Constraint-based analysis of civil work delays in utility-scale solar farm construction: a multiple-case study in Thailand

Purpose - This study examines how different types of constraints influence delay outcomes in utility-scale solar farm civil construction and proposes a constraint-based taxonomy comprising Physical, Socio-environmental and Technical-operational categories. It also introduces Earthwork Volume Intensity (EVI, m³/MWp) as a terrain-normalised indicator of earthwork demand. Design/methodology/approach - A multiple-case study design is applied to eight solar farm projects in Thailand (169 MWp total) delivered under standardised equipment and contractual conditions. Primary schedule data are analysed through cross-case pattern analysis and exploratory statistical methods, including Pearson correlation, ordinary least squares regression and the MannWhitney U test. Findings - Within hillside terrain, EVI provides a more consistent explanation of delay severity than project scale, indicating a direct link between terrain-driven workload and schedule performance. Hillside projects experienced substantially higher earthwork delays (36- 57%) than flat-terrain projects, while foundation delays were effectively mitigated through a change in construction method (driven concrete to steel screw piles). The apparent correlation between project scale and delay reflects outlier leverage rather than a causal mechanism. Research limitations/implications - The sample size (n = 8) and single-organisation data source limit statistical power and generalisability. EVI is positioned as a planning-oriented diagnostic rather than a statistically validated predictor; validation across larger, multi-organisation datasets is required. Practical implications - The constraint taxonomy and EVI metric provide a structured pre-construction decision framework supporting differentiated contingency allocation by terrain type, agricultural-calendar coordination for socio-environmental risks and geotechnical-informed foundation selection. Owner’s Engineers managing multi-site portfolios can apply the framework using routinely collected schedule data. Originality/value - This is the first study to apply a constraint-based delay taxonomy to solar farm civil construction and to introduce EVI as a terrainderived schedule risk indicator. The single-organisation controlled design isolates terrain effects from confounders typically present in multi-organisation construction delay studies.