Homogeneity and the integrity of structural components or elements are the important features of structural engineering. The load carrying capacity of the building depends on the capability of structural elements to survive against the applied load without failure or collapse.
Whenever the building collapse, the first question that arises in one’s mind is
What is the age of building?
Everyone assumes that building might have collapsed due to age.
But sometimes even the new and under construction or even old building may fail due to the excavation in adjoining building.
Here are the reasons of collapse of building due to the excavation carry out near the existing building. Here we have also discussed the precautions to be taken during excavation work near the building.
How the Buildings Collapse due to Excavation in Adjoining Properties?
Nowadays in urban areas, everyone wants to maximise the effective use of the building by expanding the size of building as close to the boundary of adjoining property. It is also people’s interest to enjoy efficient use of land and not leave the narrow space between the buildings.
In old cities where 100% ground coverage is allowed and where land is of high value, the margins between two buildings are practically nil or very less. In such cases, construction work definitely causes some disruption to the adjoining property at the time of excavation of new building and excavating foundation just adjacent to the foundation of existing building is a major challenge.
The existing building will definitely fail when the new excavation is carried out beyond the depth of the foundation of existing building. In this case, the soil below the existing foundation/footing will move from its original position which will result in immediate settlement and hence building collapse. The soil below the foundation must not move. If it moves or slides, the footing above would settle/sink/tilt immediately, as it will lose the support below. Thus, deep excavation near the existing building will result in building collapse.
Precautions before Commencing Excavation Work near Existing Property:
Though the building owner has right to excavate close to the boundary line, however, the adjoining property has to be protected in its actual state. The adequate care should be taken to protect the adjoining property from collapsing, tilting or eroding away. The excavation should be done in such a manner that the soil of adjoining property will not liable to give away or should be supported by artificial means. It is necessary to contact your structural designer before you start the construction work as they will guide you about all precautions before commencing the excavation work.
Following are the precautions which help to commence excavation work near existing adjoining property:
01. Do geotechnical study of excavation area before commencing excavation work near adjoining building. I.e. study type of soil, layers and water table.
02. Never do entire excavation at a time. Do diggings of columns one by one, possibly alternate so that the adjoining building is exposed least.
03. Never go beyond the depth of excavation of foundation of existing building.
04. If you cannot avoid it, consult your structural engineer or do diaphragm wall, etc.
05. Complete the entire work of excavation, PCC, footing, columns casting, filling and refilling the excavated earth in the least possible time i.e. one or two days. Complete all the work with minimum vibration.
06. Do necessary shoring and shuttering to prevent the collapse of excavated stuff.
07. Do not water the excavated pit.
08. If the existing adjoining building is old wooden framed or load bearing, provide additional supports in it, so as to relieve loads from the foundation, which will also take care if there is minor slippage of earth.
09. Keep adequate props, shuttering materials and refilling materials like sand, earth, etc. ready at the site to take care of emergencies.
In conclusion, to avoid the failure of building, first, start with the study of soil which is adjacent to an existing foundation. It helps to know the soil condition of the site which helps in designing the shoring and underpinning system. Second, carry out excavation work alternately with adequate shoring to the soil of adjacent building and ensure that the construction work is carried out in proper way with minimum vibration in least possible time.