Harvard is paying for the replacement of a large-diameter stormwater interceptor sewer that runs from N. Harvard St. east to the Charles, south of Western Ave. The sewer had a major failure near N. Harvard St. last year. This is about a fifth-mile of new sewer, and to replace the sewer (work is underway) Harvard had to tear down a building at 90 Seattle St. The sewer work is being done for/by the city as the sewer is the city's responsibility, --Harvard is simply paying for it.
(This ^^^^ is from the Harvard Construction Mitigation website.)
IMO, if Parsons Brinckerhoff was overkill for a demolition project, they are overkill for engineering a relatively small-diameter sewer alignment. With all the civil engineering firms in Boston, the first name that comes to Harvard's mind for a short, uncomplicated stretch of sewer is Parsons?
Harvard, much to the chagrin of many, thinks and plans long-term, and while there are no plans for a transit tunnel in the next ten years, Harvard might want to preserve the option of such in a more distant timeframe. And if Harvard intends building on the old Charlesview site in the next ten years, it might want to preserve/provide for a tunnel alignment before building whatever it intends to build on the old Charlesview site.
I think a definitive answer for Parsons' presence would be found if Harvard were cutting in a tunnel alignment on the east side of the science complex. Harvard's contractor has been working around the completed foundation of the science complex for about a year, which seems to be a rather prolonged period for a mothballed site. This week's work on the east side of the science complex.