Construction Problems with the Underground Structures

Greetings!
Here are Lecture notes on the construction problems related to the construction of the underground structures.
The major problems which we come across while underground excavation and construction of underground structures are classified into following 3 groups:

1. Stability of Excavation
2. Dewatering
3. Effect on adjoining structures

In this post, I am going to discuss only the introduction of the first part, i.e. stability of excavation. In the subsequent posts, I shall discuss it in details and other parts as well.

1) Stability of Excavation: Excavation is done manually or by chemical means depending upon the quantity of earth work involved. In case of manual excavation, the size of foundation is limited. The mechanical excavators and haulers can be used for gentle side slopes such as in road construction, thus the area involved will be larger.

Sloped excavation - for underground construction

Depending upon the space availability at the construction site, the excavation with a side slope as shown in figure, or a braced cut can be adopted, however the sloped excavation can be adopted only for a stable slope & free space in vicinity of the structure.

If, however there is an existing building in close vicinity, then excavation with the side slopes will not be feasible and economic. 

There in built up & crowded areas, braced cut is a viable proposition. Braced cuts consist of making the vertical walls in the soil and suitably propping them by driving steel struts as the excavation is proceeded.

Excavation with Braced Cut

At the final excavation level, the foundation is cast and the soil is backfilled to restore the original ground level, as the struts are successively pulled up. 

For shallow footings and raft foundations, the depth seldom exceeds 2 m and hence, no extensive supporting system is required, because earth pressure is not much. If however, heterogeneous soil exist near ground surface, side protection can be applied using timber planks or small struts may be used. 

Deep excavations exceeding 2 basements i.e. excavation depth greater than 6 m, will require thick, adequate lateral support by using diaphragm walls or continuous bored  piles and struts. 

In case of large excavation widths, it is not possible to have horizontal struts, because they may bend under their own weight. 

In such cases, inclined props or struts may be provided in between the diaphragm wall and base.  Also, a series of H-pile(Heavy I -sections) may be driven throughout the excavation at close intervals. The inclined props support the diaphragm wall in the wide cuts.

To be continued to next post.... 

Thank you!