Strength Characteristics of DMJ Piles Based on Indoor Tests

Abstract

The Deep Cement Mixing Integrated Drilling, Mixing and Jetting (DMJ) method represents a novel approach to the construction of slurry piles in the Yellow River Floodplain, offering the potential to enhance the quality of these structures. In order to investigate the pile strength and its distribution characteristics under different conditions, an unconfined compressive strength test was conducted on DMJ pile core samples. The Kolmogorov-Smirnov (K-S) test was employed to evaluate the normal distribution characteristics of the strength, and the fluctuation of the pile strength was evaluated by the autocorrelation function method to elucidate the distribution characteristics. Moreover, a resistivity non-destructive test was conducted to ascertain the correlation between strength and resistivity and to perform supplementary strength testing. The distribution of the pile body strength is normal at the 5% level of significance. A clear correlation was observed between the strength of the pile core and depth, while the correlation between the strength of the pile outer side and depth was less pronounced. Additionally, a positive linear correlation was observed between resistivity and strength, which can be used to evaluate the strength of DMJ piles.

1. Introduction

The majority of highway engineering projects in China are located in areas with soft soil, which often results in inadequate bearing capacity. This can result in several engineering issues, including excessive settlement of the roadbed, differential settlement between new and old roadbeds and other complications. The reinforcement of foundations represents a pivotal aspect of highway engineering construction. Since the 1970s, deep cement mixing technology has been used in Chinese highway construction. In some projects in the Yellow River Floodplain, the high consolidation degree of powdery clay and clay presents a significant challenge to disruption, resulting in suboptimal quality of the deep cement mixing piles. In the context of drilling, mixing and jetting piles, high-pressure jetting (10–20 MPa) is employed, with the jetting nozzle relocated from the drill pipe to the extremity of the mixing blades. This technique is otherwise referred to as DMJ piles [1], and this method effectively tackles quality challenges associated with slurry piling in the Yellow River Floodplain. The pile body, formed by DMJ [2], comprises high-pressure jetting soil on the outer side of the pile and slurry mixing soil on the inner side of the pile. The strength of the cemented soil within the pile body displays considerable variability.

Several scholars have identified spatial variability in cemented soil strength as a pivotal phenomenon within the scope of their research. Chen et al. [3] observed considerable variability in the core samples of cemented soil mixing piles, with values ranging from 1 MPa to 5 MPa. This observed variability can be attributed to a variety of factors, including the degree of homogeneity of the cement–soil mixture, differences in the nature of the in situ soil and the environmental conditions under which the cemented soil is maintained. These factors can all contribute to significant spatial variability in the strength of cement–soil mixtures [4].

Different scholars have established a clear correlation between the strength of cemented soil and the duration of maintenance, which exerts a notable influence on the hydration reaction of cement [5]. The hydration process is significantly influenced by the presence of water, which exerts a notable effect on the development of strength. The strength of stabilized soils can be increased at a reduced water content and over a longer curing period in a manner that is analogous to that observed in concrete and mortar. The strength of stabilized soils has also been found to be significantly affected by the clay fraction, plastic limit and liquid limit [6]. It has been observed that an elevated water–cement ratio is associated with a reduction in the strength of cemented clay [7].

Furthermore, the specific type and quantity of cement employed also exert a significant influence on the strength of cemented soil [8]. Taki et al. [9] observed that cement-stabilized sandy soils exhibited considerably greater strength than cement-stabilized pulverized soils and clays. In contrast, Chian et al. [10] conducted laboratory experiments involving the addition of sand to cement-stabilized Singapore marine clay and kaolin, which resulted in a reduction in the strength of cemented clay. The initial hypothesis was that this was due to the presence of excess free water affecting the hydration process. The aforementioned studies have demonstrated that the strength of cemented soil is influenced by a multitude of factors and that the impact of these factors on the strength of cemented soil is not uniform.

