15.4 Soil Classification Systems (USCS & AASHTO) and Soil Effects on Equipment

Soil Classification Systems (USCS & AASHTO) and Soil Effects on Equipment

Why Operators Need Classification Vocabulary

Plans, specifications, and geotechnical reports describe soils with standard group names. A specification might say "subgrade is A-6 clay; undercut 2 feet and replace with A-1-a material" or "no CH, OH, or PT soils in embankment fill." An operator who understands these labels knows which material is acceptable, which must be hauled to waste, and how each will behave under the machine.

These engineering systems are different from OSHA's Type A, B, and C soil classes in Subpart P (Chapter 18). OSHA's system decides how an excavation must be protected. The USCS and AASHTO systems describe what the soil is made of and how well it will perform as fill or subgrade.

Grain Sizes and Sieves

Soil particles are sorted by passing a sample through a stack of standard sieves:

Silt and clay particles are too small to separate by sieve, so they are told apart by how they behave with water.

Atterberg Limits

• Liquid limit (LL): the water content at which a soil changes from a plastic state to a liquid state.
• Plastic limit (PL): the water content at which soil just crumbles when rolled into a 1/8-inch thread.
• Plasticity index (PI): the range of water content over which the soil is plastic.

$$PI = LL - PL$$

A soil with LL = 45 and PL = 20 has PI = 25, which means a sticky clay that swells when wet and shrinks and cracks when dry. Silts have little or no plasticity.

The Unified Soil Classification System (ASTM D2487)

USCS group symbols use two letters. The first names the main soil type: G gravel, S sand, M silt, C clay, O organic, and PT peat. The second describes gradation or plasticity: W well graded, P poorly graded, L low plasticity, and H high plasticity.

Clean coarse soils with less than 5 percent fines get a gradation letter (GW, SP). Coarse soils with more than 12 percent fines get a fines letter (GM, SC). Soils between 5 and 12 percent fines receive dual symbols such as SP-SM.

To separate clays from silts, engineers plot LL and PI on the plasticity chart. Soils above the A-line, where $PI = 0.73\,(LL - 20)$, are clays; soils below it are silts or organic soils.

The AASHTO System (AASHTO M 145)

Highway agencies usually classify soil with AASHTO groups, which rate soils as road subgrade:

A group index in parentheses, such as A-6 (12), refines the rating; the higher the number, the poorer the subgrade.

How Soil Type Affects Equipment

Traction

Usable pull equals the coefficient of traction times the weight on the drivers. The Caterpillar Performance Handbook lists:

Rolling Resistance

Rolling resistance is the force needed to keep wheels rolling on a surface, expressed as a percentage of gross machine weight. Caterpillar's table for bias-ply tires ranges from 1.5 percent on a very hard, smooth road to 4 percent on a rutted dirt road with about 1 inch of tire penetration, 8 percent with 4 inches of penetration, 10 percent in loose sand or gravel, and 20 percent on a very soft, muddy road with about 12 inches of penetration. A 100,000-pound loaded truck on an 8 percent road needs about 8,000 pounds of rimpull just to keep rolling, four times the 2,000 pounds needed on a maintained 2 percent road.

Practical Behavior by Soil Group

• Clean gravels and sands (GW, GP, SW, SP; A-1, A-3): drain quickly and compact well with vibration. Dry loose sand gives tires poor traction.
• Silts (ML, MH; A-4, A-5): dusty when dry and can turn soft or "quick" when wet and vibrated. Their compaction is very sensitive to moisture.
• Clays (CL, CH; A-6, A-7): strong and hard when dry but sticky when wet. Wet clay builds up in buckets and on tracks and causes ruts. Padfoot rollers compact them best.
• Organic soils and peat (OL, OH, PT): compressible and unsuitable for structural fill. They are usually removed and replaced.