What material are you classifying?
Select the primary material type to begin the identification process.
🟫 Soil
Unconsolidated material — gravels, sands, silts, clays, organics, frozen ground
🪨 Rock
Consolidated bedrock or large intact fragments — igneous, sedimentary, metamorphic
Does the material contain visible organic matter?
Look for dark color, woody fibers, root material, or a noticeable organic odor. Even small amounts affect engineering behavior.
No organic content — <2% organics
No odor, no fibers, no dark staining attributable to organics. Ash content 98–100%. No significant change to soil properties or behavior. Classify under standard USCS.
Slightly organic — 2–5% organics
Subtle odor possible, may have fine woody particles or faint dark staining. Ash content 95–98%. Minor influence on properties — retain base USCS name with "slightly organic" modifier. Recommend wet/dry Atterberg prep if fine-grained.
Organic — 5–15% organics
Noticeable odor, dark color, elevated moisture. Ash content 85–95%. Engineering properties measurably affected — specify both wet and dry preparation for Atterberg limits and max density tests. Use "organic" prefix with USCS name.
Highly organic — 15–75% organics
Strong organic odor, very dark color, spongy or fibrous feel. Ash content 25–85%. Compressible, very low strength, high moisture content. Use "highly organic" prefix with USCS name. Strength and consolidation testing critical.
Peat (Pt) — >75% organics
>75% organics, ash content <25%. Dark brown to black, spongy, pronounced odor, fibrous to amorphous texture. Extremely compressible. USCS: PT. May be fibric (Hemic), hemic, or sapric depending on humification and fiber content.
Highly organic material identified
Pt
Peat
Alaska Field Notes — Organic Soils
Organic soils in Alaska are commonly associated with muskeg, tundra, and low-lying glaciolacustrine areas.
Peat accumulates in poorly drained depressions where cool temperatures and anaerobic conditions retard decomposition.
Alaska's peat deposits range from thin active-layer organic mats (<1 ft) over permafrost to deep muskeg systems
exceeding 10–15 ft in the Kenai Lowlands and Cook Inlet basin.
Where organic soil deposits exceed 4 feet in thickness, increase boring frequency and consult the geotechnical
engineer. Vane shear and consolidation testing are often critical for embankment design over organic soils.
Always specify both wet and dry Atterberg and compaction test preparation for organic fine-grained soils.
Estimate the fine-grained fraction
Visually estimate the percentage of silt and clay (particles too small to see individually). >50% fines = fine-grained soil.
Coarse-grained — <50% fines
Individual particles visible. Gravel or sand dominates. Silt/clay fraction is minor or absent.
Fine-grained — >50% fines
Particles too small to see individually. Cohesive or silty feel. No gritty texture when rubbed.
Which coarse fraction dominates?
Compare the percentage of gravel (retained on #4 sieve, >4.75mm) versus sand (passing #4, retained on #200). The dominant fraction names the soil.
Gravel dominant
More gravel than sand. Particles range from 4.75mm up to 75mm (3"). USCS groups: GW, GP, GM, GC.
Sand dominant
More sand than gravel. Particles 0.075mm–4.75mm. USCS groups: SW, SP, SM, SC.
Estimate the fines content in the gravel
Fines are silt/clay particles. <5% = "clean" gravel. 5–12% = borderline, dual symbol. >12% = fines significantly affect behavior.
<5% fines — Clean gravel
Minimal silt/clay. Free-draining. Gradation characteristics (Cu, Cc) determine GW or GP classification.
5–12% fines — Dual symbol
Borderline clean/with fines. Dual symbol applies (e.g. GW-GM, GP-GC). Drainage partially impaired. Frost susceptibility increases — important for Alaska subgrade evaluation.
>12% fines — Silty or clayey gravel
Fines content controls classification (GM or GC). Significant impact on drainage, strength, frost susceptibility, and compaction behavior.
Assess the gravel gradation
Well-graded = wide range of particle sizes, good representation of sizes. Poorly graded = narrow range (uniformly-graded) or missing intermediate sizes (gap-graded).
Well-graded (GW)
Particles span a wide range of sizes with good representation throughout. Dense, interlocking structure. Coefficient of uniformity Cu >4, coefficient of curvature Cc between 1 and 3.
Poorly graded (GP)
Uniformly-graded (one size), gap-graded (missing intermediate sizes), or narrow range. More void space, less stable under load.
Characterize the fine fraction (5–12%)
With 5–12% fines a dual symbol applies — gradation + fines type. Rub wet fines between fingers. Clay is sticky and plastic. Silt is floury or gritty and non-plastic.
Silty — ML or MH fines → GW-GM or GP-GM
Non-plastic or low plasticity. Floury texture when dry. Cannot roll a thread. Highly frost susceptible — critical consideration for Alaska subgrade and fill evaluation.
