Bored Piling

D&C Portal

The art of online construction estimating

1.1 Introduction

Cost is important to all industry. Costs can be divided into two general classes; absolute costs and relative costs. Absolute cost measures the loss in value of assets. Relative cost involves a comparison between the chosen course of action and the course of action that was rejected. This cost of the alternative action - the action not taken - is often called the "opportunity cost".

The accountant is primarily concerned with the absolute cost. However, the forest engineer, the planner, the manager needs to be concerned with the alternative cost - the cost of the lost opportunity. Management has to be able to make comparisons between the policy that should be chosen and the policy that should be rejected. Such comparisons require the ability to predict costs, rather than merely record costs.

Cost data are, of course, essential to the technique of cost prediction. However, the form in which much cost data are recorded limits accurate cost prediction to the field of comparable situations only. This limitation of accurate cost prediction may not be serious in industries where the production environment changes little from month to month or year to year. In harvesting, however, identical production situations are the exception rather than the rule. Unless the cost data are broken down and recorded as unit costs, and correlated with the factors that control their values, they are of little use in deciding between alternative procedures. Here, the approach to the problem of useful cost data is that of identification, isolation, and control of the factors affecting cost.

1.2 Basic Classification of Costs

Costs are divided into two types: variable costs, and fixed costs. Variable costs vary per unit of production. For example, they may be the cost per cubic meter of wood yarded, per cubic meter of dirt excavated, etc. Fixed costs, on the other hand, are incurred only once and as additional units of production are produced, the unit costs fall. Examples of fixed costs would be equipment move-in costs and road access costs.

1.3 Total Cost and Unit-Cost Formulas

As harvesting operations become more complicated and involve both fixed and variable costs, there usually is more than one way to accomplish a given task. It may be possible to change the quantity of one or both types of cost, and thus to arrive at a minimum total cost. Mathematically, the relationship existing between volume of production and costs can be expressed by the following equations:

Total cost = fixed cost + variable cost × output

In symbols using the first letters of the cost elements and N for the output or number of units of production, these simple formulas are

C = F + NV

UC = F/N + V

1.4 Breakeven Analysis

A breakeven analysis determines the point at which one method becomes superior to another method of accomplishing some task or objective. Breakeven analysis is a common and important part of cost control.

One illustration of a breakeven analysis would be to compare two methods of road construction for a road that involves a limited amount of cut-and-fill earthwork. It would be possible to do the earthwork by hand or by bulldozer. If the manual method were adopted, the fixed costs would be low or non-existent. Payment would be done on a daily basis and would call for direct supervision by a foreman. The cost would be calculated by estimating the time required and multiplying this time by the average wages of the men employed. The men could also be paid on a piece-work basis. Alternatively, this work could be done by a bulldozer which would have to be moved in from another site. Let us assume that the cost of the hand labor would be $0.60 per cubic meter and the bulldozer would cost $0.40 per cubic meter and would require $100 to move in from another site. The move-in cost for the bulldozer is a fixed cost, and is independent of the quantity of the earthwork handled. If the bulldozer is used, no economy will result unless the amount of earthwork is sufficient to carry the fixed cost plus the direct cost of the bulldozer operation.

Figure 1.1 Breakeven Example for Excavation.

t0579e0w

If, on a set of coordinates, cost in dollars is plotted on the vertical axis and units of production on the horizontal axis, we can indicate fixed cost for any process by a horizontal line parallel to the x-axis. If variable cost per unit output is constant, then the total cost for any number of units of production will be the sum of the fixed cost and the variable cost multiplied by the number of units of production, or F + NV. If the cost data for two processes or methods, one of which has a higher variable cost, but lower fixed cost than the other are plotted on the same graph, the total cost lines will intersect at some point. At this point the levels of production and total cost are the same. This point is known as the "breakeven" point, since at this level one method is as economical as the other. Referring to Figure 1.1 the breakeven point at which quantity the bulldozer alternative and the manual labor alternative become equal is at 500 cubic meters. We could have found this same result algebraically by writing F + NV = F' + NV' where F and V are the fixed and variable costs for the manual method, and F' and V' are the corresponding values for the bulldozer method. Since all values are known except N, we can solve for N using the formula N = (F' - F) / (V - V')

1.5 Minimum Cost Analyses

A similar, but different problem is the determination of the point of minimum total cost. Instead of balancing two methods with different fixed and variable costs, the aim is to bring the sum of two costs to a minimum. We will assume a clearing crew of 20 men is clearing road right-of-way and the following facts are available:

1. Men are paid at the rate of $0.40 per hour.
2. Time is measured from the time of leaving camp to the time of return.
3. Total walking time per man is increasing at the rate of 15 minutes per day.
4. The cost to move the camp is $50.

