Types of Soil in Excavation
Types of Soil in Excavation: Classification, Properties & Safety Tips
A method of categorizing soil and rock deposits as types A, B, and C in decreasing order of stability. Soil type is determined by analysis of the soil’s properties and how it performs under exposure to the elements and super imposed loads.
In excavation work, classifying types of soil is crucial for determining safety measures, shoring requirements, and the method of excavation. Both Indian standards (IS) and international standards (like OSHA and ASTM) classify soils based on physical characteristics such as cohesiveness, grain size, moisture content, and shear strength.
Type A:
Cohesive soils with an unconfined compressive strength of 1.5 ton per square foot (tc) (144kPa) or greater are classified as Type A. Examples of cohesive soils are: clay, silky clay, sandy clay, clay loam and in some cases, silky clay loam and sandy clay loam. Cemented soils such as caliches and hardpan are also considered Type A.
However, no soil is Type A if one or more of the following conditions are true:
- The soil is fissured.
- The soil is subject to vibration from heavy traffic, driving, or similar effects.
- The soil has been previously disturbed.
- The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H: 1V) or greater.
- The material is subject to other factors that would require it to be classified as a less stable material
Type B:
Soils classified as Type B are:
- Cohesive soils with an unconfined compressive strength greater than 0.5tsf (48 kPa) but less than 1.5 tsf (144 kPa).
- Granular cohesion less soils including angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silky clay loam and sandy loam clay.
- Previously disturbed soils except those which would otherwise be classed as Type C soil.
- Soil that meets the unconfined compressive strength or cement action requirements for Type A, but is fissured or subject to vibration.
- Dry rock that is not stable.
- Material that is part of a sloped, layered system where the layers dip into the
- excavation on a slope less steep than four horizontal to one vertical (4H: IV) but
- only if the material would otherwise be classified as Type B.
Type C:
Soils classified as Type C are:
- Cohesive soils with an unconfined compressive strength of 0.5 tsf (48kPa) or less.
- Granular soils including gravel, and loamy sand.
- Submerged soil or soil from which water is freely seeping.
- Submerged rock that is not stable.
- Material in a sloped layered system where the layers dip into the
- Excavation or a slope of four horizontal to one vertical (4H: IV)
Stable Rocks
Stable rock is the most secure type of soil for excavation work and is often excluded from requiring protective systems like shoring or benching under certain conditions.
Definition of Stable Rock
As per OSHA (US Standard – 29 CFR 1926 Subpart P):
“Stable rock means natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.”
Characteristics:
- High compressive strength.
- No signs of weathering, cracks, or fissures.
- Can maintain a vertical cut without collapsing.
- Requires mechanical means (or blasting) for excavation.
Examples of Stable Rock Types:
- Granite
- Basalt
- Dolerite
- Quartzite
- Chert
- Gneiss (if not weathered)
- Limestone (compact and unweathered)
Tests for Identifying Stable Rock:
- Unconfined Compressive Strength (UCS): > 1.5 tsf (144 kPa)
- Rock Quality Designation (RQD): > 75%
- Slake Durability Index
- Visual Inspection for joints, fractures, weathering
Field Identification Tips:
- Smooth, clean break lines.
- No soil-like consistency.
- No moisture seepage or loose material.
- Sound when struck with a hammer.
1. Indian Standard (IS) Soil Classification for Excavation
Indian standards primarily use the IS:1498 – 1970 (Reaffirmed 2002), which classifies soil based on grain size and plasticity:
Major Soil Types (IS 1498:1970)
Soil Types in Excavation (as per IS & GFR 2016, CPWD)
Excavation works in Indian contracts (PWD, CPWD) often use:
- Hard Rock (blasting required)
- Soft Rock (no blasting, but difficult)
- Hard Soil (requires picks or jumpers)
- Ordinary Soil (can be excavated with spade/shovel)
- Moist/Slushy Soil (needs pumping/draining)
- Black Cotton Soil (expansive, needs caution)
2. International Standards (e.g., OSHA, ASTM)
OSHA Soil Classification (29 CFR 1926 Subpart P – Excavations)
OSHA classifies soils into four categories:
ASTM Standards
- ASTM D2487 – Classification of Soils for Engineering Purposes (Unified Soil Classification System)
- ASTM D4318 – Liquid limit, plastic limit, and plasticity index
- ASTM D2488 – Description and Identification of Soils (Visual-Manual Procedure)
Comparison: Indian vs International Classification