Clayey — CL or CH fines → GW-GC or GP-GC
Plastic fines — sticky when moist, rolls into a thread. Reduces drainage, increases shrink-swell potential. Can cause significant strength loss when wet or thawed.
Characterize the fine fraction
Rub a small amount of wet fines between your fingers. Clay feels sticky and plastic. Silt feels gritty or floury and non-plastic.
Silty — ML or MH fines (GM)
Non-plastic or low plasticity. Floury texture when dry. Frost susceptible — important in Alaska subgrade and fill evaluation.
Clayey — CL or CH fines (GC)
Plastic, sticky when moist. Rolls into a thread. Can cause significant strength loss when wet or frozen and thawed.
Describe the particle angularity
Angularity reflects the history of the material — whether it has been transported, abraded, or mechanically processed. Select all that apply to most particles.
Gravel Size (dominant fraction)
Fine gravel = 4.75–19mm (¾″), Medium gravel = 19–50mm (2″), Coarse gravel = 50–75mm (3″)
Angularity
Select the best description for the majority of particles
Field Interpretation
Particle Shape (optional)
Particle Composition (optional)
Cobbles / Boulders Present?
Alaska Note
Cobbles and boulders are common in glacial terrain. Their presence is a known source of earthwork claims.
Log drilling action carefully — bouncing or slow penetration may indicate large particles even when not directly sampled.
Use the DOT&PF cobble/boulder percentage procedure (backhoe sample + spring scale) where warranted.
Additional descriptors
These supplementary descriptors complete the soil description for the field log.
Moisture Condition
Color
Color Interpretation
Cementation
Classification complete
USCS Gravel Group Reference
| Symbol | Name | Criteria |
|---|---|---|
| GW | Well-graded gravel | Cu≥4, 1≤Cc≤3, <5% fines |
| GP | Poorly graded gravel | Does not meet GW criteria, <5% fines |
| GW-GM | Well-graded gravel w/ silt | 5–12% ML/MH fines |
| GW-GC | Well-graded gravel w/ clay | 5–12% CL/CH fines |
| GP-GM | Poorly graded gravel w/ silt | 5–12% ML/MH fines |
| GP-GC | Poorly graded gravel w/ clay | 5–12% CL/CH fines |
| GM | Silty gravel | >12% ML/MH fines |
| GC | Clayey gravel | >12% CL/CH fines |
Estimate the fines content in the sand
<5% fines — Clean sand
Free-draining, non-cohesive. Gradation drives SW or SP classification.
5–12% fines — Dual symbol
Borderline clean/with fines. Dual symbol applies (SW-SM, SP-SC, etc.). Fines begin to reduce permeability and increase frost susceptibility significantly.
>12% fines — Silty or clayey sand
Fines content dominates behavior. Characterize fine fraction (SM or SC).
Assess the sand gradation
Well-graded (SW)
Wide range of grain sizes. Cu≥6, Cc between 1 and 3. Good structural properties when compacted.
Poorly graded (SP)
Uniform grain size or gap-graded. Beach sands, dune sands, and wind deposits are often uniformly-graded SP.
Characterize the fine fraction (5–12%)
With 5–12% fines a dual symbol applies — gradation + fines type. Rub wet fines between fingers. Clay is sticky and plastic. Silt is floury or gritty and non-plastic.
Silty — ML or MH fines → SW-SM or SP-SM
Non-plastic fines. Floury feel when dry. Cannot roll a thread. Highly frost susceptible — one of the most problematic materials in Alaska subgrades.
Clayey — CL or CH fines → SW-SC or SP-SC
Plastic fines — sticky when moist, rolls into a thread. Reduces permeability, increases cohesion. Susceptible to strength loss on thaw.
Characterize the fine fraction
Silty — SM
Non-plastic fines. Floury feel when dry. Highly frost susceptible — one of the most problematic materials in Alaska subgrades.
Clayey — SC
Plastic fines. Cohesive when moist, hard when dry. Susceptible to strength loss on thaw if ice-rich.
Describe additional sand characteristics
Grain Size (dominant)
Angularity
Field Interpretation
Moisture
Color
Classification complete
USCS Sand Group Reference
| Symbol | Name | Criteria |
|---|---|---|
| SW | Well-graded sand | Cu≥6, 1≤Cc≤3, <5% fines |
| SP | Poorly graded sand | Does not meet SW criteria, <5% fines |
| SW-SM | Well-graded sand w/ silt | 5–12% ML/MH fines |
| SW-SC | Well-graded sand w/ clay | 5–12% CL/CH fines |
| SP-SM | Poorly graded sand w/ silt | 5–12% ML/MH fines |
| SP-SC | Poorly graded sand w/ clay | 5–12% CL/CH fines |
| SM | Silty sand | >12% ML/MH fines |
| SC | Clayey sand | >12% CL/CH fines |
Assess the plasticity of the fine-grained soil
Roll a small moist sample into a 1/8" thread. Nonplastic soils crumble immediately.
