METHOD TO CONSTRUCT LARGE-SCALED AND DEEP BASEMENT (CONSTRUCTION ARRANGEMENT)

• Using traditional ways such as cut and fills (open cut method) or bottom up method (cut and cover method because it is relatively economical and effective.

• Basement can be constructed using :a) Open cut arrangementb) Cut and cover arrangement/bottom up arrangementc) Top-down arrangement

a) Open cut arrangement• Very old method and simplest method• Used in open space area where there is no obstruction & no

building/structure nearby• Beyond 6m deep, uneconomical because it will involved a massive

excavation work and will require retaining wall• no additional support operations and equipment, it is the economical choice• The sides of the excavation are possible slope protection – to ensure the

stability of the solid mass • Constructed from bottom to upwards.• After completion, area between basement and side slopes must backfilled.

i. Application• Limited constraint of site• Site must large enough to apply this technique.• Suitable for clay to sand soil – low soil settlement• Allow working space – provide stability to soil• Speed of construction – lowest performance – bottom upwards• Construction and consider backfill work• Depth – not exceed two storey height.• Low cost method – not have complicated action

ii. Sequence of construction

iii. Advantages• Simplest and most straight forward method of arrangement• Allow working space to easy the construction work• Low risk from avoiding the construction collapse• The most safe method for the workers to carry out the construction• The light condition on site is bright• Simplest protection for the slope of excavation• Economic method as it construct from bottom to upwards• No need strutting for this method

ii. Disadvantages• amount of free site space required• Additional cost for plant, machine and manpower• Cost increase for transport and backfilling• Easy to flood occur on site• Need another cost for disposal of water during flood on site• Cannot constructed in congested and urban area• Beyond 6m deep, allow for temporary support and make it uneconomical

ii. Open Cut Method

a) Cut and cover arrangement

• This technique is usually employed in constrained sites where ground movement to the adjacent surrounding has to kept to the minimum

• Retainining wall are required to support the excavation with the provision of bracing as the excavation proceeds downward until the deepest basement level.

• The sides of the excavation are supported with heavy lateral bracing (strutting), which installed at various depths with the subsequent progress of excavation.

• The main functions of strutting are to provide temporary support to the walls and also as working platform for the mechanical plants to operate on.

• The basement is then constructed in the conventional way, bottom upwards in sequence with the removal of the temporary struts.

i. Selection• suitable for clay, sand, silt• and mud soil• no allowance for working• space or sloping• can apply to all site area

i. Advantages• suitable for congested area and not• congested area.• low cost method.• less complicated construction.• less damaging because only shallow• grading operation are executed

i. Disadvantages• suitable for small and medium site only• the surface finish and quality of excavation of• the wall is depend on the sub-soil condition.• the water resistance of the concrete and• the joints maybe inadequate.• high risk for worker.• have many strutting.• not suitable for deep basement.• area to disposal of soil is limited.

i. Sequence of construction

ii. Cut and cover method

a) Top-down arrangement• This technique similar to the cut and cover technique, permanent perimeter

walls are first constructed.

• Excavation proceeds without the need for strutting to support the excavation as the slabs act as the permanent horizontal supports.

• Prefounded columns are then constructed,followed by the construction of the ground floor slab

• Boreholes are formed to the hard strata• Steel stanchion/ H sections are inserted and concrete pumped in to slightly

over the lowest basement slab level.• The holes are then backfilled with soil• Excavation then proceeds downward and basement slab are constructed

while construction of the superstructure proceeds simultaneously• Temporary opening are provided at various strategic locations on the

basement floors to provide access for removal of excavated earth as well as delivery of excavation machinery and materials for the construction of the substructure.

• When the formation level is reached, pile caps and ground beams are constructed.

i. Flow of construction

ii. Advantages• Suitable for basement of large size with complex environment• The superstructure can proceed upwards from ground level• simultaneously with the excavation downwards.• Strutting is not necessary• It allows early enclosure of the excavation which would permit• work to be carried out even in adverse weather condition.• Ground movement to the adjacent area is minimized as• excavation is always strutted during construction.

i. Disadvantages• Limited headroom for excavation• Restricted access for material handling• Dust and noise problem• High risk to workers• Cost of construction is high as it involves installation of more• sophisticated temporary support such as profounder column.• Provision of mechanical ventilation and artificial is necessary• during construction.

i. Selection• Used for deep excavation projects where tie back installation was not

feasible and soil movements had to be minimized.• Commonly used in hilly areas and suitable for basement of large size.• Construct using permanent perimeter walls.• Using small excavation at certain place.• Ground floor slab as a supporting.• Pre founded columns are structural columns/ piles formed before

basement excavation.

i. Top and down arrangement method