Basement

Design

Waterproofing system

Any single system or a combination of two or more systems are selected as per various facts: usage, nature of soil, ground water level, presence sulphates or aggressive chemicals, depth of structure, feasibility of application of waterproofing system etc. There are three major waterproofing systems adopted:

  1. Type A (tanked): concrete or masonry structure with no special water tightness. Performance depends on continuity of the barrier of waterproofing element, such as liquid applied membrane (LAM) and prefab sheets or roll.
  2. Type B (structurally integrated): RC or prestressed concrete with reduced or low permeability.
  3. Type C (drained): the walls of RCC or masonry have cavity that collects in which any water entering the basement is drained off.

Structural concrete

The walls and slabs constitute the structural member. SS CP 82 recommends only reinforced concrete for the outer wall. However, for drained cavities, the inner non-load bearing wall of good quality masonry. Functionally, reinforced or pre-stressed concrete for type A must withstand the imposed loads, including the maximum external water head without showing signs of movement or cracking other than at predefined movement joints. The structure should be adequately stiff at all stages of construction to minimize the occurrence of differential movements especially for pile foundations. In the case of Type B or structurally integrated system, apart from strength, the concrete should be dense, impervious and perform as monolithic element. Type C system is built along with Type A or Type B systems and hence will accordingly follow the specifications.

Waterproofing membrane for tanked or Type A system

Waterproofing membranes are essential components of tanked protection. Tanking can be mainly of 4 types, namely, (1) external or on positive side; (2) reverse or on surface of trench; (3) sandwiched or in between two construction layers and (4) internal or on negative side. Positive tanking is the most common among all. Usually tanking is not applied to exposed floors as they are vulnerable to weaning and cannot resist external hydrostatic pressure. Hence for large deep basements, Type A system is not suitable.

For tanking, either liquid applied membrane (LAM) or preformed membrane is used. Material for waterproofing membrane should be selected based on: long term water tightness, durability in exposure, ease of application, curing period, flexibility, tensile property, puncture resistance, compatibility with substrate etc.

Waterstop for integrated or Type B system

For deeper basements, where positive side tanking is not economically or technically viable due to limited access, Type B system is adopted. RCC or pre-stressed concrete structure minimize water penetration or to prevent water ingress. For 100% water resistance, additional passive protections are needed such as Type A or Type C system and waterstops at vulnerable points of RCC such as construction or movement joints. By crossing the joints; it provides a barrier or longer path for water. However, an incorrect type or positioning may reduce water resistance of the concrete. Waterstops are usually made of rubber, PVC or hydrophilic material of various preformed shapes.

Drained cavity or Type C system

Drained cavity method when used along with Type A or B, provides a high precision of protection. Its success relies on ability of cavity to prevent passage of water. Hence inner walls are usually free standing masonry and the cavity should be kept clean and well ventilated. Masonry blocks can be either burnt clay and shale bricks or precast concrete blocks. Water may move through masonry under the action of gravity, evaporation and capillary action. Hence the main aim is to keep it segregated from such moisture path. Durability depends on soundness of blocks and its compatibility with mortar.

Drainage system

It is essential to reduce hydrostatic pressure around the basement structure and prevent the entry of stormwater. An effective surface water drainage plan serves the first purpose (details in BS EN 752-3 and PUB Code of practice on surface water drainage. Second one is achieved through proper detailing in the basements. Where such ingress is unavoidable, the catchments contributing to the ingress shall be kept to the minimum and an adequate pumped drainage system should be arranged.

Flooring

Basements can have various types of usage starting from basic to special. Usually the lowermost level is allotted to carpark and plant rooms exposing the floors are exposed to moderate abrasion from rubber tyre traffic. The flooring in upper levels has usage alike any intermediate floor of building and hence, for the purpose of basement maintainability only lowermost level is considered. Such floors are designated as ‘AR4/WS’ in BS 8204-2 with a maximum test wear depth of 0.4 mm and designed to accommodate moderate abrasion for light duty industrial and commercial application. Three different types of wearing floors are common:

  1. Direct finished (DF) concrete.
  2. DF with upgraded base by dry shake/sprinkle finishes.
  3. Wearing screeds (WS) or granolithic topping – the WS is laid while the base concrete is still plastic. Separate bonded construction of WS is no more recommended by codes.

Wall finishes

Unlike façade, the internal finishes of basement walls are not subjected to harsh elements. On the contrary the walls are prone to biological staining (especially fungi) due to absence of natural lighting and presence of humidity. The finishes should be durable against water seepage, cracking and dirt accumulation for a better performance.

