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

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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.

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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')

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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.

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Foundation

 

Bored Piles

CFA Bored Piles

Driven H Piles

Advantages

Bored pile are used to support multi-story building or bridges which can producing heavy vertical loads

They are quick to install and have

no requirement for temporary or permanent casings

Driven piles are driven to a set in variable site conditions to achieve uniform minimum capacity with high reliability

Methodology

Pile drilled / soil removed and replaced with reinforced concrete

Auger drilled into ground and replaced with concrete as the auger is removed

Steel section driven into the ground

Design

Effect on adjacent ground

No displacement of the soil but the potential for relaxation / softening adjacent ground, dependant upon the soil and bore support used

Typically no displacement with good construction controls

Localised densification of loose non-cohesive soils.

Small cross sectional area and hence minimal soil displacement or potential improvement

Typical size ranges

450-2500mm diameter

450 – 1200 mm diameter depths up to 32m

150 – 350 UC’s, UBPs

Capacity

- Shaft friction

Medium

Medium

Medium

- End bearing

Very high with enlarged base

Medium

High

- Structural

Very high structural capacity and

stiffness achievable

Cage insertion can limit tensile

and flexural capacity at depth

Driving stresses often govern the steel section required

Durability

Conventional concrete in the ground design

Permanent liner in highly aggressive conditions

Conventional concrete in the ground design

Sacrificial thickness of steel above

low groundwater level

 Construction

Typical / Plant

Hydraulic or crane mounted piling rig, handling crane, casing, vibro with powerpack and / or drilling support fluid plant

Hydraulic piling rig, concrete pump and possible handling crane

Crane, vibro hammer or hydraulic hammer with powerpack or drop hammer and leaders or guide frame

Piling productivity

16m deep - 600dia @ 2No/day in soft material including a 3m soft rock socket depth. Detailed production rates

16m deep - 600dia @ 11No/day in soft material including a 3m soft rock socket depth

16m deep - 350 UC’s @ 22No/day in soft material

Material to Plant

Concrete, reinforcement cages and method dependant material

Concrete and reinforcement cages

Steel sections

Materials storage

Casing and cage lay down area

Cage lay down area

H pile lay down area

Noise

Machine only unless driven casing

Machine only

Yes, if vibro used hammer used to obtain pile set

Vibration

No, unless driven casing used

No

Yes

Spoil

100% Nett volume

100% Nett volume

None

Other

Plunged columns can be placed into the top of the pile to structural positional tolerances

Fast installation process with real time monitoring systems for construction control and records

Full strength welded splice used at connections

Predrilling can be used to overcome obstructions

Driven Tubes Piles

Precast Concrete Piles

Vibro replacement

Advantages

They are ideally suited for marine and other near shore applications with a very high end bearing capability

Precast driven piles can be environmentally friendly when construct temporary trestles in wetland

Stone piles are a very effective technique, for resolving issues with liquefiable soils, that fall within the typical grain size range

Methodology

Tube driven using external or internal hammer and filled with reinforced concrete

Pre cast section driven into the ground

Soil displaced or removed and replaced with stone

Design

Effect on adjacent ground

Large displacement of plugged tubes resulting in densification of non-cohesive soils and enhanced capacity

Large displacement resulting in densification of non-cohesive soils and enhanced capacity

Large displacement with densification of non-cohesive soils surrounding the stone column which enhances the capacity

Typical size ranges

350 – 750 mm diameter

250 – 600 mm square

600 – 1200 mm diameter

Capacity

- Shaft friction

Medium

Medium

Low

- End bearing

Very high

Very High

Low

- Structural

Tubes can be reinforced concrete filled to enhance capacity

Lifting, driving and jointing can limit capacity

Stone quality & confinement in the soil limit the capacity

Durability

Sacrificial thickness of steel and

internal reinforced concrete

Conventional concrete in the ground design Review potential corrosion at joints

Weathering / degradation of stone typically not an issue

Construction

Typical / Plant

Crane, vibro hammer or hydraulic hammer with powerpack or drop hammer, leaders

or guide frame

Crane, hydraulic hammer with powerpack or drop hammer, leaders or guide frame

Crane, vibro probe with power pack,

water pumps, compressor and front loader

Piling productivity

16m deep - 600mm Dia piles @ 22No/day

16m deep - 300mm square piles @ 20No/day

12m deep @ 6No/day in soft material

Material to Plant

Steel tubes, reinforcement cages and concrete

Precast concrete piles unless manufactured on site

Stone

Materials storage

Tube and cage lay down area

Precast pile lay down / curing area

Stone stockpiles

Noise

Yes if top driven but limited if bottom driven

Yes

Machine only

Vibration

Yes

Yes

Yes

Spoil

None, but ground heave possible

None, but ground heave possible

20 - 100% Nett volume

Other

Predrilling can be used to overcome obstructions Enlarged bases can be formed to enhance capacity

Variable pile founding depth can lead to high wastage levels and jointing expensive

Top feed “Wet” process requires water circulation system and settlement ponds to contain silts

 

 

 

Sheet Pile Wall

Secant Pile Wall

Diaphragm Wall

Advantages

Sheet piles are best suited for the following applications temporary retaining walls, cofferdams and other temporary structures

This is a permanent solution which provides increased wall stiffness compared to sheet piles  

Diaphragm walls tend to be used for retaining very deep excavations as they can be designed to take very high structural loads

Methodology

Clutched sheet piles driven into position.

A series of piles installed so that they overlap to form a wall.

A series of interlocking reinforced concrete panels.

Construction

Establishment

Cranes, vibros and hammers and / or pile jacking plant

50-60T self erecting hydraulic drilling rigs and handling crane.

50T crane + grab, handling crane,

mud conditioning plant, mud storage

Piling productivity

16m deep - 600mm wide sheet piles @ 22No/day (in clay or sand materials) Detailed production rates

16m deep - 600dia @ 4No/day in soft material including a 3m soft rock socket depth. Detailed production rates

16m deep by 800mm wide @ 14-40m3/day of completed wall per rig  per day

Materials to site

Sheet Piles

Concrete, reinforcement cages

Bentonite, reinforcement cages or concrete panels

Work face access

Plant & Materials delivery

Plant & Materials delivery

Plant materials and pipelines for mud circulation

Noise

Yes, unless jacked in

Machine only

Machine only

Vibration

Yes, unless jacked in

No

No

Spoil

No

100% nett volume

100% nett volume

Product

Wall Movement

Flexible, can be increased with clutched

king piles. More props or anchors can be

used to reduce movements

In-situ wall with ground supported

throughout construction. Very stiff.

Ground supported throughout excavation. Stiffest option given wall thickness

Watertightness

Good with joint treatment

Groundwater control over pile length and satisfactory performance with some seepages

Excellent over full depth of the wall with

waterbar across panel joints.

Connections

Welded below capping beam level

Drilled & grouted bars into piles,

shear & bending capacity possible

Full moment & shear connection via box-out and pull-out bars

Durability

Internal painting and sacrificial

thickness of steel

Conventional concrete in the ground design. Internal lining for long-term seepage

Conventional concrete in the ground design. No internal lining necessary

Load Capacity

Low end bearing capacity

Capacity can be enhanced by increasing the length of some piles

Wall has a large bearing area and individual

panels can be extended

 

 

 

 

Soldier Pile Wall

Bored Pile Wall

Soilmix/Slurry Wall

Advantages

Soldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They are also very easy and fast to construct

Low cost and speed of construction for temporary and permanent retaining walls and soil support 

Excellent resistance to contaminated groundwater. They have abilityto adapt to ground movements such as earthquakes

Methodology

Constructed using piles timber infill panels

(timber, steel or concrete)

Series of bored piles installed relatively close together with shotcrete arches

Steel or precast concrete elements placed in fluid soilmix / slurry

Construction

Establishment

50-60T self erecting hydraulic drilling rigs and handling crane

50-60T self erecting hydraulic drilling rigs, handling crane and concrete pumps

50T crane + grab / CSM, handling crane / grout plant with screw feed silos, high pressure pumps

Piling productivity

16m deep - 300mm square piles @ 18No/day

16m deep - 600dia @ 4No/day in soft material including a 3m soft rock socket depth. Detailed production rates

16m deep by 800mm wide @ 20-50m3/day of completed wall per rig  per day

Materials to site

Concrete, reinforcement cages, steel or precast concrete panels

Concrete, reinforcement cages

Cement, bentonite, steel or precast concrete panels

Work face access

Plant & Materials delivery

Plant & Materials delivery

Plant, materials and pipeline delivery of slurry

Noise

Yes, if driven sections

Machine only

Machine only

Vibration

Yes, if driven sections

No

No

Spoil

Dependant on installation method

100% nett volume

30%-80% Nett volume

Product

Wall Movement

Ground unsupported allowing relaxation prior placement of panels and backfilling Stiffness depends on structural section

and backfill compaction

Ground unsupported allowing relaxation prior to concrete

Finished product stiff

Ground supported with stiffness dependant on steel section.

Precast panels can increase stiffness.

Watertightness

Permeable with no groundwater control

below excavation. Seepages long term

Permeable until shotcrete in place with no

groundwater control below. Seepages long term

Good temporary performance due to

replacement with CB slurry but some seepages

Connections

Numerous connection options dependant on materials used

Drilled and grouted bars into piles, shear

and bending capacity possible

Welded to steel sections, shear & bending capacity possible.

Durability

Conventional concrete in the ground

design or sacrificial steel thickness given

long term seepage potential

Conventional concrete in ground design

Sacrificial thickness of steel and internal

lining wall for long-term groundwater seepage

Load Capacity

Capacity can be enhanced by increasing the length of piles.

Capacity can be enhanced by increasing the length of some piles.

Capacity limited by penetration of steel beams

 

 

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