Penetration mechanism of CPT probe in the dense sand assisted by ultrasonic vibration

2.1. The design of the ultrasonic CPT probe

The structure of the ultrasonic CPT probe is shown in Fig. 2. The external dimensions of this probe are identical to those of the conventional dual-bridge CPT probe, with a total length of 475 mm and a diameter of 43.7 mm. The cross-sectional area and the cone angle of the tip are 15 cm² and 60°, The outer diameter of the thread is 39 mm. The ultrasonic vibration unit is integrated within the CPT probe. The ultrasonic CPT probe consists of the cone tip, extension sleeve, ultrasonic transducer, ultrasonic generator, friction sleeve, metal rod and a sensor for measuring the total penetration resistance. The extension sleeve is a 110-mm-long stainless steel tube that connects the cone tip to the main body of the probe, with the ultrasonic transducer installed inside the extension sleeve. The ultrasonic transducer consists of front and rear cover plates, four PZT-8-type piezoelectric ceramics with an outer diameter of 25 mm, electrode slices, wires, and bolts. When powered, the ultrasonic generator rectifies and filters the 220 V (50/60 Hz) AC supply to 310 V DC, which is then converted into a specific high-frequency AC drive signal. The high-frequency signal is amplified to several kilovolts by the high-frequency transformer to drive the ultrasonic transducer. Under high-voltage, high-frequency excitation, the piezoelectric ceramic discs deform via the inverse piezoelectric effect, electrical energy is converted into mechanical vibrational energy. The transducer transmits the ultra-high frequency axial vibrations to the cone tip, thereby intensifying soil agitation around the tip during penetration, reducing the penetration resistance. The ultrasonic generator is equipped with a control module that tunes the operating frequency and power for penetration. To prevent the ultrasonic wave transmit to the probe body, a nylon threaded was installed at the outer layer of the cone tip, which concentrates the energy at the cone tip. Thereby minimizing measurement interference and improving the accuracy of the force sensor [40, 41].

Fig. 2Structure of the ultrasonic CPT probe. The photos were taken by Zhehong Shen at Shaoxing University on October 23, 2024

To measure the total penetration resistance, a sensor is mounted at the top of the probe. The sensor has a 50 kN capacity with an accuracy of ±0.5 %. The sensor operates based on resistive strain-gauge force measurement. When an axial load is applied to the load-bearing member in the sensor, the electric resistance is changed. According to the relationship between axial strain (Eq. 1), the strain is obtained. Subsequently, according to Eqs. (2-3), the penetration resistance is measured:

where $R$ is the nominal resistance, $∆R$ is the change in electric resistance, $GF$ represents the gauge factor. $A$ is the effective area of load-bearing member, $E$ is Young’s modulus of the load-bearing member. $\epsilon$ and $\sigma$ are strain and stress, respectively. $F$ is the total penetration resistance. To convert the small resistance variation into a measurable electrical signal, the gauges are wired in a Wheatstone bridge. With an excitation voltage $V_{ex}$, the bridge output voltage $V_o$ can be approximated for small strains as:

where $K_b$ is a configuration-dependent coefficient (for a full-bridge configuration, $K_b\approx$1). Substituting Eq. (3) yields the force-voltage relation:

where, $C_F$ is the conversion coefficient provided by the manufacturer through static-load calibration, which is used to convert the output voltage of the acquisition system (after signal conditioning/amplification and data acquisition) into the total penetration load $F$.

Due to the presence of the cone tip, the vertical penetration load is resolved on the cone surface into a normal component and a frictional component; the cone surface is subjected to the normal stress ${\sigma }_n$ and friction characterized by the cone-soil friction coefficient $\mu$, as shown in Fig. 3. The probe shaft is subjected to the horizontal normal stress ${\sigma }_h$ and the corresponding shaft friction $f_s$. Under simplified assumptions (Coulomb friction and an equivalent mean stress acting on the contact surface), the force equilibrium together with the probe geometry yields the following relationship between the cone resistance $q_c$ and the normal stress on the cone surface:

Considering an effective contact area at the cone tip (taken as the circular area with probe diameter $D$), the total cone-tip resistance is:

The unit shaft friction can be expressed in a Coulomb form as:

Fig. 3Forces acting on the probe

Accordingly, the total penetration load $F$ acting on the probe can be expressed as the sum of the cone-tip and shaft contributions:

It should be noted that the force sensor in this study directly measures the total penetration load $F$ (in N), which is used to evaluate the resistance-reduction effect of ultrasonic vibration.

