Introduction

Deep mixing (DM) method has been widely used for improving soft soil1, which can notably increase the bearing capacity2,3, mitigate both ground total and differential settlement4, and improve slope stability of soft soil5. With expansion of application fields, this method has been extensively applied in various projects, including buildings2,6, roadways7,8,9,10,11, ports12,13,14,15 and airports16,17,18. In order to meet the demands of various application fields, there has been significant development in DM equipment, including single-axis DM equipment19, dual-axis DM equipment20,21 and triple-axis DM equipment22,23, as illustrated in Fig. 1. Table 1 presents the principal technical parameters for these three types of DM methods. The single-axis DM equipment utilizes a single drilling rod that mixes the surrounding with and injects binder slurry at its base. The dual-axis DM equipment involves two drilling rods24. The dual drilling rods are used to mix the soil. Outlets of binder slurry are installed on the shafts at different levels close to the mixing blades, so that the outlets are not blocked by the soil. The triple-axis DM equipment comprises three drilling rods to produce three round piles partially overlapped each other. In this method, binder slurry is injected only from two outer rods, and compressed air is injected from the middle rod.

Fig. 1
Fig. 1
Full size image

Conventional deep mixing equipment.

Table 1 Principal technical parameters of DM equipment.
Full size table

The widespread application of engineering practices has revealed limitations in the current DM equipment, particularly with regard to construction efficiency and quality control31,32. For single-axis and dual-axis DM equipment, the limited number of drilling rods and mixing blades (as illustrated in Fig. 1) restricts the mixing efficiency and homogeneity, leading to relatively low strength development. Laboratory tests have demonstrated that increasing the number of auger blades (e.g., from 4 to 6) significantly enhances the unconfined compressive strength and modulus of elasticity of the stabilized soil33. In terms of construction efficiency, the production rate for land-based DM machinery typically ranges from 100 to 150 m³ per day per machine34, which is considerably lower than that of multi-shaft systems used in marine applications (exceeding 1000 m³ per day). The triple-axis DM equipment, as shown in Fig. 1, has the binder slurry injection outlets equipped only to the two outer shafts, and soil binder mixture produced at outer mixing shafts should flow into the central shaft, leading to the nonuniform quality of the central pile from the two outer piles1. In addition, field observations have indicated that the installation procedure of DM piles by triple-axis DM equipment significantly impacts the adjacent environment, leading to increases in lateral displacement and ground heaving35. The DM pile installation procedure includes penetrating, mixing and injecting, exerting expansion and shear forces on the surrounding soils36. Subsequently a plastic zone and high pore water pressures are formed in the soil surrounding the mixed pile, leading to ground displacements in construction. With the increasing demand for urban underground space development in recent years, the construction technology of DM piles has been advancing towards deeper and more digitized methods. Furthermore, an increasing number of construction activities are being undertaken in close proximity to existing infrastructures, such as subway tunnels and bridge foundations. To mitigate the impact of construction process on the adjacent existing structure, the displacement control is strict. For instance, in soft soil areas where a construction is proceeding adjacent to subway tunnels, the lateral displacement control standard is often strictly limited to 5 mm37. For bridge foundations, the allowable horizontal displacement may be restricted to as low as 4 mm38. Moreover, the installation of soil-cement columns itself can induce considerable lateral displacements in surrounding soils, as reported by Chai and Carter39. Consequently, the displacement control during DM pile construction is becoming increasingly stringent. In addressing this challenge, conventional approaches frequently involve the modification of construction parameters. As evidenced in the case histories in Shanghai, a prevalent strategy entails the compromise of construction efficiency. Specifically, to mitigate soil displacement, practitioners often adopt a lower penetration speed combined with a higher water-cement ratio, as reported in studies by Chen et al.40 and Pan et al.23 However, this approach has been observed to result in an augmentation of water usage and a concomitant diminution in pile strength. It is evident that the primary focus of these mitigation measures is on process improvement rather than on addressing the underlying cause through the design of advanced equipment. Nevertheless, advancements have been made in the field of jet grouting. Yuan et al.41 developed an automatic pressure-control jet grouting system. The system deploys a technologically advanced drilling rod and monitoring apparatus, which functions to automatically regulate the internal stratum pressure and oversee the controlled discharge of spoil. The system has been developed for the purpose of proactively minimizing ground displacement. This innovation indicates a development path for DM equipment that is likely to be fruitful.

The integration of digital control systems into DM technology has advanced significantly, with a focus on real-time monitoring and data visualization. Representative systems, including the “3D Pile Viewer"42 and the construction information visualization system43, have been developed to effectively leverage GNSS, tilt sensors, and current measurement to guide positioning and visualize construction parameters. Subsequent studies conducted by Yu et al.44 developed an intelligent driver incorporating frequency grouting based on real-time current feedback, enabling a degree of slurry adjustment according to soil layer hardness. Despite these advancements, several technical limitations remain to be addressed. First, existing systems lack the capability for automated control of key operational parameters, such as the number of blade rotations, which compromises the assurance of mixture homogeneity. Second, the water-cement ratio cannot be adjusted during construction, adversely affecting the consistency of slurry properties and the ultimate strength of the soil-cement columns. Third, and most critically, there is no active control over the internal stratum pressure during installation. The inability to monitor and regulate the pressure exerted on the surrounding soil results in largely unmanaged and unpredictable impacts on adjacent structures or the environment. These limitations collectively constrain the digital advancement of deep mixing equipment.

