Relative Density or Consistency, Hardness
Use the following charts to determine the density or consistency and hardness of material encountered.
Relative Density/Density (Cohesionless) | Uncorrected SPT N-values | Legacy TCP Blowcounts | Field Identification |
|---|---|---|---|
Very loose | Less than 4 | 0 to 8 | Easily penetrated by rebar many inches (> 12) ½ inch rebar, pushed by hand |
Loose | 4 - 10 | 8 to 20 | Easily penetrated with ½ inch rebar several inches, pushed by hand |
Medium Dense | 10 - 30 | NA | Easily to moderately penetrated using ½ inch rebar driven by 5-pound hammer |
Slightly compact | NA | 20 to 40 | Sample can be imprinted with considerable pressure |
Compact | NA | 40 to 80 | Sample can be imprinted only slightly with fingers |
Dense | ~30 - 50 | 80 to 5in / 100 | Penetrated 1 foot with difficulty using ½ inch rebar driven by 5-pound hammer Sample cannot be imprinted with fingers but can be penetrated with pencil |
Very dense | > 50 | 5in / 100 to 0in / 100 | Penetrated a few inches with ½ inch rebar driven by 5- pound hammer |
Consistency (Cohesive) | Uncorrected SPT N-values | Legacy TCP Blowcounts | Approx. Undrained Shear Strength, Su (Tons per Square Foot)* | Field Identification |
|---|---|---|---|---|
Very soft | < 2 | 0 to 8 | < 0.125 | Sample (height twice diameter) sags under own weight. Squeezes between fingers when fist is closed. Easily penetrated several inches by palm or fist. |
Soft | 2 - 4 | 8 to 20 | 0.125 – 0.25 | Sample can be pinched or imprinted easily with finger. Easily penetrated several inches by thumb. |
Medium Stiff | 4 - 8 | NA | 0.25 – 0.50 | Molded by strong pressure of fingers. Can penetrated several inches by thumb with moderate effort. |
Stiff | 8 - 15 | 20 to 40 | 0.50 – 1.0 | Sample can be imprinted with considerable pressure. |
Very stiff | 15-30 | 40 to 80 | 1.0 – 2.0 | Sample can be imprinted only slightly with fingers. |
Hard | 30-60 | 80 to 5in/100 | >2.0 | Sample cannot be imprinted with fingers but can be penetrated with pencil. |
Very hard | >60 | 5in/100 to 0in/100 | Sample cannot be penetrated with pencil. |
*Pocket Penetrometer and unconfined compression tests yield qu, within clay, Su = qu / 2
Hardness (Relative Rock Hardness) | Mohs’ Hardness Scale | Examples | Approx. SPT Values | Legacy TCP Blowcounts | Field Identification |
|---|---|---|---|---|---|
Very hard | 5.5 to 10 | Sandstone, chert, schist, granite, gneiss, some limestone | SPT Refusal > 100 blows | 0 in./100 to 2 in./100 | Rock will scratch knife. Core requires many blow of hammer to fracture or chip. Hammer rebounds after impact |
Hard | 3 to 5.5 | Siltstone, shale, iron deposits, most limestone | SPT Refusal > 100 blows | 1 in./100 to 5 in./100 | Rock can be scratched with knife blade or pick with difficulty |
Moderate Hard | N/A | Shale, some limestone | SPT Refusal > 100 blows | 2 in./100 to 5 in./100 | Cannot scratch with fingernail but can be peeled with knife. Fracturing with single blow of hammer. |
Soft | 1 to 3 | Gypsum, calcite, evaporites, chalk, some shale | 80 blows to 100 blows | 4 in./100 to 6 in./100 | Rock can be scratched with fingernail or knife. Crumbles under firm blow with hammer. Grains from sandstones and mudstones/shales can be rubbed off with fingers. |
Very Soft | ~1 | shale | < 80 blows | < 100 blows | Can be indented with fingers or crushed with fingers. Can be excavated easily with point of geologic hammer. |