Minneapolis Sewer System 427 



has here been sketched, have grown to a total length of over 200 miles 

 of sewers at a cost of $5,371,567.— Their total capacity is 2260 cubic 

 feet of 16,942 gals, per second with about 1-3 more when overflows are 

 in use. Some idea of the magnitude of this sewer service can be 

 gained when this capacity is compared with a river. This total sewer 

 capacity equals 45% of the average flow of the Mississippi river; or 

 it would form a river 300 feet wide and 3 feet deep, flowing at a 

 velocity of 214 feet per second or 1.75 miles per hour. 



Some appreciation of the present and future necessities of the 

 city can be realized when it is found that the 53 sq. miles of drainage 

 surface within the present city limits is larger than more populous 

 cities like Paris with its 30 sq. miles. 



The original construction material of the sewers was sewer brick, 

 except for the small branches from 9 to 15 inches, where cement pipe 

 was used until 1892. Since then vitrified clay pipe has been used in- 

 stead of cement pipe and has also been found preferable to brick for 

 sewers up to 2 feet in size. The late use of reinforced concrete con- 

 struction was applied to our 22d Av. N. E. 6^^ foot sewer in 1904, and 

 since then has been found less expensive and more satisfactory than 

 brick construction for all sewers larger than 4 feet. Brick is there- 

 fore now used only for sewers between 21/^ and 4 feet and in all man- 

 holes. For catch basins we have also lately began to use concrete. 



In connection with the records of our sewer construction for the 

 past 35 years there has accumulated much interesting and valuable 

 scientific data as to the geological foundation of our city, the stratum 

 of blue lime stone . From many records of the elevation of the lime 

 stone it is seen that it varies on the East Side from 133 ft. above city 

 datum at Central and 15th Avs. N. E. down to 102 ft. at Central and 

 Main St., to 75 ft. at the foot of Oak St. and 83 ft. at the foot of Ham- 

 line Av. Likewise on the West Side the elevations descend from 120 

 ft. at 1st St. and 3d Av. N. to 73 at the foot of 11th Av. S. and the 

 same also further south at the foot of 4th St. S., below the Wash. 

 Av. bridge. 



There is thus shown a dip in the lime stone ledge from North- 

 west to Southeast, and of some 60 ft. difference in altitude within the 

 city limits. The thickness of the ledge also varies from 9 to 20 ft. in 

 the N. E. section to 30 ft. in the S. E. and So. section. 



The average level of the surface plain of the city near the river 

 is maintained on this sloping rock by the complementary increase 

 of the soil deposits on top of the rock. For, while the lime stone 

 ledge comes quite to, or within a few feet of, the surface in N. E. Mpls. 

 (where the many quarries are), the sloping rock has been covered in 

 the S. E. section and along the west bank of the river with some 40 

 ft. of sand and gravel. 



