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i31G 148 METROPOLITAN WATER DISTRICT In the drawing of the steel-supported section two kinds of sup- port are shown: On the right, the standard liner plate, used ex- tensively in the early period of tunnel excavation; on the left, rib steel as subsequently developed for the Coachella tunnels and ex- tensively used there and in other tunnels because of its low cost and ease of erection, The 12 by 12-inch timber set shown on the drawing was subject to variation to suit conditions. The most widely used dimension timber was 10 by 10 inches with an excavated section proportion- ately smaller than shown. In heavy ground timber sets up to 16 by 16 inches, skin-tight, have been used, in which case the entire section has been enlarged to pl'Ovide for placing concrete lining up to 24 inches thick inside of the timber. Specifications The rate of progress in tunnel construction under the best of conditions is much less than can readily be maintained in the con- struction of surface conduits of comparable size or capacity. Therefore the tunnels were the first of the aqueduct features to be started. In general the tunnel schedules consisted of units up to approximately two miles in length, which could be economically constructed from the proposed point or points of access. In the longer tunnels, however, particularly those under high mountains where access was limited except at heavy cost, the schedules were of greater length. All bids were called for on the basis of unit prices for the esti- mated quantities of each of the various items of work comprising a schedule. Lump sum bids were sometimes called for on single items of a schedule, as for example the access work, but never for an entye schedule. The bidder included in his unit prices all costs of plant and materials except reinforcement steel and other permanent metal work, steel tunnel support, and cement for gunite, grout, and tunnel lining, all of which were supplied by the District. The first tunnel specifications were issued October 19, 1932 in- viting bids for the construction of San Jacinto tunnel to full size and capacity. Bids were opened November 29th and a contract was awarded December 12, 1932 to the Metropolitan Engineering Cor- poration on the basis of its low bid. This corporation defaulted on its proposal, however, and thereafter the next low bid was rein- stated with the consent and at the request of the bidder. On Febru- .ary 10, 1933 a contract for the work was awarded this bidder, i