There are some nine square miles involved in the territory, and we have figured that the assessment when made would cost from $25.00 to $30.00 per ordinary town lot in that district. There is, however, a large amount of that property that is still under what we call farming regulations, and we cannot tax as fully as you can property that is laid out, so that the project is somewhat in abeyance until that is taken care of. Under the direction of the State Board of Health the Department is now engaged in making a comprehensive survey of the needs of the city with the idea in view of bringing together all the sewer and sanitary outlets, and carry them down the river to some point outside of the city where proper disposal plants can be instituted and we can then take care of that ever-increasing subject of the disposal of our waste matter in the city. I thank you. (Applause.)

TUNNEL AND TIMBER WORK IN LARGE SEWERS

By George M. Shepard, Chief Engineer, Department of Public Works, St. Paul, Minn.

Before beginning the special description of timber and tunnel work an outline of the general nature of the sewer system of which the tunnel work is a part will be given.

The general system known as the Belt Line, Phalen and Hazel Park Sewer System is shown in Plate No. 1 and comprises approximately 14 miles of various sizes of sewer, the maximum being a 12-foot horseshoe section, this system being part of a $7,000,000.00 program of relief and trunk sewer construction being carried out by the City of St. Paul. The district of six square miles which this system serves is of glacial drift formation without any well-defined drainage courses. The succession of depressions and knolls impose a condition of sewer depths varying from slight fills to as much as 90-foot cuts. This condition is shown by Plates 2 and 3. As a result approximately 1000 feet of tunnel for the 12-foot section was necessary and 2300 feet of tunnel for the 8-foot section. In addition to tunneling for the greater depths several of the main highway thoroughfares and one set of railway tracks were tunneled so as not to interfere with traffic.

The material encountered is glacial drift consisting of varied. materials, the change from one material to another frequently taking place very abruptly. A fine sand was encountered principally on the larger tunnel and a water-bearing sandy clay with small amount of gravel, also some water bearing fine sand in the smaller tunnels. The problems involved in timber work in this class of material are not new, such methods having been used in the mines of northern Minnesota for a great many years. Had greater lengths of tunnel work been necessary undoubtedly improved methods with shields and air could have been used. The work as carried on, however, is interesting to the average engineer and with this in mind the following notes have been prepared:

Plate 4 shows the general sewer and tunnel section. Plates 5, 6 and 7 show various phases of the timber construction previous

to lining with concrete. Plate 8 shows several sections and elevations giving the various progressive steps of the work.

The construction follows the general plan of heavy timber "sets" placed between four and five-feet centers with poling boards driven or placed in advance of the heading. Where water bearing seams were encountered at the crown of the section or above the spring line it was necessary to bulkhead the upper portion of the heading and in some cases to pack with hay or straw as soon as the material was removed.

Figure 1 is a longitudinal section showing the appearance of the tunnel immediately after the set "A" has been placed. The poling boards "B" are supported by the temporary cap and posts "C" and the breast boards "D" are braced back against the following set by braces "E."

Figure 2 is a cross-sectional view showing the heading under the same conditions as Figure 1. The next operation is to transfer the support of the poling boards to the new set. This is done by placing the transverse timber "F", Figure 3, directly above the cap and blocking and wedging between the boards and cap. The blocking between the timber "F" and the cap "A" is of such thickness that the opening left will permit the entrance of a poling board "B." Then the center breast board is cut through by drilling and the poling board driven a short distance. A portion of the breast board is then removed and the earth under the poling board excavated. Other poling boards are introduced in a similar manner on each side and the excavation is widened and deepened.

As soon as the excavation has proceeded to such a point that the poling boards require support, temporary posts "C" are placed as shown in Figure 4. In sand there may be several sets of temporary posts used. Short posts are then progressively replaced by longer ones as the process of widening and deepening the heading proceeds. The face is held in place by breast boards "D" which are braced against the preceding set by braces "E." Where the soil will stand up for a short time the entire face to about a man's height is dressed down before placing the breast boards. In sand small breast boards must be placed as soon as the face is reached and these must be gradually replaced by larger boards, as the face is widened and deepened. Three poling boards on each side of the top and eight along each sloping side are made fan-shaped as shown in

Figure 1 so that they will completely retain the earth as the area widens out.

Figure 5 is a transverse view of the heading. The left half shows the appearance after all poling boards have been placed. The next operation is shown on the right half of Figure 5. A trench is carried downward and progressively sheeted by the boards "G" and "H" braced by "J" until the sill "K" may be placed. After both sills have been placed the posts, segments and caps of the next set of timbering is placed.

Line is carried forward from several plumb lines suspended from sets to the rear. Grade is carried on each side of the tunnel by small white boards nailed to the posts at a predetermined number of feet above top of sill. As each sill is placed it is brought to proper elevation by sighting back along the grade boards.

Concreting was done by hauling the concrete in cars into the tunnel from a mixer located outside, the track being elevated as much as possible above the floor of the tunnel in order to save handling. Wooden forms were used for the 12-foot tunnel and steel forms for the 8-foot tunnel. The entire space between the timbering and forms is filled with concrete, the concrete on top of the forms being shoveled into place and rammed back to fill the entire section between the forms and the timber work.

Approximate cost figures indicate that for the 12-foot tunnel the total cost of driving, including excavation, was approximately $54.00 per foot of which cost $19.00 per foot was for timber. These figures do not include the contractor's profit. The approximate cost of concreting was $71.00 per foot, this being on the basis of $17.00 per cubic yard in place, the total cost for the tunnel complete being $125.00, not including contractor's profit.

For the 8-foot tunnel section only about 500 feet out of a total of 2300 feet has been completed. 7x10 shafts were used, the total cost per foot of sinking the shafts being approximately $43.00. The approximate cost for driving, including labor and timber for the amount already driven and a proportion of cost for hoisting equipment is $45.00 per foot of which amount $8.00 is for timber.

On account of water bearing sand seams yielding 50 gallons per minute, it was necessary to install pumps at each of the two main shafts of the 8-foot tunnels. Hoists with approved safety devices were installed and operated by hoisting engineers from the same boilers as the pumps. The cost of driving tunnel includes a proportion of the cost of the hoists and general equipment.