Creating a Zero-Rework Site with Preliminary Quantity Design

One of the factors threatening productivity on construction sites is rework (redoing work) during construction. Rework, which is a primary cause of schedule delays and cost overruns, is often the result of human errors stemming from insufficient information at the design stage or mistakes in quantity calculations—precontract planning deficiencies. Conversely, these issues are largely preventable with appropriate countermeasures.

A promising approach to minimize such rework is the method known as preliminary quantity design. This approach estimates the volumes of earthworks and quantities of materials required early on and reflects those estimates in the design and cost estimation, thereby reducing quantity mistakes on site. If accurate quantities are understood during the planning stage, the need for additional work or redoing work during construction can be greatly reduced. This article explains what preliminary quantity design is, the effects it brings to the field, and concrete procedures to carry it out accurately. It also discusses mechanisms to improve consistency between design and construction, the connection with ICT construction and as-built management, and finally highlights the importance of quantity design based on on-site surveying by introducing the "Simple Survey" feature of the latest tool LRTK as an example.

What is preliminary quantity design

Preliminary quantity design, as the name implies, is a method of designing based on approximate quantities. It involves estimating in a simple way, in advance, the volumes of excavation and embankment (earthworks) and the quantities of materials to be used, and using those approximate values to prepare design documents and cost estimates. It is characterized by omitting or simplifying detailed working drawings at the time of order and adjusting the design during construction as necessary. In other words, instead of determining exact quantities from the outset, the approach sets provisional design quantities using standard cross-sections or averages derived from past experience, then revises and finalizes them later to match site conditions.

This concept of preliminary quantity design has recently attracted attention as a measure to streamline estimating and design work. For example, in water supply and sewer pipe projects, some municipalities now issue only plan views of piping routes and standard cross-sections at the time of ordering and carry out bidding using preliminary quantities based on pipe length. After contract award, the contractor conducts detailed on-site investigations and, considering site conditions such as the positions of buried objects and ground elevations, prepares accurate piping drawings and quantity tables. This reduces the amount of design work before construction while allowing post-contract updates to the design to reflect actual conditions, thereby reducing unnecessary design changes and rework. In this way, preliminary quantity design aims to both streamline the entire procurement and construction process and prevent rework on site.

Actual situation of quantity mistakes and rework on site

Quantity mistakes on construction sites are troublesome issues because even a minor calculation error can lead to significant rework. No matter how smoothly construction proceeds, an incorrect estimate of required quantities will inevitably exact a cost later. The following cases actually occur on sites:

• Errors in earthwork quantity calculation: Mistakes in calculating excavation or embankment volumes lead to shortages or surpluses of soil. If soil is insufficient, additional soil transport or re-excavation is required; if surplus occurs, disposal costs increase.

• Errors in material ordering quantities: Mistakes in the required counts or quantities of rebar, formwork, piping, etc., result in material shortages on site and work stoppage. If additional procurement takes time, the schedule is delayed and may affect delivery deadlines.

• Unit mix-ups: Basic mistakes such as misreading dimensional units during calculations—computing in mm where m should be used—create digit order errors. Large quantity excesses or shortages result, forcing a rewrite of construction plans.

• Inconsistencies between drawings and specifications: Quantities shown on design drawings and those in the specifications may conflict, leaving the site uncertain which to follow and potentially causing construction errors. For example, if pipe lengths differ between drawings and reality, additional fittings and work may be required.

• Omission in quantity aggregation: Some structural elements or work items may be omitted during the design phase and thus not included in estimates. If it is discovered during construction that required quantities were not accounted for, emergency design changes or additional orders become necessary.

When such mistakes come to light, additional rework and schedule changes are unavoidable on site. Equipment already installed may need to be removed and reinstalled, or night work may be required to make up for deficiencies, generating extra effort and cost. Rework that should have been preventable lowers staff morale and can damage trust with the client.