The operating procedure of ultrasonic CPT is similar to that of the conventional CPT. During the test, the ultrasonic CPT probe is connected to the ultrasonic generator, and the CPT probe is steadily advanced into the soil with a rate of (20±5) mm/s. Total penetration load are monitored by sensors. When hard soil layers are encountered, the ultrasonic generator is activated to induce high-frequency vibrations at the cone tip. Depending on the geological conditions, parameters such as ultrasonic frequency, amplitude, and power can be adjusted accordingly to optimize penetration performance.

2.2. Ultrasonic CPT in dense sandy soils

2.2.2. Experimental equipment

In this study, a series of ultrasonic CPTs were conducted to examine the effects of sand relative density and ultrasonic vibration on the penetration load of the ultrasonic probe and particle-breakage mechanism in sand. The experimental setup comprises an ultrasonic CPT probe, a model box, a loading system and load-monitoring system, as illustrated in Fig. 5. The model box is 40 cm in diameter and 50 cm in height. The loading system includes a reaction frame and a hydraulic jack with a maximum stroke of 600 mm. The hydraulic jack applies normal force to the ultrasonic CPT probe. The load-monitoring system consists of pressure sensors, a computer, and data-acquisition software.

Fig. 4The particle distribution curve of the river sand

Fig. 5Schematic diagram of the experimental setup

2.2.3. Experimental process

The ultrasonic CPT procedure for dense sandy was as follows:

(1) The sand was compacted in 10 layers to the target relative density until the total height reached 400 mm. After that, the sample was then allowed to stand for 12 hours to achieve a uniform stress distribution in the sand (Fig. 6(a)).

(2) The ultrasonic CPT probe, ultrasonic generator, and computer were connected. The sensor sampling frequency was set to 10 Hz, and the ultrasonic frequency was set to 30.71 kHz before loading.

(3) After the preparation, the ultrasonic CPT probe was advanced into the sand by the hydraulic jack at a rate of 2 mm/s. At the same time, the ultrasonic generator and load-acquisition software were activated simultaneously. The sum of the cone resistance and sleeve friction was recorded. When the penetration depth reaches 160 mm, the test terminated, and the ultrasonic generator and load-monitoring system were switched off (Fig. 6(b)).

(4) After penetration, sand samples surrounding the ultrasonic CPT probe were collected (Fig. 6(c)). As shown in Fig. 7, regions around the cone tip were defined at distances of 85-125 mm (A), 45‐85 mm (B), 0‐45 mm (C), –20‐0 mm (D), and –20 to –40 mm (E) from the tip. The sand in A, B, C, D, and E were collected using ring cutters with a diameter of 61.8 mm. To facilitate sampling, the sand was moistened before the penetration of the ring cutters. The collected samples from each region were then placed in an oven at 70 °C and dried for 24 hours until completely dry.

(5) Sieving tests were carried out on the sand samples. The microscopic of adhering to the probe sidewall in regions A, B, and C was observed under a stereomicroscope (Figs. 6(d-e)).

Fig. 6Schematic of ultrasonic CPT in sand

a) Layered compaction

b) Penetration

c) Sampling the sand around the ultrasonic CPT probe

d) Sieving tests

e) observation of the microstructure of sand grains

Fig. 7Schematic diagram of excavation regions

3.1. Penetration load under different relative densities

The variations of the penetration load with penetration depth for different relative densities are shown in Fig. 8(a), All curves exhibit a similar pattern trend across different relative densities, and can be divided into a linear stage and nonlinear stage. As $D_r$ increases, the penetration load increases gradually, with a greater rate of increase at higher $D_r$. Taking a depth of 140 mm as an example, Fig. 8(b) plots penetration load versus $D_r$. When $D_r=$ 30 %, 45 %, 60 %, 75 %, and 90 %, the corresponding penetration loads were 215.6 N, 519.4 N, 1372 N, 2508.88 N, and 3635.8 N, respectively.

