Loading...
Memo- Honor Award . .\ . I Public Works Department I Memorandum To: Honorable Mayor and Members of City Council Steven J. Stadler, Public Works Director ~ From: Date: March 31, 2000 Subject: Honor Award - Year 2000 Consulting Engineers Council of Minnesota 1999 Street & Utility Improvement Project The City received an Honor Award from the CECM for the storm sewer improvements completed as a part of the City's 1999 Street & Utility Improvement project. An unconventional design was used to meet stormwater management needs while reducing costs by nearly $300,000 when compared with the more conventional method. An existing 88"x 64" deteriorated, corrugated metal trunk storm sewer pipe was replaced with dual 54" high density polyethylene pipes on special pipe bedding. A more detailed write-up on the project is attached. Another cost saving technique used on the project was the stabilization and strengthening of the 1 st Street South clay soil subgrade with flyash. Flyash is the combustion byproduct from burning coal. It is made up of extremely fine particles that bind with the clay soil particles to stabilize it. The cost of purchasing and placing the flyash was about 1/3rd the cost of removing and replacing the clay soils and also could be done in much less time. An attached photograph shows a motor grader working the spread flyash into the clay subgrade soils. . . . Project Description: 1999 STREET.AND UTILITY IMPROVEMENTS Hopkins, Minnesota BOLTON & MENK,INC. Project Engineer Role of Entrant Firm Bolton & Menk, Inc. When the City of Hopkins, Minnesota, targeted the reconstruction of a major storm sewer trunk line for inclusion in its 1999 street and utility improvement plans, it appeared to be a fairly straightforward engineering endeavor. The existing line, a 5OD-foot, 88" x 64" corrugated metal arch pipe, was deteriorating. Furthermore, it lacked the capacity required by the City's long-range storm water management plan. The proposed solution was to replace the existing metal arch pipe with a 72" diameter reinforced concrete pipe. As the design process unfolded, however, a geotechnical exploration of the site revealed a substantial layer of organic peat extending along 400 feet of the proposed 500-foot align- ment, just below its invert. This material would not support the new reinforced concrete pipe. Armed with this knowledge, designs for the storm sewer were revised to include a set of 50-foot concrete-filled steel friction piles at each end of each pipe section. Estimated construc- tion costs for this length of storm sewer, including a large vault manhole at each end, were now approaching a half million dollars. Project Description Bolton & Menk, Inc. Page 2 of 4 Working closely with City of Hopkins staff, Bolton & Menk, Inc., developed an alterna- tive design .that would prove to address the problems associated with the subsurface and reduce overall construction costs by roughly 60 percent in the exchange. . Other Consultant's Role in the Project GME Consultants of Minneapolis served as soils consultants on the Hopkins project. GME conducted the soil borings that revealed the peat material beneath the sewer alignment and developed foundation design details for the plastic pipe alternative. Barbarossa & Sons, Inc., of Osseo, MN, was the prime contractor on the construction project. Bolton & M enk's Contribution to the Project Bolton & Menk considered a number of design alternatives to address the soil condi- tions, but any solution involving concrete pipe brought with it the need for friction piles. Construction costs of these designs were exceeding Hopkins' budget. At this point, Bolton & Menk began exploring a different approach, one not unheard of but considerably less common than typical concrete pipe construction. The new design utilized dual 54" high density polyethylene (HDPE) pipes. Dual pipes were used because the maximum manufactured size of HDPE pipe is 60". Much lighter weight than a single concrete pipe, the HDPE