The study of spatial variability in cemented soil is significant to engineering design. Nishida et al. [11] observed that the design strength of cemented soil for field construction is often lower than the strength of cemented soil tested in an indoor setting. This typically necessitates that field core samples exhibit a certain percentage of excess over the design value as a means of ensuring the requisite level of accuracy. On projects with zero-tolerance quality requirements, it is occasionally necessary for the strength value of all core samples to exceed the design strength. This is tantamount to utilizing the lowest strength of the core samples as the design strength, which is also equivalent to employing the lowest strength of the soil as the design value. This inevitably results in avoidable expenditure on engineering costs.

Furthermore, existing research lacks comprehensive insights into the spatial variability of DMJ piles. It is thus imperative to conduct a precise evaluation of the strength of the deep cement mixing DMJ piles, taking into account its spatial variability.

This study evaluated the strength distribution characteristics and influencing factors of DMJ core samples through the implementation of indoor tests. Moreover, the study investigated the correlation between the strength of the core samples and depth by employing the autocorrelation function method and the electrical resistivity non-destructive testing method to assess the core strength.

2. Test Materials and Methods

The site test was carried out at a highway foundation reinforcement project in Dezhou, Shandong Province, China. The engineering geological profile of both test sites is presented in

Table 1. The engineering properties of soil in pile construction tests.

Depth (m)	Soil Type	Density (kN/m3)	Plasticity Index	Yield Stress (kPa)	Internal Friction Angle	Young’s Modulus (MPa)
0–5.5	Silty Clay	18.9	14.3	22.8	14.6	6.66
5.5–8	Mucky Silty Clay	18.6	17.1	32.9	8.9	4.44
8–10	Silty Clay	19.1	15.9	26.2	14.3	5.18
10–20	Silty Clay	19.2	11	23.5	15.5	7

below. The profile shows that the site is characterized by a typical Yellow River Floodplain clay and silty clay interbedded structure. Following an age of 28 days, the pile head was cleaned, and core sampling was conducted. The coring locations for the field tests are shown in Figure 1. For each pile, three points were selected for core sampling at the pile center and symmetrical positions on the left and right sides of the pile, with a distance of 25 cm between the center and the outer side of the pile. The site coring situation is illustrated in Figure 2, and the three pictures from left to right are taken from the left point, center point and right point of the pile, respectively.

The cores were retrieved and stored for unconfined compressive strength testing for up to 90 days. Preserve at a temperature of 25 °C and air humidity of about 45 percent, which are the usual indoor temperature and humidity values. The depths of the various piles and the locations of the cores were recorded during the test to facilitate data processing. The primary objective of this test is to evaluate the performance of the DMJ Pile 1 with 25 MPa grouting pressure and the DMJ Pile 2 with 15 MPa grouting pressure. All design parameters are the same for both piles except the grouting pressure.

3. Coefficient of Variation

Prior research and engineering practice have demonstrated that the strength of deep cement mixing piles exhibits significant inhomogeneity. This variability is typically quantified by the coefficient of variation (COV), which effectively reflects and compares the degree of variability between disparate data series. The COV is widely employed to assess the homogeneity of soil-mixed piles. The COV is calculated using the following formula:

$$\mathrm{COV}={\displaystyle \frac{\mathsf{\sigma }}{\mathsf{\mu }}}$$

where σ denotes the standard deviation of the sample and μ denotes the mean of the sample.

The strength dispersion and mean values at different points were calculated, and the number of samples, mean value, standard deviation and coefficient of variation at each point are presented in

Table 2. Strength statistics of core samples.

Position	Strength Maximum (MPa)	Strength Minimum (MPa)	Sample Number	Strength Average (MPa)	Strength Standard Deviation	COV
Pile 1
Left	16.63	2.49	22	7.88	4.02	0.51
Center	11.8	3.9	23	7.69	2.46	0.32
Right	20.17	1.59	21	7.6	4.42	0.58
Total	20.17	1.59	66	7.72	3.7	0.48
Pile 2
Left	16.26	2.87	23	6.18	2.85	0.46
Center	7.81	3.41	25	5.32	1.28	0.24
Right	15.6	3.66	13	7.66	3.73	0.49
Total	16.26	2.87	61	6.14	2.73	0.44

.

