The geotechnical investigation project for Terminal 3 of Guiyang Longdongbao Airport Phase III Expansion Project is located in Nanming District, Guiyang. Terminal 3 is a landmark building in Guiyang, standing alongside the existing Terminals 1 and 2. It is a key construction project of Guizhou Province's 13th Five-Year Plan, bearing the historical mission of 'building an important aviation hub in western China and supporting Guizhou's leapfrog development'. The First Company of China Southwest Geotechnical Investigation & Design Institute Co., Ltd. undertook the geotechnical investigation task for Terminal 3 of Guiyang Longdongbao Airport Phase III Expansion Project.

Terminal 3 is designed and constructed to meet the usage demand of 30 million passengers per year by 2025. The completion of the project will further enhance Guiyang Airport's status as a regional aviation hub, improve its comprehensive support capacity, accelerate the construction of a modern comprehensive transportation system, promote opening-up, and drive steady and rapid regional economic development.‍

Forging Ahead on the Hard Road to the Sky

Terminal 3 is designed with a main building plus concourses, forming a 'T' shape. It spans 522m from north to south and 360m from east to west, with a total construction area of 167,460㎡ (including 153,415㎡ above ground and 12,583㎡ underground, and a total site area of 59,000㎡). The main building height is 38.300m, and the concourse height is 33.298m. Terminal 3 has large column grid spans and high settlement requirements; single column loads are large, requiring high foundation performance and high pile bearing capacity, with a designed ultimate bearing capacity of a single pile reaching 76,000kN.

Deep Fill

The deep high-dump fill on site reaches a maximum depth of 60m, making natural foundations for the terminal impossible. However, the negative skin friction of the fill greatly affects pile foundation design and project cost. It is necessary to identify the source, origin, spatial distribution, composition, particle size, density, uniformity, etc. of the deep fill, and to determine its engineering characteristics.

Peculiar Karst

Karst development follows no regular pattern, but it directly relates to the smooth progress of pile foundation design and construction and to engineering risk control. It is necessary to identify the size, height, planar distribution, filling conditions, and other karst development characteristics and engineering properties. 

Difficult Rock Determination

With deep fill, super-long piles, large negative skin friction, and high design bearing capacity of pile foundations, the pile tip bearing stratum is required to be moderately weathered limestone with a compressive strength of no less than 26MPa. Limestone fractures are relatively developed and generally filled with cohesive soil, making weathering degree difficult to determine. It is necessary to ensure that the pile tip bearing stratum meets the design requirements.‍

Drawing Blueprints with Forge Ahead Spirit

Investigating Fill

By fully collecting original topographic maps, satellite images, construction records, and other data, combined with site reconnaissance, high-density electrical resistivity method was mainly used to preliminarily determine the fill depth and guide on-site investigation drilling.

A pioneering patented new technology, 'Visualized Glass Casing Investigation for Deep Fill', was first created to accurately identify fill composition. The fill composition on site is complex, drilling and sampling are difficult, and identification of fill composition, origin analysis, and engineering evaluation are challenging. Using glass casing and visualization technology, the fill composition and distribution on the borehole sidewall can be observed in situ, providing reliable support for the conduct, analysis, and evaluation of investigation work in deep fill areas.

Exploring Karst

High-density electrical resistivity and ground penetrating radar were used for preliminary identification of karst on site, and targeted drilling verification was carried out in the preliminarily identified karst areas, making drilling work more targeted.

For the first time in the Guiyang area, the patented new technology 'A Sonar Detection Device and Method for Karst Caves at Pile Bottom' was applied to identify karst development at pile tips.

A pioneering hole-mouth drilling method was created to solve the technical difficulties of in-hole drilling. During pile foundation construction, geological anomalies such as 'half rock, half soil' and karst caves may cause the original borehole depth to fail to meet the bearing stratum control requirements. Backfilling boreholes is costly; drilling inside the pile hole is impossible because there is no constraint around the drill pipe; drilling outside the hole is costly, time-consuming, and affects the turnover use of full casings. To address these factors, the team creatively proposed the 'Guided Cylindrical Four-Corner Tower Technology for Hole-Mouth Drilling of Ultra-Deep Suspended Piles in Karst Areas', adopting the 'hole-mouth drilling' method for investigation.

For independent foundations in the excavation area, the comparison and verification of drilling and ground penetrating radar geophysical results were strengthened during the investigation stage. During the foundation trench inspection stage, 'small-diameter boreholes' were arranged in the rock foundation trench to verify karst, controlling the risk of karst at the foundation base.

Man Descending into Hole

Taking advantage of the 'natural advantage of full casing steel liner wall protection', the team creatively proposed a 'manual hole entry' scheme for foundation trench inspection under mechanical hole formation, to accurately determine the foundation bearing stratum.

In the preliminary 'geological survey and mapping', the rock attitude was 60°∠9°. Manual hole entry inspection revealed that the rock stratum at the hole bottom is gently inclined, nearly horizontal, and qualitatively judged to have good stability as a bearing stratum. At the same time, the sliding stability of the pile foundation near the free face was quantitatively calculated. Considering unfavorable combinations, its anti-sliding stability meets the requirements.

'Ground penetrating radar + small-diameter drilling' for karst verification after foundation trench excavation‍

Innovation Tailored to Local Conditions

For the first time, the concept of 'pile foundation design with variable cross-section pile diameter, thick at top and thin at bottom' was proposed. Since rotary drilling rigs encounter large resistance from the upper fill during construction of ultra-deep piles and drilling in bedrock is difficult, the project team separated the fill section with casing and then used a variable-diameter method for drilling the lower rock-socketed section, effectively solving the construction difficulty of ultra-deep piles.

For the first time, the concept of 'equivalent positive side friction thickness in karst development sections' was proposed. According to the preliminary calculation by the design unit, the rock surface of ultra-deep bedrock piles is around -90m. Considering the influence of negative skin friction, if the side friction in karst development sections is ignored, the characteristic value of foundation bearing capacity is only 3500kPa, which cannot meet the design requirements. The project reasonably considered the positive contribution of pile side friction in the 'half rock, half soil' strata in karst development sections, and included this part of side friction in the bearing capacity after equivalent reduction. This considers both the adverse effects of karst fillings and the side friction in non-karst sections, and provides the equivalent positive side friction thickness.

Achieving Both Quality and Efficiency

Starting from deep fill and soluble rock, the project opened up new investigation ideas, aiming at foundation type selection and effectively reducing engineering karst risks. For the ultra-deep bearing stratum of some bored piles, hole-mouth drilling was proposed, and refined design was carried out based on the geological conditions observed through manual hole entry inspection. Suggestions for variable-diameter design were put forward, and pile bearing capacity was checked considering the pile tip bearing capacity at the variable-diameter location and the contribution of non-rock-socketed side friction, optimizing the design pile length. During pile foundation construction, the quality bottom line of 'inspect every pile' was strictly implemented to prevent blind excavation and construction by the contractor, saving nearly 6 months of construction time and nearly 20 million yuan in cost. Monitoring results of the main structure show that during the settlement deformation monitoring period, the deformation rate of each monitoring point of the main building is within the stability standard specified in the code.

Terminal 3 is the core component of Guiyang Longdongbao Airport Phase III Expansion Project. It faces a complex engineering geological environment rarely seen in Guizhou Province and even in the national industry, with high-dump deep fill exceeding 60m in maximum thickness and densely developed concealed karst, etc. The site complexity is Grade I. The maximum diameter of foundation piles is 3200mm, the maximum pile length exceeds 100m, and the foundation grade is Grade I. The successful implementation of the project will serve as a strong reference and guide for similar stratum projects.‍