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HomeMy WebLinkAboutSection 32 4040 "B" STREET ANCHORAGE, ALASKA 99503 (907) 278-1551 March 25, 1979 W.O.# A18787 Alaska Land Development 4546 Business Park Blvd. Anchorage, AK 99503 Services Attention: Mr. Bill Tucker Subject: Wallace Homestead Within Section 32, T15N, RIW, SM, Ak (Eagle River-Chugiak) Sir: As agreed, we have prepared a preliminary analysis of the existing geologic conditions as they relate to residential and possible commercial development on the Wallace Homestead property. Our analysis is based on A) test hole logs performed by Construction Test Lab dated September 1973, May 1974 and November 1974, B) water well drilling logs from A&L Drilling Company completed in 1974, C) the 1974 edition of "Geology and Ground Water for Land-Use Planning in the Eagle River- Chugiak Area, Alaska" by Zenone, Schmoll & Dobrovolny. D) Stereo Pair - Aerial Photos, E) test hole location maps as supplied by Bob Johnson, F) Municipality of Anchorage Togpography Maps and G) previous knowledge of the general area gained by Mr. Jim Mack during the 1973 and 1974 test hole excavations. The topics discussed in this preliminary report are: 1. General Description of Surficial Geology - Sheets 2 . thru 3 .' 2. A Review of the' Previous Water Well Testing and Future Impact of Development on the Water Resources - Sheets 3. thru 3 .' 3. On-Site Sewage Disposal Systems and Their Impact on Adjacent Lands - Sheets 4. thru 5 · '4. Geologic Hazards - Sheets 5 . thru 8 .' Alaska Land Development Services March 25, 1979 Page 2 Generalized Geology Description of Eagle River - Chugiak Area The surficial deposits are comprised of unconsolidated materials that have accumulated on the land surface above the bedrock within the last 1 million years, during which time the general area was glaciated several times. The glaciers eroded some of the bedrock and much of the older unconsolidated materials and deposited till that formed moraines and other glacial land forms. During and after glaciation, colluvium accumulated on the mountain slopes. Colluvium is a general term for material on or at the bases of slopes that has moved down from higher areas largely by gravity. Because of the wide range of geologic processes which produced the surficial deposits and the complexitiy of the area's glacial history, the deposits vary widely in grain size and sorting. Although the glaciers were large masses, most the individual streams that reworked the glacial deposits were relatively small. Consequently, few individual geologic units have any great thickness or lateral extent, and many combinations of interbedding and mixtures of materials occur. In addition, erosion removed much material; redeposi- tion resulted in a complex juxtaposition of older and younger deposits. The surficial deposits range in thickness from a feather- edge at steep bedrock slopes of the Chugach Mountains to at least 300 feet (90.1 m) on the alluvial fan of Meadow Creek. Also, there are local variations in thickness which have at least three causes: (1) the irregular bedrock erosion surface beneath the unconsolidated deposits, (2) uneven topography of glacial deposits resulting from accumulation on and adjacent to glacier ice which subsequently melted, and (J) repeated erosion and dep0sition of unconsolidated material by streams. The surficial deposits which occurs on the Wallace Homesteads consist of (7) coarse grained deposits of glacial alluvium, (2) mixed coarse and fine grained deposits of younger and older glacial morines, and (3) colluvium (slope deposits). There are areas on this property where bedrock is exposed or occurs at shallow depths below surficial deposits. Bedrock was found in TH#I~5, 2~0 and 22A of the September 1973 Construction Testlab soils report and in test hole #9 of the November 1974 Construction Testtab soils report. Shallow bedrock occurs on this property - especially thru the center Alaska'Land Development Services March 25, 1979 Page 3 of the property in a east-west directign. Bedrock may also occur locally in the north half of Section 32. There is one deep test hole in this area (TH#25), however, the remaining test holes are relatively shallow (4-1/2' to 7' in depth). These test holes should be extended to 16 or more feet in depth and additional test holes should be placed to supplement and verify bedrock conditions. The surficial deposits on the Wallace Homestead vary in depth from nothing at exposed bedrock to as much as 168 feet at water well #6 drilled by A&L Drilling Co. The test pits excavated by CTL indicate' that the majority of surficial soils are relatively permeable. The majo~ soil type found in the previous soils investigations appear to be sandy gravels (GW-GP). There are isolated areas of sand (SW), silty sand (SM) and shallow layers of silt (ML). Water Supp!