UNIT 1 - EARTHWORKS AND DRAINAGE
Section outline
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Geotechnical testing:
An essential step before commencing any construction or structural engineering project is extracting and testing soil sections at the building site. This process allows for the determination of various soil properties such as its structure, composition across different layers, the top layer of the load-bearing subsoil, and the level of the groundwater table.
Soil Improvement:
When the bearing capacity parameters of the soil are not optimal, several techniques can be employed for soil improvement. One approach is soil consolidation, which may involve:
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Compaction through tamping or vibration
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Grouting using cement-based or chemical materials
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Heat treatment via soil freezing or incineration
If necessary, soil replacement can be carried out, wherein layers of compacted crushed stone replace soil with insufficient bearing capacity.
Drainage:
When there are high groundwater levels, it is necessary to dewater the work area to enable construction below ground level. This can be achieved through methods such as open sump pumping, groundwater drawdown, electro-osmosis, or soil freezing.
Earthworks:
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Clearance work, involving the removal of vegetation.
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Excavation of the topsoil (humus), typically 20-50 cm in depth.
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Extraction of soil blocks, often done mechanically.
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Disposal of excavated soil.
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Shoring and strutting.
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Backfilling and compaction.
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Landscaping.
Work Trench Shoring:
With Slope:
Shoring can be set up to match the natural slope angle of the soil, but there are depth limits to this approach. Typically, different soil types have recommended depths: 50 cm for silty soils, 75 cm for sandy-gravel soils, 100 cm for medium compact soils and 150 cm for dry clay soils. Beyond a depth of 200 cm in hard clay soils, it is recommended to use shoring techniques.
With Strutting:
In narrow working pits, shoring is accomplished using wooden planks held together with straps on both sides of the trench. The straps are connected by horizontal struts so the opposing earth walls essentially support each other. The planks can be placed before or after excavation work begins. While post-shoring can be horizontal or vertical, pre-shoring is only feasible in a vertical configuration. Stepped shoring is employed in work pits deeper than 3 metres.
For wide trenches, single-sided or double-sided shoring may be used. Single-sided shoring typically involves vertical straps supporting horizontal planking, while the straps are supported by inclined struts or anchored to piles. Double-sided or framed shoring is used for rectangular trenches with a maximum width of 6-7 metres. In this case, horizontal straps support the vertical planking, creating continuous frames.
Specialised shoring methods include Siemens shoring, which utilises I-beams for support, and prefabricated metal shoring structures consisting of interconnected shoring panels fixed onto support frames interconnected by tubes.
For deep excavations, steel sheet piles or concrete piles are driven into the ground with a pile driver. These piles are interconnected with flanges to form a completely closed wall. Steel piles are easily driven in and removed leaving the concrete piles in place to become permanent structures even after construction is completed.
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