Question

A. An earthwork contract requires to cut and move excavated rocks. The site is a typical limestone rock formation in a cut section, which is 15,000 ft long, 85 ft wide, and 3 ft deep. Seismographic tests indicate a seismic wave velocity of 5,000 fps for the rock layer material. The contractor proposes to rip the rock with a 370-hp crawler tractor. Answer the following questions.

(a) Calculate the production in bcy per hour, for full-time ripping, with efficiency based on a 45         -min hour. Assume that the ripper is equipped with a single shank and that ripping conditions are average (i.e., intermediate between extreme conditions). This is classified as heavy ripping.

(b) Estimate the ripping unit-production cost in dollars per bcy using your calculated hourly production. The normal O&O cost per hour, including operator, for a tractor w/ ripper $130.00. The operator’s wage with fringe is $33.00 per hour.

(c) What is the total direct cost of the cut quantity?

(d) What is the total duration of the rock cut project assuming 10 hours per day work?

B. As a Construction Manager for this site development project, prepare detailed cost estimate for the project, and bid documents.

      Assume 10% time value cost of money, bonds, insurance, and storage over total direct cost, contingency at 2% of total direct cost, indirect overhead cost at 5% of total direct cost (additional on top of the equipment and operator costs), and 10% fee (on subtotal total of direct and indirect cost).

Answer 1

Detailed solution is as follows,

a)Production in bcy/hr

For the given seismic wave velocity,ie., 5,000 fps and using 370 hp tractor ,referring the tractor production chart,

Productivity =560 bcy/hr

Applying efficieny factor i.e., 45 mins efficiency/hr

Productivity =75% of 560

                     =420 bcy/hr

b & c)Ripping cost/bcy and Total direct cost

Given Length =15000 ft

Width = 85 ft

depth =3 ft

Volume of rock = 15000 x 85 x 3

                       =3825000 cft

No of hours required to complete the work = total volume/productivity per day

                                                            =3825000/560      

                                                            =6830 hrs

Total Direct Cost

Cost per hour for tractor and Ripper as per the given detail = 130 + 33 = $163/hr

Total direct cost =Total hrs of work x rate per hr

                             =6830 * 163

Total Direct Cost =$ 1113290

Cost per unit

Cost per bcy =Total direct cost/Total qty

                        =1113290/3825000

                        =$ 0.29/bcy

d)Total duration of cut

Per day productivity =productivity per hour x no of hours worked in a day

                             =560 x 10

                             =5600 cft

No of days required to complete the work = total volume/productivity per day

                                                            =3825000/5600       

                                                            =683 days

B)Estimatiion

The normal cost for the tractor per hour = $ 163 per hour(including operator charges)

Total direct cost = 163 x 6830 =1113290

Other costs are as follows

s.no	Description	Percentage of direct cost	Amount
1	Direct Cost		1113290
2	Bond,insurance and storage	10%	111329
3	Contingency	2%	22265.8
4	Indirect cost	5%	55664.5
	Total direct and indirect cost		1302549

Cost for fee =10% of Direct and Indirect cost

                 =10% of 1302549

                =130254

Total Estimation =(Total Direct +Indirect cost) + Cost for fee

                           =1302549 + 130254

                           =$1432803