Re: Why a 'duct
Date: 05-03-2021 - 20:33
Depending on how close to 5 the tracks are, virtually every interchange and grade separation along 5 would have had to be replaced. Either that, or the tracks would have had to go over, high, a lot of roads. There would be places where the tracks have to diverge from 5 because the freeway alignment goes into the hills, or else hills would need to be leveled and bridges built to maintain the vertical alignment of the tracks. There are creeks along the west side that would have to be crossed, most of which are subject to flash floods and mudflows so the crossing would not be a simple pipe culvert in a fill. Watch carefully when driving along 5 - yes, there are some culverts, and they're fairly big, but mostly it's bridges, and bridges need pilings for foundations, where there isn't competent rock (which there isn't, in the San Joaquin Valley).
In the case of HSR, no grade crossings are allowed. All roads must be crossed with bridges on either the tracks or the road. Because of the high speeds, the vertical curves must be very gradual, so there are long embankments and bridges to get up and over when the tracks do the bridging. There are lots of roads to cross on both the east and west sides. Other considerations on the west side are seismic issues and ground deformation: there are active folds, areas of active subsidence due to groundwater withdrawal, and fairly frequent moderate thrust-fault earthquakes that affect ground elevation, from at least Firebaugh south. That could result in some interesting engineering challenges, and "engineering challenge" associated with "CAHSR" generally means "cost overrun denominated in $billions" no matter how well you plan for it.