Accordingly, a novel digital minor-disturbance deep mixing technique, referred to as the DMP method, has been developed. The primary objectives of the DMP method are to achieve significantly reduced disturbance of the subsoil compared to conventional deep mixing methods, enhance control inner stratum pressure, and enable construction with variable binder content. This paper introduces the core configuration, key techniques, and construction process of the DMP method. A field test is further given to verify the construction quality and impact on the surrounding soil.

Configuration of the DMP method

Equipment composition

Figure 2 illustrates the primary configurations of the DMP method, including a rig frame platform, mixing system, digital construction system, air supply system and an automatic slurry plant. The mixing system is installed on the rig frame, the digital construction system is situated inside the control room on the rig frame platform, and the air supply system is positioned at the rear of the platform. The automatic slurry plant is connected to the mixing system via slurry pipes.

Fig. 2
Fig. 2
Full size image

Configuration of DMP equipment.

Rig frame platform

The DMP method utilizes a rig frame platform with high mounting capacity and stability. The total height of the rig frame is 55 m, the motor power is 75 kW, and the total weight is approximately 200 t. Due to the height and motor power of this platform, the maximum drilling depth reaches 48 m. The DMP method adopts increasing the overall height of the rig frame instead of extending the drill rods during construction, as the later technique is less efficient.

Mixing system

The mixing system comprises a power system located at top of the drill rods, and a drill bit system at the bottom of the drill rods. The power system consists of an electric motor and a gearbox. The electric motor serves as the primary power source, which can offer sufficient power during DM installation. The power head of the DMP equipment is connected to two vertical variable frequency motors through a coupling. The motors are linked to a planetary gearbox inside the gear housing. Such arrangement enables the transmission of power to the flange joint, drill rods, and drill bit, facilitating drilling and mixing operations.

The drill bit is designed with two sets of grooves at angles of 60° and 75° to the horizontal, allowing for the installation of blades at different angles to accommodate various soil layers. In addition, free blades are incorporated into the mixing drill bit, which remain stationary during the installation process by preventing the mixed soil from adhering to and rotating with the mixing blade45. When the mixed soil adheres to and rotates with the mixing blade, the efficiency of mixing binder slurry and soil could be low1. Consequently, the mixture of binder slurry and soil could be more uniform in the DMP method. In order to enhance construction efficiency, a dual-mode operational strategy is adopted. During penetration, a low-speed and high-torque mode (typically 15–25 r/min with 40–35 kN m) is applied in order to ensure sufficient cutting capacity against soil resistance. During withdrawal, a high-speed and low-torque mode (typically 25–35 r/min with 30–20 kN m) is employed to improve mixing uniformity by facilitating more effective slurry dispersion.

Digital construction system

The digital construction system consists of a control subsystem, positioning subsystem, and monitoring subsystem. It offers functions of automated control of pile installation, automatic data collection and display, as well as monitoring and early warning, as shown in the Fig. 3.

Fig. 3
Fig. 3
Full size image

Framework of the digital construction system.

The control subsystem enables input and output of installation parameters, linkage between the control room and slurry plant, automated pile installation, and real-time data uploading to cloud servers. After the construction parameters are inputted into the equipment through the Human Machine Interface (HMI), the Programmable Logic Controller (PLC) automatically controls the DMP equipment to install the DM pile, the control subsystem is responsible for regulating the construction process, including adjustments to rod rotation rate and slurry flow rate through converters and valves. The latter comprise a range of components, including rotation frequency converter, pump frequency converter, compressed air throttle valve, winch proportional valve, pneumatic electric butterfly valve, and others. The corresponding monitoring components include a frequency converter, flow meter, barometer, speed sensor, and pressure sensor, among others. It is of paramount importance that the control subsystem ensures the interaction between the core parameters, particularly in the context of quality control of DM piles. The coordinated parameters primarily encompass the slurry flow rate and penetration/withdrawal speed, the drill rotational speed and mixing times, and the air volume and underground pressure. The relationship between slurry flow rate and penetration/withdrawal speed is expressed by the following Eqs. (1) and (2):

$${{\text{L}}_{\text{p}}}=\frac{{a \times {{\text{a}}_p} \times (1+{\raise0.7ex\hbox{$W$} \!\mathord{\left/ {\vphantom {W C}}\right.\kern-0pt}\!\lower0.7ex\hbox{$C$}})}}{{{\rho _{\text{s}}}}}{v_p}$$
(1)
$${\rho _s}=1+\frac{1}{{{\raise0.7ex\hbox{$W$} \!\mathord{\left/ {\vphantom {W C}}\right.\kern-0pt}\!\lower0.7ex\hbox{$C$}} \times {\rho _{\text{c}}}+1}} \times 2$$
(2)

