Basement excavation: working space, unsuitable material and bulking
A two-level basement for an apartment block in Nicosia is excavated over a plan area of 50.0 m × 37.0 m to formation level 4.10 m below existing ground. The permanent works require a 300 mm working space along the four sides. At 2.80 m depth the engineer identifies 420 m³ of unsuitable organic silty clay below formation, orders removal to 600 mm below formation and replacement with compacted 0/40 crushed stone. Of the suitable excavation, 1,350 m³ is reused as fill; the remainder is carted away. Groundwater is encountered at 3.60 m and pumping runs for six weeks. The contract measurement rules state: excavation is measured to net in-situ dimensions; working space is not measured unless instructed; unsuitable material is measured separately; cart-away volume uses a 1.25 bulking factor; fill is measured as compacted volume. The contractor has submitted a claim for working space, for the unsuitable material at the bulked volume, and for disposal of all excavated material at the bulked volume.
1. Calculate the measured excavation, working space, replacement stone and cart-away quantities, and state which items are actually measured under the contract rules.
Read the worked answer
Start from net in-situ dimensions, not from bulked or loose volumes. Ordinary excavation to formation is 50.0 × 37.0 × 4.10 = 7,585 m³. The 420 m³ of unsuitable material is below formation and is measured separately, so total excavation is 8,005 m³, of which 7,585 m³ is ordinary excavation and 420 m³ is unsuitable material.
Working space would physically be ((50.6 × 37.6) − 1,850) × 4.10 = 52.56 × 4.10 = 215.5 m³, but the rule says it is not measured unless instructed. The contractor’s claim therefore fails at this stage.
Replacement stone is measured as compacted volume in the void, so it is 420 m³ - not 420 × 1.25. Bulking applies only to cart-away.
Cart-away: suitable material not reused is 7,585 − 1,350 = 6,235 m³; add the unsuitable 420 m³, giving 6,655 m³ in situ. Bulked for cart-away: 6,655 × 1.25 = 8,318.75 m³. The reused fill is not carted away and must not be bulked.
2. The contractor argues that because the working space is necessary to construct the basement, it must be paid, and that all 8,005 m³ should be bulked for disposal. Advise the QS.
Read the worked answer
The QS should separate necessity from measurement. A method-related volume is not automatically a measured item. The contract expressly says working space is not measured unless instructed, so the contractor carries that cost in its excavation rate unless a variation or instruction changes the basis. The QS should reject the working-space claim but note that if the engineer later instructs a wider working space, it can be valued as an addition.
On disposal, the error is applying bulking twice or to the wrong volumes. Excavation is measured in situ; bulking is a conversion for transport only. The 1,350 m³ reused as fill never leaves the site. The unsuitable 420 m³ is excavated material and does leave, so it is included in cart-away, but it is still measured in situ before bulking. The correct disposal quantity is (7,585 − 1,350 + 420) × 1.25 = 8,318.75 m³, not 8,005 × 1.25 = 10,006.25 m³.
The QS should also check that the 420 m³ is not double-counted: it is below formation, so it is additional to the 7,585 m³; if it were within the formation depth, it would be a substitution within the net volume, not an addition.
3. If the engineer instructs the 300 mm working space to be measured, how should it be measured and what is the effect on cart-away?
Read the worked answer
Measure the instructed working space to the specified 300 mm width and to the actual depth over which it is available. On the four sides, the plan area is (50.6 × 37.6) − (50.0 × 37.0) = 1,902.56 − 1,850 = 52.56 m²; at 4.10 m depth the volume is 215.5 m³. Add this to the excavation quantity as instructed working space, but keep it separate from ordinary excavation so that the rate can be scrutinised.
The cart-away effect depends on whether the working-space material is reused. If it is suitable and not reused, the in-situ addition is 215.5 m³, and the bulked addition is 215.5 × 1.25 = 269.4 m³. If it is reused as fill, it is not carted away, so only the fill volume changes. The QS should not automatically bulk the whole instructed volume; the disposal item must follow the actual destination of the material.