If the camp is moved each day, no time is lost walking, but the camp cost is $50 per day. If the camp is not moved, on the second day 15 crew-minutes are lost or $2.00. On the third day, the total walking time has increased 30 minutes, the fourth day, 45 minutes, and so on. How often should the camp be moved assuming all other things are equal? We could derive an algebraic expression using the sum of an arithmetic series if we wanted to solve this problem a number of times, but for demonstration purposes we can simply calculate the average total camp cost. The average total camp cost is the sum of the average daily cost of walking time plus the average daily cost of moving camp. If we moved camp each day, then average daily cost of walking time would be zero and the cost of moving camp would be $50.00. If we moved the camp every other day, the cost of walking time is $2.00 lost the second day, or an average of $1.00 per day. The average daily cost of moving camp is $50 divided by 2 or $25.00. The average total camp cost is then $26.00. If we continued this process for various numbers of days the camp remains in location, we would obtain the results in Table 1.1.

TABLE 1.1 Average daily total camp cost as the sum of the cost of walking time plus the cost of moving camp.

Days camp remained in location	Average daily cost of walking time	Average daily cost of moving camp	Average total camp cost
1	0.00	50.00	50.00
2	1.00	25.00	26.00
3	2.00	16.67	18.67
4	3.00	12.50	15.50
5	4.00	10.00	14.00
6	5.00	8.33	13.33
7	6.00	7.14	13.14
8	7.00	6.25	13.25
9	8.00	5.56	13.56
10	9.00	5.00	14.00

We see the average daily cost of walking time increasing linearly and the average cost of moving camp decreasing as the number of days the camp remains in one location increases. The minimum cost is obtained for leaving the camp in location 7 days (Figure 1.2). This minimum cost point should only be used as a guideline as all other things are rarely equal. An important output of the analysis is the sensitivity of the total cost to deviations from the minimum cost point. In this example, the total cost changes slowly between 5 and 10 days. Often, other considerations which may be difficult to quantify will affect the decision. In Section 2, we discuss balancing road costs against skidding costs. Sometimes roads are spaced more closely together than that indicated by the point of minimum total cost if excess road construction capacity is available. In this case the goal may be to reduce the risk of disrupting skidding production because of poor weather or equipment availability. Alternatively, we may choose to space roads farther apart to reduce environmental impacts. Due to the usually flat nature of the total cost curve, the increase in total cost is often small over a wide range of road spacings.

Figure 1.2 Costs for Camp Location Example.

t0579e0x

Bored Piling

: Category: Bored Piling; Hits: 25081

Pre-Drilling - Site Investigation

Pre-boring will be carried out at each location to ascertain the target founding level.
Rotary rig drilling operations incorporate the use of a steel core that is used when a core sample is required. Another type of boreing is called “Wash Bore” or “Percussive Bore” which simply means that a bore is flushed out and is used when no soil sample is required. Typical rates are...

Type of Strata	Plant Used	Rate (m/hr)
Top soft deposits of site fill	Percussive / Rotary	10.00
Grade V/IV rock deposits	Wash Boring	3.15
	Triple Tube Sampling	0.75
Grade III/II rock deposits	Rotary	0.50
	Triple Tube Sampling	0.50

With a standard rig, working a 12 hour shift, a typical output rate of 66 hrs/hole/rig is possible.

As-Built Information. On Central Reclamation, Contract UA11/91, borehole drilling completed with the following results...

Average depth of 61.15 m, total depth of 428 m
Average rock socket depth of 4.65 m
The durations ranged from 4 to 8 days
Average of 5.85 days, total duration of 41 days.

The Target Founding level

This is defined as the required socket into the bedrock, which is defined as moderately decomposed rock grade III or better with a core recovery greater than 85% (allowable bearing capacity of 5 mPa). The continuity of the founding rock is demonstrated by continuing the pre-bored hole a maximum of 5 metres or 3 times the pile diameter, whichever is the greater.
The cores are logged, stored and photographed and submitted together with proposed founding levels for approval.