where \({{\text{L}}_p}\) is the flow rate of slurry at the penetration stage, a is the amount of cement per meter of pile, \({{\text{a}}_p}\) is the proportion of penetration slurry to total slurry volume, W/C is the water/cement ratio, \({\rho _{\text{c}}}\) is the density of cement, and \({{\text{v}}_p}\) is the penetration speed of the drill bit. From the above Eq. (1), it can be observed that there is a linear relationship between the flow rate and the penetration speed. Consequently, in order to achieve a faster penetration speed, it is necessary to increase the slurry flow rate in a corresponding manner. The relationship between withdrawal speed and slurry flow rate is identical to that described by the aforementioned equation. Prior to the commencement of construction, the pertinent parameters, including the water-cement ratio and cement dosage, are entered into the digital construction control system. During the course of construction, the control subsystem will adjust the slurry flow rate and withdrawal speed in accordance with the aforementioned equation. The control logic for the number of mixing times is delineated in Sect. 3.1, while the control logic for the jet volume and ground pressure is discussed in Sect. 3.3.

The positioning subsystem is responsible for calculating the coordinates of each drill pipe. This is achieved through the use of a satellite positioning module, which is mounted on the frame and includes positioning, directional, and radio antennas. During the positioning process, the coordinates of the four known points on the site are entered into the positioning system in order to calibrate the equipment position. Once the verticality of the equipment frame has been calibrated, the coordinates of the equipment will be collected. Thereafter, the positioning system will obtain the center point of the first and fourth drill rods, which will serve as the pile center. Subsequently, the coordinates of the pile center are compared with the coordinates of the designed pile position. Construction is only permitted within the specified design error, with the general deviation being less than 5 cm.

The monitoring subsystem is comprised of a variety of sensors, including depth sensors, flow sensors, stratum pressure sensors, inclinometers, and others. The depth of the pile is determined by the depth sensor. The monitoring subsystem employs a methodology whereby the speed of the winch drum is measured and the length of the wire rope lowering is calculated based on parameters such as the diameter of the drum and the wire rope. This process enables the determination of the pile length. The flow sensor is situated at the outlet of the slurry pump, and is equipped with an LCD display that allows for convenient viewing by site inspectors. Given that the drill bit is continuously immersed in the slurry-soil mixture throughout the construction of the DM, the fluidity of this medium is markedly poor and its viscosity is exceptionally high. Consequently, the use of conventional pressure sensors is likely to result in the medium entering the pilot hole, thereby introducing a significant degree of error into the sensor test data. The DMP equipment employs a flat film pressure sensor, with the surface of the membrane piece reinforced to ensure resilience to damage from the hard sand present in the ground. The sensor has a range of 0 to 60 MPa, which meets the requirements for 50 m of DM construction. The inclinometer used in DMP equipment is a full-temperature complementary high-precision dual-axis inclinometer with a built-in three-axis gyroscope and high-precision magnetometer. This provides high resolution and enables the inclination measurement of the DM drilling bit position.

DMP equipment is equipped with an early warning function that is triggered when an abnormal alarm is detected regarding the drill bit penetration speed during the construction process, accompanied by an abnormal power head current. In such instances, the drill bit speed is observed to be lower than the pre-established value. In response to this occurrence, the digital construction control system initiates an automatic increase in the slurry flow rate, which is calibrated according to the parameter set prior to construction. This action is undertaken with the objective of lubricating the drill bit and reducing the resistance encountered. If the slurry flow rate is increased for a sufficient duration, the rate of penetration of the drill bit will return to its normal state, and the slurry flow rate and the rate of penetration will be synchronized once more. In the event that the drill bit continues to function suboptimally, despite the implementation of an increased slurry flow rate, it is essential to ascertain whether there are any underlying issues. These may include the presence of obstructions within the pile-forming area, the functionality of the equipment, the suitability of the soil layer, and other potential factors.

The digital construction control system is capable of recording the data associated with the entire construction process in accordance with the specified collection frequency and subsequently uploading it to the cloud server. The cloud server categorizes and manages the data according to the project name, equipment number, pile number, satellite positioning, and other pertinent information to form the construction record chart required for the project. The relevant data can be displayed, viewed, and exported as needed on the computer terminal and cell phone applet. During the process of piling, the data is updated in real time, which is convenient for all parties involved in the construction to grasp the construction situation in a timely manner and to facilitate the digitization of construction management.

Air supply system

Each drill rod of the DMP equipment is equipped with a channel to transmit compressed air. The compressed air helps reduce resistance during penetration stage of the DM installation process, resulting improving the uniformity of the soil-cement mixture. Meanwhile, the transmission of compressed air enables controlling the pressure acting on the surrounding soil due to drilling. The outlet of the air compressor is connected to the proportional control valve. The digital construction system can control the valve to adjust the flow rate of the compressed air. During the DM installation process, the flow rate of the compressed air is automatically adjusted based on the depth variation and the monitoring values of the stratum pressure sensor, in order to maintain the pressure within the required range.

