DEEP EXCAVATIONS IN CONSTRUCTION

There can be different types of constructions involve deep excavation work. In foundation construction for high rise buildings, basement construction, foundations for bridge construction, underground railway stations, canal constructions, water or drainage pipe laying and many other construction works which were not listed here involves deep excavation work in construction industry.

Shoring

There are several methods or techniques used in shoring works for deep excavation such as;

1. Sheet piling with lateral bracing
2. Diaphragm walls 
3. Secant pile walls
4. Contiguous bored pile walls
5. Soil nailing and shotcreting
6. H iron and wooden plank shoring

etc.

Diaphragm wall after excavation, supported with ground anchors.

Secant pile wall after excavation, supported by ground anchors 

Sheet pile wall after excavation, supported with lateral steel bracing system.

Installation of lateral bracings using universal steel sections

Excavation

Long arm excavators will be more useful in deep excavation work due to its capability to reach long distances. In confined areas where excavators or long arm excavators cant reach, mini excavators can be used with the aid of long arm excavators to load to trucks in ground level.

Number of excavators, their capacities and types, number of dump trucks, etc. are solely depend on the scale of the excavation work.

Dewatering

Dewatering is a critical activity in deep excavation work where water table is higher than the excavation bottom level. The water seepage can be very high with the ground water pressure, and also rain water stagnation will also result in need of dewatering.

For deep excavation works, deep wells are installed prior to start excavation work. and several pipes (2", 4", 6") can be connected to connection pits prior to send to sedimentation tanks and disposal to outside drains or canals. pure water should be sent out from the site.

An ECM plant can be connected with sedimentation tanks to clean muddy water efficiently with percolation process by adding chemicals.

Installation of a deep well

Pile hacking and testing

Pile hacking is done by using pneumatic breakers connected to a compressor. Multiple number of breakers can be connected to a single compressor. The number of breakers and manpower is based on the quantity of piles to be hacked, time program, space availability and the other environmental restrictions such as affect of noise and vibration generation to public.

Pile testing (i.e. PIT, Sonic logger test, PDA test, etc.) will be done after hacking is completed to cutoff level. The number of tests to be conducted is vary with the project to project and will be based on project specification.

Basement or foundation construction work (rebar work and concreting)

Lean concreting, rebar work for foundation & concreting work, basement walls concreting will be done after completing excavation, pile hacking and testing. with the progress of the concreting work, supporting system can be removed gradually (i.e. struts have to cut and remove, ground anchors have to remove or cutoff). If sheet piles used as the shoring material, those sheet piles can be removed after basement work and backfilling being completed.

Instrumentation and monitoring 

Instrumentation and monitoring is very important in a deep excavation construction process, especially when the excavation is nearby the structures in use (buildings, roads, bridges, etc.). The following are a set of comprehensive instrumentation and monitoring process in a general deep excavation work near buildings. The required testing will be change with the condition of the project and surrounding.

1. Vibration monitoring
2. Noise monitoring
3. Inclinometer installation and monitoring
4. Piezometer installation and monitoring
5. Water stand pipe installation and monitoring
6. Tilt meter installation and monitoring
7. Ground settlement gauges installation and monitoring
8. Building settlement gauges installation and monitoring
9. Crack monitoring of nearby structures (if necessary)
10. Installation of strain gauges and monitoring (if using steel struts)

Pre-construction survey

Generally a pre construction survey of the condition of the nearby structures is carried out in large projects where accidental damages can happen due to project activities. A third party organization, recognized by the government authorities will be appointed to access the condition of the nearby structures and to provide a report. Normally a copy of the report is given to all stakeholders or representatives covering all stakeholders prior to start work.

Safety

A risk analysis report should be attached to the method of statement of the deep excavation in submitting to consultant/ employer or relevant authorities. Apart from the general hazardous expecting from excavation and construction work, care should be taken on confined space working hazardous (sufficient oxygen, poisoned gases), sound pollution inside, light condition, etc. when working in a confined area of deep excavation.

you may also read

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https://engcivilstuff.blogspot.com/2020/04/piling-pile-foundations.html