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Exp 1 Answers - Practical on substructures and its load capacities
Construction Technology 2 (Substructure) (300721)
Western Sydney University
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(Mohr) =
= tan-1 (typical rise/typical run)
= tan-1 (7/9)
= 38
(Coulomb) = 0
Over burden effects the frictional resistance of the soil particles in the Mohr test.
Clay soil particles are generally smooth as they are flat platelets meaning they are able to easily slide over each other. The sliding action on the clay particle are caused by the negative charges on the faces. On the other hand, sand particles are rougher and therefore interlock together better. As the sand particles are bigger, there is less interaction between them meaning there is a less chance of slippage. Further, the sand particles have little chemical bonding between each other (Western Sydney University, 2018).
When excavating a trench in clayey soil, the key safety precaution would be to prop the trench as you excavate. The advantage of this trench in clayey soils is that the propping doesn’t need to be continuous. However, it is important to ensure that as you excavate downwards that you periodically as you keep on excavating. The main consideration as well is that the props are strong enough to take the lateral loads from the soil (Western Sydney University, 2018).
On the other hand, when excavating a trench in sandy soils it is best to use sheet piling before the excavation process as in this case, the soil could be too sandy or moist and has an adverse effect on the effectiveness of the shoring as it will most likely flow through the joints. An addition to this would be more continuous shoring.
From the experiment above, the failure load for the surface-mounted footing is 400kg while for the overburden-loaded footing is 650kg. As you could tell, the difference in the failure loads is 250kg. This could be caused by many factors but the main reason is that the footing in the overburden-loaded case, the footing is embedded more deeply in the ground. In terms of Terzaghi’s zones this means that Zone 3 will increase and Zone 2 will carry more weight and the frictional resistance of the soil will increase (Western Sydney University, 2018). These could cause for there to be a difference in the failure loads between surface-mounted footings and overburden-loaded footing. Furthermore, in the first case there was no surcharge while in the second case there was surcharge and zone 3 had to push more soil out.
Doubling the width of the strip footing increases the bearing capacity of the foundation and this is shown mainly through changes in the zones established by Terzaghi. When expanding the size of the footings, you also increase the size of the zones. The main zones impacted by this are zones 2 and 3. In Zone 2, the slip surface is larger and the presence of frictional and cohesive force on the surface before the footing fails will be increased. In terms of Zone 3, the amount of soil that would be carried upwards and sideways as the footing fails will increase. Furthermore, Zone 1 is also impacted as its size gets slightly increased when the footing is doubled in width (Western Sydney University, 2018).
The impact of doubling the width of the footing on the zones
References:
Western Sydney University, 2018, Module 1 - The Strength of Foundations, viewed 23 August 2018
Western Sydney University, 2018, Module 2 –Bearing Capacity of Shallow Footings, viewed 25 August 2018
Exp 1 Answers - Practical on substructures and its load capacities
Course: Construction Technology 2 (Substructure) (300721)
University: Western Sydney University
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