Automatic slurry plant

The automatic slurry plant comprises four frequency slurry pumps, a stirring system and a cement tank. The plant enables the production of slurry based on inputs of binder content and binder-cement ratio in the digital construction system. The frequency-controlled slurry pumps facilitate the pumping of a proportional slurry volume, adjusting to the penetration and withdrawal speed of drill rods.

Table 2 compares the main technique parameters of dual-axis DM equipment and triple-axis DM equipment with the DMP equipment. Compared to the conventional dual-axis and triple-axis DM techniques, the DMP technique has significant improvements and enhancements. The DMP equipment has a high level of digitalization. It is equipped with digital construction system, and several sensors, including earth pressure cell and verticality sensor. Each drill rod is equipped with three channels, i.e., two slurry channels and one compressed air channel. For the DMP equipment, there are two slurry channels and one compressed air channel in each drilling rods, thus, there are eight slurry channels and four compressed air channels in total. Regarding the upper and lower nozzles, the vertical spacing of 4 m between the upper and lower nozzles, as indicated in Fig. 4, is now explicitly stated in the text. Furthermore, the operational pressure ranges for the injections have been provided: 0–2.5 MPa for slurry and 0.2–1.0 MPa for compressed air. The DMP equipment can reach a maximum improvement depth of up to 48 m.

Table 2 Main technique parameters of DM techniques.
Full size table

Innovative techniques

Triple-channel hexagon-shaped rod

In order to enhance construction quality and efficiency while also mitigating its impact on the environment, the rod is designed to have a triple-channel hexagonal shape, as shown in Fig. 4. In the cross section of the rod, there are three channels, i.e., two slurry channels and one compressed air channel. The grout nozzle of one slurry channel is equipped near the tip of drill rod, and the grout nozzle of the other slurry channel is equipped 4.0 m distance from that of the former slurry channel. The lower slurry nozzle is located at the tip of the drill bit, while the upper slurry nozzle is positioned directly below the mixing blades. This configuration releases the injected slurry directly into the active cutting and mixing zone, thereby minimizing potential slurry loss along the rod-soil interface. During penetration, the lower nozzle remains open and the upper nozzle closed, enabling the slurry to be mixed with the soil via blade rotation. While the penetration process is underway, the lower nozzle remains open and the upper nozzle is closed, allowing the slurry to blend with the soil as the blades rotate. While the withdrawal process is proceed, the lower nozzle is closed and the upper nozzle is opened, as a result, the ejected slurry can still be mixed with the soil. As a result, grouting and mixing can be proceed during both the penetration and withdrawal process, meanwhile air injection helps to decrease resistance during the penetration process. This design can enhance the uniformity of cement slurry and soil mixing, especially when encountering hard clay and sandy soils.

The rod is designed with a hexagonal cross-section, which leaves a gap between the borehole and the rod during the drilling process. This hexagonal profile applies only to the drill rod sections above the drill bit. The drill bit itself, approximately 8 m in length with a circular cross-section, is fitted with mixing blades and maintains a tight fit with the borehole wall in the mixing zone. The primary function of this gap is to serve as a conduit for the dissipation of excess air pressure, thereby minimizing disruption to the surrounding soil. Slurry pressure, regulated by a variable-frequency grouting pump, is kept relatively low (typically 0–0.4 MPa) to maintain a consistent design flow rate rather than to apply high pressure on the formation. Together, controlled slurry pressure, nozzle placement within the mixing zone, and the tight seal provided by the circular bit ensure that most slurry is retained and effectively utilized for soil mixing.

Fig. 4
Fig. 4
Full size image

Triple-channel hexagon-shaped rod (unit: mm).

The quality of DM pile is related to the blade rotation number46,47. The BRN can be determined based on the following equation:

$${\text{BRN}}=\sum {M\left( {\frac{{{N_{\text{p}}}}}{{{V_{\text{p}}}}}+\frac{{{N_{\text{w}}}}}{{{V_{\text{w}}}}}} \right)}$$
(3)

where BRN is the blade rotation number; M is the total number of mixing blades in a rod; Np and Nw are the rotational speeds of blades during penetration and withdrawal process, respectively; Vp and Vw are the penetration and withdrawal velocities, respectively. The penetration or withdrawal speed are correlated with the slurry flow rate, and the rotation speed of blades is correlated with its torque that reflects the power for cutting the soil during penetration. Therefore, the BRN affects the key parameters in construction process such as slurry flow rate and drill motor current. The European standard (EN14679:2005) recommends the blade rotation number (BRN) should be greater than 350. In the DMP equipment, there are 14 pieces of blade in a rod, the rotational speeds of blades during penetration and withdrawal process are typically 20r/min and 25r/min, respectively, and the corresponding penetration and withdrawal velocities are typically 0.5 m/min and 1.0 m/min, respectively. The blade rotation number is determined to be 910r/m, which is significantly higher than 350r/m required by the European standard (EN14679:2005). Moreover, experimental studies have shown that a higher BRN improves the mixing uniformity and strength of the treated soil. Porbaha et al.34 reported that a BRN exceeding 360r/m resulted in a lower coefficient of variation in unconfined compressive strength, indicating better homogeneity. In a similar vein, Kitazume45 proposed that a BRN exceeding 270 is essential to ensure adequate mixing in Japanese wet and dry deep mixing methods. In a recent study, Ge et al.48 conducted a series of model tests and demonstrated a clear positive correlation between BRN and strength, as shown in Fig. 5. Furthermore, the study supported the use of BRN as a reliable indicator of mixing quality. Consequently, the uniformity of cement slurry and soil mixing can be significantly enhanced.

