These  plates  are  bolted  together,  forming  a  circular  sheeting;
inside  of  this  long  timbers,  12  in.  X  12  in.,  slightly  bevelled,  as
in  arch  stones,  are  driven  with  a  light  hammer,  close  together
and  resting  against  the  iron  sheeting.  The  space  thus  enclosed
is  ready  then  to  be  pumped  out.  This  certainly  forms  a  strong
dam,  even  without  interior  bracing,  when  properly  constructed,
but  in  considerable  depths  radial  bracing  will  be  found  neces-
sary to  prevent  the  bottom  of  the  timbers  from  pressing  in-
wards.    The  objections  or  defects  arise  mainly  from  driving


I $6  A    PRACTICAL    TREATISE  ON  FOUNDATIONS,

the  piles,  as  it  is  difficult  to  keep  them  in  contact  for  their  full
length,  and  they  will  almost  always  separate  or  spread  at  their
lower  ends ;  this  can,  at  least  in  part,  be  overcome  by  cutting
grooves  in  the  piles  and  driving  filling  pieces  of  iron  or  wood
in  the  grooves,  as  previously  described.  Owing  to  the  form  of
the  dam,  the  excavation  is  confined  to  a  narrow  area  in  the
direction  of  the  length  of  the  pier,  coming  close  to  the  sides  of
the  dam  along  one  diameter,  and  leaving  broad,  unexcavated
spaces  on  either  side.  In  addition,  the  excavated  material  is
for  convenience  thrown  on  top  of  the  undisturbed  earth ;  the
tendency  of  this  is  to  bulge  the  sides  out,  and  the  great  excess
of  outside  pressure,  in  a  direction  at  right  angles  to  this,  tends
to  force  this  part  of  the  dam  in,  and  aids  in  forcing  the  other
•  parts  outwards  This  distorts  the  entire  dam  ;  a  portion  of  the
dam  comes  in  and  another  portion  goes  out,  causing  leaks  that
are  hard  to  be  controlled.  In  the  writer's  experience  these
dams  gave  great  trouble  and  caused  much  delay ;  but  in  one
case,  at  the  Schuylkill  River,  this  was  used  where  an  ordinary
coffer-dam  had  failed  entirely,  and  under  circumstances  pecu-
liarly trying  to  a  cofler-dam.  This  will  be  alluded  to  in  another
paragraph.