~ The well' logs submitted seemed to indicate that, at least in the areas tested, sufficient water was available for any on- site domestic requirements. We would point out however that the wells which have been drilled all lie within the ~E ~/4 of the property. Due to the highly variable depth of bedrock on the property, it is possible that dry holes could.be 'encountered in some areas. If on-site sewage disposal is to be used as well, the highly permeable nature of the soils combined with the very steep slopes existing on the property and the relatively shallow depths of wells, could combine to increase the danger of contamination occurring in the wells. From the log of Well No. 6, it appears that formations exist on the property which could produce sufficient water to operate a public water system for the development. In view of the highly variable and possibly uncertain nature of the groundwater resource over the entire property, we would suggest that the installation of a community system be seriously considered. Due to the topography of the pro2 perry, however, it would be necessary to carefully design a water main network with pressure reducing stations located such that proper service pressures are maintained %o all properties. Since the area of the highest apparent ground- water potential is near the upper end of the gradient of the property, it would be possible to serve the entire subdivision by gravity from a reservoir located on high ground near the east edge of the property. Alaska-Land Development Services March 25, 1979 Page 4 Sewage Disposal Although a large part of the property has not been defini- tively tested to show bedrock, and water table depths and soil conditions, indications from the locations which have been tested indicate that most of the unconsolidated materials are very permeable thus being ideally suited to the construc- tion of on-site sewage disposal facilities. Some test holes, however, indicated that water table or bedrock exists sufficiently near the surface to preclude the installation of on-site sewage disposal facilities at that particular site. Due to the steep gradients and varied nature of the terrain, however, we feel that the~e condi'tions may be sufficiently isolated that a system could be installed within a short distance of these particular test sites. As mentioned in the water supply section above, if shallow wells are used for water supply, the steep gradients and permeable nature of the materials may combine to cause contamination problsms to occur. If commercial or multi-family structures are proposed for this area, they should be constructed in areas where the subsurface soil conditions will support larger on-site sewage systems without danger of contaminating surface or subsurface waters. There are several selected areas.on this 'property which are suitable for these type of structures and they should be thoroughly investigated prior to forming the final subdivision plat. Areas favorable for these structures would not have 1) surface or shallow subsurface ground water conditions, or 2) shallow bedrock conditions. The subsurface soils must have good percolation rating and they should be fairly extensive in depth. The impact of the subsurface onsite sewage systems on adjacent lands should be tolerable if each system is constructed properly in accordance with current Municipality of Anchorage specifications. These particular systems should be insp%cted very carefully during their construction to insure the highest level of design and workmanship. On site systems should not be constructed on very steep slopes or in areas' of shallow bedrock, especially in watershed areas. Constant surveillance of stream channels by health department officials should be maintained to assure that surface waters are not contaminated from sewage systems within this land tract. Alaska Land Development Services March 25, 1979 Page 5 If the sewage systems are constructed in accordance with current approved standards, there should not be any health hazards imposed on adjacent lands. Geologic Hazards Landslides & Slope Stability Slope stability is the ability of the surface of the ground to remain fixed in position and to resist failure by land- sliding or other earth movement. A primary criterion for determining stability is the degree of slope; steeper slopes generally are less stable than more gentle slopes. Stability also depends considerably on the geologic materials underlying the slope. Slopes underlain by metamorphic rocks are generally more likely to be stable than slopes of the same steepness underlain by coarse- grained surficial deposits. The latter slopes are, in turn, generally more likely to be stable than slopes of the same steepness underlain by fine-grained surficial deposits. Factors which modify slope stability are: (1) exposure of earth materials to running water which may cause either direct erosion or loss of lateral and vertical support because of erosion; (2) chemical and physical changes in the material, caused by exposure to the atmosphere (weathering) which may result in loss of cohesiveness, (3) increase in water content which may cause the material to lose strength, and (4) excavation or other activities of man which may induce slope instability. Major instability may be triggered by ground shaking during an earthquake. Because all of south-central Alaska lies in a zone in which large-magnitude earthquakes can be expected, the likelihood of earthquake-triggered instability is relatively high throughout the area, but the probability of occurrence of such large-scale earthquakes is low. Ground movement along a fault, either during an earthquake or by nearly imperceptible creep, can also cause instability. The slope stability of this particular land tract varies from a generally high stability to moderate stability depend- ing on localized geologic conditions. Alaska Land Development ~ervices March 25, 1979 Page 6 Where the terrain is flat to moderately gentle and is underlain by surficial deposits, or where moderate slopes are underlain by surficial deposits, or where the steep slopes are underlain by metamorphic bedrock, the slope stability is rated generally high to moderate. In these areas there is little likelihood of significant downslope movement, except in areas of steep slopes where bedrock may be covered by loose colluvium or surficial deposits. Some downslope soil movement may occur on steep slopes because of one or more of the following circumstances: 1. The natural ground cover is disturbed. 