Fig. 5
Fig. 5
Full size image

Relationship between BRN and unconfined compressive strength (UCS).

It is evident that DMP has the capacity to furnish a higher BRN. However, it is acknowledged that this process may result in an increase in unit energy consumption when compared to more conventional methods. Economic analysis derived from field applications in Shanghai demonstrates that the energy cost constitutes a negligible proportion (approximately 4–5%) of the total unit price for deep mixing. The substantial enhancement in production rate (for instance, 500–600 m³ per 12 h for DMP as opposed to 250–300 m³ for conventional triple-shaft mixing) accomplished by this system leads to diminished overall project expenses through decreased construction time. Consequently, the selected BRN symbolizes a balanced compromise that guarantees design quality whilst sustaining elevated economic efficiency.

Variable binder content mixing

The subsoil typically consists of several strata. The DMP equipment can set up construction parameters by taking into account the different soil layers. In the digital construction system, the penetration and withdrawal process can be divided into a maximum number of 16 phases, comprising 8 phases in the penetration process, 2 phases for the process of repeating stirring near the pile tip, and 6 phases in the withdrawal process, as shown in Fig. 6. The maximum of 16 phases provides operational flexibility. In practice, the number of phases is adjusted based on the complexity of the soil profile and design requirements. In scenarios where soil conditions are relatively uniform or where specific design considerations are absent, the process can be streamlined into a reduced number of phases (e.g., one phase for penetration, one for repeated stirring at the pile tip, and one for withdrawal). In each phase, the construction parameters, including slurry flow rate, penetration and withdrawal speed, can be independently established according to the soil layers within the improvement zone. The number of phases as well as the construction parameters in each phase are determined according to the soil layers. When encountering weak soils, such as clay and soft soil, a high slurry flow rate can be selected, while when improving slit and sand, the slurry flow rate can be appropriately reduced. In order to mitigate potential weak zones at phase boundaries due to parameter transitions, a precautionary measure is implemented. Specifically, favorable construction parameters (e.g., higher binder content) are extended approximately 0.5–1.0 m into adjacent zones with less demanding parameters. This overlap helps account for uncertainties such as geological variability and ensures a more uniform treatment, thereby reducing the risk of weak interfaces.

Fig. 6
Fig. 6
Full size image

Schematic of drilling process.

Inner stratum pressure control

During the penetration process in DM pile construction, the shearing force of the mixing blades and the slurry pressure squeeze the surrounding soil49. This impact leads to an increase in pore water pressure and lateral displacement in the surrounding soil. Shen et al.50 indicated that the increment of excess pore water pressure caused by the shearing force of mixing blades is approximately 20–40% of the in-situ shear strength. Chai et al.51 pointed out that the lateral displacement of soil is influenced by slurry pressure, but they only considered the pressure at the outlet of the pump. The pressure at the outlet of the pump should be different from that in the borehole. The slurry is pumped from the supply plant to the drilling rod through a tube over a distance. Such transport process could cause loss in slurry pressure. Meanwhile, the slurry pressure in the borehole would also influenced by the drilling and rotational speed, and the injection of compressed air.

In the DMP equipment, a pressure sensor is installed in the rod to monitor the slurry pressure inside the borehole (also called inner stratum pressure) during the drilling process. The relationship between this inner stratum pressure and the resulting soil displacement can be conceptualized using the framework of cylindrical cavity expansion theory. The fundamental elastic solution, derived from Hooke’s law and equilibrium equations, provides a first-order approximation of the pressure-displacement relationship36:

$$P=\frac{{rE{\mu _r}}}{{(1+v)R_{0}^{2}}}$$
(4)

in which, P is the pressure inside the borehole wall, E is the elastic modulus of the surrounding soil, ur is the lateral displacement at the radial distance of r from the center of the borehole, R0 is the initial radius of the borehole, v is the Poisson’s ratio of the surrounding soil. It is acknowledged that natural soft soils exhibit elastoplastic behavior. Therefore, the purely elastic solution of Eq. (4) serves primarily as a conceptual basis. In order to account for soil plasticity in a practical manner, the theory can be extended by considering the soil’s failure criterion and the volume of injected slurry. This enhanced approach establishes a direct correlation between the controlling pressure and critical soil strength and deformation parameters. Accordingly, the inner stratum pressure within the borehole can be theoretically determined, and the subsequent criterion is adopted to regulate the inner stratum pressure within the borehole during construction:

