Single Basement

Syllabus: Unit-3: Single basement construction along with waterproofing details, alternative
ways of providing and constructing access and provisions to be made for ventilation.

Assignment: Manually drafted scaled drawings of Single basement construction with various
types of waterproofing Techniques.
Information on materials and methodology for waterproofing should be included in the journal.

Introduction: Basement is defined by National building code as ‘The lower storey of a building,
below or partly below ground level.’ It also states that:
‘Every basement shall be in every part at least 2.4 m in height from the floor to the underside of the roof slab or
ceiling’

‘The height of the ceiling of any basement shall be minimum 0.9 m and the maximum, 1.2 m above the average
surrounding ground level. However, in case of parking, mercantile or business occupancy at ground floor,
minimum height of the ceiling of the basement may be 0.3 m above the average surrounding ground level
subject to mechanical ventilation being provided.’

Since basement is to be wholly or partly to be constructed below local ground level, Construction
of basement involves;
1. Excavation of the entire footprint of the basement floor.
2. Retaining of the soil during the construction and permanently after construction.
3. Making arrangement for Preventing the ground water from entering the basement or
exert hydrostatic pressure on the retaining wall and floor of the basement.

Basements can be used for various purposes like parking spaces, installation of air-conditioning,
water tanks etc or even habitable spaces like offices, retail outlets etc. Depending on the type of
use the basement is put to, provisions are also to be made for;
1. Making appropriate provision to provide access and emergency exits to basement for
people, vehicles etc.
2. Ventilation of spaces by appropriate number of air changes per unit time and quick
removal of harmful gases accumulated due to vehicular exhaust or radon seepage.
3. Lighting
4. Signage and assistance to way finding.
5. Installation of fire detection and fire fighting.

Construction of basement becomes complicated in its methodology as it gets dipper in the

ground. Excavating dipper, as an open hole in the ground would involve extensive structural
efforts to retain the sides of excavation from collapsing, also the pressure of the soil on the
external wall of the basement will be substantial, calling for appropriate structural system to
resist it. Providing access, ventilation , lighting and emergency evacuation also becomes
substantially complex. Thus the methodology of constructing single basement is very
different than that of constructing very deep basement or multi-basements. For example
excavating to a deeper level and laterally supporting the high perimeter wall of the basement
is more economically done by using a ‘top down’ method of construction instead of
conventional ‘bottom up’ method used for single basement. The ‘top down’ method
involves a construction of diaphragm wall along the perimeter of the basement followed by
a construction of ground floor slab which laterally supports the diaphragm. The excavation
is then taken up for first level of basement along with construction of top basement floor
slabs. After the construction of top basement floor slab, excavation for lower basements is
taken up for each level of basement along with floor slab of each lower basement
constructed sequentially. This is exactly a reverse process of construction to that of
‘bottom up’ which is more economical for single basement. Also ventilation of multi-
basement is essentially by mechanical installation and appropriate provisions of ducts and
 shafts would be required to cater to it whereas, single basement do have an option of having
natural lighting and ventilation . Hence for making it simpler to understand, basement
construction is discussed in two parts; Single basement and multi-basement.

Single Basement: construction of single Basement below ground floor would involve
comparatively less depth of excavation which would minimize the problems of excavation,
retaining of surrounding ground and ground water. As per the definition of National
building code, for single basement, minimum excavation required is around 1.5 mts when
the ground floor slab is 1.2mts above surrounding ground level and the minimum height of
the basement is 2.4 mts. However the basement constructed for different purposes may have
greater floor height and may involve excavation dipper than that.
Excavation: Single Basement construction begins with excavation and construction of
retaining wall.
Selection of appropriate excavation method should be done in consideration to the following
aspects;
1. Extent of the available site beyond the basement line.
2. Type of strata
3. Existence of adjacent excavation
4. Condition of surrounding buildings
5. foundation types of surrounding buildings
6. Construction budget
7. Allowable construction period
8. Availability of excavation Equipments.

Shallow excavations for single basements are generally carried out by full open cut
excavation method, along with strutting of the excavated sides or sloped profile of
excavation. The slope method does not require constructing the retaining wall prior to the
process of excavation. However it involves substantial increase in the amount of excavation
to be done so as to retain the profile of the slope to an angle lesser than the angle of repose
of the excavated material. The amount of extra excavation required to be done is dependent
on the type of strata. This method is most cases the cheapest method of shallow excavation
and saves the cost of strutting.