2. Man-made excavations for any purpose are not constructed properly in accordance with accepted design procedures. 3. Water erosion due to surface runoff especially in steep gullies. Permafrost In reviewing previous test hole excavations, it is not eviden~ that permafrost occurs on this land tract. It is possible, however, to find localized frozen soils near the ground surface, especially in the wetter-silty areas. These frozen soils would probably be classified as seasonal frost rather perennial frost. The water well driller drilled six deeper borings without encountering deep-seated frozen soil conditions, however, these test well locations only reflect the soil conditions on a small portion of the property. Additional deeper (30 foot) test borings may be required in marginal areas where deepseaCed perennially frozen soils could occur. Floods The probability of floods occurring from natural causes in this land tract is minimal. Some localized flooding could occur in the vicinity of Carol Creek, in the upper portion of the property, if watershed drainage is not properly designed and maintained. The remainder of this land tract appears to have very good natural drainage. Stream Erosion Stream erosion can occur along Carol Creek or along other gullies located in the northern portion of this land tract (Mike Wallace's Homestead) whenever natural soil cover is 'disturbed or removed or adjacent soils are disturbed by Alaska. Land Development Services March 25, t979 Page 7 excavations. Any development which occurs adjacent to these stream channels should be carefully designed and controlled, otherwise silt and other undesirable stream contamination could occur in lower adjacent lands and especially in the Upper Fire Lake area. Past experience by the owners should be reason for utmost caution in this regard. Ground Water Based on the available information, it appears that high ground water does occur in a small portion of this land tract, particularly to the east of Carol Creek and south of the existing roadway (refer to test hole #4). This particular area is without much elevation relief for proper drainage, hence a high ground water table occurs in the existing soils. Other test holes (#1, lA, 3, 5, 6, 8, 9, 10, 13) indicate an average ground water depth of 9.8 ±1.2 feet below existing ground l'evel. No ground water was found in the remaining 20 of the 29 test holes excavated (to the depths investigated). Additional test excavations should be performed, especially in the northern half ~f this land tract, to establish if high ground water presents a geologic hazard. · High ground water is an undesirable geologic condition for construction of residential or commercial development. On- site sewage systems must be constructed a minimum of four feet above the water table and they should be covered with to 2.0 feet of soil. It is evident from these figures that the~subsurface ground water level should be deeper than 8 feet below finished ground surface to be legally constructed. Deep Peat Deposits From the available information supplied, it appears that there are no significiant accumulations of deep organic deposits. The deepest layers of organic deposits found was 3 foot in thickness (TH#5). It is not probable that deeper peat deposits will be found on this land tract. Avalanche For a snow avalanche to occur there must be a avalanche path where there is a past history of avalanches or it is suspected that future avalanches could occur. An avalanche must have a "starting zone" of 30° steeper and must receive large amounts of snow. Gullies and bowls are efficient snow collectors and are usually the most active "starting zones". Alaska Land Development Services March 25, 1979 Page 8 The primary source of avalanche history information on the Wallace Homestead would be the present owners or the U.S. Forest Service. tf there is no clear ~vidence of avalanche history from these sources then any planning or development must rely on the topography, climate and avalanche damage to local vegetation. Summary The development of a subdivision on the Wallace Homestead will depend on well-engineered foundations, roadfills and on-site sewage systems to prevent any undesirable impact on adjacent and lower land areas. Local bedrock and ground water conditions should be further investigated so that sound engineering recommendations may be made. The surficial deposits where they are are not at steep grades, appear to be sewage systems. extensive in depth and adequate for on-site The intent of this report is to provide a general description of the local, geologic conditions and impact of subdivision development. If you have any questions, please contact our office. very trul yours, ALASKA TE S TLAB James D. Mack, CET Laboratory Supervisor Approved by: Melvin R. Nichols, PE Laboratory Manager JDM:mfm