$${\sigma _{{\text{h}}0}} \leq {P_{\text{s}}} \leq {\sigma _{{\text{h}}0}}+P$$
(5)

in which, Ps is the controlled inner stratum pressure inside the borehole, σh0 is the initial horizontal earth pressure of soil, P is the additional pressure applied inside the borehole wall which results in lateral displacement of surrounding soil. For an ease of use in practice, Eq. (3) is simplified as the following expression:

$${P_{\text{s}}}=\xi {\gamma _w}z$$
(6)

in which, ξ is the modified coefficient of inner stratum pressure, z is the depth, and γw is the unit weight of water. This simplified formulation is predicated on a conceptual basis that is analogous to the pressure-control principle that is employed in advanced jet-grouting techniques. In such techniques, the pressure within the inner stratum is balanced against the overburden pressure, with the objective of minimizing ground displacement41. The coefficient, designated as “ξ”, functions analogously to empirical factors in extant methods, encapsulating deviations from ideal hydrostatic pressure due to the combined effects of slurry pressure, air pressure, soil-structure interaction, and soil properties. At a construction site with strict displacement control, a maximum soil lateral displacement of 5 mm is acceptable. Based on such criterion, the previous field date showed that ξ could be in a range of 1.2–1.6. This recommended range was primarily derived from preliminary test pile data and construction experience at sites in Shanghai with soft clay deposits. Lower values within the range (e.g., 1.2–1.4) are generally associated with softer, weaker layers, while higher values (e.g., 1.5–1.6) are applicable to stiffer layers. This empirical coefficient thus implicitly incorporates the influence of key soil characteristics. During the DM pile installation, the inner stratum pressure can be controlled based on the input value of ξ. When the inner stratum pressure exceeds the control value, the digital construction system first adjusts the air pressure. If adjusting the air pressure is not sufficient to reduce the inner stratum pressure, the construction system will automatically adopt the drilling and rotation speed.

Construction procedure

Figure 7 shows the general construction procedure using the DMP technique. The DM pile is formed as the following four steps:

(1) Step 1: Move the DMP equipment next to the installation location. Make precise adjustments to the installation location using the positioning subsystem. Input the construction parameters into the HMI. The DMP equipment initiates the penetration process. The lower grouting nozzle and the compressed air nozzle are remained open, while the upper nozzle is closed. Rods #1 and #3 rotate clockwise, and rods #2 and #4 rotate counterclockwise. The rods penetrate the soil according to the set construction parameters, as illustrated in Fig. 7a.

(2) Step 2: Once the DMP equipment reaches the designated elevation of the DM pile tips, it begins the repeated stirring process. The rods are lifted up to 4 m, while the ejection of slurry and air and the rotation of blades remain active. During this process, depicted in Fig. 7b, drill rods #1 and #3 rotate counterclockwise, while drill rods #2 and #4 rotate clockwise.

(3) Step 3: The rods penetrate again to complete the repeating stirring process. The rotation directions are reversed again, as shown in Fig. 7c.

(4) Step 4: After the repeating stirring process is complete, the DMP equipment starts to proceed the withdrawal process. The upper grouting nozzle and the compressed air nozzle remain open, while the lower nozzle is closed. The rods rotate and lift until reaching the ground. In this process, the rotation directions are reversed again, as shown in Fig. 7d.

Fig. 7
Fig. 7
Full size image

Construction procedure of DM pile using DMP technique.

It is noted that the rotation directions of rods are changed in each process. Such move can increase the uniformity of slurry and soil mixing46. Rotating the drill rods in the same direction can lead to the formation of soil clumps, making it difficult for the slurry to penetrate. The repeating stirring process near the pile tips are performed, as a result, the uniformity of slurry and soil mixing near this region can be ensured.

Fig. 8 shows the cloud interaction platform. The cloud platform allows for real-time updating and synchronization of construction data. Using this platform, it is convenient for all project participants to monitor construction progress and conduct QC/QA management43.

Fig. 8
Fig. 8
Full size image

Construction cloud platform.

Field test

Project background

The DMP method, founded on the inner stratum pressure control principle delineated in this study, has been successfully implemented in over 30 major projects across Shanghai and neighboring cities. These include large-scale infrastructure projects such as the expansion of Shanghai Pudong International Airport and a transportation hub in Ningbo, representing a diverse range of demanding construction environments. To verify the quality of DM pile using the DMP technique and analyze its impact on the surrounding soil, field experiments were conducted. Test site is located in Baoshan District, Shanghai, China. The subsoil on the test site consisted of a 3.3-m-thick crust, a 1.3-m-thick silty clay, a 5.2-m-thick soft silty clay, a 7.2-m-thick soft clay, underlain by a silty clay. The groundwater level was at a depth of 0.5 m. Figure 9 shows the soil profile and properties.