Grading for waterproofing system selection

Factor Grading criteria Grade
Usage Usage Performance Yes: 5
No:1
Grade 1 (basic utility): parking, plant rooms except electrical, workshops Some seepage and damp patches tolerable
Grade 2 (better utility): workshop & plant room of drier condition, retail storage No water penetration, but water vapour tolerable
Grade 3 (habitable): ventilated residential, working areas, office, restaurants Dry environment
Grade 4(special):  archive & stores requiring controlled environment Totally dry environment
Note: the use vs. performance should be cross checked with design brief. For example, ‘some seepage’ may not be acceptable in basement with basic utility depending on the building status.
Feasibility of application For shallow basements usually open cut method and for deep basement usually cut & cover or top-down method are adopted. For type A, waterproofing barrier should be on positive side. Masonry structure is not considered suitable as per SS CP 82. Yes 5
No: 1
Basement depth Type of water proofing system for various use
Grade 1 Grade 2 Grade 3 Grade 4
Shallow (≤ 3 storey)
  • RCC+(A/C)
  •  B
  • sheet pile+C
  • RCC+ (A/ C)
  •  B
  • PCC+A+C
  •  B+ (A/ C)
  • B+A+C+ preferred active system
Deep (> 3 storey)
  • B
  • Piled wall + C
  • B
  • B+C
  • B + (A /C) +may need active system
  • B+A+C + active control
Compatibility with soil condition
  • Presence of sulphate & aggressive chemicals. It determines material quality and system selection as per BRE Special Digest (2005), BS 8110 and BS 6349.
  • Ground water table calculated w.r.t. the depth of basement. The actual site data if different should prevail the following thumb rule
 Yes: 5

No: 1
Basement depth Water table level for design purpose
0.00 −1.33 m 1.00 m above base level
1.33 −4.00 m One quarter of the basement depth below ground level
> 4.00 m 1.00 m below ground level

Grading for design of structural concrete

System Protection provided by Significance of concrete quality Grade based on requirements met*
All Most No
Type A (positive tanking) Membrane Moderate 5 4 3
Type A (negative tanking for repair jobs mainly) Membrane but seepage may cause deboning High 5 3 1
Type B Structure Extreme 5 1 1
Type C Cavity. Some seepage thru’ outer wall is acceptable Moderately high 5 3 1
* Requirements = CONQUS21 specifications applicable for basement as follows:
Factors Recommendations for good structural concrete
System / const. method selection
  • Functional requirements as resistance to water, dampness, or aggressive soil condition.
  • Structural requirements including dead load, live load and the changes in vertical loading imposed by the movement of soil during construction
  • Feasibility of construction methods and sequences to accommodate provision of access, ground water control, other parallel activities etc:

 
Const. type Const. method Floor Wall Water & vapour resistance
Primary Secondary
RC box Open cut Monolithic Integral Conc. of low permeability Tanking
RC box Temp. sheet pile Monolithic Integral -do- Drained cavity
Diaphragm wall Excavate later Become strut As cast / faced -do- -do-
Secant pile -do- -do- Facing Facing -do-
Contiguous pile -do- -do- Subst. facing Substantial facing -do-
Design (mix design & rebar)
  • Design of concrete as per SS CP 65 or BS 8007 or BS 8110 as applicable.
  • Reduced permeability by using admixes (e.g. water reducing agent, pozzulanic products, pore refiner etc.).
  • Should be dense, free of crack or any differential movement. .
  • Sharp changes in cross section of floors and walls should be avoided.
  • Minimum wall and floor thicknesses should be 250 mm with suitable % of rebar to negate serviceability crack width.
  • Use of galvanized, epoxy coated, or stainless steel rebar in special cases where 250mm section thickness is not feasible.
  • Special consideration for rebar design at the structural joints due to flexure.

Note: Crack in any RC structure is unavoidable. A crack which is acceptable as per mix design, may allow seepage which is unacceptable for a basement. Hence water resistance should be given higher priority in design.
Joint detail Water tightness depends on how monolithic is the construction. Hence number of structural or construction joints should be minimized by:

  • Floor slab / walls should be cast in bays with transverse induced contraction joint. Closing pours should match with lapping of 1500 mm of waterproofing membrane.
  • No sharp changes in cross section at junction wall.
  • Special reinforcement detailing at the joints.
  • Provision of waterstop is desirable externally or internally on case by case basis.
Penetration details Service penetrations are weak points and vulnerable to leakage.

  • Services should be grouped, pre-planned and boxed out to minimise penetration through water proofing. Installing penetrations in cast-in-situ sleeves allow independent movement of pipes and reduce coordination between different trades.
  • No congestion around pipes for easy pouring and vibration of concrete.
  • Additional reinforcement may be required to counteract concentration of shrinkage stress especially at corners of openings.

