2012 SRS First Crest Raise Foundation Report.pdf

Mount St. Helens Toutle River SRS Spillway Crest Raise

Subject: RCC Spillway Crest Structure Foundation Condition Geologic Mapping: David H. Scofield, P.E., P.E.G.

Date: 15Oct2012

Purpose: The purpose of this memorandum is to document the foundation conditions exposed and prepared for the RCC spillway structure. The spillway structure is designed to be raised in increments as conditions warrant up to 30 feet in height.

River Diversion: The Contractor diverted the low late-summer flow through the southern half of the spillway approach channel. The northern portion of the spillway was protected by temporary dikes on the upstream and southern (river) side. Once the northern portion of the structure which includes the low flow channel and stilling basin was constructed, the Contractor diverted the river to the north and over the new spillway crest. The Contractor then completed the southern portion of the structure.

Original Foundation Excavation: Originally, the rock removed from the spillway was blasted and excavated to the designed grade lines. Because it was not a foundation for concrete structures, no special precautions were taken. The rock was blasted and excavated in mass. Hence, the top of rock / grade line was highly broken/fragmented from blast damage and excavation. Photographs of the upstream edge of the stilling basin cut shows the typical mantling of loosened/fragment rock overlying irregular contact of more intact rock.

Photo 1 and 2. Upstream edge of the stilling basin excavation. Photo shows the mantling of loosen and fragmented rock mantling more intact bedrock. Stilling basin foundation is fragmented rock.

Foundation Excavation, Clean Up, and Preparation: Since, original construction, the sediments have covered the approach channel and built up to the spillway crest. The Contractor excavated this sediment down to the original grade lines/bedrock with conventional equipment. Grade lines for the dam raised were adopted from the 1986 spillway design drawings. The actual top of rock line was at or slightly below the design grade lines. Foundation rock exposed was highly broken up and fragmented most likely from the original construction with varying amount of sediments deposited into the voids.

Excavation further mixed the sediments and rock fragments exposed. Because of the original heavy

Mantle of Fragmented Rock

Intact Rock

Stilling Basin blast damage, there was little true intact in-place rock exposed at the designed foundation grade. The Contractor was directed to stop excavating at the designed grade line and not to continue to excavate to intact rock below. The fragment rock condition was not anticipated, structural stability was rechecked to verify that the exposed foundation conditions was adequate for the planned full 30-foot raise.

Photo 3. Final foundation of the northern half of the spillway crest. Spillway crest is on the right portion, stilling basin in the center and the low flow channel on the left.

The mixed sediment and fragmented rock condition caused difficulties in foundation clean up.

Mechanical equipment just continued to loosen more rock deeper and mix the sediments in. The eventual solution was to let the expose foundation dry out as much as possible and then clean the foundation with sweeper on a Bobcat and high air pressure stream (Photograph 4). The goal was to clean the foundation to at least where 50 percent of the exposed surface area would be rock fragments with the remaining surface exposed as sand and silt sediments.

Photo 4. Sweeper on a Bob Cat used to clean final foundation.

The foundation was cleaned as best could and was judged to be adequate for the intended structure.

Foundation ranged from intact massive flow breccias to highly fractured basalt, fragmented rock with less than 50 percent sediment, and fragmented rock with more than 50 percent sediment. All foundations were firm to walk and drive heavy equipment over. Photographs of typical levels of cleaning are shown.

Example of >50% Sediment and Fragmented Rock Example of <50% Sediment and Fragmented Rock

Example of Intact Basalt Platey Basalt. Even intact bedrock still has some loosened fragments and sediments.

Photographs 5 through 7. Typical levels of cleaning foundation.

Foundation Mapping: Field mapping was conducted on 23Sep2012 for the northern half and 8Oct2012 for the southern half. The foundation was prepared and mapped in two stages. Previous mapping of the approach channel in 1986 did not different the exposed rock. Geologic mapping, generally differentiated the areas of intact basalt rock, areas of flow breccias, and areas of fragmented rock. All contacts are gradational, so contacts should only be considered approximate. Field mapping was done by waling over the surface and measuring with a cloth tape. Final compiled geologic map is shown in Figure 1. The most notable feature in the foundation was a dewatering trench from drainage operation that was left in place at station 0+195 167’LT. This trench was about 7 feet wide by 3 feet deep. It was backfilled with broken rock. The location of this trench was measured from the first foundation drain coming up out of the stilling basin.

Foundation Drains: Original design intended foundation drains to be drilled 5 feet into the underlying bedrock on 10 foot spacing beneath the RCC structure. Because of the highly broken nature of the foundation, there was concern that seepage may be greater than anticipated. To address seepage concerns, foundation drains were drilled on foot spacing. The northern foundation drains were observed with pool up to the new spillway crest. No water was observed coming out of the above water drain holes. This suggests that through seepage beneath the structure is low enough that the fragment rock foundation provides a natural drainage blanket.

Foundation drains flowing water in bend of structure at the south end.

Foundation Backwards Erosion Beneath Structure: A concern was identified that backward erosion of the broken rock could initiate at the stilling basin where the RCC cover is the thinnest. To forestall this possibility, the optional over excavation and constructing an 8 foot thick RCC layer at the upstream side of the stilling basin was executed.

Future Problems: No future problems are anticipated. It should be noted that higher drainage flow may be observed in one or two foundation drains located where the drainage ditch was backfilled with rock and left in place beneath the foundation. This will not expected to cause an internal erosion problem because of the large rock size and low hydraulic gradient.

Future Considerations: Although, the foundation is not ideal, it is adequate for the designed structure.

For future consideration, the foundation beneath the spillway raise should be considered a mixture of sediments and fragmented rock or a “pit-run rock”. If the spillway is ever raised above the planned 30 feet, new geotechnical investigation will have to be done. For future stability analysis, a continuous upstream-downstream layer of mixed soil/fragmented rock should be considered.

1997 RCC

1997 RCC

Flow Breccia

RCC

Platey Rock

Platey Rock

Trench (7’ wide x 3’ deep) backfilled with broken rock located 41 feet from drain hole at top of stilling basin

Platey Rock

Flow Breccia

>50% Soils with Rock Fragments

>50% Soils with Rock Fragments>50% Rock

Fragments, Dense

>50% Rock Fragments, Dense

Waste Pile Unmapped

>50% Rock Fragments, Dense

>50% Rock Fragments, Dense

Flow Breccia

>50% Rock Frag.

And Intact Platey Rock

>50% Rock

Fragments, Dense