September September Consulting Structural and Civil Engineers. 100 St John Street London EC1M 4EH. akt-uk.com

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1 Consulting Structural and Civil Engineers 100 St John Street London EC1M 4EH T +44 (0) F +44 (0) akt-uk.com A The Joseph Kings Bloggs Road Cultural Centre SHORT STRUCTURAL REPORT ASPECTS TITLE OF BASEMENT September 2007 WATERPROOFING September 2013

2 2 Contents 1 INTRODUCTION 2 DEFINITIONS 3 GRADE OF ENVIRONMENT 4 FORMS OF CONSTRUCTION 5 DESIGN RISK ASSESSMENT GUIDE 6 DESIGN CONSIDERATIONS 7 REFERENCES A PRELIMINARY Revision Date Status Prepared by: Alex Widdison Checked by: David Watson Approved by: Stelio Papastylianos

3 3 1 Introduction This report covers the structural aspects of providing water tightness to the basement areas of the development. It covers the basement floor and walls only with respect to protection against groundwater. BS 8102 (1990) Code of Practice for protection of below ground structures against water from the ground states that a clear understanding of the degree of water tightness is required by the Client before detail design commences. The information provided in this report together with architectural requirements and recommendations ventilation and drainage implications a full comparative cost assessment will allow recommendations on the final choice of basement waterproofing system to be made to the Client. Once a decision on the appropriate waterproofing system has been made AKT II will design and take responsibility for that part of the system which is an integral part of the main structural elements, this would cover concrete water retaining structures and any designed joints within the concrete substructure. The design of any water proofing additives to concrete will be the responsibility of the Supplier. The design of any water proofing system that is not an integral part of the main structure will be to the design and under the responsibility of others.

4 4 2 Definitions This report refers to water-resisting properties rather than using the terms waterproof and watertight, which are commonly misunderstood in relation to concrete construction. Water-resisting describes the range of ability to resist water but not necessarily to the degree of being impervious, or absolute, in the prevention of water penetration. The definitions associated with water, vapour and moisture adopted in this report are given below (based on current guidance from the references in section 6). The distinctions made in these definitions deal with properties that are comparative rather than absolute. Vapour Water in its gaseous form Moisture Water in the form of vapour as well as liquid Waterproof Impervious to water, not permitting water to penetrate Watertight Waterproof Water resistant Has a high resistance to water penetration Water resistance The extent to which a material prevents water penetration Vapour proof Impervious to water and vapour, not permitting water or vapour to penetrate Damp The condition of a material with a higher water content than dry air, but not wet Damp proof Impervious to moisture, not permitting moisture to enter Damp proof membrane A moisture and water vapour resisting material Hydrostatic Water pressure, expressed as an equivalent depth of water Passive precautions Fixed methods or materials are used in the construction of a basement to provide varying degrees of moisture resistance to control the internal environment. They do not respond to changes in conditions. They are generally difficult, if not impossible, to replace or alter after construction. To be defined by the Architect and Structural Engineer. Active precautions Method, or processes, used to control the internal environment, capable of a limited range of response to variations of external conditions. Methods include pumped drainage, heating and ventilation. They require inspection, maintenance and possible replacement during the life of the building. To be defined by the Architect and Services Engineer.

5 3 The most important parameter which needs to be agreed is the grade of environment required in the basement. The following table gives guidance on the functional requirements for various types of basement usages which Grade of environment need to be considered by the client and design team as the grade of protection chosen influences the architectural treatment of the basement and also the active measures necessary to control the environment. These are beyond the structural aspects of basement waterproofing. GRADE OF BASEMENT BASEMENT USAGE (BS 8102) RELATIVE HUMIDITY TEMPERATURE Guidance on the functional environmental requirements for basement usage (CIRIA guide 139 and BS 8102) PERFORMANCE LEVEL DAMPNESS WETNESS FORM OF CONSTRUCTION (BS 8102) Kings Road 5

6 4 Forms of construction Kings Road BS 8102, the protection of below ground structures against water from the ground, identifies three specific methods of water-resisting construction: Tanked Protection Structurally Integral Protection Drained Protection The following table gives guidance on the advantages and disadvantages of each form of construction. ADVANTAGES DISADVANTAGES ALTERNATIVES GUARANTEE 6

7 7 5 Design risk assessment guide The following table gives guidance on level of risk of water, moisture and water vapour penetration associated with each form of construction under different hydrostatic conditions on site. It takes into account design and construction difficulty in reaching the design purpose. It also indicates what grade of basement is likely to be achieved as the water table level and construction method changes. Achieves Grade 1 Achieves Grade 2 or lower Achieves Grade 3 or lower Suitable for Grade 3 with enhanced active ventilation / de-humidification (previously Grade 4 under BS ) * Design to 0.2 mm crack widths except for Low hydrostatic head ** Alternative design to BS EN mm crack width for low or variable hydrostatic pressure Subject to prevailing soil conditions Plus tanking or drained cavity Low INCREASING RISK Water table Variable High Not recommended Grade 1 Grade 2 Grade 3 Grade 3 with enhanced ventilation / de-humidification Low risk Tanked Plus external land drainage (at base of wall) BS EN * Plus tanked BS EN ** Plus tanked BS EN * BS EN ** Plus moisture barrier DECREASING RISK Type A Tanked, no integral protection Type B Structurally integral protection (water resistant concrete) Type C Drained cavity

8 8 6 Design considerations 6.1 Site issues The water levels on the site are assumed to be generally above the level of the basement construction at this stage, but this will need to be determined by a site specific geotechnical investigation. The code of practice BS 8102 (1990) Protection of Structures against Water from Ground requires further consideration of the effect of waterlogging in a storm condition. The hard ground surfaces and surface water drainage on site reduce the risks of this occurring compared to a typical greenfield site. There is currently no data on the perched water table local under the site, however this will be addressed during the geotechnical investigation. The code states that the water table might need to be considered to be at the existing ground level, under certain unfavourable conditions (Section 6.1 BS ). The loading condition generated by this head of water would result in an extremely onerous design case for the basement slab. The basement slab will therefore not be designed for this condition. The retaining walls could however be subject to this type of loading during a storm or accidental event and will thus be designed for this pressure. This basis for design is judged to provide a reasonable level of safety with little cost implication. It is anticipated that the existing surface water drainage around the site is sufficient to avoid such a build up of water across the site as a whole. The introduction of an external drainage system as an additional line of defence to drain water away from the structure will place less dependency on the requirement of the water / vapour proofing membrane, however this would not be an efficient solution unless installed at or below the basement level. The issues relating to the rising groundwater in the deep Aquifer below London are outside the scope of this report. It is recognised that these issues are concerned more with the geotechnical aspects of this scheme than the waterproofing. 6.2 Requirements of the building regulations The internal environment must at least satisfy the performance requirements of the Building Regulations Where an external wall or floor is subject to ground water pressure the Building Regulations refer to BS 8102 (1990) Protection of Structures against Water from Ground for recommendations. This document has been referred to in consideration of the structural aspects of this report. The remaining requirements of the Building Regulations are considered to fall within the brief of the architect and services engineer. Selection of the most appropriate form of passive water and vapour protection is the preliminary step prior to considering the active precautions required to achieve the desired internal environment and whether the basement would comply with ventilation, thermal transmission and resistance to moisture penetration requirements.

9 9 7 References BS EN Concrete: Liquid retaining and containment structures BS 8102 Code of practice for protection of below ground structures against water from the ground BS EN Concrete: General - Common rules for buidlings and engineering structures BS EN Steel: Plated structural elements CIRIA report 139 Water resisting basements