Effects of early quantity design

What effects can be achieved by thoroughly implementing such quantity design early in the project? The main benefits include:

• Improved design accuracy: Calculating quantities with consideration of construction conditions and site circumstances increases the accuracy of design documents, enabling plans without inconsistencies or omissions. Identifying quantitative contradictions before construction reduces the need for later drawing revisions.

• Stabilized cost and schedule: Accurate estimating based on quantities improves the precision of construction cost estimates. It prevents situations such as “the budget is insufficient” or “the schedule will be extended” after contract award, making it easier to proceed within planned budget and schedule.

• Optimized resource use: By understanding cut-and-fill balances and required material quantities early, it is possible to adjust cut and fill to minimize surplus soil disposal or imported fill. Materials can be ordered in appropriate amounts, reducing excess inventory and waiting losses.

• Increased construction efficiency on site: Construction teams that have accurate quantity information in advance can proceed with better organization. With fewer unexpected reworks, work flows without interruption, improving overall construction efficiency.

• Enhanced reliability and safety: Plans supported by quantities increase client and stakeholder confidence. Reduced anxiety about “things not going as planned” and feasible plans contribute positively to site safety management. As a result, rework risk is dramatically reduced and a sense of security emerges on site.

Appropriate early quantity design thus contributes not only to rework prevention but also to overall project quality and productivity improvement. It aligns with the Ministry of Land, Infrastructure, Transport and Tourism’s advocated “front-loading” (thorough early-stage planning and stakeholder coordination), and careful quantitative checks before construction are now emphasized across the construction industry.

Procedures for accurate quantity design

Below are the basic procedures to follow to advance accurate quantity design.

• Conduct site investigation and surveying: Perform a thorough investigation of the planned site to understand the latest conditions. Carry out topographic surveying to digitize ground elevations and terrain shapes, and confirm the locations of buried objects and existing structures. Conduct geological surveys and surrounding environment interviews as necessary to accurately identify the preconditions for quantity calculations.

• Create plan and cross-section drawings: Based on site survey data, consider the layout of the planned structures and earthworks. For roads or pipelines, determine the route on a plan and create longitudinal and cross-sections at key locations to depict ground lines and design lines. This allows visual understanding of cut-and-fill cross-sectional areas and the dimensions of necessary structures. When applying standard cross-sections, verify that the shape is reasonable for the actual terrain.

• Calculate quantities and volumes: Compute specific quantities from the prepared drawings. For earthworks, determine the cross-sectional area for each section, calculate the volume per section using the average cross-section method, and sum to obtain total earthwork volumes. Extract concrete volumes, rebar weights, and pipe lengths from structural drawings and aggregate them in spreadsheet software. Pay attention to unit conversions and factors (e.g., changes due to compaction) to ensure no required quantities are overlooked.

• Verify and adjust calculation results: Check whether the calculated quantities are reasonable from multiple perspectives. Cross-reference drawings with calculation results to confirm nothing was missed. For earthworks, review cut-and-fill balance and reconsider the design plan if significant surplus or deficit is found. Also confirm consistency with other trades and identify any design errors related to quantities. Problems discovered at this stage should be resolved through design revisions or additional investigations.

• Reflect in design documents: Reflect the finalized quantities in design documents and cost estimates. Add quantity-based notes to plan and cross-section drawings as needed, and prepare quantity tables and progress measurement sheets to share with stakeholders. Use the confirmed quantities to estimate construction costs and incorporate that information into specifications and schedules. In this way, design information handed over to the site contains accurate quantity data and minimizes the risk of rework during construction.

Mechanisms to improve consistency between design and construction

To eliminate information gaps between design and construction phases and ensure work is carried out according to design intent, it is essential to build mechanisms that enhance consistency between design and construction. One approach is to reconsider the way the project is executed. For example, adopting a design–build arrangement, where design and construction are handled by the same team or company, allows contractor know-how to be reflected in the design and reduces incompatibilities such as “cannot be built as drawn.” Also, involving construction management personnel in drawing checks from the design stage, or conducting thorough pre-contract coordination among client, designer, and contractor (so-called front-loading), can detect and eliminate issues that would lead to rework during construction.

