Subgrade & Frost Evaluation — Alaska Geotechnical Tools

❄ Stefan Equation — Frost/Thaw Depth

Estimates frost or thaw penetration depth using the Stefan equation: Z = C × √(FI) where C is a soil-type coefficient and FI is the freezing or thawing index.

Analysis Type Design Freezing Index (°F·days) Soil/Material Type Moisture Content (%, for latent heat) Dry Unit Weight (pcf)

📍 Alaska Regional Freezing Index Reference

Region	Area	FI Range (°F·days)	Notes
Southeast	Juneau / Ketchikan	200 – 800	Maritime, mild
Southcentral	Anchorage / Mat-Su	2,500 – 5,000	Most DOT&PF design work
Southcentral	Kenai / Homer	1,500 – 3,500	Coastal moderation
Interior	Fairbanks	9,000 – 14,000	Permafrost common
Interior	Delta Jct / Tok	8,000 – 12,000	Continental extreme
Interior	Nenana / McGrath	7,000 – 11,000	Discontinuous permafrost
Southwest	Bethel / Dillingham	4,000 – 8,000	Low-lying, wet soils
Northwest	Nome	7,000 – 10,000	Coastal tundra
Arctic	Kotzebue / Barrow	12,000 – 20,000	Continuous permafrost
Arctic	Prudhoe Bay area	15,000 – 22,000	Deep frost, permafrost table near surface

⚠ Use site-specific climate records for design. Values above are approximate historical ranges; 100-year design FI may exceed these bounds. Source: AKFPD Manual / WRCC climate records.

📐 Stefan Equation Results

Enter inputs and click Calculate to see results.

🚦 Spring Thaw Load Restriction Guidance

Alaska DOT&PF imposes spring load restrictions (SLR) when thawing weakens subgrade. Timing and severity depend on frost susceptibility class and local thaw conditions.

Frost Class	Typical SLR Duration	Axle Reduction	Trigger Condition
F1 — Negligible	None required	—	Non-frost-susceptible; no SLR needed
F2 — Low	2–4 weeks	Reduce by ~20%	Thaw front in upper 12"; reduced bearing
F3 — Medium	3–6 weeks	Reduce by 40–50%	Active frost heave / thaw weakening observed
F4 — High	4–8+ weeks	Reduce by 50–70%	Significant heave; subgrade may lose bearing entirely during thaw

SLR schedules vary by route, region, and year. Verify current restrictions with Alaska DOT&PF regional offices. This table is a planning reference, not an official restriction schedule.

📊 Freeze–Thaw Cycle Effects on Subgrade

❄Ice lens formation in frost-susceptible soils causes heave, disrupts pavement surface, and disrupts drainage
💧Thaw releases excess moisture — subgrade temporarily saturated and very weak (CBR may drop to <3)
📉Resilient modulus (M_R) during thaw can be as low as 10–20% of normal summer value
🔁Repeated cycling causes progressive weakening, rutting, and differential settlement in fine-grained soils
🏔Permafrost thaw adds long-term settlement risk beyond seasonal frost effects — see Permafrost Risk tab

1Grain Size Data

2P200 & P0.02

3USCS Class

4Frost Class

🔬 Grain Size & Fines Input

Enter percent passing at key sieve sizes from your gradation analysis. The Casagrande criterion evaluates the percent finer than 0.02 mm (0.02 mm = No. 635 equivalent / hydrometer).

Percent Passing No. 4 (4.75 mm) — % Percent Passing No. 40 (0.425 mm) — % Percent Passing No. 200 (0.075 mm) — P200, % Percent Finer than 0.02 mm (from hydrometer) — %

Liquid Limit (LL) — % Plasticity Index (PI) — % USCS Classification (optional — confirms routing)

🏷 Frost Susceptibility Classification

Enter grain size data and click Evaluate to classify.

📖 Frost Susceptibility Class Reference

Negligible

Gravelly soils with <3% finer than 0.02 mm; clean gravels

Low

Sands <3% passing 0.02 mm; gravels 3–10% finer than 0.02 mm

Medium

Gravels >10% <0.02 mm; sandy and fine-grained soils with moderate fines

High

Silts, CH/MH clays, varved clays, organic soils; >3% passing 0.02 mm in fine-grained soils

Classification per Casagrande (1931/1938) as referenced in USACE TM 5-818-2 and AKFPD Manual guidance. The 0.02 mm threshold is the primary Casagrande criterion. AKFPD also applies P200 limits for material acceptance.