Fig. 9
Fig. 9
Full size image

Soil profile and properties on the test site.

A trial test was conducted to install three DM piles of 18 m long. Figure 10 shows the layout of the piles. During construction, the piles had an overlap of one drilling rod to form a deep mixed wall. Due to the soil conditions at the test site, the subsoil soil within the improved depth can be roughly classified into three soil layers: layer 1 (0–4.6 m), layer 2 (4.6–16.8 m), and layer 3 (16.8–18 m). The binder content in the layers 1 and 3 was set at 13%, while that in the layer 2 was 15%. The modified coefficient of inner stratum pressure was set at 1.3. Table 3 summarizes the main construction parameters inputted into the digital construction system.

Air pressure in the DMP process is regulated based on both theoretical principles and field experience. The pressure must exceed the lateral earth pressure of the soil-binder mixture in the borehole to prevent backflow into the air pipes. The engineering experience shows that for typical treated soil up to a depth of 50 m in Shanghai, an air pressure range of approximately 0.2–1.0 MPa is generally suitable, with lower pressures applied in shallow zones and higher pressures in deeper zones. In this study, with a maximum treatment depth of approximately 18 m, a maximum air pressure of 300 kPa was adopted as a conservative limit, based on empirical depth–pressure correlations and site-specific soil conditions.

Table 3 Construction parameters of test DMP piles.
Full size table

Figure 10 illustrates the monitoring points. Settlement monitoring points were installed on the ground at distances of 1 m, 2 m, 3 m, 4 m, and 6 m from the centerline of the DMP piles, arranged along a line perpendicular to the wall axis. Inclinometer casings were installed at distances of 2 m, 4 m, and 6 m from the centerline of the DMP piles to a depth of 22 m. The depth of 22 m for the inclinometers was selected, as a result, the bottom of the casings was embedded into a stiffer silty clay layer to provide stable fixity. Furthermore, during the test period, no other construction activities, dewatering, or significant environmental changes (e.g., substantial groundwater level fluctuations) were observed within a 100 m radius of the test site, which helped minimize potential external influences on the monitoring results.

Fig. 10
Fig. 10
Full size image

Layout of monitoring points: (a) Plan view; (b) Cross sectional view.

Quality control

Figure 11 depicts the injection of slurry volume with depth recorded by the digital construction system. In order to perform a quantitative evaluation of the control accuracy, a statistical analysis of the flow rate data was carried out. The key metrics are summarized in Table 4. The findings suggest that the mean flow rates across all phases remained remarkably close to their respective set values. The fluctuations, quantified by the coefficient of variation (CV), were generally minimal and exhibited a clear correlation with the soil conditions and construction phases. It is noteworthy that higher CV values were observed in phases corresponding to stiffer soil layers (e.g., Phases 1–2, 8), whereas lower CV values were associated with softer soil layers (e.g., Phases 3–7). This phenomenon can be attributed to the increased and more variable resistance encountered by the tool in stiff layers, leading to more pronounced fluctuations in the pumping pressure required to maintain advance. Furthermore, during the phases of remixing and withdrawal, the CV values were found to be remarkably low (e.g., Phase 16, CV = 0.7%). This enhanced stability can be attributed to the pre-computation of the soil within the pile column into a slurry-soil mixture, resulting in a more uniform and significantly reduced resistance to grouting and mixing.

Fig. 11
Fig. 11
Full size image

Slurry flow rate in each phase.

Table 4 Statistical performance indicators of the slurry flow rate during different construction phases.
Full size table

Figure 12 shows the recorded air pressure and inner stratum pressure with depth. The variation of air pressure was within the controlled range and gradually increased with increasing the depth, as the lateral earth pressure increased with depth. Due to the adjustment of air pressure by the digital construction system, the monitored inner stratum pressure increased with increasing the depth and controlled well as compared with the required stratum pressure.

Fig. 12
Fig. 12
Full size image

Variation of air pressure and inner stratum pressure with depth: (a) air pressure; (b) inner stratum pressure.

Following the 28-day curing period, the deep mixing (DM) piles were cored and sampled. The unconfined compressive strength (UCS) variation with depth is shown in Fig. 13. To quantitatively assess the uniformity of the treated soil and evaluate the effect of variable binder content, a statistical analysis was performed. As summarized in Table 5, the coefficients of variation (CV) for all three piles are below 10% (7.1%, 6.7%, and 8.7%), demonstrating high consistency. Based on established engineering practice, a CV below 10% indicates uniform strength in DM piles28,52, thus confirming the quality stability along the pile depth. With a mean UCS of 1.16 MPa, the result exceeds the design requirement of 0.8 MPa, confirming sufficient strength for the intended structural purpose.

In addition, the UCS data were analyzed with respect to the designated binder content: 13% in Layers 1&3 and 15% in Layer 2. As detailed in Table 6, an independent-samples t-test conducted on the strength data from these two groups revealed no statistically significant difference (t(16) = 0.662, p = 0.517). This result indicates that the increased binder content in Layer 2 effectively compensated for the differences in soil characteristics, resulting in a statistically uniform strength profile throughout the pile. Therefore, the ‘variable binder content mixing’ technique successfully achieved strength equalization across heterogeneous soil layers, optimizing the design for uniformity and cost-effectiveness.

