Five Major Benefits of Approximate Quantity Design That Improve Operational Efficiency and Accuracy

In infrastructure fields such as waterworks, sewerage works, and general civil engineering construction, it is required to efficiently carry out a wide range of tasks from design and estimating to construction management. Cost reduction and improvements in construction accuracy are also ongoing challenges. However, with traditional contracting methods, the drawings developed during the design phase often do not match actual site conditions, and repeated design changes during construction frequently cause cost increases and schedule delays due to rework. One approach that has attracted attention in recent years to address these issues is approximate quantity design. This is a method in which planning and estimating are performed using rough quantities at the design stage, contracts are placed based on those quantities, and final quantities are determined through detailed design and surveying at the site. Trial implementations are currently progressing in water and sewerage projects and other areas. The national and local governments are also promoting the introduction of this “approximate quantity design contracting method,” and it is expected to contribute to streamlining design and estimating work and facilitating smooth project execution. In this article, we first explain the definition and mechanism of approximate quantity design, and then concretely introduce the five benefits it brings.

What is approximate quantity design?

Approximate quantity design is a design method in which the quantities required for a construction project are calculated approximately before detailed design, and the estimated construction cost and contracting are based on those approximate quantities. For example, in water pipe installation work, the initial design quantities for piping materials can be estimated by multiplying a standard unit quantity derived from past project data by the length (pipe length). At this stage, the creation of detailed route drawings and material quantity tables can be omitted, and after contracting the contractor (the awardee) will finalize detailed piping drawings and accurate quantities based on on-site test excavations and survey results. In other words, in the initial stage planning is done approximately based on standard cross-sections on drawings and quantities per unit length, and during the construction stage those are updated to confirmed quantities according to actual conditions.

To summarize the difference from the conventional method simply: the key point is that detailed design drawings are not prepared before contracting, and bidding is done on approximate quantities, with detailed design completed after contract award. The flow compares as follows:

• Conventional contracting method: The client finalizes detailed design drawings and design quantities → construction cost is estimated and bids are placed based on those quantities → the contractor constructs based on the provided design documents

• Approximate quantity design method: The client calculates the design amount and bids based on approximate quantities → after contract award, the contractor prepares and finalizes detailed design drawings and quantities based on site surveys → construction and contract settlement are carried out based on those finalized quantities

The aim of approximate quantity design is to reduce the effort required to produce detailed design drawings, thus reducing workload in design and estimating and enabling earlier contract placement and smoother construction. Another major feature is that because the contractor participates in detailed design, design and construction can be advanced in an integrated manner. This approach is being actively adopted particularly in linear infrastructure works such as water main installation and sewer pipeline construction, and expansion into other fields such as road improvement work is expected. So what concrete benefits can be obtained by adopting approximate quantity design? The five main advantages are explained below.

1. Cost reduction

By introducing approximate quantity design, the efficiency of design and estimating processes and the elimination of waste can lead to significant cost reductions. For the client side, simplification of detailed drawings can reduce the time and labor costs required for design work. Initial estimates can also be performed quickly based on approximate quantities, shortening the lead time to bidding and contracting, and yielding cost benefits from shortened overall project schedules. For example, if the creation of detailed design documents is no longer necessary, the cost of outsourced design can be significantly reduced.

On the contractor side, cost reduction effects are also expected. Since the contractor can produce detailed construction drawings after contract award, optimal design and construction tailored to actual site conditions become possible, reducing material excesses or shortages and rework. For instance, even if design changes or additional work occur, because the contract anticipates flexible quantity adjustments from the start, large cost increases associated with changes can be restrained. Reducing waste during construction improves overall construction cost performance.