In this study, 127 samples were obtained from two distinct working conditions for analysis. Of these, 66 were taken from DMJ Pile 1, and 61 were taken from DMJ Pile 2.

It can be observed that the mean strengths of the three sampling points of Pile 1 are approximately equivalent. The mean strength at the center of Pile 2 is 5.32 MPa, which is less than that of the core samples extracted from the side of the pile. This is attributable to the fact that the quantity of cement utilized in Pile 2 was less than that employed in Pile 1.

The standard deviation and coefficient of variation of the strength at the pile core are smaller, indicating a reduced variability of strength and a more uniform distribution of strength. The variation in strength values at the outer side points is more pronounced, with a higher maximum strength value and a more uneven strength distribution. This is due to the fact that the central part of the DMJ pile is better mixed with cement and soil by the secondary mixing of the mixing rod, resulting in a more uniform strength. In contrast, the outer side, which consists of a high-pressure injection of slurry mixed with soil, is more affected by the difference between slurry injection and soil in the depth direction, leading to a greater difference in strength.

In order to eliminate the potential impact of soil quality variations on strength distribution, the core samples were stratified into three depth groups: 0–5.5 m, 5.5–8 m and 8–10 m, in accordance with the prevailing soil layer conditions. The table below illustrates the frequency distribution of strength for the two pile types across these three depth ranges.

As can be observed from the data presented in

Table 3. Strength statistics of core samples in different depths.

	Depth (m)	Strength Maximum (MPa)	Strength Minimum (MPa)	Sample Number	Strength Average (MPa)	Strength Standard Deviation	COV
Pile 1	0–5.5	8.33	2.84	29	5.54	1.43	0.26
	5.5–8	20.17	6.46	20	11.66	3.30	0.28
	8–10	11.14	1.59	17	5.98	2.23	0.37
Pile 2	0–5.5	7.81	3.64	29	5.15	1.37	0.27
	5.5–8	16.26	2.87	17	7.47	3.17	0.43
	8–10	8.70	3.26	15	4.71	1.45	0.31

, the coefficients of variation for Pile 1 in the depth ranges of 0–5.5 m and 5.5–8 m are 0.26 and 0.28, respectively. Additionally, the coefficient of variation for Pile 2 in the depth range of 8–10 m is 0.37. The distribution of pile strength is more uniform in the depth ranges of 0–5.5 m and 8–10 m, with coefficients of variation of 0.27 and 0.31, respectively. In contrast, the distribution of pile strength in the depth range of 5.5–8 m is less uniform, with coefficients of variation of 0.27 and 0.31. The coefficient of variation is 0.43, while the coefficient of variation of strength within the same soil texture is evidently diminished, which suggests that soil texture is a significant factor influencing the strength of cemented soil.

Cao [12] conducted a statistical analysis of the coefficient of variation (COV) of the strength of deep cement mixing piles employed in a number of soft-ground composite foundation reinforcement projects. The findings of this analysis are presented in Figure 3. The different symbols in the figure represent the range of COVs counted by different researchers

The COV exhibited a range of 0.20 to 0.79, with an average value of 0.42. It can be observed that the dispersion of DMJ piles is less pronounced than that of the majority of conventional cemented soil mixing piles and is situated below the average value [13]. Furthermore, it is evident that the DMJ pile process can effectively enhance the mixing uniformity of cemented soil and the uniformity of pile quality.

4. Statistical Analysis of Strength

The statistical analysis of the frequency of occurrence of strength ranges was conducted for Piles 1 and 2 based on the field test coring results. The frequencies of occurrence of the different strengths of the two pile types were counted separately, and the data were plotted as bar graphs, as illustrated in Figure 4. Subsequently, the data were fitted with normal distribution curves.

