Case 2
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Drainage
Design
Basement walls must be protected from a hydrostatic head of water and must also be able to control the movement of moisture by diffusion and capillarity from the soil into the basement.
The standard procedure is to install a drainage tile or perforated plastic pipe and crushed stones around the basement so that water will not reach the under side of the basement floor slab. Usually, the drainage mat is located outside the footings, with the bottom of the tile at the same level as the base of the footings and connections are made through the footings to the crushed stone under the floor slab. The tile or pipe is normally covered to a depth of at least 6 inches with crushed stones or other coarse granular material which acts not only as a filter to exclude fine grained soil particles but also, with the pipe, as a drain.
The landscape outside the basement should be designed to a fall in order to provide proper draining of water. A drainage system should be effectively designed to prevent water from the open planters to build up hydraulic pressure against the wall. During the construction stage, supervision is crucial to ensure compliance with design.
Waterproofing
Design
BS 8102 has identifies 3 forms of constructions as
Type A as tanked protection – The protection is dependent on the application of a continuous waterproofing barrier system applied to the structure. It comprises of a system that forms a closed tanking, surrounding the structure on all sides [3-4] (Figure 3).
Type B as structurally integral protection – The structurally integral protection system comprises only the reinforced or prestressed concrete structured designed to either SS CP 65 or BS 8007 [2-4] (Figure 4).
Type C as drained cavity protection – The drained cavity protection system comprises cavity floors and walls with drainage channels leading to sumps, from which any water penetrating into the basement can be pumped away (Figure 5).
However, depending on the purported usage of the basement, the expected level of performance must be consistent with the inherent constraints of the 3 forms of construction identified (Table 1) [4].
|
Grade
|
Basement Usage
|
Performance Level
|
Form of Construction
|
Comments
|
|
1 | Car parking, plant rooms (excluding electrical equipment) and workshops. | Some seepage and damp patches tolerable. | Type B -reinforced concrete design to BS 8110 (normal concreting standards. | Groundwater should be checked for aggressive chemicals. A cementitious waterproofing system may be used to improve water-resistance. |
|
2
| Workshops and plant rooms requiring drier environment, retail storage areas. | No water penetration but moisture vapour tolerable. | Type A. Type B – reinforced concrete design to BS8007 (water resisting concrete) | Careful supervision of all stages of construction is necessary. Membranes can be applied in multi-layers with well lapped joints. |
|
3
| Ventilated residential and working areas including offices, restaurants etc, leisure centres. | Dry environment. | Type A. Type B – reinforced concrete design to BS8007. Type C – with wall and floor cavity and damp proof membrane below floor screed. | As grade 2. |
|
4
| Archives and stores requiring controlled environment. | Totally dry environment. | Type A. Type B – reinforced concrete design to BS8007 plus a vapour proof membrane. Type C – with ventilated wall cavity with vapour barrier to inner skin wall and floor cavity with damp proof membrane below floor screed. | As grade 2. As grade 1. |
Table 1: Performance expectations for basements, source [4]
Construction
Backfilling should be done with a granular material compacted to about 88% Modified Proctor Density [5]. Care should be taken not to over specify-compaction. Experience showed that less stress would be induced in walls during backfilling and better drainage is achievable with 88% compaction. Compaction that is less than 88% would be likely to induce some settlement.
Protection of waterproofing membrane during back fill is very important. The exact location of a punctured membrane is difficult to determine. Once the membrane is punctured, the migration of water within the concrete would be difficult to detect (Figure 6).
Dampness
Design
The material selection for exposed surfaces, and their regularity and surface texture should be detailed to
prevent moisture retention. Areas prone to high moisture exposure shall be made impervious with adequate waterproofing systems. Also such surfaces should be without impediments (e.g. built-in cabinet, fixed equipment/services) to ensure access for inspection and ease of cleaning. Access for adequate cleaning should be provided in accordance with BS 8221- 1, SS 509-1 or equivalent. Ventilate to prevent moisture retention on floor/wall/ceiling. For natural ventilation, opening > 5% of floor area. For mechanical ventilation, air exchange rate > 20 air changes per hour, in accordance
with “CP on Environmental Health”.
Construction
Exterior surfaces of porous building material (e.g., cellulose, brick, stone, cement rendering) can develop biological growth. Avoid such growth as much as possible with treatments of anti-algae/anti-fungus solutions and allow to dry before painting/repainting (SS 652: B.5.2.1). Improve ventilation and remove sources of dampness to dry out the substrates as thoroughly as possible during painting works in
accordance with BS 6150, SS 542 or equivalent. Ensure access ducts are connected to the mechanical
ventilation system. The exhaust sy
Ventilation
Design
A proper ventilation system is very important for the basement. It is necessary to remove the exhaust from carpark and pump in fresh air for the people in basement. Therefore, engineers should pay special attention to ensure that the system chosen is effective.