Highly plastic soils roll easily and re-roll after reaching the plastic limit.
Silt (ML/MH) feels floury or gritty; clay (CL/CH) feels smooth and sticky.
Nonplastic — Silt (ML or MH)
Cannot be rolled into a thread at any moisture content. Floury or gritty feel. Dilatancy rapid. No strength when dry. USCS: ML (low plasticity) or MH (elastic silt).
Low plasticity
Thread barely rolls, lump cannot be formed when dry. Falls in lower portion of plasticity chart. May be lean clay (CL) or silty with some plasticity (ML).
Medium plasticity
Thread rolls easily, cannot be re-rolled after plastic limit. Lump crumbles when dry. Lean clay (CL) range.
High plasticity — Fat clay (CH)
Considerable effort to roll. Thread re-rolls several times. Lump forms without crumbling when dry. Very high dry strength. Highly compressible, significant swell potential.
Describe the fine-grained soil
Consistency (intact sample)
Use thumb pressure test on undisturbed sample
SPT Blow Count Equivalent
Moisture
Color
Color Note
Soil Structure
Frozen?
Classify the frozen soil condition
Per ASTM D4083. Describe the ice content and structure. Do not use blow counts to determine density in frozen soil.
Ice Content / Bonding
Nf — Poorly bonded or friable
No excess ice. Soil particles weakly held by ice. Easily chipped or broken. Usually thaw-stable. Density similar to unfrozen state.
Nb — Well bonded
No excess ice. Particles strongly held. High resistance to chipping. Usually thaw-stable.
Vx — Individual ice crystals / coatings
Excess ice visible as individual crystals or coatings on particles. May be thaw-unstable.
Vc — Random ice formations
Irregular or randomly oriented ice bodies. Thaw-unstable — expect settlement and strength loss on thaw.
Vs — Stratified / oriented ice
Ice lenses, layers, or veins with distinct orientation. Thaw-unstable. Significant settlement potential.
ICE — Substantial ice strata (>1 inch)
Dominant ice mass with soil inclusions. Severely thaw-unstable. Foundation and embankment design critical.
Classification complete
USCS Fine-Grained Reference
| Symbol | Name | Key Characteristics |
|---|---|---|
| ML | Silt | Low plasticity, rapid dilatancy, low dry strength |
| MH | Elastic silt | Medium plasticity, slow dilatancy, medium dry strength |
| CL | Lean clay | Medium plasticity, no dilatancy, medium-high dry strength |
| CH | Fat clay | High plasticity, no dilatancy, very high dry strength |
| OL | Organic silt/clay | Low plasticity with organics, odor present |
| OH | Organic clay | Medium-high plasticity with organics |
Identify the primary rock genesis
Check texture first: phaneritic (grains visible) vs. aphanitic (grains not visible).
Check for directional structure (foliation, banding).
Check grain hardness with pocketknife (most rock-forming minerals >5.5).
Igneous
Formed from cooling molten rock. Crystalline texture, no foliation, no layering. May be intrusive (coarse-grained) or extrusive (fine-grained or glassy).
Sedimentary
Layered or bedded. Clastic (fragmental grains) or chemical (crystalline or amorphous precipitate). May effervesce in HCl if carbonate.
Metamorphic
Altered by heat, pressure, or fluid. Foliated (schistose, slaty, gneissose) or non-foliated (marble, quartzite, hornfels).
Igneous rock texture
Can you see individual mineral grains with the naked eye?
Phaneritic — grains visible
Coarse (>5mm), medium (1–5mm), or fine (<1mm) but distinguishable. Intrusive origin — cooled slowly deep underground. Examples: granite, diorite, gabbro.
Aphanitic — grains not visible
Microcrystalline or glassy groundmass. Extrusive origin — rapid cooling at surface. Examples: basalt, rhyolite, andesite.
Porphyritic — mixed grain sizes
Large crystals (phenocrysts) set in finer groundmass. Two-stage cooling history. Examples: granite porphyry, rhyolite porphyry.
Glassy / vesicular
No crystalline structure. May have gas bubbles (vesicular) or be entirely glass (obsidian). Pumice is highly vesicular and lightweight.
Pyroclastic — fragmental volcanic
Ash, lapilli, blocks, or bombs cemented together. Tuff (<2mm), lapillistone (2–64mm), volcanic breccia (>64mm).
Describe the igneous rock
Color Index (% dark minerals)
Likely Rock Type
Chromatic Color
Alteration / Weathering
Engineering Significance
Special Features (optional)
Sedimentary rock grain size / texture
Coarse clastic — Conglomerate / Breccia
Rounded fragments (conglomerate) or angular fragments (breccia) >2mm. Cemented by silica, calcite, or iron oxide matrix.
