Stability of slopes in open excavation in different soil condition along with the factors that control slope stability in open excavation are discussed.
Fig.1: Various Excavation Condition
Factors Affecting Slope Stability in Open Excavation
- Types of soil
- Time during which the excavation is required to be open
- Allowable degree of risk of slipping which is determined based on the existing structures and new constructed buildings around the excavation area.
If the surrounding facilities are significant, then it is mandatory to eliminate slippage danger since it could deteriorate structures close to the excavated area. However, a certain degree of slippage danger can be adopted if the surrounding structures are not important.
Excavation Slope Stability in Cohesive Soils
In this section, slope stability in open excavation in different types of cohesive soil will be discussed:
Slope Stability in Normally Consolidated Soils
It is theoretically proven that, open excavations in ordinary compacted soil with vertical wall can stand without the need for any supports provided that the excavation wall height does not surpass critical height.
In the case of exceeding the critical height, the stability of the soil would vary with time due to variations in pore water pressure behind the face of the excavation wall after release of lateral pressure.
The critical height of an open excavation is calculated by dividing four times un-drained shear strength of soil over its density.
If it is required to have stable excavation in normally consolidated soft to firm clay for considerably long time, then the safety factor is specified based on the severity of the risk that imposed by major slip.
The use of low safety factor would be adequate unless the major slip of the excavation wall leads to the loss of life and damaging properties at the vicinity of the construction site.
For deep excavations, it is specified to take the expense of removing considerable mass of slipped clay from the excavated area into consideration while the safety factor of the excavation is evaluated.
Finally, it is recommended to place the soil, which removed from excavation, away from the top of the slope since it could increase the possibility of the slippage. Therefore, this factor is also required to be considered while the safety of the excavation is analyzed.
Slope Stability in Stiff Clay
It can stand almost vertically with small soil mass fall due to erosion and frost damage from sandy lenses in the clay. However, if pocket lenses of water bearing sand and gravel are present in clay or when the excavation is dug steeply and cuts fissures in the clay, then a major risk would be highly likely and hence the excavation is massively unstable.
It is proven that, the spread of fissures in stiff clay would pose serious issues to the stability of slope in excavations. This is because the pore water pressure variations cannot be anticipated when the overburden pressure is removed.
The slope slippage in stiff fissured clay is either small falls due to crumbling or slipping along fissure plan or rotational shear slide of sizable mass of clay soil. When the slipping does not impose server risks to the surrounding structure, then it would acceptable to use a slope of 1:0.5.
This slope will not eliminate the risk of slippage completely but the fallen soil mass should be smaller and clearing operation should not be difficult. If the slipping creates serious risks to the structures close to the excavation area, then a slope of 1:2 to 1:2.5 should be adopted or the face of excavation wall should be supported using suitable techniques.
Finally, if the excavation is not open for long time, then it is advisable to use a sheet layer such as polyethylene or tarpaulin to the steeply excavated face to prevent the penetration of water into the trench wall and destabilize the excavation.
Excavation Slope stability in Cohesionless or Partially Cohesive Soil
In this section, the slope stability of excavations in various types of soil such as dry sand and gravel, dump sand, sandy gravel, water bearing sand, water bearing sandy soil, silt and silty sand, dry silt, and wet silt.
Dry sand and gravel
They are cohesion less soil that able to stand at a slope equal to their angle of repose disregard of the excavation depth.
Damp sands and sandy gravel
These are partially cohesive soil that able to stand vertically for a period of less than a month.
The slope stability in this type of soil may be kept through the use of protective layer like cement mortar to the surface.
Factors such as erosion due to surface water and wind or degradation die to construction works are the sources of instability of steep slopes in such soil.
Water bearing sand
As far as water bearing sand is concerned, open slope excavation in such soil is substantially unstable specifically steep slope in which water seeps from the excavation wall face at the toe and soil would collapse at the wall upper part till the stable angle is realized which ranges from 15-20 degree.
The stability of water bearing sand is more problematic when thin layer of silt or clay are present.
This is because clay or silt layer may bleed from the face and consequently jeopardize the stability of other strong layers.
Dry silt soil
It can stand vertically and the depth of excavation of more than 15m with slight cementing on the face can be achieved.
If such slopes are not cemented, then vibration would easily disturb their stability. Another undesired factor that lead to destabilize slopes in silt is the erosion due water.
So, stability of slopes in wet silt is considerably difficult because erosion due to water lead to the collapse of the excavation until a stable angle is reached.
Excavation Slope Stability in Rocks
The stability of vertical slopes in rocks is not free from problems since it is dependent on the angle of bedding plane and the extent of shattering of shattered or deteriorated rocks.
If the bedding plane slope is steep and toward the excavation area, then the slope would be unstable especially in the presence of ground water that lubricator rock planes and hence facilitate slipping.
However, if the slope of bedding plane is away from the excavation area or horizontal, then vertical slope of the excavation wall would be stable.
With regard to shattered rocks, it could lead to the collapse of the excavation wall. For example, of the disintegrated rock falls, then the intact rock rest on the shattered one would fall as well and eventually total collapse are likely to occur.