Setting Out

Before starting excavation at the pile position the following steps are taken:

Survey and record the existing ground level at the pile position
Setout the pile location from the reference points and in order to monitor the position of the steel casing, control pins are usually established at two orthogonal positions, offset from the centre of the pile.

Pile Tolerances

In the case of out of position casings, adjustment can be made to keep the vertical alignment and plan position within the limits of no more than 75 mm off-centre on plan position and not deviating by more than 1:75 from the vertical axis.

Pile Excavation / Casings

The shaft of the pile is excavated within a temporary steel casing with an outside diameter of say approximately 200 - 300 mm greater than the pile diameter. The casing is used mainly in areas of unstable ground and are driven using hydraulic casing oscillator attached to a crawler crane or a casing vibrator.
Shaft excavation is carried out using a single or double hammer grab supported by crawler crane. The steel casing toe is kept in front of the excavation level until it is 0.5 metres above the pile cut-of level. The pile shaft is often flooded with bentonite or water and excavation proceeds to the top of the CDG.
Excavation then proceeds by reverse circulation drilling (RCD) using large diameter drilling heads with special rock cutters and flushing by air lift. Bentonite or water levels are always maintained above the ground water level to ensure stability of the shaft.

Calculation Of Bored Pile Construction / Excavation Time

Piling times can be reduced by the use of service cranes for reinforcement and concreting activities.
An additional extended shift would often be required for certain piles, as would RCD down-time.

Forecast construction times can be derived by using output rates (hours per item)...

Operation	Element	Details	Hours
Add or Remove	Reverse Circluation Drill Plant (RCD)	incl drill bit	2 hrs
Add or Remove	RCD drill bit	(incl assembly of drill string)	5 hrs
	RCD bell-out bit	(incl drill string & stabilisers)	5 hrs
Installation	Airlift tremmie tube		5 hrs
	Reinforcement cages	(time for joining each cage)	2 hrs
Cleaning Time	Initial airlift cleaning	(after finishing excavation)	8 hrs
	Final airlift cleaning	(after fixing steel cage)	2 hrs
Concreting	Incl extract casing	( < 70 m deep)	12 hrs
		( > 70 and < 95 m deep)	14 hrs
		( > 95 and < 135 m deep)	48 hrs
Curing Time	Only required prior to removal of telescopic casings		72 hrs
Cycle Time	Move piling setup to next location		2 hrs

Shaft Excavation	Strata	Plant Used	Rate (m/hr)
	General Fill (upper ground levels)	Grab	3.50 m/hr
	Sand, Minor Rubble	Grab	2.10 m/hr
	Marine / Alluvium Deposits	Grab	2.50 m/hr
	CDG < 150	RCD/Grab	1.50 m/hr
	CDG > 150 < 200	RCD	1.00 m/hr
	CDG > 200, Compacted Gravel	RCD	0.50 m/hr
	CDT	RCD / Grab	0.50 m/hr
	Corestones	RCD / Chisel	0.50 m/hr
	Rock Socket - Grade IV/V	RCD	0.25 m/hr
	Rock Socket - Grade II/III	RCD	0.125 m/hr
	Rock Socket - (Tendering Rate)	RCD	0.10 m/hr

Forecast excavation or cycle times can then be derived by analysing ground conditions. Site investigation will provide the depths / types of strata which can then be matched to production output rates (see above).
Note - A pile’s diameter has negligible effect on production time and as such is ignored.

Example - For a pile founding on rock at 60 m deep...

(a) Calculate Allowance For Plant Time / Other Elements (hrs)....

Element	Hours
Set up RCD	5.0
Excavate Time	See Below
Remove RCD (including drill bit, string and stabilisers)	5.0
Setup / Remove Airlift Tremmie Tube	5.0
Initial Post-Excavation Airlifting	5.0
Place reinforcement (5 No cages @ 12m = 5 x 2 hrs)	10.0
Setup / Remove Airlift Tremmie Tube	5.0
Final Post-Reinforcement Airlifting	2.0
Concrete and remove casing	12.0
Move to next location	2.0
Calculate Total Construction / Plant Time	52.0 hrs

(b) Calculate Allowance For Excavation Time (hrs)...

