A construction project is planned for a site that is underlain by 20 feet of sof

Question

A construction project is planned for a site that is underlain by 20 feet of soft to medium stiff clay (unconfined compressive strength ranges from 0.25 to 0.75 tsf). The clay is underlain by 15 feet medium dense (average SPT N-Value = 35 bpf) well-graded sand with gravel, cobbles, and boulders. The well-graded sand is underlain by sandstone bedrock with an RQD of 65%. Groundwater was encountered close to the top of the well-graded sand layer. The proposed building will have maximum column loads on the order of 500 kips. Which of the following foundation options would you recommend (1= most suitable; 4 = least suitable).

options :

Mat foundations

shallow spread footing

Augur cast pile bearing in medium dense well -graded sand

H- pile bearing on sandstone bedrock

Explanation / Answer

The first layer is 20 feet clay, The second layer is 15 feet sand, the bedrock layer is sandstone

The total length of the soil=35 feet

RQD=65%, This means that the rock is moderately weathered rock in the bed rock

Load=500 kips, so it is a very heavy column

The foundations are laid based on the type of construction, soil available and the GWT

The last layer of the foundation is sandstone bed rock, deep foundation is definitely required, the last choice will be shallow foundation, because the depth is very huge

As the type of construction project is not specified it may be assume that it is a residential building and the column loads are acting in the foundation below

The first type of choice cannot be directly specified because there is no clarity in the type of project, if it is a resedential building then auger cast pile bearing in medium dense well graded sand would be preffered, if it is a commercial project like any shopping mall or a hotel with multiple levels then mat foundations will be preferred, it is preferred because it can accomodate a large number of columns and withstand heavy loads

The third choice will be H-pile foundations, but if these are used they become expensive, they are used for projects like bridges and where a pile cannot penetrate, but here as the top layer is clay, it can easily penetrate into that depth.

Shallow foundations are usually not preferred for big projects and heavy loads

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