Mitchell Bay Interim Corrective Measure Remediation of Docks 6 and 7
Significant Project Features
- Installation 500 LF of near-shore Open Cell sheet pile wall 50 feet off the shoreline
- Mechanical dredging of 8,000 CY of petroleum-contaminated sediment from Mitchell Bay. Sediment transferred to staging pad for processing
- Amended material with Portland cement to 10 psi and placed solidified sediment behind sheet pile wall. Installation of a 3,500 CY multi-layer low-permeability cap over processed material
- Collection and treatment of all water and wastewater generated during remediation with 500 gpm water treatment system
- Achieved 19,159 workhours without a lost-time incident. Level D PPE
History & Location Details
The ExxonMobil Baytown Refinery is located in Baytown, Texas. The Project Site is within an armored shoreline and shallow near-shore tidal zone located between two active refinery docks along the Houston Ship Channel. Historic operations deposited petroleum-containing sediments, some of which had settled below the water level in Mitchell Bay. This settled portion was subsequently covered by silt from the Houston Ship Channel system through natural sedimentation processes over the years. On the shore side, contaminated sediments had been covered by fill material and armored with Gobi-Mats and riprap. The project site overlays clay and sand deposits.
Prior to initiating dredging operations, site preparation began with the installation of an Open Cell™ sheet pile wall which extended approximately 50 feet offshore. A single tier sheet pile template was spudded at the cell location. A crane and a ground Release Sheet Pile Shackle (GRSPS) was used to hoist a pile from a material barge and set along the template. Pile was interlocked with the preceding pile, using a STABCAT sheet pile threader. Tail wall and anchor piles were then installed. A vibratory hammer drove the pile to target depth.
Approximately 1,750 CY of rip rap was removed and relocated from behind the sheet pile wall. Gravel was placed along with a seepage collection system within the sheet pile wall. After the sheet pile wall was installed, gravel was installed on top of the geogrid under submerged conditions. Backfilling above this gravel layer proceeded under dry conditions. All water pumped out of this area was pre-treated to remove visible sediment and non-aqueous phase liquid and discharged to the refinery lift station 219, which transferred it to the plant wastewater treatment system. The maximum allowable flow rate was 500 gallons per minute. The area was prepped to receive Foundation Layer material by relocating the rip rap and then rough grading and removing unsuitable materials including obstructions and vegetation that interfered with proper construction of the foundation layer. Approximately 3,500 square yards of geogrid was installed before commencement of backfilling activities. A 4,000 CY gravel layer was installed above the geogrid. Material for the gravel layer was identical to Erosion Resistant Soil.
Following site preparation, Sevenson mechanically dredged of 8,000 CY petroleum-contaminated sediment from Mitchell Bay. Sevenson used a Komatsu PC400 LF excavator to mechanically dredge sediment in order to maximize solids content in dredge material. A low-water generating bucket (Cablearm Environmental Bucket) was used to make precise and accurate cuts, which was necessary for this project. The dredge was equipped with GPS technology and sonar for accurate horizontal and vertical control during dredging. Dredge material and imported fill was solidified with cement to achieve the required compressive strength, then placed behind the sheet pile wall. The dredged material was then capped with 3,500 CY multi-layer low-permeability cap with a draining slope, followed by 1,200 CY compacted clay containment cap with erosion-resistant layer. Removal of water and free non-aqueous phase liquid (NAPL) present behind the new sheet pile wall was necessary to place the solidified dredge material. Sevenson added an extra treatment step (organoclay) to capture any additional oily waste or emulsions.
Restoration of the site included grading smooth existing surfaces that remained but were disturbed by the operations. Sevenson utilized an ExxonMobil approved surveyor to demonstrate the final grading has been achieved. Material was smoothed beyond final cover to achieve surrounding grade. Damaged grades, elevations, and slopes were repaired or re-established to complete site restoration.