Amd 01 W9128F25B0005 Spec 31 11 00.pdf

Lower Brule – Shoreline Protection Project – Grassrope Gravel/Sand Quarry Geotechnical Investigation and Geologic Interpretation

Geological Interpretation Goal; To assist mapping of the cohesive versus non-cohesive material in the Grassrope Sand/Gravel Quarry.

Summary: The Grassrope Sand/Gravel pit (study area), located on USACE land, contains a range of cohesive to non-cohesive material. The study area with 9 test pits (up to 15 ft depth; Figure 1) were excavated around the quarry and sampled various lithologic/material units. USGS (United States Geological Survey) mapping shows this area as alluvial/fluvial terraces associated with glacial outwash transported by the Missouri River: USGS description: Terraces and Outwash (Water-laid material deposited in streams and lakes formed by the melting glaciers or deposited on terrace cut by earlier stages of the Missouri river and its tributaries. The material ranges in size from clay to gravel and has a maximum thickness of more than 100 feet; Howells, 1974). The terraces can be illustrated using a map and cross section which show the change in elevation of the upper terrace to the lower terrace and the modern Big Bend reservoir (former Missouri river; Figure 2).

Caveat: the following cross section interpretation is based on Google Earth derived elevations for the Test Pit locations. If the elevations deviate, the interpretation may change.

Notes on Compacted surface material: Excavator Notes concerning very hard, dense material at the surface likely due to compaction by heavy equipment: TP-01, TP-06.

Notes on collapsing pits: TP-03; sidewall collapses at 2.5’ depth and continued after 15’ depth reached.

TP-06; trench collapsing at 10’.

The following maps and cross sections as well as facies models can be used to understand spatial variations in materials across the quarry (Figure 3).

In general: 2-3 basic facies were documented during the excavation and description of the test pits. The lithologic descriptions include poorly graded sand with gravel to well-graded sand (non-cohesive) and silts/clays (cohesive). The nature of these deposits (sedimentology) and stacking pattern (stratigraphy) are consistent with an interpretation as fluvial bar (bar head and bar tail- non-cohesive) as well as overbank (cohesive) (Figure 4). I.e. Braided Stream Deposits (Figure 5). This interpretation is consistent with the photos that show cross bedded sands and gravels as well as geomorphic evidence as a terrace of the Missouri river. The upper terrace appears to contain at least 2 to 3 vertically stacked braided stream belts.

1) Well-graded sand with gravel and/or silt, little cobble, trace boulder (SW)g interpreted as Bar Head

2) Poorly graded sand with gravel: (SP)g interpreted as Bar Tail

3) Silt or silty lean clay (ML) interpreted as Overbank

Summary:

Included cross-sections and maps can be used to guide future material selection for excavation and use in shoreline protection projects. In general, there are 2 terraces within the quarry area. The lower terrace (to the west) contains surficial non-cohesive deposits that are up to 15 feet thick. The upper (higher elevation) terrace (majority and eastern portion of the quarry) contains areas with surficial

Am-0001 cohesive deposits and areas with surficial non-cohesive deposits. The following map highlights the areas with surficial cohesive deposits. The area of the quarry that is not highlighted with a green color is expected to have non-cohesive sands and gravel at the surface.

Figure 1: Location Map

Figure 2. Big Bend map and cross section of Missouri river terrace geomorphology.

Figure 3: Cross section and perspective map to show the location and spatial orientation of cohesive versus non-cohesive material.

Figure 4: Geologic interpretation of cross section and perspective map view illustrating the stacking pattern of different lithologic units.

Figure 5: Facies diagrams modified from Menzies, 2002.

References

Howells, L.W., 1974, Geohydrology of Crow Creek and lower Brule Indian Reservations, South Dakota;

USGS Hydrologic Investigations Atlas HA-499. 1:125,000

Menzies, J., 2002, Modern & Past Glacial Environments; Reed Elsevier Plc Group, 543 p.