Substructure Groundworks - Hazards Substructures are construction activities which take place below ground level. You will learn how substructures are constructed safely, understand what is used to do this and why. Recognise hazards in the process. Sub-structure involves all the construction works below floor level, including the foundations and associated activities. A very important operation at this stage is excavation. This means digging the ground so that foundations can be constructed.

Hazards associated with Groundworks

Control of Groundwater Sub-soil water is the water present below ground. It is also known as groundwater. When designing and constructing sub-structure, by law sub-soil water must not be allowed to enter the building and damage it. There are two different ways of controlling water, one temporary and one permanent. Depending on the site, sub-soil water and surface water might just need to be controlled temporarily during excavation. This is called simple sump pumping , because the water collects in a sump or pool and is then pumped out. Some sites might need permanent control of sub-soil water. This is known as land drainage . There are several methods of land drainage.

Land Drainage Land drainage involves burying a permeable pipe under gravel for water to enter it. This is then fed into a main drain or culvert for removal to a river. This is essential in areas built upon flood plains and areas with a high water table.

Earthwork Support This is the support of the sides of excavation. There are different methods of earthwork support depending on the needs of the site and the type of soil you are excavating, including: https://www.youtube.com/watch?v=4FrzFIX5svY Trench support Video Steel Trench Sheets https://www.youtube.com/watch?v=DqWt4l643M4 Video Can be easily fitted and interlocked. They can also be easily removed and reused. Used extensively in underground constructions such as tunnelling. Can be cut to size in confined spaces.

Hydraulic Trench supports https://www.youtube.com/watch?v=0wmcD3aM8X4 Trench Cave-In Video Aluminium walling trench Supports/Boxes. http://www.mabeyhire.co.uk/groundworks/trench-boxes/aluminium-trench-shoring/ A great website to look through. https://www.youtube.com/watch?v=W2SsmTyA2a4 Trench Box construction https://www.youtube.com/watch?v=3sS24ggTTlU digger using a trench box

Soil Types Soil and stability Some soils are more stable than others. The type of soil is one of the factors that determine the chance that an excavation will cave in. There are three basic soil types that you may encounter • Type A – very stable. Clay is an example. • Type B – less stable than type A soil. Crushed rock, silt, and soils that contain an equal mixture of sand and silt are examples. • Type C – less stable than type B soil. Gravel and sand are examples. Soil has other qualities that affect its stability. These include granularity, saturation, cohesiveness, • Granularity refers to the size of the soil grains; the larger the grains, the less stable the soil. • Saturation means how much water soil will absorb. • Cohesiveness means how well soil holds together; clay is a cohesive soil.

Plant machinery close to an excavation Excavated soil, called spoils, piled too close to the edge of an excavation can cause a cave-in. So can heavy equipment. Keep spoils and heavy equipment at least two feet from the edge; when possible, use vertical shores or shields that extend above the top of the excavation to restrain spoils. Spoils and heavy equipment that exert an excessive load on ground adjacent to the excavation could cause a cave-in. A dangerous spoil next to an excavation.

Safety in confined spaces Working in confined spaces can be deadly. The common hazards present are: Gas – build up of gasses which lay in a tunnel or trench and are invisible. Water – sudden flooding from rain or a water main leak. Collapse of the excavation. There must be: Full training of the workers in safety procedures. Clear entrance and exits to the confined space Full risk assessment Full Personal Protective Equipment PPE

Overburden Avoiding Services Overburden is an unwanted top layer of soil that must be stripped away to open access to useful construction materials buried beneath it. Overburden being bulldozed away from the excavation. The overburden must be stored safely so that it will not collapse. Avoiding Services Before excavations take place plans of current piping, drains, gas, electricity and water should be consulted and all contractors made aware of their locations.

1. List six hazard in excavations and explain the possible risks. 2. How are each of these risks best controlled to minimise injury? 3. You are on a site carrying out groundworks. You are told that there is groundwater in the trenches as well as a gas leak in the confined space of the excavation. a) Identify two risks for each of the two hazards mentioned above.

3b) For each risk named in question 3a, list one control measure that could be used. 4. Explain how three kinds of earthwork supports work. 5. Explain how soil type can affect its stability in excavations. 6. Explain the dangers of plant and spoils near excavations