Grading for design of tanked protection (Type A) system

Factor Grading criteria Grade
Location Application on positive side so that hydrostatic pressure can push the membrane against the basement wall. For open cut method it is easy to apply. 5
Positive waterproofing for confined sites, which uses soil retaining systems (sheet / contiguous piles) special provision needed. 3
Application on negative side has risk of debonding. Hence recommended only for remedial action. For cast-in-situ construction, this method is not suitable and needs additional aid of water management system or active system. 1
Application
  • Continuous from lowest part to at least 150mm to finished ground level.
  • Extended at least up to 300mm above the maximum ground water level for non-cohesive soil and proposed ground level for cohesive soil.
 Yes: 5
No: 1
Material selection Liquid applied membrane: Hot: 5
Cold 4
Pros
  • Easy application on both horizontal and vertical surfaces. Seamless for complex geometry and protrusions.
  • Fast curing, reinforced and thick – very good puncture resistance.
  • Built–in primer allows application for both substrate and screed.
  • Better dry film thickness for hot application systems.
Cons
  • Needs protection and well-prepared dry substrate.
  • High TVOC content.
Self-adhesive membrane: 2
Pros
  • Moderate flexibility and tensile property.
  • Can be applied on both horizontal and vertical surface.
Cons
  • On exposure to UV ray, suffers from blistering and debonding.
  • Substrate priming and pressure rolling required.
Bentonite or hydrophilic system: 2
Pros
  • Self-healing, hence no protection required.
Cons
  • High vapour permeability on partial hydration.
  • Limited application (only for constant hydrostatic pressure). Mainly suitable for reverse tanking in deep excavation.
Built–up membrane: 1
Cons
  • Labour intensive, field fabricated product.
  • Slippage, puncture, splitting, defective flushing is common
Cementitious: 1
Cons
  • More suitable for simple application like residential basements.
  • Needs sufficient thickness to render existing cracks and cannot cater to the movements of live cracks.
Material property Required material properties of suitable standards but not restricted to the following list should be considered:

  • Permeability, water absorption.
  • Adhesion to substrate, resistance to leakage at joint.
  • Tensile strength, elongation to break, crack bridging.
  • Durability against exposure condition.
  • Resistance against perforation and laceration.
  • Durability: preferably have useful life same as design life of building.
 Yes: 5
No: 1
Joint details Flashing or line of joints between two structural surfaces is vulnerable to weathering and mechanical damage.

  • For slabs abutting basement, water proofing should turn below the soffit of ground slab for min 600mm.
  • Special attention given to sections, where basement has different depth or sharp changes in section.
 Yes: 5
No: 1
Penetration details Penetration for services affects the continuity of protective membrane. Hence the design aim is to minimise the number of penetration by grouping and boxing out the services. Details as follows:

  • A min 150 mm double coat at fold junction, min lapping of 150 mm and angle fillet at sharp turns is must.
  • Minimum gap of 6” between two penetrations should be kept for an effective flashing. Else the penetrations should be of min 1” dia for proper sealing with a pourable grade over a sheet of metal form of same diameter.

Penetration through water/ vapour proofing membrane

 Yes: 5
No: 1

Grading for design of waterstops (Type B)

Factors Grading criteria Grade
Shape selection Selection depends on: type of joints, and location. A suggested guideline as follows: Yes: 5
No: 1
Water-stop Characteristics of joints
Ribbed flat & dumbbell Construction or contraction joints where little or no movement is expected. Ribbed one is a better choice.
Base seal Slab-on-grade joints or walls to be back-filled. Easy fixing.
Labyrinth Vertical joints where little or no differential movement is expected. Does not require split forming and adds a key to the joint. Difficult to be used in horizontal joints.
Split Eliminates the need for split form bulk head.
Ribbed/ dumb bell with centre-bulb Most versatile type. The centrebulb accommodates lateral, transverse and shear movement. Larger bulbs have higher tolerance for movements.
Tear Web For large movements. U-bulb ruptures and deforms without putting the material in tension during joint movement.
Various types of water stop (Kubal, 1999)
Material Should have lifespan same as design life of building and easy to install.
PVC: Most commonly used for easy installation and available in many specialized sections. 5
Rubber: More suitable for movement joints (less likely to be required in basements). It is difficult to install at site due to the need of hot vulcanised process to form an effective joint. 1
Hydrophilic:  joint width should not be very big or very small compared to the swelling property. A slight delay in concreting renders the waterstops useless. 1
Detailing 1.      Should be fully continuous system at all joints (except settlement joints, where longitudinal force pulls out the waterstop).

2.      Internal placement in horizontal direction should be avoided as concrete cannot be compacted around along the full length.

