"For some thirteen hundred feet, it seemed to break through
the prairie and sank a maximum of about thirty feet."

Letter from N.P. President H. Elliott to S. Rea, V. President, Pennsylvania Railroad, November 30, 1907.

Construction of Stockwood Fill trestle, ca. summer, 1906..

S.P. Wange photo, © Clay County Hist.Soc.

[Driving piles for Stockwood Fill]

Except for persistent problems with labor, construction of the Stockwood Fill progressed satisfactorily. A major railroad re-grading cut effort through the glacial uplands to the east yielded gravel that could be hauled by trains onto the trestle, where it would be dumped.

[Burial of trestle

Burial of trestle with gravel hauled by dump trains, ca. August, 1907. The sag in the embankment has just started to develop.

S.P. Wange photo, © Clay County Hist.Soc.

The first signs of any problems with the integrity of underlying geology were realized on about August 1, 1907. At that time, settlement of up to 2 ft was noted at one region, with the ground to either side of the embankment rising by about an equal amount. By August 4th, settlement had increased to 8 ft, and the adjacent ground had risen 7 ft.

[Graphic of fill settlement] Cross-section of Stockwood Fill, showing rate of
settlement between August 7 - 13, 1907.

Resketched from S.A. McCoy's profiles, August , 1907).

Over the next week, the embankment settled rapidly. On August 13th, McCoy wrote, "The bank was practically complete to grade on the 1st, and between August 1st and 8th we unloaded within the 600 feet . . . a total of 71680 cubic yards, endeavoring to keep our track to grade; since August 8th to date, we have unloaded in the same hole 43540 cubic yards, and the condition is growing worse daily. . . . The incline has become so steep that it is almost impossible to unload trains in the hole with the present equipment" (letter to W.L. Darling, N.P. Chief Engineer).
Associated with each settlement event was the appearance and growth of a pair of compressional ridges, flanking each side of the Fill. (In their origins, these ridges are somewhat similar to "squish marks" produced when one steps into mud).

Gentleman is standing on a compressional ridge in region of station #1245, Stockwood Fill, October 15, 1907. Note prominent sag in embankment, where trestle has been buried by fill. View is toward the west-southwest.

Northern Pacific Railway Company, Engineering Records, © Minnesota Historical Society

[Compressional ridge]

On August 8th, McCoy initiated an investigation of the geologic conditions underlying the Fill. These revealed a complex package of sands, quicksands, gravels, and clays. McCoy repeatedly dismissed any problems induced by the quicksands: "While the above soundings indicate that there is quicksand throughout the entire Stockwood country, I do not think that we need fear any serious results therefrom" (letter to W.L. Darling, August 24, 1907).

[Graphic of subsurface geology]
Subsurface geology in region of Stockwood Fill, as interpreted by S.A. McCoy, 1907.
Archway was located at present position of Minnesota Highway 9 underpass.

Resketched from S.A. McCoy's profiles, Fall, 1907.

Soon, however, the tracks on either end of the sag became too steep for the work trains. McCoy built and filled a second trestle over the problem region, but that also sank within days. His crews doggedly continued to dump fill on the sink, only to watch it disappear.

[Construction of second trestle]

Construction of second trestle across sag region near station #1245, Stockwood Fill, September, 1907

Northern Pacific Railway Company, Engineering Records,
© Minnesota Historical Society

By December, 1907, the embankment was no more complete than it was in August. McCoy sought employment elsewhere, and the N.P. placed its construction superintendent, F.L. Birdsall, in charge. In the spring and summer of 1908, Birdsall had a 3000 ft long, permanent bridge constructed over the sink, extending westward over the archway (= N.P. survey station #1262). Construction of this bridge allowed dump trains to access the west end of the embankment and to progress eastward from there in burying the trestle. Work progressed rapidly, but in May two more zones of settlement developed. Eight hundred feet of track sank 5 to 8 ft per day for a week. Birdsall extended the width of the fill outside of the slope of the trestle, hoping that a wider "mattress" would help support the embankment.

Overview of sag (left side of photo) developing where trestle has been filled, October, 1907. View is toward the southwest. Archway is visible at right-center of photo. Rise in unburied trestle shows the original, intended grade of the embankment.

Northern Pacific Railway Company, Engineering Records,
© Minnesota Historical Society

[View facing southwest, October, 1907]

The strategy seemed to work, at least for a while. Birdsall reported no settling through early October, and it appeared that the N.P. might complete the embankment by November 1st. However, on October 10th, a fourth zone of settlement developed. Cracks opened on either side of the embankment over a length of one mile. Birdsall reported that "indications are now that the entire fill for this distance will go down as we widen it out" (letter to W.C. Smith, N.P. Chief Maintenance-of-Way Engineer, October 25, 1908). Then on October 31st, Birdsall wrote Smith: "Had to stop work this P.M. on account of the permanent bridge opposite the Stockwood Depot settling so badly that we were not able to get over it."


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