However for dipper excavations or in strata with very shallow angle of repose this method may
not always prove economical. Also in consideration to the bulb of pressure of the footings of
surrounding buildings it may not be advisable in certain situations. In such situation perimeter
retaining wall is constructed first and then the excavation is carried out. The simplest way of
doing it for the excavation up to 3.00 mts is called Dumpling method. This is used where there
are buildings or street in the proximity. The method is to construct a series of retaining wall in
trench, section by section, around the site perimeter ,leaving a centre Called "dumpling"
When the perimeter walls are in place, excavation may start at the centre of the dumpling, until
exposing a section of the wall. Then the wall may be side supported by struts, shoring or soil
anchor etc., again section by section in short length, until the excavation is all completed. Once
the floor slab of ground floor is constructed it acts as a lateral bracing between the retaining wall
facing each other.
This method does not require much heavy mechanical equipment and thus cost of work is
relatively lower. It can excavate up to a maximum depth of about 3m. Sometimes in very poor
soil or in waterloggedd ground, interlocking steel sheet pile may be driven to confine the area to
be excavated .After that excavation can be done in section and properly supported similar to that
mentioned above.

Excavation and Construction of Shallow Basement using Dumplin

Dumplingg Method

Perimeter Trench Method

 Construction of perimeter retaining wall: Type of retaining wall and the material used to
construct it depends on the methodology of excavation, Height of the retained material at
the back of the wall, Structural grid of the superstructure, intended use of the basement etc.
For retaining wall of up to 1.2mts height, Mass (gravity) retaining masonry wall using
modular concrete blocks, brick
brick, mass plain concrete or stone wall of appropriate thickness
can be used. For slightly
htly more height reinforcement can be introduced in the masonry to
give the wall the required tensile strength against the lateral loading of the retained soil.
However masonry retaining wall may not be a good choice from the point of view of
waterproofing the wall since the mortar joints between the masonry modules may be
difficult to seal against the ingress of water. Also the joint on the floor between the wall and
the RCC raft may pose a difficult area to seal due to non
non-monolithic
monolithic nature of the junction.
junctio
Nevertheless, such construction can be economically constructed if proper care is taken of
employing appropriate water proofing techniques like external tanking or drained cavities.

Different options of constructing perimeter retaining wall

RCC retaining wall is the most convenient material to construct the perimeter wall since it can be
cast monolithically along with the RCC raft and the
therefore the position and number of
construction joints can be controlled and appropriately placed.
The retaining
ing wall can be designed as;
1. Cantilever retaining wall having fixed end at the base
2. Propped retaining wall with the top being laterally propped by the slab of ground floor.
3. Fixed at both ends, with raft at the bottom and with the ground floor slab at the top.

Action of each type of retaining wall under pressure of retained soil

 Masonry retaining wall can be propped and the RCC retaining wall can be having fixed
top end
Retaining wall can also be designed to span horizontally as counterfort retaining wall or between
the perimeter row of columns. In this case the main reinforcement will be placed horizontally
and across the column. Column at the basement level, in such case will have to be designed for
resisting moments due to earth pressure in addition to resisting load from floor above. The slab
or wall behind the column and against the retained earth can then be comparatively thin than
cantilever or vertically designed wall.

Counterfort retaining wall as the perimeter wall of the basement

Retaining wall spanning between the columns

Care must be taken regarding the backfilling behind the wall keeping in mind the type of
retaining wall constructed. In case of cantilever retaining wall, backfilling of the excavated
material or percolation medium should be done after the construction of ground floor slab since
it will act as a prop or the top fixed end for the wall and support the wall laterally.

Backfilling behind the propped/fixed retaining wall

Foundation:

Not always
appropriate
The construction method shown above for column footing and the floor of the basement may be
feasible in some cases of shallow basement with water table lying at considerable depth.
However, it do have some inherent problems arising from the behaviour of strata below the
basement floor and regarding the monolithic sealing of the basement floor against the ingress of
water. The plinth constructed above ground is a compacted section of filled-up material while the
soil below the excavation will tend to swell due to the release of burden of soil which is now
removed or due to the seasonal rise and fall of the water table level. This behaviour of the strata
at the excavated depth will necessitate reinforced RCC slab as the floor of the basement to
counter any tensile forces. The water proofing layer below the floor slab of the basement will be
intercepted at the location of the columns going through the floor slab. Hence in most cases it is
preferred to construct raft footing for the buildings with basement as the lowest floor. In case of
the strata with very poor bearing capacity at the floor level of the basement, the raft can be
supported by group of piles transferring the load to dipper layers of safe bearing capacity.
Raft Foundation: The raft to be designed can be simple reinforced slab of a required thickness,
beam and slab raft or even cellular raft where ever necessary. Beam and slab raft is sometimes
preferred to have up-stand beams so that a continues plane is available on the underside of the
raft to install a un-intercepted layer of water proofing. Also the cells that are formed on the upper
side can be filled up which helps in pressing down the raft against the upthrust of the water table
before the superstructure is constructed and it load is imposed on the raft. This becomes
especially necessary, where the water table rises seasonally above the floor level of the
basement. However extra work of compacting the filled material and laying of PCC for the floor
is required. Another way of relieving the up thrust of the rising water table is to provide small
nozzles to the raft so that to allow water to percolates to the top of the raft. Once the
superstructure is constructed and its load is imposed on the raft, the nozzles are sealed by
grouting.
Down stand beams for beam and slab raft foundation for basement will have an advantage of
having the floor ready to put on finishes directly on the raft but will have difficulty in placing the
waterproofing layer to the underside of the raft. Also depth of excavation required will be
somewhat less than that required to construct raft with up-stand beams.