3.      Use of factory made specialized sections instead of site fabricated one.

 All: 5
Most: 3
No: 1

Grading for design of cavity (Type C)

Factor Grading criteria Grade
Cavity wall design 1.      Ties to stabilize the inner skin should be nonferrous and dimpled or twisted to shed any water collected on them.

2.      Provision of access opening for maintenance of channels.

3.      Well ventilated by low level vent. Mechanical ventilation for limited air flow.

 Yes: 5
No: 1
Masonry block & mortar selection 1.      For a good quality work, the masonry blocks should be strong, dense, impermeable to water and of uniform dimension.

2.      Based on mix, water permeability, strength and ability to accommodate movement, mortars can have four designations: (i), (ii), (iii) and (iv).  The selection depends on types of masonry blocks:

3.      Clay bricks: all four.

4.      Calcium silicate unit, aggregate concrete bricks or blocks: (iii), (iv).

5.      Note: For ‘designation (iv)’ mortar, all masonry units, mortar and masonry under construction should be protected fully from saturation.

 Yes: 5
No: 1
Cavity floor selection Existing condition Type of cavity Yes:5
No: 1
Seepage is less No-fines concrete of min 300mm thickness
High vapour ingress / less space for excavation Proprietary system (profile drainage sheet, purpose -made tiles) are shallow and act as vapour barrier
Pumps on raised floor need access to drainage channel Wider cavities made of precast concrete planks
Drainage Water collecting points, channels or scupper drains laid to 1:100 fall and discharged into sumps. Yes: 5
No: 1

Grading for design of drainage

Factor Grading criteria Grade
Sump pump 1.      A sump pump of sufficient capacity to be provided.

2.      Pumps that run intermittently have higher chance to break down, so standby pump should be provided along with emergency power source.

3.      Automated device to start the pumping operation with storm water ingress, but equipped with manual control.

 Yes: 5
No: 1
Catchments 1.      The catchments should be totally segregated from surface drainage by:

2.      Access to the basement should have a crest level min. 150mm higher than the platform level.

3.      Runoff from the roofs, rainwater downpipes and all premises at and above ground level should be channelled into surface gravity drains.

4.      Cut-off drains across the access way.

 Yes: 5
No: 1

Grading for design of flooring

Factor Grading criteria Grade
Material A17 , A20 1.      All material should work as monolithic unit to comply with requirements such as, durability same as design life of building, strength, dimensional stability, resistance to wear, slip and dampness.

2.      For both DF and WS flooring screed, mix should be cohesive (low water cement ratio) to prevent excessive bleeding and allow finishing within the specified time. Ready mix concrete provides better quality.

3.      Careful selection of fines (sand). Its quality, grading, particle shape and strength are critical contributors for flooring durability.

4.      No service pipes should bury in the screed.

 Yes: 5
No: 1
Additives 1.      Conscious use of air-entraining superplastisizing or set retarding ad mixers, low heat Portland cement or in-surface organic resin sealer is beneficial.

2.      Penetrating resin in-surface sealers and surface hardeners can be used.

3.      Alkyd based paints should be avoided. It causes saponification.

 Yes: 5
No: 3
Finishing 1.      Falls (max 1:40) on base concrete for good drainage.

2.      Fall should be in finishing not in screed.

3.      To avoid cracking, induced contraction joints should be provided at about 6 m to 8 m centers in both directions by:

4.      Sawing.

5.      Grooves made by vertical strip to a depth of 1/4 -1/3 of the slab thickness.

6.      Plain vertical butt joints with load transfer devices at day work positions.

 Yes: 5
No: 1

Grading for selection of wall finishes

Factor Grading criteria Grade
Paints selection 1.      Basement walls are subjected to wetness and dampness. Paint with breathable system can reduce the amount of trapped moisture.

2.      Alkyd based paints should not be used on concrete surface. The fatty acid and alkali from concrete react (saponification) to form oily patches.

3.      Fungus resistance and resistance against dampness are required.

 Yes: 5
No: 2
Tiles selection 1.      Porosity should be less to keep water absorption under control.

2.      Resistance to cracking from soaking or transverse elongation.

3.      Uniform dimension and thickness to match joint dimension or alignment.

4.      Adequate thickness to resist cracks form direct impact.

5.      Proper key at back for a good bonding.

 Yes: 5
No: 2

Grading for consideration for ancillary facilities

Factor Grading criteria Grade
Coordination among professions Two-way communication:

1.      The requirements of basements are explained to designers / contractors of other professions.

2.      Basement design incorporates requirements of other professional.

 5
One-way communication – poor co-ordination. 3
No communication and coordination 1