2.3 The manual tests: hands on the soil
The manual analysis is the requirement that separates crews who classify soil from crews who guess. It takes a handful of spoil, about two minutes and no equipment, and it is mandatory: Appendix A (c)(2) requires the classification to rest on at least one visual and at least one manual analysis. This chapter is the five acceptable manual tests, in the standard's own words, and how to run them where the crew can see.
1 Why hands, and why now
Appendix A (d)(2): "Manual analysis of soil samples is conducted to determine quantitative as well as qualitative properties of soil and to provide more information in order to classify soil properly." Quantitative is the operative word. Type A and Type B are divided at an unconfined compressive strength of 1.5 tsf; Type B and Type C at 0.5 tsf. No visual observation delivers a number like that.
Timing matters as much as technique. The thumb penetration test in (d)(2)(iii) is to be conducted on an undisturbed soil sample, such as a large clump of spoil, as soon as practicable after excavation to keep the effects of drying to a minimum. And the standard is explicit about what changes the answer afterward: "If the excavation is later exposed to wetting influences (rain, flooding), the classification of the soil must be changed accordingly."
2 Test 1: plasticity — the 1/8-inch thread
Appendix A (d)(2)(i): mold a moist or wet sample into a ball and attempt to roll it into threads as thin as 1/8 inch in diameter. Cohesive material can be rolled into threads without crumbling. The standard gives the pass mark: "if at least a two inch (50 mm) length of 1/8-inch thread can be held on one end without tearing, the soil is cohesive."
This is the fastest test on the list and the one to teach a whole crew. Roll it on your palm, pick it up by one end, and watch. Two inches of thread hanging from your fingers means cohesive material; a thread that tears under its own weight, or never forms at all, means you are holding granular soil — which is Type C.
3 Test 2: dry strength
Appendix A (d)(2)(ii) reads the behavior of dry soil, and it distinguishes three outcomes:
| What the dry sample does | What it indicates |
|---|---|
| Crumbles on its own or with moderate pressure into individual grains or fine powder | Granular — any combination of gravel, sand or silt |
| Falls into clumps that break into smaller clumps, but the smaller clumps break only with difficulty | May be clay in combination with gravel, sand or silt |
| Breaks into clumps that do not break down further and can only be broken with difficulty, with no visual indication of fissuring | May be considered unfissured |
That third row is doing quiet work. "Unfissured" is a precondition for Type A: soil that meets Type A strength but is fissured drops to Type B under Appendix A (b) Type B (iv). The dry strength test is one of the two places in the appendix where you can support a claim of unfissured material — the other is the drying test below.
4 Test 3: thumb penetration — the strength estimate
Appendix A (d)(2)(iii) gives the field estimate of unconfined compressive strength, and notes that it is based on the thumb penetration test described in ASTM D2488.
| Result | Strength | Class |
|---|---|---|
| Readily indented by the thumb, but penetrated only with very great effort | 1.5 tsf | Type A range |
| Easily penetrated several inches by the thumb, and can be molded by light finger pressure | 0.5 tsf | Type C range |
| Between the two | 0.5 to 1.5 tsf | Type B range for cohesive soil |
Two conditions the standard attaches, and both get skipped: the sample must be undisturbed — a large clump, not a handful of loose fines — and tested as soon as practicable after excavation. A clump that has sat in the sun on top of the pile since 7 a.m. tests stronger than the ground it came from.
(d)(2)(iv) offers instruments for the same job: estimates of unconfined compressive strength can also be obtained with a pocket penetrometer or a hand-operated shearvane. Both are inexpensive and both remove the argument about whose thumb is strongest. Note from the appendix's definitions that saturation, or near saturation, is necessary for proper use of instruments like a pocket penetrometer or shear vane.
5 Test 4: the drying test
Appendix A (d)(2)(v) exists to answer a specific question: is this cohesive material fissured, unfissured, or actually granular? The procedure: dry a sample roughly one inch thick and six inches in diameter until it is thoroughly dry, then read it.
- 1If the sample develops cracks as it dries, significant fissures are indicated.
- 2If it dries without cracking, break it by hand. Considerable force needed to break it means significant cohesive material content — classify as unfissured cohesive material and determine the unconfined compressive strength.
- 3If it breaks easily by hand, it is either fissured cohesive material or granular material. Pulverize the dried clumps by hand or by stepping on them: clumps that do not pulverize easily are cohesive with fissures; clumps that pulverize easily into very small fragments are granular.
The drying test needs time, which is why it is the least used of the five. On a long job with a stable soil profile, it is worth doing once properly: it is the appendix's own method for defending an unfissured, and therefore possibly Type A, classification.
6 Putting a defensible classification together
A classification that would survive a serious conversation has four parts, and none of them takes long:
- One visual analysis — which of the seven observations you made, and what you saw.
- One manual analysis — which test, on a fresh undisturbed sample, and the result in the standard's own terms ("two inches of 1/8-inch thread held without tearing").
- The disqualifiers you ruled out — fissured, vibration, previously disturbed, dipping layers. If you are claiming Type A, this is where the claim lives or dies.
- A name and a date, on the inspection record, so the class can be inherited and challenged.
Appendix A also permits other recognized methods: (c)(2) accepts tests adopted by ASTM or the USDA textural classification system. If your company uses a geotechnical report or a laboratory result, that satisfies the requirement — provided somebody carries the result to the trench, and provided the report describes the ground actually in front of you.
- At least one manual analysis is mandatory, not optional — Appendix A (c)(2).
- Plasticity test: two inches of 1/8-inch thread held on one end without tearing means the soil is cohesive.
- Thumb penetration: Type A at 1.5 tsf is readily indented but penetrated only with very great effort; Type C at 0.5 tsf goes in several inches and molds under light finger pressure.
- Test an undisturbed clump as soon as practicable after excavation; rain or flooding afterward forces the classification to change.
- A pocket penetrometer or hand shearvane does the same job as the thumb and settles arguments.
- The dry strength and drying tests are how the appendix supports a claim that cohesive material is unfissured — the precondition for Type A.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.