ST8804-CS 910201GEO ,~.CONS!II,TING SPECIALISTS IN:
-' Eovironmcntal Assesslnent.s
T Enviromnel~tal Engineering
Cons~ction Materials Testing
I NC. Commercial & Industrial lns~6on
"Excellence by E~erience'
February
Dowdey & Associates
16250 Dallas Parkway
Suite 100
Dallas~ Texas 75248
Attn.: Mr. Bill Anderson
Re: Pipe Imbeddment and
Backfill Requirements for
Underground Utility Construction
Dear Mr. Anderson:
In response to your question concerning the above referenced matter~
we offer the following observations and opinions.
Customarily, the pipe is laid on a crushed stone or gravel base in the
bottom of the trench. Thi~ process provides a stable seat and a
~onstant gradient. Next, the pipe im cradled by a clean, free
draining granular imbeddment m~terial (cushion sand> in order to
maintain alignment. This imbeddment material completely surrounds the
pipe up to approximately one (I) foot above the top of the pipe. The
material i~ carefully placed simultaneously on both sides o~ the pipe
in order to~ not only maintain alignment, but also to eliminate any
void pockets in the imbeddment material. The specified nature of the
material helps to eliminate any bulking or pocketing in the pipe
trench. Once the imbeddment material is in pla~e around and over the
pipe, normal trench backfill operations begin.
As to compa~ion requirements, w~ have 0bm~rved in the pa~t that the
imbeddment material is rarely tested. Testin~ beside, or immediately
above~ the pipe i~ very difficult and o~ten produces erratic resullts.
It is a consensus of opinion that obtaining required density on the
initial li{t of backfill material above the cushion mand will also
achieve required compaction in the imbeddment material around and
above the pipe.
The thickness of the ~irst trench backfill lift is mlso a
consideration. On storm sewer applications where the pipe has great
crushing strength, the firet lift may be on the order of one (1) foot.
However, on applications where PVC pipe is placed~ the initial lift
~hould be thicker in order to prevent crushing of the pipe. The
imbeddment material will compact, and the utilization of hi.~ tech
compaction equipment could cause needless harm to softer pipe provided
ov~?--~ompa~ion i~ 8thieved.
1939 E~t Cominen~! Boulev~d · Sou~lake, Tex~ 76092 * (817) 329-0281
GEO CONSULTING SI'~:CIALISTS IN:
" '-" ~'~ P. nvironmcnlal As&es.~mcnts
T Environmental Engineering
Construction Materials Tesling
l ac. Co.me.iai & Indus~al Inaction
"~cellence by Experience"
Underground Utility Construction
Page Two
I trust this is the information you desire; and if we can be of
further ~rvice~ pleasm call on us.
Respectfully submitted,
Geo-Technique~ Inc.
/;,.e'..:..' '.
l:)'.
· .*i.... E~.. ':*~
t. oui~ L. Ha~i~, P.E.
President
1939 East Continental Boulevard · Southlake, Texas 76092 * (817) 329-0281