Proposed Collector/Drop Manhole on Ex. Deep Interceptor Sewer

[MarshfieldTimC]

I have a project that requires connecting a collector sanitary sewer (12") to an interceptor sewer. The existing interceptor is 60" reinforced concrete and approximately 25' deep. I would like build the proposed manhole as an outside drop manhole. I am considering a poured-in-place, reinforced bottom section that does not disturb the integrity of the existing 60" pipe, except that the top of the pipe will be removed. (The manhole would be formed around the existing 60" pipe.) The remaining risers would be 48" precast concrete. Anybody have any experience with a project/manhole such as this?
(Thanks in advance for all responses.)

 

[alexcmmi]

To me, it sounds like the most technically challenging part of that project is the excavation to the sewer. Might need to put in some sheeting, or else you will need a pretty wide excavation to get down 25'

Any reason you dont want to use a precast doghouse over the existing interceptor? I am sure a local precaster can make that larger size up for you, and I still think you would be able to tranport it from the precaster to the site (it'd prbably be around a 12' wide base, just guessing???? < this of course would be the big question). I would think it would be much easier to drop a precast bottom section down a 25' hole rather forming something way down there (see above) and then using elephant trunks to pour concrete 25' below grade. Just a quick thought... Of course I have no idea what your site constraints are

 

[MarshfieldTimC]

Thanks for the reply (alexcmmi).
Yes the depth is a challenge, but soil and groundwater conditions are usually favorable in our area (tight clay soils, minimal water pumping, no buildings nearby, etc.
In previous projects, contractors have been able to form/pour manholes of this size for less than the cost of installing a large precast. Although our soils usually do not require major groundwater pumping during excavation, groundwater can weep into leaky sewers and manholes, and the 'doghouse' type manhole can be difficult to make water tight. I have had luck with incorporating a water stop gasket and mortar/concrete seal on pipe connections of a doghouse manhole on a smaller sewer (27"), but am hesitant to try this on a 60".