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Annex  I    

 Design  of  Pervious  Pavement  System

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PERVIOUS  PAVEMENT  SYSTEM  DESIGNS   1  PERMEABLE  BLOCK  PAVING  DESIGN   4  HYDRAULIC  DESIGN   4  STRUCTURAL  DESIGN   5  POROUS  CONCRETE  PAVEMENT  SYSTEM  DESIGN   6  POROUS  ASPHALT  PAVEMENT  SYSTEM  DESIGN   9  

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Pervious  Pavement  System  Designs  This  document  describes  how  the  three  pervious  pavement  systems  under  investigation  

were  designed.  In  order  for  a  fair  comparison  of  environmental  impacts  to  be  assessed,  

each  option  is  designed  to  fulfil  equal  functions.    

 

Structural  Function:  

The   pervious   pavement   systems,   including   all   subsurface   layers,   are   designed   for  

pedestrian  and  light-­‐vehicle  use   in  an  urban  environment.  The  design   life   for  all   three  

pavement   systems   shall   be   20   years   as   recommended   by   The   SuDS   Manual   (CIRIA,  

2015).   Structural   maintenance   will   be   considered   on   an   individual   basis   due   to   the  

different   durability   characteristics   of   each   system.   The   surface   and   subsurface   layer  

thicknesses  must  be  calculated  for  each  pavement  type  under  intended  loading.    

 

Hydraulic  Function:  

Pervious  pavement   systems  provide   the  additional   function  of  water  management.  As  

such,   the  design   rainfall   intensity  must  be   stipulated   to  ensure  adequate  depth  of   the  

attenuation  layer.  The  Environmental  Agency  and  SEPA  suggest  that  a  20%  increase  in  

the  1   in  100  year  rainfall   intensity   is  used  for  design  to  allow  for  more  severe  rainfall  

events  as  a  result  of  climate  change  (Interpave,  2010).  An  assumption  will  be  made  that  

the   pervious   pavement   systems   are   to   be   constructed   in   the   City   of   Glasgow   in   the  

United   Kingdom.   The   map   shown   in   Figure   1   was   developed   by   HR   Wallingford  

(Kellagher   and   Laughlan,   2005)   to   define   eight   hydrological   zones   within   the   United  

Kingdom.    

• ‘M100  –  6hr’  is  the  1  in  100  year,  6  hour  duration  rainfall  

Therefore  it  can  be  seen  that  the  1  in  100  year,  6-­‐hour  duration  rainfall  for  the  City  of  

Glasgow  is  71mm.  It  shall  be  more  useful  to  have  rainfall   intensity   in  units/hour,   thus  

the   1   in   100   year,   1-­‐hour   duration   rainfall   for   the   City   of   Glasgow   is   12mm.   With  

allowance  for  climate  change,  the  design  rainfall  intensity  shall  be  15mm/hr.    

 

 

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Figure  1  –  100  Year,  6  Hour  Rainfall  Depths  for  Hydrological  Regions  Across  the  UK  (Kellagher  and  Lauchlin,  2003)  

Glasgow  

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Functional  Unit  (FU)  including  all  subsurface  layers:    

1m2  of  SuDS  pavement    for  pedestrian  /  light-­‐vehicle  use  in  an  urban  environment  

with  infiltration  rate  ≥  15  mm/hr  design-­‐life  =  20  years  

with  annual  maintenance    

For   each   pervious   pavement   system,   the   required   thicknesses   for   the   surface   and  

subsurface   layers   shall   be   determined   based   on   both   hydraulic   and   structural  

considerations.  The  maximum  of  the  two  design  methodologies  shall  be  implemented.    

 

For  the  purposes  of  this  investigated,  the  native  soil  has  been  specified  as  a  well-­‐graded  

sand  with  CBR  =  10%.   In  practice,   it   is  necessary   to  carry  out   laboratory  CBR  tests   in  

accordance  with  BS  1377-­‐4:1990  to  ensure  adequate  support  of  the  pavement  structure  

on  a  site-­‐specific  basis  (CIRIA,  2015).  

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Permeable  Block  Paving  Design  The  permeable  block  paving   system   shall   be  designed   in   accordance  with   Interpave’s  

‘Permeable  Pavements  –  Guide  to  the  Design,  Construction  and  Maintenance  of  Concrete  

Permeable   Pavement’   (Interpave,   2010).   This   guidance   is   largely   similar   to   that  

specified  in  ‘The  SuDS  Manual’  but  provides  greater  insight  to  permeable  block  paving  

design  in  particular.    