• Design cost reduction: Omitting the creation of detailed design drawings and quantity summaries can substantially save time and effort for design and estimating

• Construction cost reduction: Design adjustments matching site conditions reduce material waste and unnecessary work, enabling efficient construction

• Indirect cost reduction from shorter schedules: Shorter project durations reduce site management fees and miscellaneous expenses, lowering indirect costs

2. Strengthened coordination between design and construction

In approximate quantity design, some design work that was previously handled by the client (for example, detailed pipeline design) is carried out by the contractor. As a result, the contractor’s know-how and ingenuity are reflected from the design stage, considerably strengthening coordination between design and construction. When the contractor prepares the detailed design, it becomes easier to create “constructible designs,” i.e., plans with high constructability, reducing inconsistencies and construction problems on site.

Also, advancing design and construction in an integrated fashion reduces communication losses and smooths information sharing. Feedback on change items and site conditions that traditionally flowed from client → designer → contractor can be reflected immediately because the contractor is directly involved in design adjustments under the approximate quantity design method. This enables quick responses to design changes and faster decision-making during construction. Furthermore, early contractor involvement facilitates VE (value engineering) proposals and the adoption of new construction methods, making it easier to present ideas that can realize higher quality and lower cost projects for the client. For contractors, participating in design work also leads to skill development for engineers, contributing to improved technical capability and human resource development for the company as a whole. An integrated design-construction system (similar to the design-build approach) also clarifies risk-sharing between the client and contractor, which has the advantage of making responsibility clear in the event of problems. Overall, lowering the barrier between design and construction and enabling the team to work together as one on the project is a major strength of approximate quantity design.

3. Compatibility with ICT construction

Another attraction of approximate quantity design is its good compatibility with ICT-enabled construction (information-oriented construction) that uses digital technologies. Managing quantity data digitally from the design stage makes it easier to incorporate ICT throughout various processes up to construction. For example, using CAD or estimating systems to calculate approximate quantities reduces human error and automatically updates quantity calculations when design changes occur. Also, the detailed design drawings prepared by the contractor after contract award are organized as digital data, which can be directly used for machine control and quality control during construction. With the momentum of *i-Construction* promoted by the Ministry of Land, Infrastructure, Transport and Tourism, ICT utilization at sites has been accelerating in recent years, and approximate quantity design has high affinity with such advanced technologies and can serve as a foundation for construction DX (digital transformation).

Combining the following ICT technologies can further advance the design and construction process:

• Use of 3D models (CIM): Create 3D models of structures and terrain from the design stage to streamline quantity calculation and construction simulation

• Drone photogrammetry and laser scanners: Rapidly measure site topography from the air or ground to obtain high-precision point cloud data and improve the accuracy of approximate quantities

• RTK-GNSS for single-person surveying: Using high-precision GNSS receivers in smartphone-equipped devices enables quality surveys that previously required multiple people to be conducted quickly by one person

• Use of machine guidance: Link digital design data with construction equipment to perform precise automated excavation and earthwork, shortening construction time

By using these digital technologies, tasks that used to take a survey team several days, such as as-built measurement, can be completed in a matter of hours with drone aerial photography or almost immediately with GNSS surveying, allowing point cloud data and coordinate values to be obtained on the same day. Furthermore, data and drawings can be shared instantly via the cloud, smoothing progress reporting to the client and consultations on design changes. Combining ICT construction with approximate quantity design enables smart project execution that consistently uses data from investigation and survey through design, construction, and inspection.

4. Clearer explanations to the client

Approximate quantity design also clarifies and simplifies explanations and contracting procedures for the client (the project owner). Traditional detailed design documents often include technical drawings and complex quantity tables that can make it difficult for the client to understand the overall picture. In contrast, designs using approximate quantities present the basis for estimates in intuitive ways—for example, “per ○ m (○ ft) of length, ○ units of pipe material”—making it easier for clients to grasp the scope and cost breakdown. Entering the contracting stage with minimal necessary drawings and quantity information allows meetings with the client to simply focus on “where and how much to construct,” speeding up consensus building. The simplification of submitted documents also reduces the time spent preparing and revising explanatory materials.