Raft Foundation with Beam and slab Raft foundation with up-stand beam
The retaining wall at the basement perimeter and the raft foundation is cast monolithically in
sections so as to take care of shrinkage during curing of concrete. Construction joints are taken
care of by introducing embedded and surface water bars. Adjacent bays of concrete floor and
wall are cast with a gap of 450mm to 600mm between them. When the bays have cured and
dried and most of the drying shrinkage in them has taken place, concrete is then cast in the space
between the bays. Because of its narrow width this second placing of concrete will suffer only
small shrinkage and nominal cracking because of it. This method of construction, without any
impermeable lining, will provide a reasonably water tight structure where ground water pressure
is low. Where there is an appreciable pressure it is necessary to use PVC or rubber water bars
(also called water stops) into all construction joints in the floor and into vertical construction
joints in the wall. In most cases, Horizontal construction joints in the wall do not require a water
bar generally the weight of the concrete wall above the joint is sufficient to provide a water tight
joint by compression.
 Casting of Raft in sections with kicker for retaining wall

Embedded and surface water stop/bar

Structural Grid: Basements have very different ambience as it cannot have openings at the
level which can allow visual connect with outside. Also light and ventilation situation are
different along with the physical access to the space. These are the reasons for using this floor of
the building for functions which are very different from the other floors. This different use of the
floor plate often requires a different layout and column grid to accommodate that layout, e.g.
vehicular parking demands a certain discipline in column grid to accommodate the parking
spaces and driveways. The same column grid may not be the most appropriate to accommodate
uses on those upper floors, hence there is a tendency to alter the column grid at basement level
by introducing girders at the basement roof level which supports the floating columns for the
superstructure. However, this is not recommended as good practice to resist the lateral
earthquake loads on the building, as stated in IS 1893(part-1) 2016, Criteria for earthquake
resistant design for buildings. It states about floating or stub column; ‘Such columns are likely
to cause concentrated damage in the structure. This feature is un-desirable, and hence should be
prohibited, if it is a part of or supporting the primary lateral load resisting system’
Such floating column also introduces stiffness irregularity and strength irregularity in the
structural configuration of the buildings as shown below. Hence it is always recommended that
the all the columns of the superstructure should be continued in the basement and to the
foundation without altering its position.

Source: IS 1893(part-1) 2016, Criteria for earthquake resistant design for buildings.
Water Proofing: Since basements are subgrade structures, the perimeter is continuously in
contact with soil and subsoil water. This water would tend to enter the basement through cracks
and capillaries in the peripheral wall unless an impervious layer is install to stop the ingress. The
problem is also further compounded because of the hydrostatic pressure of the whole body of
water in the retained soil which would be seasonally variable . This hydrostatic pressure can be
relieved to a certain extent by collecting the water in the perforated pipes laid along the periphery
of the wall below raft level and draining it elsewhere by gravity or pumping. This is referred to
as French drain.

Perforated pipe peripheral drain (French drain) to relieve hydrostatic pressure on wall.
Depending on the amount of likely hydrostatic pressure built-up at the back of the wall, Two
approaches are taken for preventing the water from entering the basement. If it is possible for the
retaining wall to structurally counter/ bear the maximum hydrostatic pressure exerted by the
subsoil water then water is stopped from entering the inside of the basement by tanking methods.
If the hydrostatic pressure is a substantial burden beyond the structural strength of the retaining
wall then, methods are adopted to relieve the pressure by allowing the passage of water through
the retaining wall and then collect this water in a sump through network of peripheral channels
on the inside of the retaining wall. This method is called drained cavities. Thus basement water
proofing can be done by one of the following methods;
1. External Tanking
2. Internal Tanking
3. Drained cavities
External Tanking: Impervious waterproofing layer is installed on the outside face of the
retaining wall and seamlessly continuous below the raft foundation. To protect the waterproofing
layer from mechanical damage during the backfill behind retaining wall, a protective wall is
required to be constructed to cover the waterproofing layer. External tanking is to be done during
the construction of the basement and should be carefully done since any damage caused to it
which tears waterproofing layer will be extremely difficult to repair after the completion of the
basement construction.
External Tanking waterproofing to basement floor raft and retaining wall

