Pros and Cons of Prefabricated vs. Cast-in-Place Piles: A Comparative Analysis

With the rapid development of the social economy, large and super-large buildings have been emerging across China like mushrooms after rain. Consequently, the basic requirements for buildings have become increasingly stringent, and the use of pile foundations has become more widespread. This is especially true for soft soil foundations, such as those found in Lianyungang, where buildings with two or more floors often require pile foundations. Pile foundations are primarily categorized into two types: cast-in-place piles and prefabricated piles.

Cast-in-Place Piles
These piles are formed by drilling mechanically or manually excavating at the pile location, then placing steel reinforcement and pouring concrete into the hole. Depending on the drilling method, cast-in-place piles can be classified into drilled cast-in-place piles, immersed cast-in-place piles, dry operation drilled piles, and manually excavated cast-in-place piles.

Compared to prefabricated piles, cast-in-place piles offer several advantages:

• Not limited by geological variations
• No need for pile connection or cutting
• Strong adaptability
• Relatively stable stress distribution
• Excellent resistance to compression and pull-out forces
• Low vibration and noise during construction

Because they do not exert negative soil squeezing effects and can penetrate various hard interlayers and rock formations to reach firm bearing layers, cast-in-place piles have flexible geometric dimensions and adjustable bearing capacities. This makes them highly suitable for a wide range of applications, especially high-rise buildings.

However, there are some drawbacks to cast-in-place piles:

• Higher cost
• Complex construction process
• Longer construction period
• Possible interruptions between foundation and superstructure construction
• Difficulty in cleaning sediment (loose soil) at the bottom of the pile, which limits bearing capacity and engineering stability

To address the sediment issue, pressure grouting technology for the bottom of drilled piles was developed in the late 1990s. This technology generally includes four methods:

1. Open Grouting: After drilling, grout and overflow pipes are embedded. Concrete is poured into the pile body, and cement slurry is injected directly into the soil at the bottom of the pile. The slurry mixes and solidifies with the sediment and surrounding soil, forming a high-strength composite.
2. Closed Grouting: Following drilling and pipe embedding, cement slurry is injected into a pre-made elastic cavity at the pile bottom. With increasing pressure and slurry volume, the cavity expands, creating a high-strength joint within the soil layer beneath the pile.
3. Post-Excavation Grouting: After manual excavation and conduit embedding, concrete is poured into the pile body. Then, a drilling machine is used to drill along the conduit into the soil at the pile end for grouting. This method also repairs defects in existing pile foundations.
4. Immediate Grouting and Vibration: Grouting and vibration are performed immediately after drilling, followed by concrete pouring for the pile body.

The first three methods are widely applied and highly effective. In recent years, engineering safety accidents related to cast-in-place pile construction quality have been almost nonexistent.

Prefabricated Piles
Prefabricated piles are produced in factories and mainly come in two types: steel piles and concrete piles. Common varieties include concrete solid square piles, concrete hollow square piles, and prestressed concrete hollow pipe piles. Steel piles typically consist of steel pipe piles and H-pipe piles.

Compared to cast-in-place piles, prefabricated piles offer:

• Lower production costs
• Lower reinforcement ratios, saving steel
• Hollow piles that are environmentally friendly
• Smaller diameters with larger specific surface areas
• Higher bearing capacity per cubic meter of concrete
• Ease of construction with relatively low technical difficulty
• Shorter construction periods
• Capability for continuous construction
• Positive effects in loose soil and unsaturated fill, helping densification and improving bearing capacity

However, these piles also have disadvantages:

• In saturated cohesive soils, the soil squeezing effect can be negative, causing issues like pile breakage and necking in cast-in-place piles.
• For precast concrete and steel piles, soil squeezing can cause the BIM-designed pile body to shift, reducing bearing capacity and increasing settlement.
• The squeezing effect may damage surrounding buildings and municipal infrastructure.
• Prefabricated piles cannot resist horizontal loads and can only serve as anti-pull piles if the prestressed hinge line or filling core strength is sufficient.
• Production of solid square prefabricated piles is limited geographically; for example, they are not widely used around Lianyungang Port due to high costs, regional restrictions, transportation, and construction challenges.

While the advantages of prefabricated piles make them popular engineering choices, accidents related to their shortcomings occur frequently. For instance, the building collapse incident in Shanghai was caused by poor shear resistance of pipe piles, which are unsuitable for resisting large horizontal loads. Soil pressure differences on either side of the building led to shear failure of the foundation piles. Despite increased awareness of pipe pile safety, especially in soft soil areas, similar accidents continue to happen in nearby buildings.

One example is a commercial and residential building designed in 2010 in Ganyu County, Lianyungang City, with a construction area of 36,000 square meters. The three main buildings have 18 floors above ground and are connected to a 4-story podium structure. The B1 pile foundation used two high-strength prestressed pipe piles with a diameter of 500mm and an effective length of 22 meters, totaling 574 piles. After construction, inspection revealed 381 piles classified as Class IV, with most piles failing quality standards. Camera probe inspections showed that most damage occurred at the weld seams between upper and lower pile sections, as illustrated in the following image.

The root causes identified were:

• The pile foundation contractor did not perform full welding as specified, instead opting for spot welding or no welding, resulting in extremely weak pile joints.
• The soil squeezing effect on precast concrete piles in saturated soil caused pile flotation, which, combined with uneven gripping forces from surrounding soil and weld seam fragility, led to this safety issue.

Although corrective measures were eventually implemented, the incident resulted in significant financial costs, manpower losses, and project delays.

Another case occurred in 2009 in Xinpu District, Lianyungang City. A development company built an office building with a total area of 31,000 square meters, 24 floors, and a height of 91 meters. The pile foundation used two high-strength prestressed pipe piles with a diameter of 550mm, an average effective length of 28 meters, and a total of 224 piles. Square full-span piles were installed beneath the raft slab with a pile spacing of 2.2 meters (4D). Due to the compressive strength of the pile concrete and building load, the specified minimum pile spacing of 4.5D was not achievable, so piles were arranged densely.

Because of tight construction schedules and the construction team’s limited experience with high-rise pile installation, the maximum observed pile uplift after driving was 18 cm, commonly around 10 cm. Repeated hammering had little effect. Testing showed no detachment between pile sections and sufficient bearing capacity, but residual safety risks remained. Over time, experience has led to improved construction practices such as controlling pile driving speed, driving from the middle towards both ends, skipping impacts, and drilling stress relief holes—all of which help reduce pile uplift. Nowadays, for buildings around 100 meters tall (precast piles are no longer permitted for taller structures), pile uplift is typically controlled below 5 cm.

The choice between cast-in-place and prefabricated piles primarily depends on construction requirements and the specific characteristics of each pile type. It is important to understand that the limitations of prefabricated concrete hollow piles, combined with insufficient experience or construction quality issues, can lead to serious quality and safety hazards. Therefore, special attention should be given to the selection and construction of prefabricated concrete hollow piles.

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