
Without looking: What are the 7 steps to classify a fine-grained soil in USCS? Start with the bouncer.
Memory chain
Chain story: 'The BOUNCER (Step 1) lets fine soils in. The LAB DOCTOR (Step 2) measures LL and PI. The ORGANIC INSPECTOR (Step 3) sniff-checks for organics. The CARTOGRAPHER (Step 4) plots the point. ARTHUR (Step 5) draws his A-line. The SPEED LIMIT SIGN (Step 6) at 50 decides L or H. The NAMER (Step 7) writes the final symbol.' Bouncer → Doctor → Inspector → Cartographer → Arthur → Sign → Namer = B-D-I-C-A-S-N.
Items to remember
- Step 1: Check % passing No. 200 sieve — if ≥ 50%, it is fine-grained
- Step 2: Determine LL and PI from Atterberg limit tests
- Step 3: Check if organic (LL after oven-drying < 0.75 × original LL)
- Step 4: Plot (LL, PI) on plasticity chart
- Step 5: Compare PI with A-line: above = C (clay), below = M (silt)
- Step 6: Compare LL with 50: below = L (low), above = H (high)
- Step 7: Combine: e.g., C + L = CL (low-plasticity clay)
Grain-Size Boundary Sizes (from coarse to fine)
Recall test
What sieve size (mm) corresponds to No. 4? No. 200? What separates gravel from sand? What separates sand from silt/clay?
Memory chain
Use the mnemonic 'Big Cats Growl — Sometimes Silently' = Boulders, Cobbles, Gravel — Sand, Silt/Clay. For sieve sizes, anchor: No. 4 sieve = 4.75 mm (sand/gravel boundary), No. 200 sieve = 0.075 mm (sand/fines boundary). The 'famous four': 75mm, 19mm, 4.75mm, 0.075mm are your FOUR KEY BOUNDARIES.
Items to remember
- Boulders: > 300 mm
- Cobbles: 75 mm – 300 mm
- Gravel (coarse): 19 mm – 75 mm
- Gravel (fine): 4.75 mm – 19 mm
- Sand (coarse): 2.00 mm – 4.75 mm
- Sand (medium): 0.425 mm – 2.00 mm
- Sand (fine): 0.075 mm – 0.425 mm
- Silt and Clay: < 0.075 mm (passing No. 200)
AASHTO Groups in Order of Decreasing Subgrade Quality
Recall test
Which AASHTO group is the best for highway subgrade? The worst mineral soil? What is A-8?
Memory chain
Story: 'First-class AGGREGATE (A-1-a) and BIG SAND (A-1-b) are TOP students. ARENA sand (A-3) is third. The A-2 squad has four sub-members (4,5,6,7). Then the problem soils: A-4 to A-7 are increasingly bad students. A-8 is expelled peat.' Key rule: NUMBER GOES UP = QUALITY GOES DOWN.
Items to remember
- A-1-a: gravel, stone — BEST
- A-1-b: coarse sand
- A-3: fine clean sand
- A-2-4 and A-2-5: silty/clayey gravel and sand (low PI)
- A-2-6 and A-2-7: silty/clayey gravel and sand (high PI)
- A-4: silty soil, low LL
- A-5: silty soil, high LL
- A-6: clayey soil, low LL
- A-7-5 and A-7-6: highly plastic clay — WORST mineral
- A-8: peat/organic — EXPELLED
Well-Graded Criteria Checklist
Recall test
A sand has Cu=8 and Cc=0.85. Is it well-graded? Why or why not? (Answer: No — Cc not in sandwich)
Memory chain
'GRAVEL needs FOUR, SAND needs SIX, BOTH need the SANDWICH (Cc between 1 and 3).' Mnemonic sentence: 'Gravel-4, Sand-6, Cc-Sandwich — ALL THREE or FAIL.' Failing either Cu or Cc criterion immediately gives you a P (poorly graded) second letter.
Items to remember
- For GRAVEL: Cu ≥ 4
- For SAND: Cu ≥ 6
- For BOTH: 1 ≤ Cc ≤ 3
- BOTH criteria must be satisfied simultaneously
- If either fails → Poorly Graded (GP or SP)
- GI if using AASHTO: higher GI = worse soil
Group Index Computation Order
Recall test
What is the GI if F=60, LL=45, PI=23? Walk through all 8 steps from memory.
Memory chain
'F first, TWO CHUNKS (LL then PI), FOUR CAPS (40-20-40-20), ADD them, ROUND and FLOOR.' Memory sentence: F-Chunk1-Cap-Chunk2-Cap-Add-Round-Floor = FCCCCARFl. Or simply: 'Two-chunk sandwich with caps, add, round, never go below zero.'
Items to remember
- Identify F (% passing No. 200) — use as a NUMBER not decimal
- Compute LL-chunk: (F−35)[0.2 + 0.005(LL−40)]
- Apply cap: (F−35) ≤ 40, (LL−40) ≤ 20, no negatives
- Compute PI-chunk: 0.01(F−15)(PI−10)
- Apply cap: (F−15) ≤ 40, (PI−10) ≤ 20, no negatives
- Add both chunks: GI = LL-chunk + PI-chunk
- Round to nearest integer
- Floor at 0 (never negative)
Diagrams
Soil Classification Overview — Master Mind Map
A complete mind map showing all four major topics of Soil Classification: grain-size analysis, Atterberg limits, USCS, and AASHTO. Use this as a visual syllabus before each study session and as a final review checklist.
USCS Classification Decision Flowchart
Step-by-step decision flowchart for USCS soil classification. Follow the arrows based on lab test results: start with percent passing No. 200 sieve, then branch to coarse-grained (G or S) or fine-grained (M, C, O) paths. Each terminal node shows the USCS symbol.
AASHTO Group Index Computation Flowchart
Detailed computation flowchart for the AASHTO Group Index formula. Shows both chunks separately, includes all four cap checks, and ends with the floor-at-zero and rounding rules. Follow this flowchart step-by-step on board exam problems to avoid common errors.
Atterberg Limits — Soil Consistency States with Increasing Moisture
State diagram showing the four consistency states of fine-grained soils as moisture content increases from left to right. The Plasticity Index PI equals the width of the Plastic State (from PL to LL). Used in conjunction with the monsoon micro-story anchor.