The practical application of granite in bridge structures can be divided into two levels: firstly, it is used directly as a natural stone in bridge components; secondly, it influences bridge foundation design as a geological bedrock. The following is a systematic introduction from an engineering practice perspective.
I. Application of Granite as a Structural Material
Granite is an igneous rock with high density (approximately 2.6–2.7 g/cm³), high compressive strength (100–250 MPa), strong wear resistance, low water absorption, and excellent weather resistance. In bridge engineering, it is mainly used in the following areas:
1. Bridge Deck Paving and Crash Barrier Facing
In highway bridges, granite is often used for the facing treatment of crash barriers (guardrails). For example, the crash barrier design of the Changshanbao Grand Bridge (1797.52 meters long) on the Shenyang-Jilin Expressway uses granite facing, effectively solving the problems of surface bubbles, honeycomb pitting, and winter salt corrosion of traditional concrete crash barriers, significantly improving construction quality and appearance.
2. Bridge Railings and Decorative Components
Granite, with its beautiful and durable natural texture, is widely used in the railings, columns, and deck decoration of urban bridges. In the main urban roads of Lijiang City, granite is used for paving bridge decks, constructing bridge railings, and decorating piers, bearing the long-term loads of vehicles and pedestrians while enhancing the urban landscape.
3. Pier Foundations and Retaining Wall Structures
Granite blocks or slabs can be used to reinforce and protect bridge pier foundations, especially in coastal or high-humidity areas. Their stable chemical properties effectively resist chloride ion erosion and acid/alkali corrosion, preventing steel reinforcement corrosion and extending the bridge's service life. Furthermore, granite blocks are often used in the construction of retaining walls and slope protection in mountainous bridge approach road projects.
4. Structural Reinforcement Materials
In the reinforcement of existing bridges, granite slabs can be used as lining materials for tunnel lining or pier cladding, improving the overall stability of the surrounding rock and preventing further damage to the structure from water erosion and weathering.
II. The Impact of Granite Geological Bodies on Bridge Foundations
In the field of geotechnical engineering, granite, as the bearing stratum, has a decisive influence on bridge foundation design, often more critical than the direct use of the stone itself.
1. End-Bearing Pile Foundations for Cross-Sea Bridges
Granite bedrock is an ideal bearing stratum for the pile ends of large cross-sea bridges. For example:
The Hong Kong-Zhuhai-Macau Bridge: The bedrock is mainly Yanshanian granite, employing variable cross-section steel pipe composite pile foundations, with a maximum pile diameter of 2.5 meters and a maximum pile length of 137 meters, designed as end-bearing friction piles.
The Pingtan Strait Road-Rail Bridge: The seabed cover is shallow or nonexistent, the underlying bedrock is mainly granite, and the water depth at the pier locations exceeds 40 meters. Large-diameter bored cast-in-place pile foundations are used, with a maximum pile diameter of 4.5 meters, designed as column-pile foundations.
2. Geological Challenges of Mountain Bridges
Constructing bridges in granite-rich mountainous areas often presents unique geotechnical engineering challenges. A bridge project case study in a mountainous area of northern New South Wales, Australia, illustrates how uneven granite weathering layers, well-developed spheroidal weathering (isolated boulders), and intrusion of weak rock layers necessitated modifications to the original enlarged foundation design, requiring the replacement with pile foundations and micropile foundations. This was further reinforced with earthen retaining walls and rockfall protection netting to ensure the stability of the bridge approach roads and abutments.
III. Granite Crushed Material as Concrete Aggregate
Crushed granite is a high-quality artificial aggregate widely used in bridge concrete and asphalt mixtures. Its high strength and wear resistance significantly improve the durability of bridge deck paving and concrete structures. Mount Airy granite in North Carolina, USA, has even been used as a foundation material for iconic projects such as the Washington World War II Memorial and the Arlington Memorial Bridge.
Granite plays a dual role in bridge engineering: on the one hand, it serves directly as a high-performance natural stone for bridge decks, railings, crash barriers, and decorative structures; on the other hand, it serves as a hard bedrock, making it an ideal bearing layer for the pile foundations of cross-sea bridges and mountain bridges. With the advancement of processing technology, the application of granite components in bridge reinforcement and durability improvement is gradually expanding. However, their large self-weight and high processing cost also need to be comprehensively balanced in engineering design.