Moreover, when quantities are finalized after contract award, comparing them with the initial approximate quantities and explaining the reasons for differences with data enables clear explanations to the client. If additional excavation is required on site, for example, one can convincingly explain that “an additional ○ m³ (○ ft³) of work is required because the actual soil volume exceeded the estimate based on the standard cross-section,” using measured values obtained from as-built management. Using digital survey data and photo records adds visual evidence to oral or written explanations, increasing client confidence.

For example, by overlaying the design model on the actual site using a tablet’s AR (augmented reality) function, the client can intuitively understand “where pipes will be buried” and “how the terrain will change after completion.” Spatial information that is hard to convey with drawings alone can be visualized using AR displays and 3D models, aiding explanations to the client and notifications to local residents. As a result, client interactions become smoother and trust is enhanced.

5. Improved on-site responsiveness

Approximate quantity design also enhances the ability to respond to unexpected situations and changes on site. By not locking in detailed design too rigidly in advance, there is flexibility to address conditions discovered during construction. For example, if underground utilities are misaligned or unforeseen obstacles are encountered, the contractor can promptly revise the design and consult with the client to proceed, preventing major stoppages or rework. Because the contract is structured from the outset to accommodate quantity adjustments and design changes, procedures for contract changes due to minor site alterations can proceed smoothly, minimizing impacts on the schedule.

In addition, progressing work while measuring as-built conditions ties operations directly to inspection and as-built management. By measuring and aggregating as-built quantities immediately after construction and comparing them with design values, quality can be continuously verified rather than leaving work unchecked. Recent advances in ICT technology have increased the tools available to site personnel to grasp as-built conditions in a short time, such as drone surveys, ground-based laser scanners, and high-precision RTK-GNSS positioning. Combining these tools with approximate quantity design enables quick, quantitative decision-making when minor adjustments or additional work are needed during construction.

Concrete examples of improved on-site responsiveness:

• Even if unexpected buried objects or obstacles are encountered underground, design changes and construction method adjustments can be considered and implemented on the spot

• Measured as-built values can be immediately reflected in contract quantities, allowing smooth processing of additional orders or deduction settlements

• If additional work arises due to design changes, prompt countermeasures can be taken on site to minimize schedule delays

Thus, approximate quantity design is robust against various on-site irregularities and makes it easier to complete the project as planned. Traditionally, when problems occurred on site, confirming and querying the designer or client took time, but in an integrated design-construction system there are more matters that can be decided on the spot, speeding up decision-making and enabling safe and smooth construction management. This capability is a significant advantage not only for normal construction but also in emergency situations such as disaster recovery, where the ability to make quick on-site judgments and responses is crucial.

Conclusion

Approximate quantity design, as described above, is a method that delivers significant effects both in terms of cost (improved productivity) and technical performance (improved accuracy). By integrating design and construction and making full use of ICT, this approach can be expected to streamline operations and improve accuracy across waterworks, sewerage works, and general civil engineering construction. It can be regarded as one approach to promoting productivity improvement and DX (digital transformation) in the construction industry.

To fully realize these benefits on site, it is essential to perform surveying and as-built management tasks easily and accurately. One reliable tool for that purpose is the LRTK "Simple Survey" function. By combining a smartphone with a compact positioning device, this solution enables site personnel to perform necessary measurements quickly without specialized surveying equipment or multiple personnel.

Specifically, it can handle photogrammetric dimensional measurement, overlaying design data via AR displays, cloud synchronization of measurement data, monopod measurement using a dedicated pole, and even volume calculation for excavation soil (volume computation) — all in a single device. For example, you can instantly measure elevation differences and clearances of buried pipes from photos, synchronize drawings and as-built data to the cloud for team sharing, and whenever needed measure, share, and move to the next action immediately using your own team.

Actively utilizing such advanced ICT tools can make the benefits of approximate quantity design more certain. By advancing digitalization at sites, it becomes possible to simultaneously achieve operational efficiency and improved precision—two objectives that were previously difficult to reconcile. Such initiatives may become the new standard for future site operations. Why not consider introducing them?