Fig. 13
Fig. 13
Full size image

Variation of unconfined compressive strength (UCS) with depth.

Table 5 Statistical results of UCS for DMP piles.
Full size table
Table 6 Comparison of UCS statistics between soil layers with different binder contents.
Full size table

Environment impact

Figure 14 shows the accumulated lateral displacements of the subsoil during the installation of the three piles. Upon completion of Pile 1 construction, the maximum lateral displacement at CX1 was 2.97 mm at a depth of 12 m, while at CX2 it measured 1.65 mm, and at CX3 it was less than 1 mm. Subsequent construction of Piles 2 and 3 led to slightly increased lateral displacements at CX1 and CX2, with minimal change observed at CX3. Upon the conclusion of Pile 3 construction, the maximum displacements measured 4.08 mm, 2.28 mm, and 1.0 mm at CX1, CX2, and CX3, respectively. The lateral displacement influenced by the pile construction remained below 5 mm, with nearly negligible impact on the subsoil up to 6 m from the construction site.

Fig. 14
Fig. 14
Full size image

Lateral displacement with the depth.

The magnitude of lateral displacement induced by deep mixing piles is intrinsically linked to the volume of grout introduced into the ground. As demonstrated by Gong et al.53, a higher cement content generally leads to a more pronounced squeezing effect and consequently larger soil displacements. A direct comparison with the data from Gong et al.53, obtained under comparable Shanghai soft clay conditions, is presented in Fig. 15. Notably, the maximum lateral displacement caused by the DMP piles in this study, even at a cement content of 13%/15%, remains lower than that induced by traditional triple-axis pile at lower cement contents of 10% and 13%.

Although long-term monitoring was not conducted, it is reasonable to expect some recovery of lateral displacement over time due to soil consolidation and the dissipation of excess pore water pressure generated by cement hydration. Consequently, the impact of DM pile installation on subsoil displacement is most pronounced in the short term.

Figure 16 illustrates the measured ground heaving during the pile construction. The construction of the piles resulted in ground heaving of the surrounding soil, particularly noticeable in the vicinity of the piles. Following the completion of Pile 1 construction, the ground heaving at a distance of 1 m from the pile measured 1.11 mm. Subsequent completion of Piles 2 and 3 led to its measurement of 3.35 mm. At a distance of 6 m, the ground heaving was recorded at 1.17 mm after the completion of all piles. The measured lateral displacements and ground heaving indicate that the DMP method minimally disturbs the surrounding soil.

Fig. 15
Fig. 15
Full size image

Comparison of lateral soil displacements induced by DMP piles and traditional triple-axis DM piles under various cement contents.

Fig. 16
Fig. 16
Full size image

Measured ground heaving with the distance.

Conclusions

Considering the limitations of the current DM methods and strict displacement control in some construction activities, this paper introduces a novel DM technique called digital minor-disturbance deep mixing technique, including the main configuration of the DMP method, innovative techniques and construction process, and a filed test. Based on the analyses and discussions, the following conclusions can be drawn:

(1) The DMP equipment consists of a rig frame platform, mixing system, digital construction system, air supply system and an automatic slurry plant. The construction process is controlled by the digital construction system, enhancing installation efficiency and quality. Through an interactive cloud platform, real-time updating and synchronization of construction data are achieved, enabling dynamic quality control of construction.

(2) Triple-channel hexagon-shaped rods are enabled to expel excessive mixture pressure and thoroughly mix slurry, air and soil, leading to mitigating construction impact on the surrounding soil and enhancing uniformity of cement slurry and soil mixing.

(3) The variable binder content mixing technique is enabled to consider construction parameters by taking into account the soil layers, dividing the all construction process into a maximum number of 16 phases. In each phase, the construction parameters, including slurry flow rate, penetration and withdrawal speed, can be independently established according to the soil layers within the improvement zone.

(4) The inner stratum pressure inside the borehole can be measured by the pressure sensor and controlled by digital construction system. When the stratum pressure exceeds the control value, automatic adjustments are made to the air pressure and installation speed, subsequently minimizing disturbance to the surrounding soil.

(5) The field test confirmed that the DMP equipment performs well in construction. The recorded construction parameters, including slurry flow rate, drilling speed, air pressure, and inner stratum pressure, were consistent with the required values. The measured lateral displacements and ground heaving, influenced by the pile construction, remained below 5 mm. The unconfined compressive strengths were uniform at approximately 1.1 MPa along the pile shaft.

The case study conclusively demonstrates that the proposed pressure-control method is highly effective in limiting ground displacements in soft clay. While the method has seen widespread regional application, the detailed validation presented herein confirms its mechanistic efficacy. Future work will systematically document its performance across the broader spectrum of soil conditions (e.g., dense sands, stiff clays) encountered in other projects to further establish its generalizability.