pipes eliminated the need for pilings, requiring only soil corrections beneath the invert. An additional three-foot subcut was made along the pipe alignment, extending through the organic mate- rial. The subcut was then lined with(("geofabric and backfilled with an open-graded aggregate. The geofabric was wrapped over the top of the . aggregate, creating an "aggregate pillow" to not only support the dual piping system but also Project Description Bolton & Menk. lnc. Page 3 of 4 protect against the migration of organic soils into the pipe bedding zone. Traditional bedding and backfilling operations of the HDPE pipe were employed above the aggregate pillow. A new vault manhole structure was constructed to connect three existing inflow pipes to the dual piping system. Besides unstable soil conditions, the project pre- sented a number of constraints to overcome. Early on in the project, upon exposing the existing storm sewer and watermain in the area, differences were discovered between the as-built information and actual pipe align- ments. Revisions to the utility plan thus required the implementation of special fittings. Given the depth and width of the dual pipe sys- tem, a substantially large trench was required for its con- . struction. To minimize this excavation, a trench box was used and backfill operations commenced as quickly as was practical, keeping the trench as small as possible. These efforts minimized danger and inconvenience to the public. Other constraints included the aforementioned lim- itation of available HDPE pipe sizes, a circumstance that truly gave rise to the dual pipe design; and a finite storm sewer budget, a reality that prompted Bolton & Menk to consider the feasibility of a rather unique design solution. The basic goal of the project was to design a pipe system capable of routing the flows required by the City of Hopkins' storm water management plan. Bolton & Menk investigated the compatibility between the increased flows accommodated by this system and the capacity of . Nine Mile Creek, the ultimate discharge point of the system. Discussions with the watershed board revealed that previous improvements to down- stream culverts along the creek would allow for the designed increase in flows by the storm sewer system. This reconstructed storm sewer trunk line serves nearly two-thirds of Hopkins' drainage area The project has successfully addressed the City's long-term storm water management needs, and has offered the City a sav- Project Description Bolton & Menk. Inc. Page 4 of 4 ings of nearly $300,000 over the original concrete pipe and pile design. In the end, Bolton & Menk's creative solution to an unforeseen problem helped the City of Hopkins turn a potential liability into an opportunity to save a considerable amount of money. . -- j 7 J -~ ...~..: ~ ~ ,'-!.,.:: ': .!;....~,..:~1-...~;.. 1.' :; ~'}2 .. _ ~,<,' '-'..", r-,;_ ".. -- ",.li; 'a ~ !,_",,~.'f;"'o~~JfI!JIJ"':...- ~ ( ,,~ -~~"-._-t''''':; . w" jC~ :__::wo.... '..."..... -\.... '>~~\l!'" . -;!J;tif!J~;;tir;'::j>.""" ' {~.... -;;. -;'-~~:~1 ,ti~ -;..- ~-- , - ~ -~ ...~ ~ '=7F-'~~ ~ ,.".. .' ~ . ~_ ~ -~...:r:. - '-~, :',' -.. ,',:" ;." '.,;" ~--;--. ,_ .r":;' '. .-'J....... ,'~ . ,; L'- ''I. , .::t .: ~_:r- ~. ~>-:r.. . '-~ . ,,;'1.1, fi..' ~. ~ - -... .",..- -. . - . . ... ...r ." , -1~<"':' ~'~A"'~' i'....~.._.,~..... 'J," . ,. " _ . ,.....,' ~.l' "' ,,~l;.' .. ~~~'::i1.. ::-' ! ":--:"#t.;=;.,~~~~:,.:~~~r~~~{..l'&~ . ~ ~ r './:tV;:; ..,. ~...'l,: ~;f ~.~(~ . . I' . - ....~ ~ ,I" ~&:.:.::::r;?i' ''a''.!'.~rJ7''':' ;V~.~~,t!. ~~.r '. .c:.. " :+- ~...-r,,: - ,~.' '; ;,~",":~'~ ''::.'''_ ~.~ ~ j..' ~.r/ ..:- ,,,--t, ~ '~c..;t~~..,~- . "'L"'1'"-;..~.- "",' . .' .. .~ 7 ' ,. ',<. ,'l""''''-l}(llf, 'l!":'".~,-\V .'~J ~ . ~"~~ ;",,';,.~~lf'-".f<;!}. )!;; ~". .,,; l c.~,-,.- " '.' { , - 't... ~.~.r!loo?...:. ,_ i' '!. ~ ,. . ;';,--:'-: . -', r ~.t",;:.. .:."~T~"r' .. .... ,,' . . ..... ,'r,. _' "'c,~~:.,..,J~9 :~r~~~~"~j:1.~,~;x~i!::~~t;' ,..~, C.)o :t:;'" .