Basement RCC retaining wall with External tanking waterproofing

Internal Tanking: Internal tanking is a process of placing the impervious layer on the inside of
the wall. The advantage of this system is that it can be done after the construction of the
basement and is accessible for any repairs after the construction of basement. The layer of water
proofing is necessary to be protected by providing backing wall against the thrust of hydrostatic
pressure. The disadvantage of placing the waterproofing layer is that the retaining wall and raft is
affected by the penetration of ground water.
Basement wall with internal tanking waterproofing
Drained cavities: This system is not really a treatment of waterproofing but rather a way of
keeping the usable floor and wall of the basement free from any wetness due to subsoil water.
This is employed in the case where the hydrostatic pressure is substantially more or widely
w
variable in different seasons. In such cases, designing the retaining wall and raft slab to resist this
magnitude of pressure would be unviable therefore this pressure on the wall and raft is relieved
by allowing the ingress of water through the wall and collected in sump through the network
net of
peripheral channels. Inside
side panel wall and cavity floor are to be installed so that the usable floor
and the wall surfaces are kept away from this moisture.
Alternative arrangement of drain cavities and the sump for collecting water from cavities
and surface drain

Ventilation: Basement spaces can be used for various purposes, ranging from habitable spaces
of offices, shops, commercial spaces, part of residences to service zones like parking spaces or
installation of STP, UGR, Machinery for air conditioning etc. Ventilation and ambient
environment of basement spaces needs to be maintained as per its use. Ventilation of basement
has Three objectives;
1. Adequate number of air changes and removal of radon gases so that that space becomes
safe for human activity and in some cases human habitation.
2. Removal of fumes/gases during the regular day to day use of space for automobile
parking or operating certain machinery.
3. Emergency removal of smoke during the instance of fire.
Air changes required in the space is as per the activity carried out in that space and also number
of people occupying the space or performing activity in that space. Parking spaces or
underground parking require minimum 6 air changes/hour (as per NBC 2017) while shops and
supermarket need 8-15 air changes/hour.
Openings in the peripheral walls of required size with proper orientation and in some case with
some mechanical assistance is sufficient in most cases to provide the required number of air
changes. Mechanical assistance is chiefly required in cases where openings are not located
appropriately or their sizes are very small. Basements which are 900mm to 1200 mm (as per the
NBC Norm) above surrounding ground level can be installed with ventilators. As per the
required air changes, passive natural ventilation or mechanically assisted by means of exhaust
fans is generally adequate. Basements which are entirely below surrounding ground level
necessarily have to be installed with mechanical system of bringing in fresh air and removal of
used air.
Removal of exhaust gases( in case of basement vehicular parking or operation of certain
machinery) and radon gases is the main concern of the ventilation provided to the basement
floor. For ventilation and removal of exhaust gases by passive natural ventilation, the ventilator
area has to be atleast 5% of the floor area of basement and atleast 50% of it has to be installed on
opposite walls. If the ventilator area is provided around 2.5%, then mechanical system is
required to be employed for removal of CO and other exhaust gasses. This method obviously
relies on a path to outside being freely available, which is not usually the case in underground car
parks - hence the need for mechanical extract systems. These systems use powered fans to
control the build-up of CO in the car park. The regulations for fume exhaust
state that the system should be capable of limiting the concentration of CO within the car park to
below 30 parts per million (averaged over an eight-hour period).
The regulations for smoke clearance state that the system should have an extract facility which is
split into two parts, each part capable of providing50% of the required duty and extracting from
both high and low level.
Natural ventilation is often not adequate for maintaining the required air quality in basement
hence mechanical ventilation systems are used to take care of this situation. Broadly twosystems
are generally used;
1. Ducted mechanical extract system.
2. Impulse system and/or Induction system.

Ducted Mechanical Extract System: These use sheet metal ductwork to transport the fumes or
smoke being extracted to the external atmosphere. The ducts must be evenly distributed around
the car park and also drop to low levels to provide the low level extract points. Accommodating
large ducts can be problematic due to the low headroom in most car parks; low level ducts can be
subject to damage from vehicles. Full capacity of Fans has to be operated irrespective of Smoke
level or CO Concentration. Fresh air inlets are either the ventilators or the shaft opening above
ground level.

