With the continuous advancement of transportation infrastructure construction, tunnel projects are increasingly common. As a crucial component of the tunnel structure, the quality of the secondary lining directly impacts the safe operation of the tunnel. Due to various influencing factors, the secondary lining is prone to different types of defects, such as cracks, water leakage, spalling, and voids. These defects not only affect the tunnel's appearance but also seriously threaten the structural safety and service life of the tunnel. This article introduces the types of these defects.

1. Cracks

(1) Temperature Cracks: Temperature variation inside the tunnel is a significant factor causing these cracks. During the day, temperatures inside the tunnel may rise due to external air temperature, operation of construction equipment, etc., and drop at night. The thermal expansion and contraction characteristic of concrete causes it to be repeatedly stretched and compressed during these temperature fluctuations. If the temperature variation range is large and frequent, the stress generated within the concrete exceeds its tensile strength, leading to temperature cracks.

(2) Shrinkage Cracks: During the hardening process of concrete, moisture gradually evaporates, and cement undergoes hydration, both leading to volume shrinkage. If this shrinkage is restrained by formwork, reinforcement, or previously set concrete, preventing free contraction, shrinkage cracks will form. In secondary lining construction, factors like the concrete mix proportion and pouring methods can influence the occurrence of shrinkage cracks. For instance, concrete with a high water-cement ratio tends to have more pores left after moisture evaporation and greater shrinkage, making it more susceptible to shrinkage cracks.

(3) Differential Settlement Cracks: Uneven settlement of the tunnel foundation subjects the secondary lining structure to uneven stress. The geological conditions where tunnels are located are often complex and variable, potentially including soft soil strata, fractured rock zones, and other unfavorable ground conditions with uneven bearing capacity. After tunnel completion, the foundation may undergo differential settlement. If the settlement difference is significant, the secondary lining structure experiences stress like twisting and stretching, resulting in cracks.

2. Construction Quality-Related Cracks

(1) Improper Concrete Mix Proportion: Incorrect ratios of cement, aggregate, sand, and water affect the strength and properties of concrete. For example, insufficient cement content results in inadequate concrete strength, making it prone to cracking under external loads; an excessively high water-cement ratio increases workability but reduces strength and increases shrinkage, also leading to cracks.

(2) Inadequate Compaction during Pouring: If vibration during concrete placement is insufficient, defects such as pores and voids remain inside the concrete. These defects reduce the integrity and strength of the concrete. Under external loads, cracks are likely to form at these weak points. Additionally, overly fast pouring speeds or unreasonable pouring sequences can also lead to inadequate compaction and subsequent cracking.

(3) Inadequate Curing: Concrete requires proper curing after placement to ensure normal strength development and volumetric stability. If the curing duration is insufficient or methods are improper (e.g., failing to maintain moisture by sprinkling water in hot, dry environments), surface moisture evaporates too quickly, causing rapid internal moisture loss and leading to shrinkage cracks.

3. Water Leakage

(1) Sidewall Leakage: The sidewall, where the secondary lining contacts the surrounding rock, is a common location for water leakage. Causes may include improper installation of the waterproofing membrane, such as insufficient overlap width, incomplete welding of seams, or insecure fastening, allowing groundwater to penetrate the secondary lining at connections or damaged areas. Additionally, inadequate concrete vibration compaction or existing cracks/defects in the sidewall area can provide pathways for water infiltration.

(2) Settlement Joint Leakage: Settlement joints are designed to accommodate deformations from foundation settlement, temperature changes, etc., during the tunnel's service life. Leakage at these joints can occur due to improper installation of waterstops, aging/damage of waterstops, or failure of joint sealants. Improper handling during construction, such as not adhering to design specifications or substandard workmanship, can also cause settlement joint leakage.

(3) Invert Construction Joint Leakage: The invert is the bottom structure of the tunnel, and its construction joints are prone to leakage. This is primarily because treating construction joints in the invert is challenging, and concrete placement is susceptible to inadequate compaction. Furthermore, groundwater pressure is often higher at the invert. If waterproofing measures at the construction joints are inadequate, groundwater will seep into the tunnel interior through these joints.

(4) Arch Crown Leakage: The arch crown is the top structure of the tunnel. Due to its elevated position, construction is difficult, and ensuring concrete placement quality is challenging. If voids, inadequate compaction, or other defects exist in the arch crown concrete, or if the waterproofing membrane is not laid smoothly or fixed securely in this area, leakage can occur. Additionally, significant surrounding rock pressure on the tunnel roof can squeeze the secondary lining, widening cracks and defects in the arch crown and exacerbating leakage.

4. Spalling

(1) Insufficient Concrete Strength: Concrete strength is a key indicator ensuring the stability and durability of the secondary lining structure. Factors like improper mix proportion, substandard cement quality, or uneven mixing can lead to insufficient strength. During service, the secondary lining bears loads from surrounding rock pressure, vehicle vibrations, etc. If the concrete strength is inadequate to withstand these loads, spalling occurs.

(2) Freeze-Thaw Cycles: In cold regions, the secondary lining is affected by freeze-thaw cycles. In winter, low temperatures cause water within the concrete to freeze and expand, damaging the concrete structure. When temperatures rise in spring, the ice melts, and water fills the pores again. Repeated freeze-thaw cycles accumulate damage, eventually causing surface spalling.

(3) Chemical Erosion: The tunnel environment may contain harmful substances like sulfates or chlorides. These can react chemically with components in the concrete, generating expansive products that create internal stress and damage the concrete structure. For example, sulfates react with calcium hydroxide in concrete to form calcium sulfate, which can combine with water to form gypsum. The volumetric expansion of gypsum causes cracks and spalling. Additionally, carbonation reduces the alkalinity of concrete, destroying the passive film on reinforcing steel and leading to corrosion, which subsequently causes spalling.

 5. Voids

(1) Inadequate Concrete Compaction: During concrete pouring, insufficient vibration time, inadequate vibrator power, or improper vibrator placement can lead to internal defects like pores and voids. Other factors affecting placement quality, such as poor concrete workability or excessive pouring speed, can also cause voids.

(2) Formwork Deformation: Formwork shapes the concrete during placement. If the formwork lacks sufficient stiffness, has inadequate support, or is subjected to external impact during construction, it can deform. Deformed formwork prevents concrete from fully filling the intended space, resulting in voids.

(3) Detachment ( voids behind lining): Detachment between the initial support and the secondary lining is a common cause of voids. Factors like an uneven initial support surface or uneven thickness of shotcrete can create gaps between the initial support and the secondary lining. If these gaps are not detected and addressed during secondary lining concrete placement, detachment occurs. Shrinkage of the secondary lining concrete during setting can also cause detachment from the initial support.

To Wrap up

In summary, the main types of defects in tunnel secondary linings manifest as cracks, water leakage, spalling, and voids. The occurrence of these defects often results from the combined effects of design, construction, materials, and complex external environments. These defects are not only interrelated and often mutually causal (e.g., cracks lead to leakage, leakage exacerbates spalling) but also severely weaken the structural load-bearing capacity and durability of the secondary lining, posing a direct threat to the long-term operational safety of the tunnel.

Therefore, fully recognizing and understanding the types and causes of these defects is the first step towards scientific maintenance, timely repair, and effective management of tunnels, and is also crucial for ensuring the long-term integrity of tunnel engineering.

Understanding the types of defects is the first step; accurately and promptly detecting these "hidden ailments" is key to ensuring tunnel safety. In the next article, we will detail the Detection Methods for Tunnel Secondary Lining Defects.