Case 1

Good Practices

Water Ponding

Design

Adopt proper drainage design (slope/outlet). Adopt the proper compaction practices of ground/soil to maintain an even settlement in accordance with BS 8004, EN 1997 or equivalent. It is presupposed that the requirements in basement construction comply with applicable statutory and regulatory requirements.

Construction

Perform proper levelling of the floor surface to avoid ponding in falls for wet areas. Establish the full specification and procedure for repair, propping and testing, and handing over with the as-built drawings.
Observe construction requirements on drainage in accordance with the Code of Practice on Surface
Water Drainage.

Joint

Design

Construction joints in the basement structure should be protected by waterstops [1-8]. Waterstops used should be compatible with the waterproofing membrane and can be placed internally or externally (Figures 1-3). In addition, the choice of waterstop design (Figure 4) should be appropriate with the characteristic of the joint to which they are applied to (Table 1) [1-2]. Generally, external waterstops should be at least 240mm in width to be effective.

Table 1: Characteristics of joints

Construction

Waterstops must be fixed and held firmly in place until concreting is completed. Otherwise, the waterstop may fold during concreting and prevent the effective functioning of waterstops (Figure 5). The following safety measures can be used:

• Internal water-stops can be secured by using wires to hold it in place.
• External water-stops are tied to the formwork of concrete works.
• Flanges of water-stops should be tied to the reinforcement steel at an interval of 300mm along the flanges.
• Bulb of the water-stop should be placed directly on to the midpoint of the joint for efficient expansion actions.
• Individual sections must be properly jointed by special junction pieces.

Hydrophilic waterstops should not be exposed to liquid prior to concreting, as it will expand and be rendered useless. Also, external waterstops should be placed in position prior to the lying of steel in order to prevent accidental abrasion and the punctuation of the waterstop.

Waterproofing

Design

BS 8102:1990 has identifies 3 forms of constructions as:

Type A as tanked protection – The protection depends on the application of a continuous waterproofing barrier system applied to the structure. It comprises a system that forms a closed tanking, surrounding the structure on all sides (Figure 6).

Type B as structurally integral protection – The structurally integral protection system comprises of only the reinforced or prestressed concrete structured designed to either SS CP 65 or BS 8007 (Figure 7-8).

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 9).

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) [2].

Construction

Care should be taken to prevent the accidental punctuation of the waterproofing from steel fixing or nails and debris. In vertical walls, 6mm thick polyethylene should be provided as a slip-sheet to protect the waterproofing treatment from being damaged due to slippage during backfilling (Figure 9).

Drainage

Design

Drainage detailing of external wall of the basement is to provide protection against direct exposure of high hydrostatic pressure from soil water on the structure. Effective drainage design should be capable of collecting, draining and discharging water away from the basement envelope. They should consist of the three fundamental items (Figure 10):

1. A perforated drainage pipe installed with perforation set downward in a bed of gravel that allows water drainage. These pipes are usually made of PVC or vitreous clay.
2. Coarse gravel installed around and over the drainage pipings for percolation and collection of water.
3. Sump pits or other drain fields installed to collect and discharge the drained water from pipes

The landscape outside the basement should be designed to a fall in order to provide proper draining of water (Figure 11).

Tiling

Material

Various types of granite tiles present different physical characteristics and chemical properties. These tiles should be fully tested to meet the performance requirements stipulated in the contract before mass installation. Table 1 shows the expected standards for granite tiles performance.

Table 1: Expected standard of granite tiles

Sealers for stones should be selected if tiles are porous. This is to prevent water from being retained in the pores. Sealers should have the following properties:

1. virtually odourless
2. non yellowing
3. deep penetrating protection
4. weather resistant
5. maximum stain resistance.

Ventilation

Adequate ventilation should be provided to enhance the drying process and reduce the possibility of dampness. Proper air circulation can be designed by natural, mechanical or a combination of both mechanical and natural ventilation systems ,in which minimum air change per hour (ACH) should be 10 [9]. However, if entirely mechanically ventilated systems, minimum ACH should be 15.