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Field surveys were made to obtain baseline profiles and to establish boring locations at the upper and lower Dickey sites and at the Falls Brook and Hafey Brook dike sites. All borings were drive-sampled in overburden, and the bedrock, where encountered, was drilled with coring diamond bits. The classification of overburden samples and description of bedrock cores recovered from borings at the selected downstream damsite and the Falls Brook and Hafey Brook dike sites are shown on record of explorations, plate No. 14. 2-08 Site selection
Explorations indicated that subsurface conditions encountered at the upper site were not favorable for construction of a high dam. As shown on geologic section, plate No. 13, the very thick deposits of soft silt and clay which occur throughout the valley bottom, would necessitate very flat slopes on the dam, and the relatively pervious materials which occur to depths of more than 40 feet would require a deep cutoff and elaborate downstream drainage. The very thick, relatively pervious materials in the left abutment extend to depths of more than 150 feet below the valley bottom, making cutoff impractical and control of seepage very difficult. Because bedrock is inaccessible in the left abutment and valley bottom, it would be necessary to concentrate the diversion tunnels, penstocks, powerhouse and spillway in a relatively limited area of the right abutment.
It was found by reconnaissance, which continued during the exploration of the upper site, that at a lower site bedrock outcrops occurred on the top and upper slopes of the prominent hill immediately above the mouth of the Allagash River and at other locations along the Allagash River bank and the south side of the valley as shown on plan of exploration, plate No. 11. On the north side of the valley, a gully was found which formed a deep reentrant in the continuous pervious terrace deposits so that cutoff to till is possible. In view of the feasibility of cutoff in the left abutment and the availability of bedrock for structure foundations at several locations, a dam alinement at this location, the lower Dickey site, was finally selected. This selected site is hereinafter designated as the Dickey damsite. 2–09 Foundation conditions
(a) Dickey Dam.-In the valley bottom below the nominal channel deposits of silty, sandy gravel, outwash consisting of silty sand, sandy and clayey silt extends to a depth of approximately 40 feet as shown on geologic section, plate No. 13. Cutoff through the river channel deposits and into the upper part of the relatively impervious outwash materials is possible at reasonably shallow depths.
Beneath the fine outwash a zone of stratified silty sands and gravels with scattered till lenses approximately 50 feet thick overlies the basal till which extends to bedrock in the bottom of the valley at depths of more than 180 feet below the river. The fine outwash deposits in the valley bottom extend up the left abutment in a deep gully to a height of more than 100 feet above the river, but throughout the gully area the outwash is blanketed by up to 40 feet of till. Upstream and downstream adjacent to the gully the prominent terraces on the abutment form very thick deposits of silty sands and gravels with beds of sandy silt and zones of till. Above the terraces the upper slopes of the abutment are blanketed with till. The fine-grained outwash which occurs in the valley bottom also extends up the right abutment of the river section to heights above the highway, but again these materials are blanketed by till with thin beds of sand and gravel so that cutoff to relatively impervious material is available at reasonable depths. Throughout the upper slopes of the right abutment the till extends to bedrock at depths estimated at about 30 feet.
On both abutments at the south section of the dam, the overburden is generally thin and consists of till composed of gravelly, silty sand. Along the lower part of the left abutment, as shown on geologic section, plate No. 13, it is expected that the thickness of the till cover increases to 40 feet or more and grades into a more silty, clay till. The bottom of the broad, flat valley is largely a peat bog with organic deposits estimated to be about 10 feet in thickness. Underlying the organic material, stratified, silty sands and gravels occur to estimated depths of 60 to 75 feet. Cutoff through these relatively pervious outwash materials can be made to the underlying till which extends to bedrock. Although some finegrained soils occur in the foundation, they appear to be dense and have relatively high shear strengths and would permit relatively steep design slopes for the embankment.
The bedrock is dark gray to black, relatively hard, slaty shale, generally
Bedrock is generally sufficiently deeply buried and blanketed with thick de-
.) Falls Brook dike. At the Falls Brook dike site which is the major remote dike, the overburden on the abutments is till as shown on geologic section, plate No. 13. On the left abutment the till cover is very thin near the bedrock outcrops bizh on the slope. On the upper slope of the right abutment, the till is also thin, bat the thickness of the till blanket increases down the abutments to the deeply filled ralley bottom, where the till rests on bedrock at depths of more than 90 feet. In the valley bottom the till is covered by outwash consisting of silty sands and gravels with minor lenses of till. The outwash is generally less than 10 feet thick. although local accumulations up to 25 feet occur. Cutoff through the relatively perrious outwash deposits to the till is therefore possible at generally shallow depths. The bedrock is the common slaty shale with characteristics similar to the rock at the damsite.