The results indicate that the penetration load increases with the increasing $D_r$. This is because both the internal friction angle and the shear strength of sand increase with relative density [42, 43], and higher $D_r$ strengthens the soil’s force-chain network, thereby increasing the penetration load. At the initial stage of loading, the penetration load increases linearly with depth, and the sand particles around the probe are compressed [31, 44]. Sandy with low relative density has higher compressibility, providing more space for particle rearrangement and reorganization during penetration, resulting in a longer linear stage. In contrast, sandy with high relative density has lower compressibility, and more tightly packed particles, making particle motion difficult and shortening the duration of the linear stage. As the penetration depth increases, the penetration load rises nonlinearly, with a gradually increasing slope. During this stage, the shear load applied by the probe causes plastic deformation and particle fragmentation in the surrounding sand [30]. According to Coulomb’s law, the shear strength of sand is proportional to the normal stress on the shear plane. Sliding friction between particles and the interlocking of rough surfaces generate frictional resistance, which increases with $D_r$. As $D_r$ increases, soil porosity decreases, the contact area between the soil and the probe increases, and the particles become more tightly packed. This increases the normal stress between particles and the internal friction angle, thereby enhancing the shear strength of the soil. Therefore, a greater penetration load is required to penetrate sand at high $D_r$. Additionally, fluctuations are observed in the penetration curves for $D_r=$60 %, 75 %, and 90 %, which can be attributed to the layered artificial compaction of the sand. At higher $D_r$, compaction is more challenging, make it difficult to ensure uniformity.

Fig. 8Penetration load acting on the CPT probe at different relative densities

a) Penetration load- depth curve

b) Relative density-penetration load curve

3.2. Penetration load under static load and ultrasonic vibration

The variations of the total penetration load with penetration depth for both the CPT and ultrasonic CPT probes in the sand at different relative densities are shown in Fig. 9. From the figure, it can be observed that the trend of the total penetration load under ultrasonic vibration is similar to that of the static CPT, the total penetration load increases with increasing the penetration depth. Furthermore, as shown in Fig. 10(a), at the same penetration depth, a higher relative density corresponds to a greater total penetration load. Additionally, under the ultrasonic vibration, the total penetration load is significantly reduced. However, as shown in Fig. 10(b), the reduction in penetration load decreases as the relative density of the sand increases. At a penetration depth of 140 mm, the total penetration load for ultrasonic CPT is reduced by 60.0 % for $D_r=$ 30 %, 41.0 % for $D_r=$ 45 %, 47.0 % for $D_r=$ 60 %, 24.0 % for $D_r=$ 75%, and 18.0 % for $D_r=$ 90 %.

Fig. 9Curve of penetration load with depth for ultrasonic vibration probe and conventional probe

a)$D_r=$ 30 %

b)$D_r=$ 45 %

c)$D_r=$ 60 %

d)$D_r=$ 75 %

e)$D_r=$ 90 %

The experimental results indicate that ultrasonic vibration effectively reduces penetration load, thereby enhancing the penetration capability of the CPT. However, compared to loose sand, the reduction in penetration resistance is smaller in high relative density sands. In both penetration modes, the probe is subjected to normal and shear stress, causing compression and shear deformation of the surrounding sand. Gong [39] utilized the DEM to clarify that under vibration, the contact normal anisotropy, normal contact force anisotropy, and tangential contact force anisotropy parameters of granular soil all decrease, leading to a reduction in the shear strength of the sand. During the vibratory penetration process, the high-frequency vibration of the cone tip causes particles in the contact area to rearrange, reducing the normal contact force between particles and the frictional force at the probe interface. The sand particles undergo fluidization [40-42], and the force chains between sand particles break [11], resulting in a local decrease in stiffness [43]. Therefore, the penetration load under vibratory conditions was observed to be lower than that under conventional penetration conditions.

Fig. 10Comparison of the penetration load of acting on the CPT probe and ultrasonic CPT probe at penetration depth of 140 mm

a) Penetration load-relative density curve

b) Reduction in the penetration load-relative density curve

Research has found that the degree of shear strength attenuation is significantly affected by the sample density. As the sample density increases, the weakening effect of vibration on the strength of sandy soil diminishes [44]. Denies [12] suggested that vibration causes particle collisions, generating a “shaking pressure” effect, which macroscopically resembles excess pore-water pressure in saturated soils. This effect reduces the internal friction angle of the sand and weakens the effective normal stress. In loose sand, the friction between the soil and the probe is lower, and the larger pores allow particles around the cone tip to move and rotate. During the penetration process, soil particles flow around the probe and collide, resulting in fragmentation. This combined effect significantly reduces the penetration load. In the penetration of high relative density sand, the friction between the soil and the probe is greater, and the influence of the cone tip vibration is limited to a smaller area. The smaller pore space and tighter particle contact restrict particle movement and rotation, increasing the strength of the force chains between particles [45-47]. This suppresses the strength attenuation caused by vibration, leading to limited particle fragmentation. As a result, the reduction in penetration load is less compared to that in low relative density sand.

