The measurement for working space in SMM7 is now given in square metres, provided that the face of the work requiring space is less than 600 mm from the face of the excavation. There is no minimum depth below which working space is not measurable; ifit is required it must be measured. At first sight this change in SMM7 might appear to be quite radical, but to the estimator it only rationalises what he has always had to do: assess for himself exactly how much working space would be required, whether or not any was given in the bill!
All previous methods ofmeasurement have treated working space as a theoretical allowance, where the distance from the surface worked upon to the face of the excavation was set in accordance with the depth ofthe particular class ofexcavation. Invariably all other factors, such as the nature ofthe soil, were ignored. Some methods ofmeasurement did try to relate some of the difficulty factors to the measurement, but the allowance was still theoretical.
The problem then that the estimator faces is twofold. It is necessary to decide first how much space the workmen will need, and secondly how many external and internal corners there are on the plan! The first problem requires answers to the following:
* To what depth will the work be carried out? * What kind ofwork is involved?
* Is the working space partly or wholly below water level and, ifpartly, by how much? * Will the backfilling be ofany special materials? Ifso, then disposal ofthe excavated
material will be required.
The second problem only occurs ifthe building is not rectangular in plan. Ifit is rectangular then there are always four corners to add to the working space excavation, no matter how many indents or cutouts there are. Should the building plan be a polygon with more than four sides then careful note of the internal and external corners should be made, so that the extra excavation, disposal and support will be properly accounted for.
Having sorted these problems out, the estimator has to decide what width ofexcavation to allow, cube it up, price it, extend a total and divide by the bill quantity in square metres to arrive at a rate. But what about that width? This is another case for the estimator's skill and working knowledge, backed up by meticulously kept records ofearlier work. Every job must be assessed on its merits.
Example 5.20
Extra over basement excavation for working space excavation starting at
bottom of shallow basement excavation and not exceeding 1 m deep. m2
Refer to Figure 5.4 for details of what causes the need for working allowance, how much is measured and how much we must excavate and backfill. The working space excavation will start below the more shallow basement excavation and go down to the top ofthe concrete slab. It will therefore be within 1.5 m maximum throw depth.
50 mm 200 mm say, 600 mm Working space required Face on which allowance is measured Mastic asphalt tanking
Volume
excavated Difficultymultiplier Manhours Rate perhour Cost
Max. throw from 1.5 m deep 1 1.25 2.2 £6.50 £17.88
Backfill 1 1 1.6 £6.50 £10.40
Dispose ofsurplus* 0.2 1 0.7 £6.50 £0.91
Total volume 1 Total cost £29.19
Girth offace ofprotective brickwork 22.00 m Actual depth ofworking space
excavation 0.80 m
Area for working space given in bill 37.40 m2
Width for working space 0.60 m
Volume ofdigging
22:00 0:80 0:55 9.68 m3
Plus corners 4 0:55 0:55 0:8 0.97 m3
Total excavation 10.65 m3
Excavation cost: total cost total excavation £310.76
Additional strutting: 10 reuses, 30 m of100 mm 100 mm £6.60
Timber used 0.3 m3at £320.00 £9.60
Labour hours setting strutting per m3 10 hours at £7.15 £238.33
£565.30
Profit and oncost 20% £113.06
£678.35
Rate per m2ofworking space £18.14
Assume that we will excavate unit length and unit width at any time. Note
In Example 5.20 * signifies that bulking must be disposed of.
Earthwork support
Earthwork support is measurable to all faces of excavation except the following:
* Faces not exceeding 250 mm high.
* Sloping faces where the angle of inclination is less than 45 to the horizontal. * Faces which abut existing structures.
In SMM7 measurement is classed in depth stages and in distances between opposing faces. It appears that the intention is to give gross areas ofsupport where the total depth of excavation falls within any depth stage.
Up to 4 m between opposing faces it is possible to span across with struts of various kinds, e.g. timbers wedged in or proprietary telescopic struts ofsteel. Over 4 m it is usual to set up raking supports to the lower level ofthe excavation, although ifthis is below water level it may be better to use steel work for the shoring, and this will add considerably to the cost.