 

As  stated  in  Section  2.6.1  of  the  Literature  Review,  the  most  common  dimensions  for  a  

permeable  block  paver   is  200x100x80mm  (length  x  width  x  depth).  The   laying  coarse  

for  a  paving  block  with  these  dimensions  is  50mm,  as  stipulated  by  Interpave  (2010).  

Hydraulic  Design    ‘M5  –  60’  is  the  1  in  5  year,  60  minute  duration  rainfall.  For  Glasgow,  M5  –  60  is  equal  to  

17mm  and   the   rainfall   ratio   ‘r’   is   equal   to  0.2.     Therefore,   from  Table  1,   the   required  

permeable  sub-­‐base  thickness  for  a  1  in  100  year  rainfall  event,  with  an  allowance  for  

climate  change  of  20%  is  270mm.  No  drainage  from  surrounding  impermeable  areas  is  

considered.    

   

                                         

Table  1  -­‐  Permeable  sub-­‐base  thickness  for  attenuation  storage  (Interpave,  2010)  

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Structural  Design  The   intended   use   of   the   pavement   is   for   pedestrians   and   light-­‐vehicle   transport.  

Interpave  (2010)  categorises  this  as  a  ‘Pedestrian  Load  –  Load  Category  3’.  The  system  

is  to  allow  for  full  infiltration  and  an  assumption  is  made  that  the  CBR  of  the  native  soil  

is   equal   to   10%.   From   the   design   chart,   the   layer   thicknesses   are   determined.   An  

excerpt   from   the   design   chart   is   displayed   in   Figure   2   and   indicates   the   layer  

thicknesses  for  ‘Load  Category  3’.    

 

The   required   permeable   sub-­‐base   thicknesses   based   on   structural   design   are   greater  

than  that  specified  by  the  hydraulic  design.  Therefore,  the  structural  design  thicknesses  

shall   be   used;   giving   a   sub-­‐base   attenuation   layer   for   the   permeable   block   paving  

system   that   consists   of   a   150mm   coarse   graded   aggregate,   overlain   by   125mm   of  

hydraulically  bound  coarse  aggregate.      

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Figure  2  -­‐  Layer  Thicknesses  for  Permeable  Block  Paving  System  for  Pedestrian  Loading  (Interpave,  2010)  

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Porous  Concrete  Pavement  System  Design  The  design  of  the  porous  concrete  paving  system  shall  be  completed  in  accordance  with  

‘The   SuDS   Manual’   (CIRIA,   2015).   The   guidance   in   this   manual   is   based   on   the  

comprehensive  design  programme  implemented  in  the  US.    

 

Table   2   contains   the   Load   Categories   for   pervious   pavement   design,   as   specified   by  

CIRIA  (2015).  The  function  of  the  systems  under  investigation  indicates  that  the  loading  

is  classified  as  Traffic  Category  5.    

 

The  typical  layer  thicknesses  for  porous  concrete  paving  systems  are  provided  in  Table  

3,  based  on  the  Traffic  Category.  These  typical  values  are  sufficient   for   the  purpose  of  

this   investigation  as   it   the  aim  is   to  determine  the  most  representative  environmental  

burden  imposed  by  pervious  pavement  systems.    

 

The   depth   of   the   porous   concrete   surface   layer   =   150mm   and   the   Coarse   Graded  

Aggregate  water-­‐retention  layer  will  have  thickness  =  300mm.    

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Table  2  -­‐  Traffic  Loading  Categories  for  Pervious  Pavement  Design  (CIRIA,  2015)  

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Table  3  -­‐  Typical  Construction  Thickness  for  Porous  Concrete  over  Subgrade  with  5%  CBR  or  Greater  (CIRIA,  2015)  

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Porous  Asphalt  Pavement  System  Design  The   design   of   the   porous   asphalt   pavement   system   is   also   carried   out   in   accordance  

with  ‘The  SuDS  Manual’  (CIRIA,  2015).  Traffic  Category  5  is  also  used  as  the  function  of  

the  pavement  systems  under  investigation  is  to  support  pedestrian  and  occasional  light-­‐

vehicle  loads.    

 

With  reference  to  Table  4  the  porous  asphalt  pavement  system  shall  consist  of  a  surface  

layer  160mm  in  depth  and  a  sub-­‐base  water-­‐retention  layer  with  a  depth  of  150mm.    

Table  4  -­‐  Typical  Construction  Thickness  for  Porous  Asphalt  over  Subgrade  with  5%  CBR  or  greater  (CIRIA,  2015)