4.1. Particle breakage of the sand under different relative densities

4.1.1. Particle breakage of the sand adhered to the ultrasonic CPT probe tip

Figs. 11(a-e) show the microscopic structure of the sand grains adhered to the surface of the ultrasonic CPT cone tip at different relative densities. Based on the experimental results, approximately 100 particles were measured for each relative density using ImageJ, from which the mean particle size and the standard deviation were obtained (Fig. 11(f)). At $D_r=$ 30 %, both broken and unbroken grains are present at the tip. Beneath the larger unbroken grains, numerous small broken grains (0.1-0.25 mm) are closely attached to the cone surface. The broken grains exhibit sharp edges (Fig. 11(a)), with an average particle size of 331.8±41.6 μm. At $D_r=$ 45 %, the overall characteristics of the sand particles are similar to those observed at $D_r=$ 30 %, with broken and unbroken particles coexisting, but the average particle size decreases to 200.3±20.3 μm (Figs. 11(b, f)). For $D_r=$60 %-90 %, particle size of the sand decreases significantly, and the grains appear more extensively crushed (Figs. 11(c-e)), with mean sizes of 71.8±11.1 μm ($D_r=$ 60 %), 64.6±14.8 μm ($D_r=$75 %), and 39.7±4.4 μm ($D_r=$ 90 %). Apart from a few larger grains, most particles have been crushed into fine powder, indicating a more uniform degree of breakage.

The results indicate that dense sand facilitates particle breakage around the cone tip. As sand relative density increases, the contact area between the sand and the CPT probe increases, Ultrasonic vibration effectively crushes the sand surrounding the probe, thereby enhancing the penetration capability of the CPT probe in sand. XRD analysis revealed that the sand consists primarily of quartz, feldspar, and mica. Contacts between the mineral particles tight packing (frictional contacts) and cemented bonds. In dense sand, particles are packed more tightly than in loose sand. Under ultrasonic vibration loading, stress is transmitted through the cementing materials and particles, leading to local stress concentrations. When the stress exceeds the tensile or shear strength of the cementing bonds, particles are broken. Additionally, repeated ultrasonic loading induces fatigue damage in the cementing bonds, ultimately increasing particle breakage (Fig. 12).

Fig. 12Breakage of sand grains under ultrasonic vibration

a) Loose sand

b) Dense sand

4.1.2. Sand particle breakage along the ultrasonic CPT probe

Under the combined effects of the penetration of CPT probe and ultrasonic wave, the sands surrounding the probe are broken. Currently, the breakage index $B_g$ is commonly used to define the degree of particle breakage. This index was first introduced by Marsal [48]:

11

$B_g={\sum }_{i=1}^n|a_i-b_i|,$

where $a_i$ and $b_i$ are the mass percentage of particles in the size range $i$ before and after the breakage, respectively, $n$ is the total number of size ranges for a particle sample. $\sum$stands for adding up the mass percentage from the largest particle to the smallest one.

After sieving the sand samples from different regions, the breakage index ($B_g$) was computed for regions along the ultrasonic CPT probe. Fig. 13(a) illustrates the variation of $B_g$ with the mean depth of regions A-E at different realtive densities. The results indicate that $B_g$ generally increases with depth, with the highest value occurring in region D, adjacent to the cone tip. Additionally, the average breakage index for regions A-E was calculated at different relative densities. As shown in Fig. 13(b), the average breakage index generally decreases as relative density increases.