🗺 USCS → Frost Class Mapping

USCS Symbol	Typical Frost Class	Key Criterion
GW, GP	F1	P<0.02mm < 3%; non-susceptible
GM, GC (clean)	F1–F2	Depends on fines; borderline
SW, SP	F1–F2	Clean sands; low susceptibility
SM, SC	F2–F3	Silty/clayey sands; moderate
ML, CL	F3–F4	Silt-dominated; high concern
MH, CH	F4	Highly plastic; very high heave potential
OL, OH, PT	F4	Organic soils; avoid in frost zones

📋 AKFPD Material Acceptance — P200 Limits by Layer Position

Alaska DOT&PF Flexible Pavement Design Manual (AKFPD, July 2020) specifies maximum percent passing the No. 200 sieve for each pavement layer position. Enter your material's P200 to check acceptance for each position.

Material P200 — Percent Passing No. 200 Sieve (%) Material P0.02mm — Percent Finer than 0.02 mm (%) (if available) PI of Fines (if applicable)

Enter P200 and click Check to evaluate layer acceptance.

📐 AKFPD P200 Limit Reference Table

Layer Position	Material Type	Max P200 (%)	Additional Limits	Notes
Surface / Leveling	HMA / Asphalt Concrete	—	Mix design per AKDOT spec	Not evaluated by P200 directly
Base Course	D-1 Gravel (Spec. 703-2.1)	8%	PI of fines ≤ 6; LL ≤ 25	Structural layer; strict limit
Subbase	Select Material Type A (703-2.4)	12%	PI of fines ≤ 6	Drainage and frost barrier function
Subbase / Embankment	Select Material Type B (703-2.5)	15%	PI ≤ 8; frost susceptibility F2 or better	Less critical structurally
Embankment Fill	Common Borrow (703-2.6)	35%	Free of organics, oversized rock	Frost susceptibility not restricted but screened
Subgrade (existing)	In-situ soil	N/A	Evaluate frost class; design for	Cannot replace — must design around

⚠ These limits reflect AKFPD Manual Table values. Always verify against current project-specific specifications. DOT&PF specs may be updated; the project Special Provisions supersede general table values.

❄ Frost Susceptibility Acceptance by Layer

Layer Position	Acceptable Frost Class	Casagrande 0.02 mm Limit	Design Approach if Non-Conforming
Base Course (D-1)	F1 required	<3% passing 0.02 mm	Reject material; source conforming aggregate
Subbase (Sel. Mat. Type A)	F1–F2 acceptable	Generally <3–10% passing 0.02 mm	Increase thickness or add non-frost layer
Subbase (Type B / Lower)	F2–F3 borderline	Case-by-case; evaluate heave potential	Frost heave analysis; may require capillary barrier
Embankment / Subgrade	F3–F4 common	Evaluate — design total section for frost	Full reduced-subgrade design; consider thaw consolidation

🌡 MAAT-Based Permafrost Screening

Mean Annual Air Temperature (MAAT) is the primary climate indicator for permafrost presence. This screening estimates relative permafrost risk and suggests investigation level.

Mean Annual Air Temperature (MAAT) — °F Terrain / Vegetation Type Surface Drainage Condition Aspect / Solar Exposure Known Historical Permafrost at Site?

🗺 Alaska Permafrost Zone Reference

Region	Permafrost Type	MAAT Range
Arctic Coastal Plain	Continuous	<14°F (<-10°C)
Brooks Range / North Slope	Continuous	14–23°F (-5 to -10°C)
Interior — Fairbanks	Discontinuous (extensive)	23–30°F (-5 to -1°C)
Interior — Delta / Tok	Discontinuous (sporadic)	28–34°F (-2 to 1°C)
Southwest — Bethel area	Discontinuous (widespread)	26–32°F (-3 to 0°C)
Southcentral — Anchorage	Isolated patches only	34–38°F (1–3°C)
Kenai / Homer	None / very rare	>36°F (>2°C)

Permafrost distribution is highly sensitive to local vegetation, drainage, and disturbance. Site-specific investigation (drilling, temperature monitoring) is required for any project in permafrost terrain.

⚠ Permafrost Risk Assessment

Enter site conditions and click Assess to evaluate risk.

📊 Simplified Ground Temperature Profile

Illustrative annual ground temperature range by depth. Actual temperatures require site instrumentation.

Run assessment above to generate depth profile estimate.

🏗 Permafrost Engineering Implications

🌡Thermal stability: Any disturbance — clearing, grading, paving — changes energy balance. Dark pavement absorbs heat; can deepen active layer or thaw permafrost table.
💧Excess ice: Thaw of ice-rich permafrost causes large settlement (thaw settlement ≠ frost heave). Volume loss on thaw can be 10–40% in silty soils.
📐Design approach: Preserve permafrost (insulation, raised grade, air convection embankments) OR design for thaw (remove ice-rich material, thaw consolidation analysis).
🔭Investigation: Require borehole temperature monitoring, undisturbed sampling for ice content, and thaw consolidation testing for critical structures.
📈Climate change: Permafrost is actively degrading across Alaska. Design life assumptions for roads should account for warming trends — especially in discontinuous zones.