foundation system of burj al arab

Unsupported Foundation Excavation in Clay Soil

In the previous post we have learned about foundation excavation in sand without any support to the slope. We learned these that sand have little cohesion and these soil is unstable in steeper slope. we have provided some regular slope for sandy soil excavation. But question is clayey soil is a cohesive soil. Is it safe to excavate this soil unsupported and can be left open for considerable period. We will find the answer of this question in this post.

Normally, foundation engineer proportions and designs foundation for given superstructural loads. This is not function of engineers to select excavation equipment for a proposed site or to provide bracing. The foundation construction of any locality normally excavates and provides support to excavation if required.
Considering Peat resting on Clay, a preparation for huge supporting to excavation in the New York Subway ConstructionHowever, engineers have to approve the construction procedure, in this fact excavation procedure and safety, for a given construction site. But for a giant project it is required to provide construction guidelines from design office.

Now we will discuss the facts with clay soil. We will learn about the factors that control to provide a steeper slope:

1. Depth of cut

2. Shear resistance of desired clay soil.

Now we have to notice the properties of clay below base level of foundation excavation. If it is soft, a flat slope is chosen. This is done to avoid rise of bottom. Now we will consider hard or stiff clays. Normally this type clay commonly has cracks or develops crack, near ground. Have there any problem with cracks? 

Even being clay soil, flat slopes with steps are provided for a giant construction project. The steps are sometimes used as transportation routes for construction materials and excavated soil as well.

Excavation support to a road excavation underlying clay soil for utility maintenance. Yes this weak joint of clay soils are penetrated by water, a hydrostatic pressure is developed and which may lead to failure of slope even in stiff clay excavation. This pressure reduces the factor of safety considered in providing a steep slope.

With the time the moisture penetrated further deep in the excavation sides’ results soft clay (previously stiff). This also reduces factor safety with time. So one should not thick an excavation in clay deposit is unquestionable.

It is possible to provide steep slope and this slope remain stand satisfactory in clay soil for certain periods. But considering above fact, it is very common to provide support to excavation in clay type soil too for a safe and sound construction progress in foundation engineering.


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