A22 - Questions and Responses.pdf

Carolina Beach Coastal Storm Risk Management Project – Questions

Q1. Is there are minimum core recovery requirement? If so, is there a minimum number of attempts to achieve the requirement?

Response 1:

A total of 150 vibracores are supposed to be drilled for this contract. Locations of and target depths for the first 70 vibracores are provided in Table 2 and Figure 1 of the Scope of Work.

While the SOW does not require a “minimum core recovery,” the contractor should fully attempt to penetrate to and recover from the target depths listed in Table 2. Additionally, according to the SOW, the results from this first round of vibracoring will be used to locate additional vibracoring locations, up to a total of 150 vibracores. Target depths for some of those vibracores will likely be 10 ft., while target depths for others should not exceed 20 ft.

According to the Scope of Work paragraph (4), Vibracore Sampling, “…Vibracore drilling shall be accomplished by vibrating a sampling tube to the maximum target depths below the ocean floor or to refusal. Penetrometer data collection shall be recorded and presented to verify vibracore refusal. Vibracore refusal is defined as the sampling barrel having a penetration rate of less than 0.2 feet over a 2-minute period. If vibracore refusal is encountered at the sea bed, the Contractor shall offset up to 20 feet and conduct at least two additional vibracore attempts.

Each location where refusal is encountered shall be recorded.”

Response 2: PER SOW, the Contractor will make a 10 or 20 foot drive. If he’s hitting refusal, offset 20 feet and attempt 2 more times. A minimum core recovery requirement is not specified in the SOW, but professional engineering/geologic judgment would necessitate re-drilling a boring if recovery was less than 50% of the run. Common preventable causes of this are due to the following driller/operator errors:

1) Worn, damaged, old sample retention baskets or “fingers” located behind the core bit/shoe

2) Poor pneumatic/hydraulic action of the vibratory head, resulting in poor penetration

3) Loss/damaged of core barrel shoe/bit

4) BLOCKED core barrel/shoe/bit (for example a large oyster shell or buried/submerged tree stump).

5) Poor placement of vibracore machine on seafloor surface resulting in tipping.

Core recovery should be visually verified prior to disembarking to the next boring location. The logger/geologist will as a matter of logging the core denote the Run and Recovery of the core in decimal feet. Non-recovered/lost sections will be denoted in the log notes and also on the USCS/stratigraphy legend column so that USACE will know what depth interval is essentially “unknown”.

Q2. If an initial attempt yields refusal and the second (20-ft offset) also yields refusal, is the boring location terminated?

If not, how many attempts (that yield refusal) are required before a location is terminated?

Response:

According to the Scope of Work paragraph (4), Vibracore Sampling, “…Vibracore refusal is defined as the sampling barrel having a penetration rate of less than 0.2 feet over a 2-minute period. If vibracore refusal is encountered at the sea bed, the Contractor shall offset up to 20 feet and conduct at least two additional vibracore attempts. Each location where refusal is encountered shall be recorded.”

Per the SOW, conduct initial attempt, then if refusal is encountered, make your offset 20 feet, make an attempt. If refusal is encountered again, document and offset 20 feet (“to other side”) make last attempt. If refusal encountered, document and denote depth/elevation of the refusal. For example:

CB-21-V-001, coordinates, Refusal encountered at -20 feet NAVD88

CB-21-V-001A, coordinates +20ft NW, Refusal encountered at -20.5 feet NAVD88

CB-21-V-001B, coordinates -20ft NE, Refusal encountered at -19.8 feet NAVD88

Terminate CB-21-V-001, A&B……continue to next boring location.

Q3. Is there a standard or FOP for creating a photographic mosaic log?

Response 1:

According to the Scope of Work paragraph (4), Vibracore Sampling, “A photographic mosaic log depicting the stratigraphic horizons relative to depth shall be produced for each core. Vibracores shall be field classified and logged in accordance with ASTM D2488 “Standard Practice for Description and Identification of Soils (Visual-Manual Procedure).” The top and bottom elevation of each vibracore and core photo shall be referenced to NAVD88 vertical datum.” This means that each core shall be photographed and shall have a depth/elevation scale beside it.

Doing so will allow for the vibracore log and the photo to be matched together.

Response 2:

USACE does not have a standard form for the photomosaic log, only the standard ENG Form 1836, drilling log. The SOW outlines the critical pieces of information required in the photomosaic logs: photograph with core number/id, depth scale beside the core, elevation of the top and bottom of the core, all clearly shown. Core photos should be clearly visible, and have enough resolution to discern basic textural and stratigraphic features. Core photos/mosaic logs that lack scales, elevation data, or are fuzzy/blurred/poor resolution will not be accepted and will be re-shot at the contractors expense.