in the immediate future. Up to the present time, however, the details of this class of work have not received as much attention as in the case of beams, floors, and similar work. In work of this class the stresses can be readily computed, and the construction can readily be tested in an experimental way. In the case of a sewer, it is difficult to compute the stresses to which the shell may be subjected, and difficult to subject a section experimentally to the strains which occur in practice. This subject, therefore, opens up an interesting field for investigation.

The Michigan Engineering Society is indebted to the courtesy of the Engineering News Publishing Co. for the use of the cuts illustrating the foregoing paper, by Mr. Pierson. See their advertisement on last page.

HASLETT PARK SINK-HOLE ON THE GRAND

TRUNK RAILWAY.

LESLIE A. WATERBURY,

INSTRUCTOR IN MATHEMATICS AND CIVIL ENGINEERING,
MICHIGAN AGRICULTURAL COLLEge.

The Grand Trunk Railway has experienced considerable difficulty in many places in Michigan, with sink-holes. One of these is located between Haslett Park and Lansing, near milepost 107.

These sink-holes are caused by small lakes which are covered with a layer of peat bog, so as to present an appearance of ordinary marsh land. The formation of such bogs is described by N. S. Shaler, in the Tenth Annual Report of the United States Geological Survey. The diagram illustrating this

FIG. 1.

(From N. S. Shaler's Report, Tenth Annual Report U. S. Geological Survey.) SHOWING FORMATION OF PEAT BOG.

is also taken from the same report. The formation of the crust BB is caused by a growth of water-moss, which requires no under soil for its roots to penetrate, the species of sphagnum being the most prominent of this class of plants. The growth begins near the shore, and gradually extends over the surface, until the entire pond has become covered with a dense mat, which increases in thickness, collecting soil washed down

from the surrounding hills, thus affording opportunity for other vegetable growths, and in this way a considerable crust is formed. In the diagram, A represents the remaining portion of the water area; CC represents muck, or a peaty mass derived from the disintegration of the surface layer; DD shows the solid part of the swamp, bearing trees near the shore, and further out bushes; EE indicates climbing bog, which may be absent; shows the original water-level, and the arrows indicate the direction of growth of the upper and under layers of peat.

In the case of the sink-hole near Haslett Park, the pond has a crust about six feet thick. The upper portion of the crust is quite firm and solid, but gets softer beneath, so that the bottom, which is in contact with the water, is soft and spongy. Upon this crust there is a considerable growth of trees and bushes. Some of the trees are of very good size, yet their. roots do not penetrate much below the surface, but spread out in the upper portion of the peat crust. As would be expected, from the character of its formation, the crust covering the lake is light, and is composed largely of organic matter. Three air-dried samples analyzed at the Agricultural College were found to be composed of the following ingredients:—

SAMPLE NO. 1.

Taken south of hole and one foot from surface.

Taken 100 feet south of hole in crust, from ground ex

posed by a tree which had been blown over.

Moisture

Organic matter

Mineral matter

Total

10. per cent

80. per cent 10. per cent

100 per cent

Pieces

The specific gravity of this crust is very nearly 1. of it which were placed in water in a frozen condition floated, but generally sank after thawing. However, the material as it is around the sink-hole has a considerable supporting power. This is perhaps due to the fact that marsh gas is generated by the decomposition, and also to the fact that the crust would withstand tension so that any load coming upon the surface would not depend entirely upon the floating power of the crust beneath for its support.

It was upon this bog that the road was originally built. To add to the supporting power of the crust, a large quantity of timber was first laid upon the surface, forming a corduroy foundation some twenty feet deep. Upon this foundation the embankment and the track were placed. The construction is shown in the diagram.

After the construction of the road, little difficulty was experienced at this point until preparations were begun for laying double track. At this time, the dirt, which was dumped beside the old track for the purpose of widening the embankment, gradually sank out of sight, leaving a pond of water, at the same time forcing the crust apart and pushing the track and right-of-way fences out of line. The plan of the sink-hole

FIG. 3.

PLAN OF SINK-HOLE SHOWING DISTORTION IN
ALIGNMENT, AND CRACKS AT SIDES.

shows the alignment of the track, and approximately that of the fences as it was Nov. 26, 1902. The track had been further out of line than the diagram shows, but had been thrown back before the displacement was measured. The broken lines show the original positions of track and fences, and the full lines show them after being distorted. The portion of the track which had been moved the most was 19 1-2 feet from its original position when measured. The portion of the fence which had been displaced most was 38 feet from its original position.

The material used for widening the embankment was till, a mixture of sand and clay, and was much heavier than the peat crust. This filling carried down with it not only the crust beside the track, but also the embankment upon which the track rested. It also broke the crust so that the track sank under the action of traffic. Whenever a train passed the track