Research on the Characteristics and Quality Control of Sand-Fill Construction in Valley-Type Urban Expansion Areas

Abstract

To solve the problem of a lack of land resources in the valley-type urban area of Lanzhou and increase space for urban development, sand-filling projects to "level mountains and create land" have been carried out in recent years. However, the poorly graded sand often used in such construction projects has many defects, such as weak particle interlock, poor compaction performance and susceptibility to differential settlement. Consequently, the engineering quality control and evaluation system for these projects urgently requires improvement. This paper studies the construction of the sandy soil filling area in the Gaolan Ecological Restoration and Industrial Development Demonstration Area, Lanzhou. According to laboratory tests (Grain Size Analysis and Heavy Compaction Test) and geophysical exploration (Surface Wave Method, Ground Penetrating Radar), obtain in-situ data such as Standard Penetration Tests, Static Cone Penetrometer Tests, Sand Cone Tests, etc., on parameters related to particle gradation, compaction characteristics, hydraulic properties and other factors influencing compaction efficiency. It shows that there is a quantitative relationship between the degree of fill compaction (λc) and in-situ mechanical indicators; for instance, when λc ≥ 0.88, N > 15 and Ps > 5 MPa. Therefore, to obtain a comprehensive understanding of the quality of sandy fill engineering, several types of test data were collected and organised. A Geographical Information System (GIS) has been employed to show the distribution of compaction and indicate high-risk areas. The calculation of the earthwork volume in the demonstration area has achieved a good balance. After process optimisation, the qualified compaction rate (λc ≥ 88%) was over 90%, the characteristic bearing capacity was between 313 and 357 kPa, and thus it met the engineering design requirements. However, the local excavation slope is steeper than the design slope, and therefore a reinforcement measure will be taken. The four modules mentioned above are combined to form a general assessment system for the quality of sandy fill construction sites, and they provide full theoretical support and a technical basis for the quality control, safe operation and maintenance of similar coarse-grained soil fill projects in river valley cities.