Depth of Strata (m)	Type of Rock	Production Rate (m/hr)	Plant Used	Time (Hrs)
0 - 20	Sand/minor rubble	2.00	Grab	10.0
20 - 35	CDG less than 150	1.50	RCD/Grab	10.0
35 - 47	CDG more than 150	1.00	RCD	12.0
47 - 57	CDG > 200/corestones	0.50	RCD	20.0
57 - 60	Rock socket	0.20	RCD	15.0

(c) Calculate Total Pile Excavation Time = 67.0 hrs

(d) Overall Pile Time

Description	Calculation	Hours
Construction / Plant Time	(“b” above)	52.0 hrs
Excavation Time	(“d” above)	67.0 hrs
OVERALL CYCLE TIME	(“b” + “d”)	119 hrs
(With 12 hr shifts)	(“b” + “d”)	9.9 days

Bored Pile Construction As-Builts

General Rule of Thumb Pile Times (days)...

Description	Rule of Thumb Pile Times (days)
Depth (m) =>	<20	<40	<70	<90	<135
Days Per Pile	4.0*	8.0	10.0	25.0	45.0

Note - due to the required plant assembly and operation times, 4 days is the minimum possible pile construction time for any situation.

Methods For Overcoming Obstructions

If the obstruction is shallow (ie 0 to 2.5 m below ground level) a backhoe-breaker will be used to form a suitable hole.
Where the obstructions are located at greater depths an oversized temporary casing is driven by the oscillator to the top of the obstruction.

If the obstruction is above water level a hand operated air hammer is used, a typical rate of = 0.8 m/hr
If below water level a down the hole hammer or heavy chisel supported by crawler crane will be used, a typical rate of = 0.5 m/hr
If a concrete “plug” is required to provide a well formed shaft wall when an obstruction or excessive overbreak or fault is encountered...

Element	Hours
Remove RCD	5 hrs
Install Tremie Concrete Tube	5 hrs
Place Concrete Plug	2 hrs
Cure Concrete	36 hrs
Replace RCD and drill string	5 hrs
TOTAL TIME LOSS	53 hrs (2.2 days or 4.4 shifts)

Cleaning of Pile Base

The hole of the pile shaft is cleaned using an airlift until the water becomes clean or negligible particles in suspension is discharged.

Reinforcement Cages

Cages are constructed in suitable sections, usually in the order of 12 m long, complete with sonic tubes and coring tubes.
Fabrication, 12 m long cage with 6 no fixers...

Description	Duration
Fabricate 1 cage	2.5 hrs
Total cages required	5 no
Overall Fabrication Time	12.5 hours

Steel Stanchion Fabrication and Installation

Stanchions are usually fabricated off-site and delivered in sections. Prior to installation the sections are welded together to form the complete stanchion. Stanchion sizes are usually in the region of 525 mm x 525 mm.
With an average length of say 28 m, the welding time would be around 5 days and are tested by an ultrasonic weld test and an MPI test.
After the installation of the reinforcing cage in to the shaft the stanchion will be lifted until it hangs vertical. It is then lowered into the excavation and clamped into position.

Concreting

Pile concreting is carried out under water by "tremie" techniques maintaining the water or bentonite head inside the casing at or above existing ground water level. The tremie tube (250mm) is withdrawn as concreting proceeds ensuring a minimum concrete head of 2 metres above the top of the tremie tube.

Sequence Of Piling

The sequence of construction of piles is chosen in such a manner that no damage can be caused to nearby piles still under construction or recently concreted (i.e. less than 3 days).
On a 12 metre grid, a normal layout would mean say having two non-worked on piles between each open excavation in the longitudinal direction (ie a 36 metre spacing) thus allowing room for the crane etc and a lesser spacing of every other pile being worked on (ie a 24 metre spacing).

Pile Testing

The workability of concrete is tested on site by measuring the slump and temperature of concreting at the time of discharge into the pile shaft. Laboratory tests are carried out in order to check the strength of the placed concrete. A number of test cubes are made and tested at 7 and 28 days.

Coring Test - Certain piles selected by the Engineer will be cored their full depth. The depth of cores into the base material (rock) will usually be at least 600 mm. Cores are placed in correct order and relative position in core boxes which clearly mark the depths of cores. The cores are usually photographed and submitted to the Engineer. The testing of the coring will provide additional information about the quality of the concrete as well as the condition of the interface between concrete and rock.
Sonic Logging Test - In order to test the quality of the concrete as well as the integrity of the pile in its overall length and pile toe condition sonic core testing is used. Sonic tubes are installed with the reinforcement cage in order to allow the lowering of the signal transmitter and signal receiver probe down the bottom of the pile. These tubes are sealed at the bottom.
Vibration Tests - This test determines the pile length and shape and the overall pile concrete quality. This is a specialist test.