SubSurface Dams - Sustainable Sanitation and Water Management ...

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4) Basal stratum of the fossil valley sedimentAt some parts between the “lower stratum” and the basement rock, a stratum of hard siltrock interposed. Its clear unconformity with the “lower stratum”, and its advancedcompaction and cementation compared with the other fossil valley sediment that had anunconsolidated or semi-consolidated state showed that this stratum of hard silt rock wasformed much earlier than the “lower stratum”.In the lower part of this “basal stratum”, a piece of angular gravel was observed. It isprobably a remnant of the fluvial deposits dating back to when the fossil valley wasformed by active erosion by the old river.5) Basement rockThe bottom and the side slopes of the fossil valley consisted of basement rock (old agerock) that had the appearance of contaminated rock composed of granite, diorite,amphibolite and slightly metamorphosed sedimentary rock. The basement rock of theright bank of the valley was covered with a hard, lateritic crust about 3 m thick.On the basement rock of the bottom of the fossil valley, a fault (argillized zone 0.5 mwide) with a strike slightly oblique to the direction in which the fossil valley extensionwas observed. On the basement rock around this fault, fractured or heavily argillizedzones were also observed.(3) Difference in hydraulic characteristics among the constituent layers of the fossil valleyThe permeability coefficients of the sand layers in the “upper stratum” and in the “lowerstratum” of the fossil valley sediment were estimated to be 10 -2 to 10 -4 cm/sec. On the otherhand, those of the silt, the cemented sand and the lignite layers fell to 10 -6 to 10 -7 cm/sec.The excavation for the construction of the subsurface dam encountered perched water(groundwater that accumulates in the lens shape above the "main" groundwater), from whicha great quantity of water sprang. The presence of this perched water can be explained by asignificant difference, as described above, in the permeability coefficients among theconstituent layers of the fossil valley.At the part where the boundary between the “upper stratum” (silty layers) and the “lowerstratum” (sandy layers) was clear, there was a space 0.5 to 3 cm wide between both strata.On the roof of this space, there was thick "water grime", which indicated that this space hadbeen a groundwater channel for a long period. (See Fig. 5.1)45
46LegendGranule-Topsoil Sandy silt layer Coarse sand layerSiltstoneClay-Silt layer Sand layer Lignite layer Basement rocksFig.5.1: Geological cross-section of the "fossil valley" at the subsurface dam site (upstream excavation profile)Note: The vertical scale of the profile is different from the horizontal scale.
Page 2 and 3: Photo: Construction of the subsurfa
Page 4 and 5: Subsurface DamSmall-Scale Surface D
Page 6 and 7: ContentsIntroduction1. General desc
Page 8 and 9: 1. General description of the Model
Page 10 and 11: Department of the Environmentand Fr
Page 12 and 13: According to the results of an anal
Page 14 and 15: A large amount of waterevaporates f
Page 16 and 17: y adopting an appropriate dam struc
Page 18 and 19: 2-5 "Fossil valley", a suitable sit
Page 20 and 21: temperature of 30 to 34 degrees and
Page 22 and 23: 3-3 Distribution of fossil valleys
Page 24 and 25: 3-4 Surveys carried out in this pro
Page 26 and 27: Satellite image of the area around
Page 28 and 29: 3-4-2 Field explorationAt the 5 sit
Page 30 and 31: 3-4-3 Geophysical surveyAt the 5 si
Page 32 and 33: Groundwater level assumed from the
Page 34 and 35: 3-5 Detailed field surveys (test dr
Page 36 and 37: Groundwater level isalmost the same
Page 38 and 39: The village is on a peneplain with
Page 40 and 41: In this project at Nare, the "earth
Page 42 and 43: DownstreamFilled back soil3mCoarseg
Page 44 and 45: All the results of the quality test
Page 46 and 47: Photo 4.3: Construction of the subs
Page 50 and 51: 6. Observation of the water storage
Page 52 and 53: Table 6.1: Features of the faciliti
Page 54 and 55: PerchedwaterImpermeablelayer“Main
Page 56 and 57: Table 6.3: Annual rainfall in the K
Page 58 and 59: 3) The reserved water level during
Page 60 and 61: Ground surface level: 275.73mGround
Page 62 and 63: 6-4 Analysis of seasonal fluctuatio
Page 64 and 65: SandysiltSiltSand(white)Sand(moist)
Page 66 and 67: (1) Relation between the rainfall w
Page 68 and 69: 4) Subsequently, the cycle in which
Page 70 and 71: 7. Other studiesIn addition to the
Page 72 and 73: these facilities:- 3 mill stations,
Page 74 and 75: Photo 7.3: Water-supply station for
Page 76 and 77: 8. Recommendations for future subsu
Page 78 and 79: advisable to arrange rainfall stati
Page 80: AcknowledgementsThe Model Project t

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