Other special conditions must be given in the bill, i.e. where the face is curved, below water level (there does not appear to be any provision for support partly below water level), next to roadways, next to existing buildings or in unstable ground, and finally where the support is left in place. The estimator is left to decide how to actually achieve the support, and he will take into account the following additional considerations:
* The most suitable material must be chosen, for example: solid timbers for walings, poling
boards or strutting; steel sheet, plywood, etc. for support; proprietary struts; spacing of all members considering the soil type; proprietary close sheeting system.
* The number ofuses must be estimated, i.e. the number oftimes each component can be
reused before being discarded. Figures are quoted from 5 to 15 uses.
* Men working in the excavations require protective clothing. * Outputs for excavation round timbering are generally halved.
* The proportion ofvarious classes ofexcavation below the water table must be considered. * Pumping, although given separately in the bill, must be realistically assessed.
Example 5.21
Earthwork support not exceeding 1 m maximum depth and distance
between opposing faces not exceeding 2 m. m2
The price for this will vary according to the nature of the soil to be supported. For fairly firm, stable soil allow poling boards at 400 to 600 centres, two walings in the height and double strutting at about 2 m centres.
Take a 12 m length oftrench and cost it out. Polings: 2 sides, 12 m long, set at 450 mm centres:
nr 53 2 55, each 1.00 0.20 0.038 m 0.418
Walings: 2 sides, 2 per side:
nr 4, each 12 0.2 0.038 m 0.3648
Strutting: at 2 m centres gives 6 1 7 pairs:
nr 14, each 2 0.1 m 0.28
Volume oftimber 1.0628 £320.00 £340.10
Waste 10% £34.01
Total Cost £374.11
Cost per use if10 uses £37.41
Labour to unload, set up, strike on completion:
1.06 m3at 10 hours/m3at £7.15 £75.99
Cost for trench £113.40
Trench 12 m long and 1 m deep requires 24 m2ofearthworks support:
Cost for 1 m2 £4.73
Profit and oncost 20% £0.95
Example 5.22
For soil such as we proposed for our imaginary building in Figure 5.3 it might be necessary to put the poling boards closer and use stronger walings than in Example 5.21. This example illustrates the differences.
Take a 12 m length oftrench and cost it out. Polings: 2 sides, 12 m long, set at 400 mm centres:
nr 60 2 62, each 1 0.2 0.038 m 0.4712
Walings: 2 sides, 2 per side:
nr 4, each 12 0.2 0.05 m 0.48
Strutting: at 2 m centres gives 6 17 pairs:
nr 14, each 2 0.1 0.125 m 0.35
Volume oftimber 1.3012 m3
Cost at £320.00 per m3 £416.38
Waste 10% £41.64
Total Cost £458.02
Cost per use if10 uses £45.80
Labour to unload, set up, strike: 1.30 m3at 10 hours/m3at £5.59 at £7.15 £93.04
Cost for trench £138.84
Trench 12 m long and 1 m deep requires 24 m2ofearthworks support:
Cost for 1 m2 £5.78
Profit and oncost 20% £1.16
Rate per m2 £6.94
Example 5.23
Earthwork support not exceeding 2 m maximum depth, and distance
between opposing faces not exceeding 2 m. m2
Assume the same loose soil as in Example 5.22. Take a 12 m length oftrench and cost it out. Polings: 2 sides, 12 m long, set at 400 mm centres:
nr 60 2 62, each 2 0:2 0:05 m 1.24
Walings: 2 sides, 3 per side: nr 6, each 12 0:2 0:05 m 0.72 Strutting: at 1.5 m centres gives 8 1 9 sets of3:
nr 27, each 2 0:15 0:15 m 1.215
Volume oftimber 3.175 m3
Cost at £320.00 per m3 £697.40
Waste 10% £69.74
Total Cost £767.40
Labour to unload, set up, strike: 3.17 m3at 10 hours/m3at £7.15 £177.20
Cost for trench £253.91
Trench 12 m long and 2 m deep requires 48 m2ofearthwork support:
Cost for 1 m2 £5.29
Profit and oncost 20% £1.06
Rate per m2 £6.35