Soil Stability

What Kind of Dirt Is That??

I'm going a little off track, but not far. I wanted to talk some on sands because we haven't covered that yet and also, I'm not quite through with clays, so the USCS will  have to suffice for the moment and then back to expansive clays.  (I did give you a heads up)

  For an in-depth coverage, let us look again at the chart of the Unified Soil Classification System Symbols. Broken down into four groups, and rated excellent to poor; we have:

Group I  (Excellent)   

GW  

• Well-graded-gravels, gravel-sand mixtures, little or no fines.
•  Allowable bearing capacity in pounds per square foot with medium  compaction or stiffness: 8000.
• Drainage Characteristics: Good.
• Frost heave potential: Low.
• Volume change potential: Low.

GP

• Poorly-graded-gravels, or gravel-sand mixtures, little or no fines.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 8000.
• Drainage Characteristics: Good.
• Frost heave potential: Low.
• Volume change potential: Low.

SW

• Well-graded-sands, gravelly-sands, little or no fines.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 6000.
• Drainage Characteristics: Good.
• Frost heave potential: Low.
• Volume change potential: Low.

SP

• Poorly-graded-sands or gravelly-sands, little or no fines.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 5000.
• Drainage Characteristics: Good.
• Frost heave potential: Low.
• Volume change potential: Low.

GM

• Silty-gravels, gravel-sand-silt mixtures.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 4000.
• Drainage Characteristics: Good.
• Frost heave potential: Medium.
• Volume change potential: Low.

SM

• Silty-sand, sand-silt mixtures.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 4000.
• Drainage Characteristics: Good.
• Frost heave potential: Medium.
• Volume change potential: Low.

Group II  (Fair to Good)

GC

• Clayey-gravels, gravel-sand-clay mixtures.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 4000.
• Drainage Characteristics: Good.
• Frost heave potential: Medium.
• Volume change potential: Medium.

SC

• Clayey-sands, sand-clay mixtures.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 4000.
• Drainage Characteristics: Medium.
• Frost heave potential: Medium.
• Volume change potential: Low.

ML

• Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 2000.
• Drainage Characteristics: Medium.
• Frost heave potential: High.
• Volume change potential: Low.

CL

• Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 2000.
• Drainage Characteristics: Medium.
• Frost heave potential: Medium.
• Volume change potential: Medium.

Group III  (Poor)

CH

• Inorganic clays of high plasticity, fat clays.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 2000. 
• Drainage Characteristics: Poor.
• Frost heave potential: Medium.
• Volume change potential: High.

MH

• Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 2000.
• Drainage Characteristics: Poor.
• Frost heave potential: High.
• Volume change potential: High.

Group IV  (Unsatisfactory)

OL

• Organic silts and organic silty clays of low plasticity.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: 400.
• Drainage Characteristics: Poor.
• Frost heave potential: Medium.
• Volume change potential: Medium.

OH

• Organic clays of medium to high plasticity, organic silts.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: -0-.
• Drainage Characteristics: Unsatisfactory.
• Frost heave potential: Medium.
• Volume change potential: High.

Pt

• Peat and other highly organic soils.
• Allowable bearing capacity in pounds per square foot with medium compaction or stiffness: -0-.
• Drainage Characteristics: Unsatisfactory.
• Frost heave potential: Medium.
• Volume change potential: High.
  

  As you well know, the gravel is key in the drainage system. It provides an unobstructed path for the water to flow away from the foundation or into a sump in a full-basement. The gravel will, further more, help to prevent pressures from building up against the foundation and curtail deflection.