Page 142 - Merton SuDS Design and Evaluation Guide

Detailed Design 9.11.5 Storage structures
Attenuation storage in underground
structures is currently utilised throughout
construction industry with may applications
being in the form of geocellular tanks. Simply
providing underground tanks should not be
confused with a full SuDS approach; however,
they can form part of the SuDS management
train.
■ Where storage is in an underground tank,
failures and blockages tend not to get
noticed, which may mean that the
consequences of failure can be
catastrophic.
■ Underground storage tanks do not have
inherent treatment capacity and therefore
require integration with a SuDS
management train.
137
London Borough of Merton SuDS D & E Guide
The introduction of geocellular structures is
still relatively recent in the construction
industry and the long term implications of
their use is still being understood. The SuDS
manual (Section 21.1) clarifies that:
■ Geocellular systems and plastic arches
tend not to be easily accessible for
inspection or cleaning, so very effective
upstream treatment is required to ensure
adequate sediment removal.
■ The structural design of geocellular
systems tends to be more complex and
there have been a number of collapses of
these systems caused by inadequate
design. (see Mallett et al, 2014, and
O’Brien et al, in press) (see C737)
© 2018 McCloy Consulting & Robert Bray Associates London Borough of Merton SuDS D & E Guide
In addition, to the statements from the SuDS Where underground storage is preferred Detailed Design
Manual the following should also be
after a full exploration of the available
considered:
options the designer should demonstrate
that:
■
There are risks of structural failure due to
construction loading, which may exceed
■
Robust silt removal has been provided
design life loading that the designer may
through means of filtration (bioretention,
not be aware of.
permeable pavement) or other source
■
There are a wide range of attenuation
not be accepted as a demonstrable form
products each with its own loading
of silt removal. The SuDS manual (Section
characteristics. Surety must be provided control SuDS components. Catchpits will
that a specified product is not swapped 4.1) clarifies that sediments within
for one of inferior quality during the catchpits can be remobilised and washed
construction phase. downsteam. Equally, gullypots are
■ suggested by Table 26.15 to provide
Guarantees and warranties are dependent negligible to zero treatment (Ellis et al,
on the survival of product manufacturers. 2012).
■ Underground structures require structural
design consideration even if they are not
receiving vehicular loading. CIRIA report
C737 outlines the design requirements for
geocellular tanks. The SuDS Manual
(Table 21.1) provides a summary of the 138
structural design requirements using a risk
classification system (Scored between
0-3). Designers should demonstrate that
the classification system has been
followed and present the appropriate level
of design information accordingly.
Design Note:
Where the stated design life of the tank does not meet the design life of the development,
the design should demonstrate how the structure will be replaced whist maintaining the
functionality of the drainage system and the scheme. Consideration should also be given to
funding mechanism for undertaking these replacement works.
© 2018 McCloy Consulting & Robert Bray Associates