Observations/Finding

FIELD OBSERVATIONS

On the above date, an AS Engineering and Consulting, LLC (ASEC) representative visited the site at the contractor’s request to observe excavation and mud mat subgrade preparation along the sheet pile wall installed for the proposed bridge on the east side of Pumpkin Vine Creek and along River Walk Manor Drive, in accordance with the project drawings, specifications, and prior coordination with the design team.

Excavation was performed along the face of the installed sheet piles to expose the underlying material for mud mat placement. Based on prior coordination with the bridge designer, the mud mat is intended to bear directly on competent, clean bedrock, requiring removal of all overlying soil and debris and preparation of a clean rock surface.

To maintain temporary support for the sheet pile system, excavation was performed in an alternating sequence, leaving portions of the existing soil in place while adjacent sections were excavated to bedrock.

TESTING AND RESULTS

A total of eleven (11) excavation sections were observed along the sheet pile alignment. Bedrock was encountered at varying elevations within each section.

• Two (2) locations exhibited sufficient depth below proposed mud mat elevation to accommodate the required minimum mud mat thickness of 5 feet.

• MUD MAT THICKNESS (T) – FIELD MEASUREMENTS
  Location	Estimated Mud Mat ThicknessT (ft-in)	Notes
  1a	2′ 8″	Does not meet requirement
  1b	2′ 6″	Does not meet requirement
  2a	5′ 3″	Meets requirement
  2b	5′ 6″	Meets requirement
  3a	-8″	Rock above bottom of footing elevation
  3b	-1′	Rock above bottom of footing elevation
  4a	>5′	Meets requirement
  4b	1′	Does not meet requirement
  5a	>5′	Meets requirement
  5b	0′ 6″	Does not meet requirement
  6a	3′ 6″	Does not meet requirement
  6b	0	At elevation
  7a	0′ 11″	Does not meet requirement
  7b	0′ 11″	Does not meet requirement
  8a	-0′ 6″	Rock above bottom of footing elevation
  8b	-1′ 6″	Rock above bottom of footing elevation
  9a	-3′ 0″	Rock above bottom of footing elevation
  9b	1′ 0″	Does not meet requirement
  10a	-1′ 6″	Rock above bottom of footing elevation
  10b	-0′ 6″	Rock above bottom of footing elevation
  11a	-0′ 6″	Rock above bottom of footing elevation
  11b	-1′	Rock above bottom of footing elevation
  —	—	“Uneven bottom” noted

• The remaining nine (9) locations encountered bedrock at shallower depths, resulting in potential mud mat thicknesses of less than 5 feet.
• In several areas, bedrock was encountered at or above the proposed bottom of footing or mud mat elevation.

The exposed bedrock surface was observed for cleanliness and preparation. The contractor utilized hand tools (shovels and brooms) to remove loose soil and debris and provide direct contact between the mud mat and the rock surface. The cleaned rock surface appeared adequately prepared for concrete placement.

The bedrock was observed to be generally competent based on visual assessment. While full continuity of the rock surface could not be verified within the excavated sections, prior subsurface exploration performed in this area indicates the presence of competent residual bedrock at this elevation.

Photographs documenting the rock profile and excavation conditions are included with this report.

DISCUSSION AND RECOMMENDATIONS

Observed conditions related to insufficient mud mat thickness in areas of shallow rock were discussed on site with Chad Clark and Brian Hughes of Reinforced Wall Systems.

Based on these observations, additional guidance from the project designer is required to address areas where the minimum 5-foot mud mat thickness cannot be achieved. Construction in these areas should not proceed until design direction is provided.