Fig. 13Particle breakage rate of sandy soil

a) Breakage indexes of sandy soil at different depths

b) Average breakage indexes of sandy soil adjacent to the cone tip under different relative densities

Fig. 14The microstructure of sand grains along the ultrasonic CPT probe (Dr= 75 %). There photos were taken by Zhehong Shen at Shaoxing University on November 8, 2024

a) Region A

b) Region B

c) Region C

d) Region D

Since particle fragmentation primarily occurs near the CPT probe, we further analyzed its characteristics by examining the microstructure of sand grains adhering to regions A-C and the cone tip with a stereomicroscope after the penetration test. As shown in Fig. 14, using sand at $D_r=$75 % as an example, in region A, unbroken sand grains with diameters of 0.2-0.5 mm were adhered to the probe surface (Fig. 14(a)). In region B, the adhering particles are generally smaller than those in region A, and broken sand grains with diameters of 0.074-0.25 mm were observed (Fig. 14(b)). In region C, numerous broken sand grains with diameters of 0.074-0.25 mm were observed, indicating significant breakage (Fig. 14(c)). At the cone tip, abundant sand powder was observed, exhibiting the smallest average particle size (Fig. 14(d)). Therefore, breakage is concentrated at the cone tip owing to the intense ultrasonic vibration.

4.2. Particle breakage characteristics under ultrasonic-static coupling

Similar to section 4.1.2, the breakage indexes $B_g$ for zones A, B, C, D, and E under CPT and ultrasonic CPT were calculated by Eq. (11), with the results shown in Figs. 15(a-e). Additionally, the average breakage indexes for zones A, B, C, D, and E for sand with different relative densities are presented in Fig. 14(f). Overall, under ultrasonic vibration, $B_g$ in zones A, B, C, D, and E gradually increases, which is consistent with the microstructures of the sand adhering to the CPT probe. However, the $B_g$ for ultrasonic CPT and CPT differ. Specifically, when $D_r=$ 30 % and 45 %, the trends of $B_g$ under ultrasonic CPT are similar to those under CPT, $B_g$ increases with depth and reaching its peak in region D. Additionally, $B_g$ under ultrasonic CPT is generally higher than that under CPT (Figs. 15(a-b)). However, when $D_r=$ 45 %, the $B_g$ in zone D (beneath the cone tip) is slightly lower under ultrasonic CPT than under traditional CPT. When $D_r=$ 60 %, $B_g$ in zones D and E (beneath the cone tip) under ultrasonic CPT is slightly lower than that under CPT (Fig. 15(c)). When $D_r=$ 75 % and 90 %, $B_g$ under ultrasonic CPT is significantly lower than that under CPT (Figs. 15(d-e)). As shown in Fig. 15 (f), when $D_r=$ 30 % and 45 %, the average $B_g$ in regions A, B, C, D, and E under ultrasonic CPT are higher than those under CPT. In contrast, when $D_r=$ 60 %, 75 % and 90 %, the average $B_g$ under ultrasonic CPT are lower than those under CPT.

5.1. Particle breakage mechanism under the ultrasonic-static coupling

According to the experimental results, particle breakage mechanism under the ultrasonic-static coupling are illustrated in Fig. 16.

(1) Ultrahigh-frequency impacts between particles and the cone tip. The large voids between particles allow high-frequency vibrations of the sand under the ultrasonic excitation, causing particle movement and rotation (Fig. 16(a)). During motion, sand particles collide with the cone tip at high frequency, promoting particle breakage. The maximum acceleration of the vibrating cone tip is given by: $a=A{\left(2\pi f\right)}^2$. Substituting $A=$1 μm and $f=$30.71 kHz, the calculated maximum acceleration $a=$3.7×10⁴ m/s², which is far greater than gravitational acceleration $g$. Under the influence of high acceleration, sand particles around the cone tip experience ultrahigh-frequency impacts, leading to localized breakage. The breakage increases sand compressibility, resulting in a reduction in penetration load.

Fig. 15Comparison of breakage index between CPT and ultrasonic CPT

a)$D_r=$ 30 %

a)$D_r=$ 45 %

c)$D_r=$ 60 %

d)$D_r=$ 75 %

e)$D_r=$ 90 %

f) Average breakage index

Particle breakage caused by ultrasonic vibration can be attributed to: (a) Stress-wave propagation: High-frequency stress waves generated by ultrasonic vibration propagate through the sand, creating localized high-pressure and low-stress zones between particles. The rapid variation in local stress weakens the frictional and cohesive forces between sand particles, thereby further promoting particle breakage. (b) Resonance effect: When the ultrasonic vibration frequency aligns with the sand’s resonance frequency of the sand, the internal vibration amplitude increases significantly, further reducing frictional forces between particles, making them more susceptible to breakage. (c) Fatigue-damage accumulation: Prolonged ultrasonic vibration induces fatigue damage in sandy soil. Under repeated vibrational loading, microcracks and pores within the sand particles gradually expand, eventually leading to structural failure of the soil. This cumulative fatigue damage process makes the sand particles more prone to breakage.