The Kolmogorov-Smirnov (K-S) test is employed to ascertain the suitability of the normal distribution as a model for the strength of the DMJ piles. The K-S test is a widely used method for evaluating the fit of a given distribution to a set of observed data.

A substantial number of soil strength statistics from the deep cement mixing pile project indicate that the strength of core samples exhibits a tendency to adhere to a normal distribution or lognormal distribution [29,30].

Figure 4a illustrates the distribution of pile body strength at varying depths within the soil layer of Pile 1, while Figure 4b depicts the distribution of pile body strength for Pile 2. It can be observed that the median value of the pile strength distribution for DMJ piles is 5 MPa. Pile 1 has a greater number of samples with high strength, exceeding 10 MPa, due to the larger quantity of cement employed. In contrast, Pile 2 exhibits a more uniform strength distribution, with a relatively lower number of high-strength samples.

Table 4. Strength statistics of core samples in different depths.

Pile No.	Coring Position	Simple Size	Form of Distribution	D	P
Pile 1	Left	22	Normal distribution	0.23	0.15
	Center	23		0.09	1
	Right	21		0.16	0.63
	Total	66		0.12	0.31
Pile 2	Left	23		0.17	0.46
	Center	25		0.11	1
	Right	13		0.23	0.42
	Total	61		0.16	0.07

presents the results of the K-S test for pile strength. The value of D represents the maximum absolute difference between the actual and theoretical values, with smaller values of D indicating a superior fit.

The value of P indicates the level of significance of the observed result, and the threshold for this level of significance is typically considered to be 0.05 [31,32]. If the value of P is greater than 0.05, the original hypothesis cannot be rejected. This implies that the overall distribution of the sample is not significantly different from the shape of the target distribution. The P-values for all tests are greater than 0.05, indicating that the distribution pattern of the strength of DMJ piles approximately follows a normal distribution at the 5% significance level.

5. Fluctuation Range

The parameters of geotechnical bodies demonstrate a certain degree of spatial variability, which can be attributed to the differences in the geological effects experienced by the bodies during their formation [33].

Two principal methods are typically employed to examine the spatial variability of geotechnical parameters: the random field method and geostatistical techniques.

The random field method employs autocorrelation functions to delineate the spatial variability of the soil body, whereas the geostatistical method utilizes semi-variational functions to characterize the spatial variability of the soil body [34].

This analysis was conducted using the autocorrelation function method. In order for the parameters of a geotechnical body to comply with the tenets of random field theory, they must adhere to the mathematical tenets of a smooth random field.

As demonstrated in the preceding section, the strength distribution of DMJ piles exhibits a normal distribution. The most commonly used autocorrelation functions are presented in

Table 5. Theoretical autocorrelation models and the corresponding SOF.

Type	Function	SOF
Mono-Exponential	$\rho \left(\tau \right)=e^{-\tau /{\mathrm{l}}_{\mathrm{h}}}$	$2{\mathrm{l}}_{\mathrm{h}}$
Gaussian	$\rho \left(\tau \right)=e^{-(\tau /{\mathrm{l}}_{\mathrm{h}}{)}^2}$	$\sqrt{\pi {\mathrm{l}}_{\mathrm{h}}}$
Second-Order Regression	$\rho \left(\tau \right)=e^{-\tau /{\mathrm{l}}_{\mathrm{h}}}(1+\tau /{\mathrm{l}}_{\mathrm{h}})$	$4{\mathrm{l}}_{\mathrm{h}}$
Exponential Cosine	$\rho \left(\tau \right)=e^{-\tau /{\mathrm{l}}_{\mathrm{h}}}\cos (\tau /{\mathrm{l}}_{\mathrm{h}})$	${\mathrm{l}}_{\mathrm{h}}$

Note: I_h is the fitting parameter, and τ is the hysteresis distance.

