Every construction project happens on a specific physical land called the construction site. Buildings and other civil engineering infrastructure are anchored to the ground by their foundations. For these foundations to be created, the ground is excavated. Depending on their magnitude for the specific project, these excavation works significantly contribute to the construction cost.
We have seen many buildings collapsing in the country. Among the reasons cited by the task forces that have investigated these matters were issues related to structural failures. Therefore, foundation works must be correctly anchored into stable ground to prevent these issues. Shortcuts for cost savings must not be allowed, but professional advice from a quantity surveyor can be sought on how to reduce the costs of these excavation and foundation works without compromising the structural integrity of the works.
That said, it is important to first understand the components of the excavations and earthworks work item and to see where the costs, as would be captured in a detailed bill of quantities, would stem from.
Components of Excavations and Earthworks
In the book “Price Determination, Costing and Planning for Construction Works”, the authors list the following as the construction work items that form the main item of excavations and earthworks in the substructures element in a bill of quantities:
- Site preparation works: cutting down trees, hedges, shrubs, turfing and excavating topsoil for re-use.
- Earthwork support: temporary support to the sides of excavations.
- Disposal of excavated materials either on/off-site.
- Filling: backfilling around foundations and making levels generally in the areas of the site using materials specified by the architect/structural engineer.
- Surface treatments: preparation of the ground surface by compaction, blinding, and levelling, to the ground surface (to receive blinding).
You will find your project QS accurately capturing those items depending on the conditions on your construction site. However, for the contractor, the builder’s quantities and the costs thereof may differ from the quantities calculated following the 2008 Standard Method of Measurement (SMM) published by the Quantity Surveyors Chapter of the Architectural Association of Kenya, as it prescribes that those quantities be measured net. Therefore, the contractor’s QS also needs to cater for these factors when pricing a competitive tender.
Let’s see below what these cost factors are.
Cost Factors: Excavations and Earthworks
1. Weather Conditions
The excavations and earthworks are exposed to weather, which cannot be predicted with any degree of accuracy. Although the quantity surveyors can rely on weather predictions and climatic data to predict the possibility of weather patterns, it becomes more problematic with the ever-changing climatic conditions.
For example, in dry weather conditions, construction plant and equipment for excavating will work best and achieve considerable savings in time and costs. Conversely, in wet conditions, working with an excavating plant becomes problematic, with its efficiency greatly reduced.
Therefore, this knowledge should help the site manager to schedule the excavations and earthworks during a period when the weather conditions are suitable for the usage of the most effective means for doing the work. This way, the contractor can make some cost savings with that element, which will contribute to the overall profitability of the project.
2. Bulking of Soil
Bulking can be explained as the change in volume of a material when it is excavated from its original in situ position. When soil is at its natural, undisturbed location on the site, it is compact and occupies a certain volume. Once it has been dug out, its particles are taken apart, and it will occupy a large volume that it did when it was compacted together in its natural, undisturbed state on the construction site. This volume change is what is referred to as the bulking of soil (excavated material).
In the bills of quantities, the quantity surveyors measure soil volume for excavations as net as prescribed by the local Standard Method of Measurement. However, after excavation, as we have seen earlier, the soil will increase in volume depending on the nature of the ground conditions. This volume change then affects the costs of items such as soil/excavated material disposal, as the builders’ quantities on the construction site are higher than the measured quantities in the bills of quantities prepared by the QS due to the bulking factor.
Therefore, a contractor’s QS building up rates for pricing these works in a competitive tender must take into account this volume change when excavations are done and adjust their unit rates to cover the extra work that will be encountered on the actual construction.
This requires experience in the construction industry to understand such phenomena. Also, such knowledge can be gained through a formal education and training in a university/college in course units teaching construction technology or civil engineering technology.
3. Type and Volume of Excavation
There are various types of excavations and earthworks in a construction project, depending on the type of building foundations used and the nature of the soils. For example, where there are firm soils, you may find less excavation being done as a firm and stable ground can be easily reached. However, in construction sites with very loose soils, it may be required to remove all the loose material, depending on the recommendation given by the structural engineer in the project.
Loose material will lead to an increased volume of excavation. Where top vegetative soil is required to be stripped and collected for later reuse in the project, the means of excavating will differ from when a contractor is required to excavate trenches to receive footing foundations. Even with these trenches, costs incurred vary based on how deep the trench is being excavated.
The project bills of quantities usually describe the type of excavation and details of depth stages. However, the contractor is at liberty to choose the approach to use in achieving the desired work results.
Considerations: how to achieve the fastest progress with the lowest possible cost; where there is insufficient volume to necessitate the use of construction plant and equipment; and existing services or barriers such as tree roots that may restrict the use of construction plant and equipment.
This restriction will raise the intensity of manual labour usage, which will make the project take longer as the efficiency of humans cannot be compared to that of machines in difficult working conditions.
4. Ground Conditions or Nature of the Ground at the Construction Site
The nature of the ground at the construction site influences the choice to use either construction plant and equipment or manual labour.
Normal ground with loose topsoil, leaves, decayed plant material, sands and loams is suitable for the use of excavating plant and equipment.
Where rock is encountered during the excavation process, it leads to increased time and costs. You will need more time to break and remove hard rock that you would need if the soils were loose.
Also, extra care is required to avoid damaging the construction plant or equipment when excavating in hard rock. Therefore, encountering hard rock produces a considerable decline in the output of manual labourers and construction plant and equipment on the site.
Also, soft, loose, unstable soils will require earthwork support, while waterlogged grounds require dewatering to remove the excess water. These extra requirements lead to a higher cost.
Therefore, both the project QS and the contractor’s commercial manager must acknowledge these conditions in their computations for unit rates when pricing the tender bills of quantities. That’s why there is always a requirement that bidders/contractors’ representatives visit the proposed construction site to acquaint themselves with the site conditions that may affect the pricing of these works.
The client, for purposes of better cost management and certainty in the planning, should engage professionals to undertake a geotechnical investigation before even the design of the building. The report produced thereof will guide on the nature of the soil conditions below the ground. This, in turn, dictates the best structural solution to be designed by the civil engineer and influences the cost of the building’s foundations and excavation works required.
5. Distance from the Construction Site to the Dumping Site
The last factor in our discussion is the distance travelled by the dumper from the excavated area to the collection site or the dumping site.
You will find some construction contract documents specifying how far the arisings should be dumped. Others indicate that this distance will be determined during the construction phase, depending on the instructions given by the construction project manager or the contract administrator.
Both situations must be carefully priced to allow the contractor to execute the works and not burn out their profits.
Where surplus soil is to be removed from the site, while allowing the contractor to choose which dumping site to use, then the farther the dumpsite from the construction site, the more the costs for transporting spoil to it.
Apart from the transportation costs, there are other costs like payments for any required permits to move materials around and to access the dumping site. These will form a significant portion of the excavations and earthworks costs in any construction project.
Conclusion
We have seen that excavations and earthworks are inevitable in a typical construction project. Because buildings have foundations anchored into the ground, these foundations need space created to accommodate them through excavating and moving materials.
Several factors impact the costs associated with these excavation works, as discussed.
These include, but are not limited to, weather conditions, bulking of excavated materials, ground conditions, type and volume of excavations, and the distance from the construction site to the designated dumping site.
Are there other factors which you have found significant in your project? Let us know in the comments section below.