Another important mechanism is using digital technologies to centrally manage and share design information. Traditionally, information loss occurred when exchanging paper drawings and quantity tables, but BIM/CIM-based 3D modeling methods that link data from design through construction are becoming more widespread. If constructors directly utilize the 3D models of terrain and structures created during the design phase and apply them to ICT machine guidance or as-built measurements, design data and the construction site become seamlessly connected. Sharing drawings and quantity data on the cloud ensures that everyone can instantly access the latest version in the event of a design change, enabling consistent instructions. Additionally, AR displays on tablets or smartphones allow overlaying design models on the site view for confirmation. Being able to visually check on site whether the work will finish as designed helps prevent mistakes caused by misunderstandings or assumptions and greatly helps close the gap between design intent and construction execution.

Connectivity with ICT construction and as-built management

The affinity between preliminary quantity design and ICT construction—construction using information and communication technology—is dramatically enhanced by digitizing quantity design. If the 3D terrain model and construction data created at the design stage are loaded into ICT machines for machine control during construction, heavy equipment can excavate or place fill automatically to the design heights and slopes. Even without relying on the intuition of experienced operators, construction machines equipped with high-precision GNSS and sensors can accurately trace the work quantities, allowing finishing to target quantities. The number of layout stakes and survey attendances previously done manually is reduced, minimizing the opportunity for errors.

Quantity design data also proves powerful for as-built management (verification of dimensions and shape of the completed work). With design reference data numerically clarified, point cloud data obtained by photogrammetry from drones or 3D laser scanners can be directly compared with the design model. Visualizing the surface differences between the as-built terrain and the design with color maps makes even slight height differences or irregularities immediately apparent. For example, if the embankment thickness falls below the standard in areas, it can be detected instantly and corrected on the spot. As-built measurement tasks that used to take half a day by tape measure and level can now be completed quickly with drone flights or mobile surveying. The Ministry of Land, Infrastructure, Transport and Tourism reports that surveying time in ICT earthwork sites has been reduced by an average of more than 30%, showing marked improvements in inspection efficiency and accuracy. Recording and reporting measurement results can be semi-automated with software and shared with the client via the cloud to expedite approval. By linking quantity design digitally through ICT construction and as-built management, the system for ensuring construction quality and aiming for zero rework becomes robust.

Importance of quantity design based on on-site surveying and LRTK’s Simple Survey

Lastly, it must be emphasized that quantity design based on on-site surveying is crucial. Calculating quantities solely from desk-based design is meaningless if it is far removed from actual site conditions. Conversely, having accurate site survey data can significantly reduce the risk of rework even with preliminary-level design. Surveying technology has advanced in recent years, enabling site managers themselves to easily perform measurements that used to require specialist technicians.

For example, the "Simple Survey" feature of LRTK (a compact, high-precision GNSS surveying device) allows rapid surveying and measurement required on site. This compact device, attached to a smartphone, can obtain position coordinates within an error range of several cm (several in). With a single unit, it supports photo positioning (automatically recording high-precision location information for photos), AR display (overlaying design models and measurement results on the site view), cloud sync (instant sharing of measurement data), as well as monopod measurement (easy height measurement using a monopod) and on-the-spot volume calculation. Without waiting for a professional surveying team, site personnel can obtain sufficiently accurate surveying data themselves and immediately reflect it in the design.

Introducing such modern tools dramatically speeds up and simplifies feedback from site investigation to design. Because the actual site conditions are accurately considered at the design stage, discrepancies in quantities that cause rework will almost disappear, bringing the project closer to an ideal construction cycle with virtually no rework. To fully realize the benefits of preliminary quantity design, data linkage between site and design is essential. In that sense, user-friendly, high-precision "Simple Survey" tools like LRTK are powerful allies that strongly support creating sites with zero rework.