846 



Popular Science Monthly 



ST net J iu/trnce 



Eliminating all but the most vital 

 technical details, the problem becomes 

 one of blasting the rock from beneath 

 the present station over small areas, 

 which are self-supporting and which are 

 subsequently shored by steel beams and 

 timber supports before any further prog- 

 ress is attempted. To allow for the 

 blasting and removal of the rock with- 

 out endangering, to any serious extent, 

 these temporary 

 supports, the ar- 

 rangement of the 

 shoring must be 

 changed from time 

 to time until 

 enough of the new 

 structure can be 

 finished safely to 

 sustain the struc- 

 ture above. The 

 support is arranged 

 temporarily until 

 the new work is 

 completed, where- 

 upon the permanent 

 footings of masonry 

 are placed. 



By referring to 

 the illustrations, it 

 will be seen that, 

 while a meshwork 

 of light steel beams 

 resting on posts or 

 other temporary 

 work serves to sup- 

 port the platform 

 and lighter portions 

 of the station, the 

 shoring of the plat- 

 form columns, each 

 of which is com- 

 puted to carry a 

 load of about two 

 hundred tons, has 

 to be advanced in 

 slow and expensive 

 stages. The cast- 

 iron bases of the 

 columns are first 

 carefully exposed 

 and enough of the 

 foundation concrete 

 and rock removed to 

 allow placing large 

 steel beams beneath 

 sides of base. 



How It Was Done 



The top diafiram shows the first step. Large steel 

 beams wore placi-d under the jiartly exposed col- 

 iiMiiis of the old subway as a shoriiiK and wedRes 

 hammered in until tlip weight was lifled. Then 

 these were replaced with a single heavy timber post, 

 directly under the original column. As the new 

 steel-work progresses and is filled up with concrete 

 all the Bhoring is taken away until at lengtli the 

 i-tructurc stands complete, one subway on another 



Wherever the headroom or working 

 space is sufficient, these beams are sup- 

 ported on well-placed timber columns 

 beneath which steel wedges are driven, 

 until there is a visible separation be- 

 tween the cast-iron base and its original 

 foundation, indicating that the full load 

 of the column above has been transferred 

 to these temporary supports. The rock 

 foundation is then removed to about a 

 foot below the roof 

 of the new subway, 

 whereupon a single 

 square timber post, 

 accurately centered 

 under the column 

 and wedged to take 

 the full load, re- 

 places the shoring 

 beams and releases 

 them for use at 

 other points. 



Working at the 

 new subway roof 

 level, small rectang- 

 ular cuts or "drifts" 

 are excavated and 

 timber posts placed 

 for the support 

 of the structures 

 above. Where the 

 character of the 

 rock permits, the 

 excavation is next 

 made to a depth 

 sufficient to as- 

 semble a portion of 

 the new structure. 

 Where the full 

 depth cannot be 

 excavated without 

 endangering the 

 supports, long "run- 

 ner" beams, upon 

 which several sets 

 of roof girders are 

 placed, are set in 

 concrete bearings 

 upon the rock. An- 

 other "pick-up" of 

 the platform col- 

 umn is now made 

 by means of a crib- 

 work of steel beams 

 and blocking, built 

 up from the perma- 

 nent roof steel. 