(c) Hafey Brook dike.--At Hafey Brook dike site the overburden on the right of east abutment is till generally about 10 feet in thickness as shown by boring FD-10. Although subsurface information is not available in the valley bottom of rest abutment, reconnaissance indicates that it is quite possible for a deeply filled valley in the bedrock to occur in this area. Immediately north of the dike kile there is an extensive outwash plain floored with sands and gravels. If a deep buriel valley does occur at the site, it is probably filled with similar sands and
Belrock encountered in the boring at the site is andesite with scattered quartz stringers and shale inclusions. The bedrock exposed in extensive outcrops immediately south of the site is slaty shale.
Campbell Brook dike.--The overburden on the abutments at Campbell Brook dike is till. It is assumed that the organic materials, probably superficial in depth which occur in the valley bottom, are underlain by thin deposits of sand and gravel resting on till.
Bedrock was not found outcropping in the vicinity of the site. Outcrops on the
(e) Blue Brook dike and Cunliffe Brook dike.-These remote minor dike sites have not been visited. It is assumed, however, that the overburden on the abutments at these locations is till. Superficial organic deposits probably occur in the valley bottoms overlying till. Bedrock at the sites is assumed to be the regional slaty shale. 2-10 Embankment and foundation designs
(a) Dickey Dam.-Typical sections of the embankment for Dickey Dam, designed for maximum utilization of available materials, are shown on plate No. 7. The section design provides for a minimum core section composed of glacial till, flanked by large relatively pervious zones. The large pervious zones not only enables full utilization of the more economically available earth borrow, but permits earth placement to be carried out over a longer construction season than possible with moisture sensitive materials. Also the internal design can be modified considerably in final design to suit the characteristics of materials actually available and adopted construction sequences without change in outer slopes. Seepage through the embankment is controlled by the arrangement of the impervious and pervious fill zones, and foundation seepage is, in general, controlled by carrying the impervious section of the dam to rock, glacial till, or other impervious stratum. At the left abutment, however, it may not be feasible to obtain complete foundation cutoff, and the cost estimate provides for inclusion of a short upstream impervious blanket and a drainage tunnel downstream for intercepting possible underseepage at this abutment.
Because of the slaty nature of rock from required excavations, and the need for processing to remove the expected large quantity of excess fines, rock slope protection is provided only on the upstream slopes. Below minimum pool, the rock slope protection will consist of processed rock from required excavation and a select granitic rock transported from the Deboullie Mountain area will be used above. Downstream slopes will be protected by gravel and cobbles from the pervious borrow and site excavations.
(b) Falls Brook dike.-The only known sources of large quantities of embankment materials in the vicinity of the dike site appear to be deposits of relatively impe ious glacial till. This dike has therefore been designed as an essentially homogeneous impervious fill section, with impervious cutoff carried through shallow pervious deposits to rock or impervious glacial till. An internal pervious wick drain and downstream horizontal pervious blanket are provided for controlling through and under seepage. Slope protection on the upstream slope above the minimum pool will consist of select granitic rock from the Deboullie Mountain area and processed gravel below. The downstream slope protection will consist of locally quarried rock.
(c) Hafey Brook dike.—Materials indicated to be available in the area for embankment construction are pervious sands and gravel sand impervious glacial till. Cutoff to bedrock or other impervious stratum does not appear feasible in the valley bottom and at the west abutment. A horizontal upstream impervious blanket connected to the core, a pervious fill drainage trench downstream of the core, and a downstream rock toe are provided for control of seepage. However, the upstream impervious blanket is eliminated where cutoff to rock or glacial till appears feasible on the east abutment. Upstream slope protection will consist of select granitic rock from the Deboullie Mountain area above minimum pool and processed gravel below. The downstream slope protection will consist of locally quarried rock on gravel bedding.
(a) Campbell Brook, Cunliffe Brook and Blue Brook dikes.—These dikes which are of relatively low height are designed as essentially homogeneous impervious fill sections, utilizing the local glacial till, with pervious downstream toes, for control of seepage. Cutoff to impervious glacial till is assumed at relatively shallow depth. Upstream slope protection for the Campbell Brook dike will consist of select granitic rock from the Deboullie Mountain area. Downstream protection will consist of processed rock from required excavations at Dickey Dam. Local rock will be used for all slope protection at Cunliffe Brook and Blue Brook dikes. 2-11 Construction materials
(a) General.—Quantity estimates of borrow materials required for construction of the dam and dikes are based on available topography and geology for the various sites and preliminary designs appropriate to this stage of investigations. Estimated volume of the main dam and dike at Dickey including rock slope protection and rockfill is approximately 56 million cubic yards. The volume of earth and rock materials in the Falls Brook dike is about 7,300,000 cubic yards.