Q_and_A_Document_Prosser_Creek_Dam_Becker_Drilling.doc

R17PS00289

Prosser Creek Dam Becker Drilling

Request for Quote

R17PS00289

Questions and Answers Response Date: February 13, 2017 Question 1:

What is the approximate date to start the work at the site?

Answer 1:

June 2017 Question 2:

Is there a water supply at the site? If yes, please provide details ie. location, type of supply etc.

Answer 2:

Typically Reclamation forces have used the river downstream of the dam for drilling water. Recommend bringing 50 to 100 feet of hose and a screened pump.

Question 3:

If refusal is reached before the estimated total depth or the thin layer of alluvial material is not completely penetrated, will the hole be pre-drilled with an open bit in order to advance the IBPT to bedrock?

Answer 3:

The program is based upon conducting closed bit iBPT testing of the alluvial layer below the zone 2 or 3 of the dam. Reclamation drilled this same area with a CME 85 using 6.5 inch ID hollow stem augers to bedrock. This location can be “bony” but nothing an open bit iBPT shouldn’t be able to advance through.

Question 4:

What is the estimated depth to ground water in the embankment holes and the holes at the toe?

Answer 4:

No water is expected to be encountered in the dam off the bench on the downstream side of the dam. Water may be encountered in the alluvium beneath the dam and at the toe (recent drilling from the bench encountered water at 75 feet).

Question 5:

Will the coarse bentonite used for backfill be hydrated to surface?

Answer 5:

Tremmie grouting with high solids bentonite is an acceptable method of completing boreholes in embankment dams. Mixes which yield 20 to 30% solids should be used. Stage up tremmie grouting methods should be used in the embankment core with the casing (i.e. hollow-stem augers, rods, etc) pulled incrementally to insure hole wall stability. The bentonite slurry should always be injected through a tremie pipe to ensure the best possible placement and most thorough borehole completion.

Neat cement grout is another acceptable method of completing boreholes in embankment dams. The best results are achieved when the mix consists of 5-7 gallons of water to one sack, 94 lbs of Type I or Type II Portland cement (using higher water contents may result in excessive shrinkage, cracking, and bleed water). The addition of up to 3% powdered bentonite by dry mass of cement is recommended for pumping ease and to reduce shrinkage and cracking after curing. As with the bentonite grout, stage up tremmie grouting methods should be used in the embankment core with the casing pulled incrementally to ensure hole wall stability. The grout should always be injected through a tremie pipe to ensure the best possible placement and most thorough borehole completion. Additives such as calcium chloride or carboxylic acid can be used to control set times, but shrinkage factor must be considered. Using type K cement or adding up to 1% gypsum or aluminum powder will give the cement expansive properties, which may be advantageous in embankment dams where internal seepage is an issue.

Question 6:

Are there restrictions on hour or days of work?

Answer 6:

Daylight hours up to 12 hour shifts, Reclamation is not expecting night work to be required.

Question 7:

Is there site security, locked gates etc.?

Answer 7:

Reclamation will provide access behind gates if required. There is no site security and equipment should be secured at the end of each shift. Reclamation had equipment at this site for over 2 months and had no problems. Reclamation will contact the local sheriff to let them know we will be on site with a contractor.

Question 8:

Can you provide the width, slope and length of the DS Bench?

Answer 8:

12 feet wide, a CME 85 made it without help. Reclamation drillers will be onsite in the spring conducting non contract explorations and will prep the road for contractors. A quick calculation from google earth the ramp is about a 10% grade.

Question 9:

Our understanding of the proposed drilling procedure for this project is as follows:

Toe Holes

i. No predrill

i. Drive closed IBPT from surface to 20 feet or refusal

iii. Trip out IBPT

iv. Redrill to target depth of IBPT hole with open bit

v. Backfill with coarse bentonite while tripping out open bit

Embankment Holes

i. Predrill with open bit to 80 feet

ii. Trip out open bit and backfill with pea gravel

iii. Drive closed IBPT from surface to 100-100 feet (until Truckee Basalt formation encountered)

iv. Trip out IBPT

v. Redrill to target depth of IBPT hole with open bit

vi. Backfill with coarse bentonite while tripping out open bit

Can you confirm that this is the basic procedure the B. of R. plans to employ for the drilling portion of the project?

Answer 9:

Generally yes Question 10:

For the Price Quotation Submission, do you want offerors to quote a list of unit rates and estimated quantities or one fixed price for the scope of work?

Answer 10:

This will be awarded as Firm Fixed Price

Question 11:

The Becker Anaylsis Program, (iBPT) is performed by U.C. Davis. (Their specialized instruments are attached to the drilling contractors drill casing to conduct the IBPT) Are offerors expected to subcontract the services of U.C. Davis, or is the Bureau of Reclamation contracting with U.C. Davis for the IBPT directly?

Answer 11:

BOR will contract directly with the iBPT provider.

Question 12:

Can you provide a description of the embankment material and how it was placed and how it was compacted?

Answer 12:

Drilling from the bench on the downstream face of the dam the following can be expected. Zone 2 material consisting of Clayey Gravel to Clayey Sand with Cobbles (GC/SC)c, Zone 3 material consisting of Clayey Sand with Gravel (SC)sc. The dam was completed in the early 1960’s and is considered well compacted according to practices at the time.