4.1.1.2 Embankment

In 4.1.0 Right of Way I discussed how a railroad company got the land that it operates on. In 4.1.1 Roadway Roadway is planned and constructed based on the roadway engineering plan. The plan identifies all the issues involving construction, to include:

1) Geography: that is the roadbed, embankment, cuts and fills, drainage,  and

2) Track structure: Ballast, Ties, Rails, Turnouts, Track crossings (Special works), and other associated materials.

On this page we will discuss roadway engineering and construction of embankments.

Prototype Information

An embankment is

an artificial mound or bank of earth or stone built to carry a road or railroad over an area of low ground. Railroad engineers see it as a low cost solution for the roadbed in preference to bridging. 

Formation of Embankments. 

Embankments are made of local materials and built in layers. Generally, stone and heavy material should form the slopes and finer material be placed in the center of the embankment. The more solid and compact the roadbed is made, the more impervious it will be and the better it will drain water and support the loads brought upon it by traffic.

Width of embankment at top

The roadbed on top of an embankment is not the standard roadbed width. Because of the possibility of shrinkage of the embankment due to poor construction material or erosion due to improper drainage, the width of the roadway is wider than the usual roadway width. Width can also be based on the height of the embankment. Without discussing the details of the formula, here are the common standard widths of embankments for the railroads in the WWSLs operational area.

• On the Great Northern Railway all single track roadbed was 14 foot wide on tangents and 16 foot wide on curves. 
• On the Northern Pacific Railway all single track roadbed was 16 foot wide.
• On the Milwaukee Road all single track roadbed was 18 feet wide.  
• On the Union Pacific the width is differentiated based on main or branch line and the standard changed over the years. In 1906 - all roadbed on embankments were 16 foot wide. In 1927 the  main line was 18 foot wide, a branch line was 17 foot wide -  with a caveat that when embankments greater than 10 foot in height the roadbed would be 24 foot width. In 1967 the main line was 28 foot wide, the branch line was 20 foot wide.

Slope of embankment

The slope of railway embankments in engineering terms is called the angle of repose That angle varies from 1 to 1 (horizontal to vertical) for rock fills to 11 to 1 for ordinary earth, and easier slopes for soft material like clay soil, when such must be used. By building up a rough dry wall on the exterior of a rock fill it may be made to stand at a slope somewhat steeper than 1 to 1. Ordinary earth will stand for a time at a slope steeper than that, but under the action of the rains, the winds and frost it will gradually wear down to about 11 to 1.

Excavation slopes.  The Union Pacific has listed four generic excavation slopes ratios based on slope materiel

• 0.25:1 for solid rock
• 0.5:1 for fractured rock
• 1:1 for common materiel
• 1.25:1 for sand

 

The WWSL

There are five embankments on the WWSL: 1) the Satsop River embankment, 2) the Polson Canyon embankment, 3) the Tunnel 1 embankment, 4) the Smith Canal embankment, and 5) the Preachers Slough Highway embankment..See 5.5.3 Embankments for specific details.