Pile foundation serves as the primary load-bearing component of structures, directly impacting their safety and longevity. However, being a concealed engineering aspect, the evaluation and judgment of its quality necessitate professional inspection methods.

  Pile foundation engineering encompasses various classifications, typically categorized by load-bearing capacity into friction piles, end-bearing piles, and combined friction-end bearing piles. Pile foundation inspection technology has evolved from the limited use of sonic echo methods in the late 1980s to comprehensive surveys employing low-strain, sonic echo, static loading tests, core drilling, high-strain testing, and other integrated approaches.

1. Low-strain Testing

1.1 Theory

The low-strain detection method involves striking the pile top with a small hammer and capturing stress wave signals from the pile through sensors attached to the top. Utilizing stress wave theory to study the dynamic response of the pile-soil system, the method involves inverse analysis of measured velocity and frequency signals to ascertain the integrity of the pile.

1.2 Testing Purpose

(1) Detecting defects and expansion positions in the pile body. Based on waveform characteristics, it is difficult to determine the severity of defects. There is little distinction in the reflected waves for various defects such as expansion, mud inclusion, concrete delamination, or pile breakage. To determine the severity of defects, we must be familiar with construction processes, construction records, geological reports, and common quality issues associated with certain types of piles. Additionally, personal engineering experience can provide rough estimations, but accurate determination requires validation through excavation or core drilling.

(2) Assessing categories of pile integrity. Integrity categories refer to the severity of defects, the proportion of defects in the pile cross-section and whether they affect the normal load-bearing capacity of the pile structure. Currently, the severity of defects can only be qualitatively assessed and cannot be quantitatively determined.

1.3 Applications :

(1) The low-strain detection method is applicable for assessing the integrity of concrete piles, including cast-in-place piles, precast piles, prestressed concrete piles, and cement fly ash gravel piles
(2)During the low-strain detection process, factors such as soil frictional resistance on the pile side, damping of pile materials, and changes in pile cross-sectional impedance affect the propagation of stress waves. As a result, the ability and amplitude of stress wave propagation gradually attenuate. Generally, before stress waves reach the pile bottom, their energy will completely dissipated, casingto the inability to detect reflected signals from the pile bottom and the inability to determine the integrity of the entire pile. Based on practical experience, it is advisable to limit the measured pile length to within 50m and the pile diameter to within 1.8m.

1.4 Advantage

High-stress testing is very easy and fast testing, as well as a cost-save testing, approximate cost is USD $10/pile

2. Crosshole Sonic Logger ( SCL)

2.1 Theory & Purpose

The Crosshole Sonic Logger method involves embedding several guiding rubes into the pile before concrete is casted. These tubes serve as channels for ultrasonic pulse emission and reception probes.Using an ultrasonic detector, sound parameters are measured point by point along the longitudinal axis of the pile as the ultrasonic pulses pass through each cross-section. Subsequently , these measured values are processed using specific numerical criteria or visual judgments. After process, the method provides information on defects within the pile and their locations, facilitating the determination of the pile’s integrity category.

2.2 Application :

Suitable for concrete piles with pre-embedded acoustic tubes

2.3 Advantages & Disadvantages

  The crosshole sonic logging method can assess the concrete quality of each cross-section along the whole length of the pile, detecting defects such as concrete delamination, mud inclusion, expansion, variations in compaction, and pile breakage. The results are more intuitive and reliable compared to the low-strain testing method. Additionally, it offers simpler on-site operation, faster detection speed, and is not limited by the length-diameter ratio or pile length.

 The disadvantage is that the tested pile requires pre-embedded guiding tubes, which increase the cost of the foundation. The cost of one meter of guiding tubes is approximately USD $2. Moreover, the testing cost of Crosshole Sonic Logger is higher than that of low-strain testing method, approximately $50/pile

3. Static Loading Test

3.1 Theory & Purpose

The static load test method for pile foundations involves applying a load to the pile top to understand the interaction between the pile and the soil during load application. Finally, by measuring the characteristics of the Q-S curve (such as the settlement curve), the construction quality of the pile is determined, and the bearing capacity of the pile is established.

3.2 Application :

(1) The static loading test method is applicable for evaluating the vertical compressive bearing capacity of individual piles.

(2) By employing the static loading test method, piles will be loaded to destructed, providing data on the bearing capacity of individual piles for design purposes.

3.3 Advantage & Disadvantage

The static loading test method for pile foundations primarily employs the slow-maintained load method. In bridge construction, due to the high bearing capacity of pile foundations and the harsh construction environment, the testing process is time-consuming and costly (approximately USD $5,500 to USD $7,000 per pile). Additionally, the associated supporting work is complex. Consequently, this method is less frequently utilized.

