Every year people excavating or working in excavations are injured and killed. If you work in excavations then there are some things you need to know and things you need to do if you’re going to stay safe.
Soils Ain’t Soils
Despite how it appears, not all soils are the same and, if you think about it, you probably already know that. Soils are mixtures of clay, sand and rock and different combination’s of these create soil with different characteristics. Here’s a rough guide to identifying the type of soil you maybe working with:
|Clay||Very Soft ClaySoft ClayFirm Clay
Very Stiff Clay
|Easily penetrated 40mm with fistEasily penetrated 40mm with thumbModerate effort needed to penetrate 30mm with thumb
Readily indented with thumb but penetrated only with great effort
Readily indented by thumbnail
Indented with difficulty by thumbnail
|Sand||Loose Clean SandMedium-Dense Clean SandDense Clean Sand or Gravel||Takes footprint more than 10mm deepTakes footprint 3mm to 10mm deepTakes footprint less than 3mm deep|
|Rock||Broken or DecomposedSound Rock||Diggable. Hammer blow “thuds”. The joints (breaks in the rock) are spaced less than 300mm apartNot diggable with pick. Hammer blow “rings”. The joints (breaks in the rock) are spaced more than 300mm apart|
The Angle of What?
A pile of excavated soil (or spoil as it’s known) will have a different natural slope according to the type of soil. This is called the “angle of repose”. The approximate angles for different types of soil are:
|Soil Type||Slope Ratio(Width to Height)||Slope Angle|
|Granular soils: crushed rock, gravel,on-angular, poorly graded sand, loamy sand||1.5:1||34|
|Weak cohesive soils: angular well graded sand, silt, silty loam, sandy loam||1:1||45|
|Cohesive soils: clay, silty clay, sandy clay||0.75:1||53|
The angle of repose is a good gauge for estimating the angle of shear planes in the soil profile – shear planes are the lines through which the unexcavated soil forming the excavation walls may break. We want to minimise the pressure on this area of potential weakness and the angle of repose allows us to estimate the distance that equipment and materials need to be from the edge of the excavation to reduce the chance of the excavation wall breaking.
For example, the angle of repose for sandy loam soil is 1:1 so equipment and materials need to be the depth of the excavation away from edge of the excavation. In a 2 metre (just over 6 feet) deep excavation in sandy loam soil equipmentand materials should be no closer than 2 metres from the edge of the excavation. If we were excavating in rocky soils the ratio is 1.5:1 so the distance is 3 metres and for clay soils, 1.5 metres.
Be aware that this angle will reduce if the soil is wet and more so if it’s saturated so always err on the side of caution.
Ground Support Systems
That’s a nice piece of jargon, so what does it mean? Essentially these are work practices to be followed where the risk of ground collapse is unacceptably high. This would include all excavations more than 1.5 metres (5 feet) deep and even lesser depths where the soil is loose such as sandy soils or when it’s wet or where there’s been previous excavations or a stack of other things that may reduce the strength of the excavation walls. There are 3 generally accepted methods for preventing excavations collapsing:
Battering involves sloping the sides of the excavation to the angle of repose thereby removing the soil that is likely to fall into the excavation.
Benching is cutting the side walls of the excavation into steps of the same ratio as the angle of repose with no vertical face being more than a metre (3 feet) high.
Shoring requires mechanical devices to be inserted into the excavation to strengthen the side walls and prevent it from collapsing. There are different types of shoring available for different circumstances and expert advice should be obtained to make sure you get the right type and its installed in the right way.
Soils can dry out or become sodden or change in other ways that increases the risk of collapse. All excavations should be inspected at least twice a day to monitor changing soil conditions and the effect this has on the stability of the walls. Some of the warning signs to watch out for are:
- TENSION CRACKS appearing in the wall of the excavation or existing cracks getting larger.
- SLIDING usually happens in loose soil and is indicated by soil from the side wall sliding into the excavation
- TOPPLING describes a situation where large blocks of soil fall from the walls into the excavation
- SUBSIDENCE AND BULGING of the side wall indicate unbalanced stresses in the soil
- HEAVING OR SQUEEZING is where the floor of the excavation starts to bulge as a result of the pressure from the walls of the excavation
- BOILING happens when the excavation has cut into the water table or the water table has risen causing water to pool in the excavation
Where these things are detected work should stop and expert advice obtained about corrective steps to take to prevent collapse.
Appearances can be Deceptive
- How a soil looks on the surface is probably not a good indication of what it is like below the surface
- Soil types can vary within an area and different soil types can be found along the length of an excavation
- Because there are no signs of previous excavation doesn’t mean there hasn’t been any. Previous excavation adjacent to where you’re digging will reduce soil integrity possibly leading to the collapse of the excavation walls.
- Not all buried services are marked (this is more so with the advent of underground boring for below ground service placement) – always locate underground services before starting to dig.
Never assume what type of soil you’re working with or that things will stay the same during the life of the job. Staying safe when working in an excavation means making sure you know what the risks are and taking the necessary precautions to prevent you becoming a story in the local news.