Ch. 7] CONSTRUCTION ON SEDIMENTARY ROCKS 133 



enous materials and free from closely spaced, steeply inclined frac- 

 tures and joints, slopes of % to 1 and even vertical are possible. In 

 the Appalachians the strata are seldom devoid of joints; consequently 

 in cuts 80 or 90 feet deep and deeper there is a growing tendency to 

 excavate benches at varying heights above grade. 



The Clear Ridge Cut, near Everett on the Pennsylvania Turnpike, 

 is one of the deepest highway cuts in North America. Here the bed- 

 rocks are interbedded sandy shales and sandstones, striking normal 

 to the direction of the road and dipping 53° from the horizontal. This 

 cut is 153 feet deep, 2,600 feet long, 380 feet wide at the top, and 88 

 feet wide at grade. Two benches, the lower 23 feet wide and 30 feet 

 above grade, and the upper 85 feet above grade, were provided for 

 protective and drainage purposes. Access from the ends of the cut 

 was provided to the benches for removal of accumulated debris. The 

 slope from grade to the lower bench is % to 1, that between the benches 

 %. to 1, and above the upper bench % to 1, flattening to 1 to 1 near 

 the crest. Maintenance for falling rock fragments on the shoulders 

 and benches has been negligible in nearly 9 years of operation. 



An unusual design involves a new cut on the Turnpike's Philadel- 

 phia Extension at the western approach to the Susquehanna Bridge. 

 Here Triassic shales and sandstones dip toward the highway at angles 

 varying from 34° to 45° and strike at an angle about 15° from the 

 line of the road. A slope of 1 to 1 or steeper was originally planned, 

 but it is apparent that such a slope would progressively cut off indi- 

 vidual layers at grade. Because weak clay shales are interbedded 

 throughout the sequence of strata, slides were invited by such a de- 

 sign. Consequently the slopes are now designed to follow up the "ap- 

 parent" dip on the "shingle-like" edges of the strata irrespective of 

 whether the beds dip 34° or 45°. In this way every stratum on the 

 slope is "toed in" below grade. A bench is planned at the top of rock 

 about 45 feet above grade, and the 20 (plus or minus) feet of over- 

 burden will be laid back on a 1% to 1 slope. 



In cuts excavated in more or less horizontal strata where massive- 

 bedded hard rock is underlain by clay or soft shales,, progressive de- 

 terioration of the clays and shales permits undermining and collapse 

 of the overlying strata. This condition is accelerated when the over- 

 lying hard rock is strongly jointed. A uniform slope design is obvi- 

 ously an invitation to rock falls unless the slopes are extremely flat, 

 which condition in cuts 60 feet and greater in depth may not be 

 economically sound. Observations in western Pennsylvania indicate 

 that a compound slope design is practicable. Here, by accident rather 

 than by intent, one cut 90 feet high shows that relatively stable slopes 