Medium clastic — Sandstone / Graywacke
0.0625–2mm grains. Predominantly quartz. Graywacke is indurated impure sandstone with clayey matrix.
Fine clastic — Siltstone / Shale / Mudstone
Siltstone: similar to sandstone, finer, thin beds. Shale: fissile, predominantly <0.002mm. Mudstone/claystone: no fissility.
Chemical / Carbonate — Limestone / Dolomite
Effervesces in HCl (limestone strongly, dolomite weakly or when powdered). Many varieties — precipitated or detrital origin.
Other chemical — Chert / Coal / Evaporite
Chert: silica, conchoidal fracture, very hard. Coal: altered plant material, black, soft to hard. Evaporites: halite, gypsum, anhydrite.
Describe the sedimentary rock
Bedding / Structure
Induration / Hardness
Weathering Grade
Color
Metamorphic rock structure
Structure is the primary basis for naming metamorphic rock. Is there a directional fabric?
Slaty — very fine-grained, splits into thin sheets
Aphanitic. Well-developed planar cleavage. Hard, brittle. Rock name: Slate.
Phyllitic — silky/satin sheen on foliation surfaces
Intermediate between slaty and schistose. Aphanitic but with visible micaceous sheen from chlorite or sericite. Rock name: Phyllite.
Schistose — visible platy minerals, well-developed foliation
Phaneritic. Parallel orientation of mica, chlorite, or talc. Rock name: Schist. Note dominant mineral (e.g., biotite schist, chlorite schist).
Gneissose — alternating light/dark bands
Coarse-grained, imperfect foliation from alternating quartz/feldspar and dark mineral bands. Rock name: Gneiss. May grade into schist.
Non-foliated — granular or hornfelsic
No directional structure. Examples: Quartzite (metamorphosed sandstone, very hard), Marble (recrystallized limestone), Hornfels (fine-grained contact metamorphic).
Mylonitic — foliated, fine-grained, crushed fabric
Cataclastic structure from intense mechanical metamorphism. Streaked and banded. May indicate fault zone. Can cause unstable slope conditions.
Describe the metamorphic rock
Grain Size
Dominant Minerals (optional)
Color
Weathering Grade
Assess rock material strength
Use a geological hammer and pocketknife. This is wall rock strength per ISRM classification (Table 2-2 in the rock guide).
R6 — Extremely Strong (>36,000 psi)
Can only be chipped with geological hammer. Fresh granite, quartzite, chert.
R5 — Very Strong (15,000–36,000 psi)
Requires many blows to fracture. Most fresh igneous and metamorphic rocks.
R4 — Strong (7,000–15,000 psi)
More than one blow to fracture. Many sandstones, limestones, schists.
R3 — Medium Weak (3,500–7,000 psi)
Cannot scrape with pocketknife; one firm hammer blow to fracture. Some shales, weathered rock.
R2 — Weak (725–3,500 psi)
Can peel with pocketknife. Shallow indentations from hammer point. Soft shales, heavily weathered rock.
R1 — Very Weak (150–725 psi)
Crumbles under firm hammer blows. Can peel with pocketknife. Extremely weathered or decomposed rock.
R0 — Extremely Weak (35–150 psi)
Indented by thumbnail. Borderline rock/soil. Chalk, very soft shale.
Rock mass description
Describe the in-situ character of the rock mass — discontinuities control engineering behavior more than intact rock properties in most slope and foundation applications.
Discontinuity Spacing
Block Shape
Roughness of Discontinuity Surfaces
Shear Strength Implication
Infilling
Seepage
Rock description complete
Common Alaska Rock Types Reference
| Rock | Type | Common Context |
|---|---|---|
| Granite | Igneous | Batholith cores, mountain ranges; strong, good foundation rock |
| Basalt | Igneous | Volcanic terranes; vesicular zones can be weak |
| Schist | Metamorphic | Very common in SE and SC Alaska; foliation planes are failure surfaces |
| Gneiss | Metamorphic | Widespread; generally strong but foliation can control stability |
| Greywacke | Sedimentary | Common in accretionary terranes; variable quality |
| Limestone | Sedimentary | Karst risk; check for dissolution features |
| Shale | Sedimentary | Weak, often closely jointed; slope stability concern |
| Mylonite | Metamorphic | Fault zone indicator; can cause unstable slope conditions |
▸ Description Builder
Material
Organic Content
Grain Type
Gradation
Fines
Plasticity
Angularity
Gravel Size
Sand Size
Moisture
Color
Consistency
Structure
Frozen Condition
Rock Genesis
Texture / Structure
Strength Grade
Weathering
Discontinuity Spacing
Block Shape
Roughness
▸ Log Entry Description