Ducted Mechanical Extract System

Ductless Extract System:

Impulse system of Exhaust:

A series of jet fans and exhaust fans are used in ductless ventilation systems for the removal of
exhaust. The jet fans provide a high velocity to the gas stream entering into it and pass it onto the
next exhaust or jet fan which then releases the gas stream to the outside surrounding while also
replacing it with fresh air from outside.

Advantages of Ductless Ventilation Systems

• The first and foremost advantage that these systems have over ducted ones is that due to

the lack of ducts, it is extremely to install ductless systems in car parking spaces. The

time and cost taken for the installation is also considerably lower.
 • With the absence of ducts, the weight of the installation is also less. The jet fans are made

of lightweight, less density corrosion resistant aluminum and the outer casting is made up

of stainless steel.

• The overall height of the basement gets restricted due to the installation height of the

ducts and then the parking space is also convenient for small and medium cars and larger

cars do not fit in. Whereas with ductless systems, one does have to face such issues.

• The jet fans in ductless systems are synced with CO2 detectors and they automatically

adjust themselves to work according to the CO2 level in the parking space. Whereas in

ducted systems, the fresh air and exhaust fans would be required to work all day leading

to more energy consumption.

• The jet, impulse or induction fans are designed to have a characteristic thrust that ensures

a relatively even distribution of air.

• The jet fans can be worked in single or in both directions, according to the requirement.

Access to basement: Vehicular access to basement from and to ground level and people access
to basement from and to ground level and upper storey of the building needs to be given
elaborate consideration for aspects of ‘ease’ and ‘safety’. Vehicular access from ground level to
basement level is generally through a ramp with appropriate gradient ( 1:10 is acceptable, 1:12 is
comfortable and in exceptional cases and for a short length1:8 is permitted.) and preferably in
straight lengths. If the ramp is with turn i.e. circular ramp or ‘L’ or ‘C’ shaped then the bents
should have proper turning radius as per the vehicles expected to access the basement. Vehicle
access to parking areas can be via:

• External ramps
• Internal ramps
• End or centre ramps, or a combination
• vehicle lifts, Mechanical parking systems

With the advent of mechanical Parking system, in certain cases elaborate provision of access
ramps can be altogether avoided.

** Please refer to the attached PDF on mechanical Parking sample manual

Similarly, vehicle circulation can be organized by:

• Combined or separate entry/exit

• One-way or two-way traffic

Parking layout of the basement should be such that it is easy for wayfinding, entry and exit are
clearly marked and on the main vehicular driveway while the secondary driveways gives access
to the individual parking spaces.

**Please Refer to the PPT attached, for the parking and ramp details
Pedestrian Access: Pedestrian access to the basement from the ground level and
upper floors should be through staircases, ramps and elevators. Separate staircase
should be provided from ground floor to basement and the staircase giving access
to upper floors should be discontinued on the ground floor. Fire escape staircases
should not be continued in the basement. The staircase in the basement should be
enclosed in the lobby. Elevators of required numbers may continued in the
basement and should have an enclosed lobby. Separate pedestrian ramps from
ground level to basement level are generally not desirable unless justified by the
specific use of the basement. Staircase bays are continuities of the space connected
to upper floors. Care must be taken that they become the inlet of fresh air instead
of places from where polluted air or exhaust gases enter the upper floors. For this
reason basement ( or at least the enclosed staircase lobbies at basement level)
should have mechanical exhaust which creates and maintains a negative pressure at
the basement level.
Other important features of basement design are;
• Appropriate Lighting
• Fire detection and fire fighting installations.
• Surface water drainage
• Signage for easy way finding

Assignment: Syllabus specifies a manually drafted drawing. A basement space for parking
should be considered from their previous architectural design proposals or a common basement
plan may be considered for the assignment. All the construction details, waterproofing details
and arrangement of ramps and other means of horizontal and vertical movement to be shown.
Provisions of ventilation ( no calculations), fire detection and fighting and lighting to be
indicated in the drawings.

Questions in the End Semester Examination:

Drafting question of showing all the provisions of basement construction as in the assignment
will be very lengthy and unnessary.
Questions related to excavation, construction of raft and retaining wall, water proofing methods
of basement should be included for drafted scaled drawings and description.
Questions based on provisions for ventilation, access, fire detection and fire fighting can be
asked for free hand sketches and notes.