Category 1 – Cohesive soils of firm to stiff consistency that are fissured (Category 1b) or unfissured (Category 1a). These soils are generally of medium to high plasticity but may also include glacial clay tills of low to medium plasticity. These soils usually have low moisture content and most often occur above the water table.
Category 2 – Cohesive soils of soft consistency and non cohesive silt soils. The cohesive soils can be of medium to high plasticity while the silt soils are of non to low plasticity. These soils typically have high moisture contents and will tend to fill voids left between the excavation walls and shoring.
Category 3 – Cohesionless soils of loose to medium dense (Category 3a) and dense to very dense (Category 3b) consistencies. The Category 3a soils are generally easy to excavate by hand and are easily disturbed by construction equipment, particularly when they are at or near the water table or become saturated.Category 3b soils are generally not easy to excavate by hand and are not easily disturbed by construction equipment, except if they are at or near the water table or become saturated.
Silty clay – soil of medium to high plasticity of primarily lacustrine origin. The silty clay can range from soft to hard depending on the moisture content and is usually brown in the upper six to 10 metrs and grey below indicating the extent of previous oxidation and weathering. Typically, the upper three meters of lacustrine clay is weathered, fissured and nuggety.
Alluvial Clay – soil of medium plasticity, although plasticity can range from low to high. Alluvial clay can vary greatly in grain size distribution and consistency, but generally, the major constituent of this soil type is silt, followed by clay and then sand. Alluvial clay can range from very soft to stiff, depending on moisture content. In a dry state, the soil may often appear to be cohesion less, while in a wet state, alluvial clay is often very soft and subject to sloughing.
Glacial clay till – heterogeneous mixture of boulders, cobbles, gravel, sand, silt and clay, generally of low to medium plasticity.Glacial clay till can vary from soft to hard,primarily dependent on moisture content and deposition characteristics.
Silt – soil that is non-plastic to low plastic. Silt ranges from loose to extremely dense depending on moisture content and deposition characteristics. Silt is seldom encountered in a pure state, but normally has a significant fine sand component and occasionally a trace of some clay
Sand – sand can range greatly in grain size and density, and is often poorly graded (sorted). Typically, saturated sand exhibits a dilate behavior (fine grained sand), unstable with respect to excavations, and is subject to sloughing.
Gravel – like sand, gravel can have a wide range of grain size distribution and density. In a dry state, gravel is generally more stablethan sand (although still somewhat unstable) in vertical cuts, but still requires sloped excavation walls. Typically, on saturation, gravel becomes unstable (although less so than other cohesionless soils) with respect to excavation, and is subject to sloughing,
Glacial silt till – soil that is non-plastic to low plastic. Glacial silt till is a heterogeneous mixture of boulders, cobbles, gravel, sand, silt and clay.
Fill – fill can be a single soil type or a mixture of various soil types such as clay, sand, gravel, organic soils, etc. and may even contain non-soil materials such as demolition rubble or wood. It can vary widely in consistency, but is often softer or looser than the surrounding native soil, and has a greater likelihood of sloughing when encountered in excavations. In particular, it cannot be relied upon to be uniform, even over short vertical and horizontal distances, and may collapse in any one of several different modes, depending on its makeup.
The limestone and shale bedrocks are typically highly weathered and fractured when at or near the natural ground surface, but often become less fractured and more intact with depth.
The shale bedrocks are generally considered to be soft rock while limestone can vary from soft to hard. Generally, limestone bedrock is stable to very stable with respect to excavations or rock cuts. Similarly, shale bedrock can also be table to very stable but, in instances where the shale is fractured or contains existing failure planes, the shale can perform poorly.
Types of Soil Collapse:
General Zone of Exposure
(the area where workers are exposed to mass soil or rock movement)
Collapse Type 1
Spoil pile slide – improper excavating procedures occur when the excavated material is not placed far enough away from the edge of the excavation. The recommended minimum distance for location of excavated soil (spoil) from the edge of the excavation is equal to or greater than the excavation depth. However, the minimum permissible distance of spoil from the edge of the excavation is 0.6 metres for every one metre of excavation depth.
Collapse Type 2
Side wall shear – common to fissured or desiccated clay-type or alluvial soils that are exposed to drying.
Collapse Type 3
Slough-in (cave-in) – common to previously excavated material, fill, sand, silt and sand mix and gravel mix where the water table is above the base of excavation, or where soils are organic or peat.
Collapse Type 4
Rotation – common in clay-type soils when excavation walls are too steep, or when moisture content increases rapidly.
Soil is test:
A competent person must conduct visual and manual soil tests before anyone enters an excavation. Visual and manual tests are a critical part of determining the type of protective system that will be used.
Visual tests: Visual testing involves looking at the soil and the area around the excavation site for signs of instability. The competent person might do visual tests such as the following:
Observe the soil as it is excavated. Soil that remains in large clumps when excavated may be cohesive. Soil that breaks up easily is granular.
- Examine the particle sizes of excavated soil to determine how they hold together.
- Look for cracks or fissures in the faces of the excavation.
- Look for layers of different soil types and the angle of the layers in the face of the excavation that may indicate instability.
- Look for water seeping from the sides of the excavation.
- Look for signs of previously disturbed soil from other construction or excavation work.
- Consider vibration from construction activity or highway traffic that may affect the stability of the excavation.
Manual testing involves evaluating a sample of soil from the excavation to determine qualities such as cohesiveness, granularity, and unconﬁned compressive strength. Soil can be tested either on site or off site but should be tested as soon as possible to preserve its natural moisture.
Plasticity test: Shape a sample of moist soil into a ball and try to roll it into threads about 1/8-inch indiameter. Cohesive soil will roll into 1/8-inch threads without crumbling.
Dry strength test: Hold a dry soil sample in your hand. If the soil is dry and crumbles on its own or with moderate pressure into individual grains or ﬁne powder, it’s granular. If the soil breaks into clumps that are hard to break into smaller clumps, it may be clay combined with gravel, sand, or silt.
Thumb penetration test: This test roughly estimates the unconﬁned compressive strength of a sample. Press your thumb into the soil sample. If the sample resists hard pressure it may be type A soil. If it’s easy to penetrate, the sample may be type C.
A support structure (shoring) is required, or the excavation walls must be sloped at an appropriate angle, before a worker enters an excavation considered to be: