IV. PRESENTACIÓN Y DISCUSIÓN DE RESULTADOS
4.2. Discusión de resultados
To complicate matters further, the estimator will also be required to consider that the material being excavated may vary in consistency from liquid to solid, that is from silt or sand below water level to hard rock such as whin or granite.
Obviously some materials are easier than others to remove, and the greatest differences are to be found in hand excavation. Table 5.2 gives a range of multipliers which may be applied to an output for normal soil to obtain outputs for various types ofhand excavation in a range ofsoil types.
Soil conditions should be defined in the preambles section of the bill for excavation and filling, but the site visit should be used as check on all ofthese points. Indeed, ifno trial pit or bore information is given the estimator might consider it prudent to obtain permission to have this done.
Outputs
Table 5.3 gives a range ofoutputs for various classes ofhand excavation in ordinary soil. The variations in the figures are a consequence ofa number offactors, the more important having been identified in Table 5.2.
Table 5.4 shows sundry outputs for various hand excavation tasks.
Examples 5.5 to 5.10 are based on an imaginary building. A section ofthe earthworks is shown in Figure 5.3. The difficulty multipliers are obtained from Table 5.2 and the man hours per m3 from Tables 5.3 and 5.4.
In these examples the volume ofmaterial to be excavated is calculated for an area ofunit length and unit width, i.e. 1 m 1 m. The depth is, in the case ofthe reduce level digging, the
average depth. For example, in Figure 5.3 the shallowest portion ofreduce level digging starts at zero and rises to 0.25 m deep, giving an average depth of0.125 m. The volume excavated is therefore 1 m 1 m 0.125 m 0.125 m3. Similarly, the deepest portion of reduce level digging starts at 1 m deep and rises to 1.2 m deep, giving a total volume of 1 m 1 m 1:1 m 1:1 m3 (Example 5.7).
Table 5.2 Multipliers for difficultydue to nature of ground and type of hand excavation
Soil type Multiplier
( hours/m3) Excavation type Comment
Vegetable soil 1.11±1.33 Because ofsmall volume for a
given area Sand 1.25±1.52 Reduced level, basement,
trench, pit Depending on degree of compaction and moisture content, a shovel will not hold as much sand as soil
Ordinary 1.00 Reduced level, basement,
trench Any soil capable ofbeing
removed with a shovel and minimal use ofthe pick 1.11±1.25 Narrow trench, pit Higher proportion oftrimming 1.11±1.33
Heavy clay 1.25±2.00 Reduced level, basement,
trench, pit Graft or pneumatic spade;
trimming is almost assured with accurate use oftools Soft rock 1.43±3.00 Reduced level, basement,
trench, pit Heavy use ofpick or wedges;
trimming is almost assured with accurate use oftools Hard rock 3.00±5.00 Reduced level, basement Wedges or compressor and
pneumatic drill 5.00±10.00 Trench, pit Trimming difficulty and
confined space
Table 5.3 Range of outputs for various classes of hand excavation in ordinarysoil (hours/m3)
Oversite <250 m 1.25±1.50
Basement and the like 2.00±2.25
Trenches (depending on width) 2.00±2.75 Pits (depending on size) 3.00±4.50
Reduce level 1.75±2.00
Benchings 1.75±2.00
Example 5.5
Excavate to reduce level, starting at ground level and not exceeding
0.25 m maximum depth. m3
SMM7 has already been criticized for requiring depth stages and starting levels in items for reduce level excavation. However, the estimator knows that this work is all above the starting level and that the subsoil is a compacted sand/gravel. In this instance, the get-out part ofthe excavation will require barrow runs, mechanical barrows or dump trucks. Provision ofbarrow runs will be included in the Table 5.4 Sundryoutputs associated with hand excavation
Volume outputs (hours m3)
Wheel 50 m and deposit 0.60±0.80
Additional 50 m 0.60±0.80
Spread and level 0.40±0.50
Re-excavate from spoil heaps 1.25±2.00
Return fill and ram 1.25±1.75
Lift one stage 1.25±1.50
Clear from side of excavation 1.25±1.50
Clear from side of excavation and segregate from backfilling 1.40±1.80
Barrowing and filling hardcore in bulk 0.80±1.00
Barrowing and filling hardcore in beds <250 mm 1.15±1.30 Mechanical ramming bulk fill in 300 mm layers 0.40±0.60
Fix and withdraw timbering 7.00±10.00
Area outputs (hours/m2)
Grading/levelling trench bottoms 0.15±0.20
Mechanical ramming hardcore in beds 0.12±0.13
Spread and consolidate blinding 0.05±0.10
Examples 5.5 & 5.11
Examples 5.6 & 5.12
Examples 5.7 & 5.13
Examples 5.10 & 5.16 Example 5.20 Examples 5.10 & 5.16 Examples 5.8 & 5.14 Examples 5.8 & 5.14
Examples 5.9 & 5.15 1.90 m 0.25 m
1.00 m 1.20 m
Figure 5.3 Cross-section through a typical excavation
general items in preliminaries, and the additional labour is reflected in the choice of difficulty multiplier.
Assume that we will excavate unit length and unit width at any time. Average depth (0 0:25)/2.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 0.125 1.11 1.5 £6.50 £1.35
Total volume excavated 0.125 Total cost £1.35
Dividing total cost by total volume cost/m3 £10.82
Profit and oncost 20% £2.16
Rate per m3 £12.99
Example 5.6
Excavate to reduce level, starting at ground level and not exceeding 1 m
maximum depth. m3
Assume that we will excavate unit length and unit width at any time. Average depth (0:25 1:00)/2.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 0.625 1.2 2 £6.50 £9.75
Total volume excavated 0.625 Total cost £9.75
Dividing total cost by total volume cost/m3 £15.60
Profit and oncost 20% £3.12
Rate per m3 £18.72
Example 5.7
Excavate to reduce level, starting at ground level and not exceeding 2 m. m3
Assume that we will excavate unit length and unit width at any time. Average depth (1:00 1:20)/2.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 1.1 1.25 2 £6.50 £17.88
Total volume excavated 1.1 Total cost £17.88
Dividing total cost by total volume cost/m3 £16.25
Profit and oncost 20% £3.25
Rate per m3 £19.50
Note that despite the maximum depth being greater than one stage throw, there is in effect no stage throw in reduce level excavation. The result is that the rate for getting out remains the same as the previous item.
The next item, basement excavation, is `down into' the ground, and throw may have to be taken into account ifthis proceeds beyond 1.5 m.
Example 5.8
Excavate basement, starting at formation level and not exceeding 1 m
maximum depth. m3
Assume that we will excavate unit length and unit width at any time.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 0.9 1.25 2.2 £6.50 £16.09
Total volume excavated 0.9 Total cost £16.09
Dividing total cost by total volume cost/m3 £17.88
Profit and oncost 20% £3.58
Rate per m3 £21.45
This rate assumes that the getting out will increase the hours/m3, as it will be done on the low side ofthe excavations and will require the spoil to be moved over to one side ofthe excavation.
Example 5.9
Excavate basement, starting at formation level and not exceeding 2 m
maximum depth. m3
Assume that we will excavate unit length and unit width at any time.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 1.5 1.3 2.2 £6.50 £27.89
Max. throw from 2.0 m deep 0.4 1.35 2.2 £6.50 £7.72
Throw one stage 0.4 m3 0.4 1 1.4 £6.50 £3.64
Total volume excavated 1.9 Total cost £39.25
Dividing total cost by total volume cost/m3 £20.66
Profit and oncost 20% £4.13
Rate per m3 £24.79
Reference to Figure 5.3 will show that although the depths are all within the 1.5 m throw stage, the fact that a deeper separate excavation exists within the shallower one makes a throw a necessity.
The volume thrown is not included in total volume.
Example 5.10
Excavate trench starting at bottom of basement excavation and not
exceeding 1 m maximum depth m3
Without a definitive starting level, this excavation could start at one of two maximum depths below formation level 1 m or 2 m. However, reference to Figure 5.3 will show that the trenches start at the bottom ofthe more shallow basement excavation. A throw still has to be added, as the initial spoil will be excavated onto the bottom ofthe basement excavation.
Assume that we will excavate unit length and unit width at any time.
Volume
excavated Difficulty
multiplier Man
hours Rate per
hour Cost
Max. throw from 1.5 m deep 1 1.45 2.2 £6.50 £20.74
Throw one stage 1 m2 ± 1 1.4 £6.50 £9.10
Total volume excavated 1 Total cost £29.84
Dividing total cost by total volume cost/m3 £29.84
Profit and oncost 20% £5.97
Rate per m3 £35.80