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.
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