4. Core Sampling

4.1 Theory & Purpose

The core sampling method primarily involves the use of a drilling machine (typically with an inner diameter of 10mm) to extract core samples from the pile foundation. Based on the extracted core samples, clear assessments can be made regarding the length of the pile, concrete strength, thickness of sediment at the pile base, and the condition of the bearing stratum

4.2 Application :

The core sampling method is applicable for assessing parameters such as pile length, concrete strength, sediment thickness at the pile base, and the condition of the bearing stratum. It is commonly employed in the inspection of rock-socketed piles

4.3 Advantage & Disadvantage

The core sampling method offers a straightforward approach. It not only provides insights into the integrity of cast-in-place piles and determines the thickness of sediment at the pile base and the condition of the bearing stratum but also serves as the sole reliable method for assessing the concrete strength of cast-in-place piles. However, this method is limited by its single-hole perspective, and may not accurately assess localized defects or horizontal cracks in the pile foundation. Generally, it is used in conjunction with other testing methods. The cost of core sampling testing is proportional to the length of the pile, approximately 10,000 yuan per pile.

5. High-strain Testing

5.1 Theory & Purpose

The high-strain testing method is a technique used to assess the integrity of pile foundations and the vertical bearing capacity of individual piles. This method involves striking the pile top with a heavy hammer weighing at least 10% of the pile weight or 1% of the vertical bearing capacity of the pile. By analyzing and calculating the obtained dynamic coefficients according to specified procedures, parameters related to the pile integrity and vertical bearing capacity of individual piles are determined. This method is also known as the Case method or the Cap-wape method.

5.2 Application :

High strain testing is suitable for piles that need to be tested for integrity and reviewed for bearing capacity

5.3 Advantage & Disadvantage

The results of high-strain testing method combine those of low-strain testing and static load testing. The cost of high-strain testing is higher than that of low-strain testing but lower than that of static load testing. The accuracy of high-strain testing method in determining the bearing capacity of pile foundations is not as precise as static load testing, with a typical error margin of around 10%. In conclusion, as evident from the above analysis, various pile foundation testing techniques have their own theoretical assumptions and are influenced by various factors, thus exhibiting certain limitations.    

Therefore, it is necessary to fully utilize the strengths of each method to address practical engineering issues.

For pile foundations that do not meet requirements in the first three types of testing or for structures with relatively complex designs, such as important bridges (with a single span exceeding 25 meters, arch bridges, cable-stayed bridges, continuous beam bridges, suspension bridges, etc.), both high-strain and static load testing methods should be employed to assess the bearing capacity of the pile foundation. The advantages and disadvantages of these two methods are clear, and the choice between them can be made based on the specific circumstances and requirements.

6. Self-balancing Testing

6.1 Theory & Purpose

The self-balancing method, as the name suggests, is a static load testing method for piles where the reaction force is provided by the weight of the pile itself, without the need for external forces. This method involves embedding pressure units between piles and applying load with a hydraulic jack. By testing the bearing capacity of the upper and lower sections of the pile, the overall bearing capacity of the entire pile can be determined.

Different from traditional methods such as the static load method and the anchor pile method, the self-balancing method involves placing a load box, manufactured according to the pile bearing capacity requirements, at the bottom of the pile during construction. A pressure oil pipe and displacement measurement device are connected to the top of the pile. After the concrete has cured to the standard age, high-pressure oil is pumped into the load box at the bottom using a top-mounted high-pressure pump to determine the bearing capacity at the pile tip and the total side friction resistance of the pile.

The self-balancing method for pile testing is an indirect static load test method based on seeking the reaction force within the pile foundation. Its primary apparatus is a specially designed load box, which is connected to the reinforcement cage and placed at the bottom of the pile. During the test, pressure is applied to the interior of the load box from the pile top through a pressure pipe. As a result, the cover and bottom of the box are pushed apart, mobilizing the soil friction resistance and end resistance around the pile until failure occurs. The lateral soil friction resistance and end resistance of the pile foundation are combined to obtain the compressive bearing capacity of the individual pile.

6.2 Features

(a) The equipment is simple, requiring no space and avoiding the need to transport hundreds of tons of materials or cumbersome reaction frames. The test is safe and environmentally friendly, without pollution.

(b) By utilizing the lateral and end resistance of the pile foundation as mutual reaction forces, the lateral resistance and end resistance of the pile can be directly measured.

(c) The preparation work for the test pile is time and labor-saving, and the testing costs are relatively low.

(d) After testing, the pile foundation can still be used for engineering purposes, with the option to grout the bottom of the pile using pressure pipes if necessary.

(e) This method demonstrates superiority in locations with limited space, such as testing piles on water, slopes, or at the bottom of excavations.