(2) Reduction in local sandy density near the cone tip due to ultrahigh-frequency vibration. Assuming the penetration velocity $v_0$ of the CPT probe. Under the vibration, the velocity of the probe varies between ($v_0-2\pi fA$, $v_0+2\pi fA$). Given the ultrasonic vibration frequency $f=$ 30.71 kHz and the amplitude $A=$1 μm, the maximum vibration velocity of the cone tip can be calculated, $v=2\pi fA=$ 0.19 m/s. When the penetration velocity $v_0\le 2\pi fA$), the relative velocity range between the cone tip and the surrounding sand falls within ($v_0-2\pi fA$, 0), indicating periodic separation between the cone tip and the sand. This separation, along with particle movement and rotation, results in a decrease in local sand density. Since the experimental penetration velocity satisfies the separation condition ($v_0\le$ 0.19 m/s), the penetration load is reduced.

(3) Vertical stress and shear stresses from the CPT probe cause sand-particle breakage. In sand with medium to high relative density, the voids between particles are smaller, and particles are difficult to move and rotate under ultrasonic vibration. Consequently, high-frequency collisions between sand particles and the cone tip are suppressed. In high-density sand, the propagation range of ultrasonic waves is reduced, meaning that the outer soil layers are less affected by ultrasonic vibrations, leading to a lower breakage rate. At this stage, the primary effect of ultrasonic vibration is to loosen the compacted sand surrounding the probe (Fig. 16(b)).

Fig. 16Schematic diagram of the interaction mechanism between ultrasonic CPT probe

a) Loose sand

b) Dense sand

5.2. Interaction between probe and sand under ultrasonic vibration

During penetration, the stress state of the sand surrounding the probe varies. According to observations by Yang et al. [24], [54], on particles in the cone-tip region, the area can be divided into three zones: Zone I (high-normal-stress-shear coexisting zone), Zone II (high-normal-stress zone) and Zone III (low-normal-stress zone) (Fig. 17(a)). Tovar-Valencia [54] found that fragmentation mainly occurs in Zones I and II during penetration. In Zone I, the sand is simultaneously subjected to both normal and shear stresses. Due to the relative movement of the surrounding sand, a shear band forms around the cone tip, within which significant fragmentation occurs [24. 54]. In Zone II, the normal stress is lower than in Zone I, and because sidewall friction is reduced, the sand in this zone is scarcely affected by shear stress. Previous studies have shown that particles are more prone to fragmentation under shear than under compression [55-57], consequently, the fragmentation rate in Zone II is lower than. In Region III, the normal stress further decreases, and it is primarily composed of unbroken sand and debris, which are migrated from Regions I and II through particle migration [54].

However, the stress state of the sand surrounding the ultrasonic CPT probe differs, and the can be further subdivided into: Zone I-1 (high normal stress-shear-vibration zone), Zone I-2 (high-normal-stress-shear zone), Zone II-1 (high-normal-stress-vibration zone), Zone II-2 (high-normal-stress zone) and Zone III (low-normal-stress zone) (Fig. 17(b)). In Zone I-1, sand beneath the cone tip experiences high normal and shear stress but also subjected to high-frequency ultrasonic vibration induced, leading to further breakage. Therefore, this zone is defined as the high-normal-stress-shear-vibration zone and corresponds to experimental regions C and D, where the overall sand breakage rate is relatively high. In Zone I-2, the stresses are similar to those in Zone I around the conventional CPT probe, sand is subjected to high normal and shear stress, but ultrasonic vibration is negligible. In Zone II-1, the sand is affected by both high normal stress and ultrasonic vibration, while the shear stress is negligible. The sand in Zone II-2 is subjected to high normal stress. Due to the limited propagation of ultrasonic waves, the stress conditions in Zone III are similar to those around a conventional CPT probe, normal stress is low and ultrasonic vibration is negligible.

Fig. 17The stress around the CPT probe during the penetration

a) Conventional CPT probe

b) Ultrasonic CPT probe