. In this study, the vertical fluctuation scale of the spatial variability of the cemented soil parameters is defined as the depth of different strength point locations from the ground surface. The autocorrelation function is of the mono-exponential type.

As the distance between two points in geotechnical body space increases, the correlation between geotechnical body parameters diminishes. When the distance in question exceeds a certain value, the correlation between the geotechnical parameters in question can be disregarded [35]. The distance in question is then defined as the scale of fluctuation (SOF). The scale of fluctuation (SOF) is an index that describes the spatial variability of soil parameters and has a significant impact on the study of geotechnical engineering reliability.

The autocorrelation function values for each intensity test point are calculated using the formula shown below:

$${\mathrm{r}}_{\mathrm{k}}={\displaystyle \frac{{\displaystyle \sum _{i=1}^n({\mathrm{h}}_{\mathrm{i}}-\overline{\mathrm{h}})\hspace{1em}({\mathrm{h}}_{\mathrm{i}+\mathrm{k}}-\overline{\mathrm{h}})}}{{\displaystyle \sum _{i=1}^n({\mathrm{h}}_{\mathrm{i}}-\overline{\mathrm{h}}{)}^2}}}$$

where h represents the strength value of each strength test point, $\overline{\mathrm{h}}$ represents the average value of the strength test points of each pile and n represents the number of strength test points, $0\le \mathrm{k}\le \mathrm{n}-1$. Moreover, r_k represents the value of the autocorrelation function at the k point.

Each sampled point is defined by three coordinates, namely x, y and z, which describe its position in space. In calculating the strength value of the cemented soil pile along the depth direction, it is ensured that the positional coordinates of its other two points remain unchanged. Furthermore, only the range of fluctuation of the strength value of its unconfined compressive strength in the vertical position is considered.

The autocorrelation function fitting curves for Pile 1 are presented in Figure 5. The SOF value of the center point of Pile 1 is 10.7, which indicates a significant correlation between its depth and strength within the depth range of 10 m. In contrast, the SOF value of the side point is 5.5, indicating a notable fluctuation in strength values at the side of the pile with no discernible correlation with depth. This finding aligns with the results of the unconfined compressive strength test. The maximum and minimum strengths of Pile 1 were both observed in the right-side core sample.

The autocorrelation function fitting curve for Pile 2 is presented in Figure 6. The SOF at the center point of Pile 2 is 74, which is markedly larger than that of the other pile types, indicating that the pile strength values at this location are significantly correlated with depth up to a correlation distance of 74 m. The SOF at the center point of pile No. 2 is 6, which is markedly larger than that of the other pile types.

The SOF of the strength of the core sample obtained by coring the side of pile No. 2 is 6, indicating that there is no discernible correlation between the strength of the core sample on the side of the pile and the depth within the depth range of 10 m. The results of the calculations demonstrate that there are notable discrepancies in the SOF between the various pile types of DMJ piles and between the different horizontal positions of the same pile type.

6. Resistivity Properties of Cemented Soils

The resistivity non-destructive testing technique is employed for the inspection of pile quality. It has been demonstrated that the strength of cemented soil exhibits a strong correlation with its resistivity, with the resistivity providing a more accurate reflection of the strength characteristics of cemented soil. Extensive research has investigated the resistivity properties of cement-stabilized soils. The development of empirical formulas or models for the resistivity of cemented soils can provide theoretical support for the application of the resistivity method in the evaluation of the strength of cemented soils [36].

6.1. Resistivity Test Methods

The core sample obtained from the field test is cut into a cylindrical shape, and its diameter and height are measured for resistivity testing. During the test, the ambient temperature is maintained at room temperature.

As illustrated in Figure 7, the resistivity tester selected was the FT-300A1, which consists of a core sample holder integrated with a four-electrode measurement system. Equipment manufacturer is Xigao Huadian Group Company Limited in Wuhan, China. Four wires are connected to the tester, with the outer two wires connected to the current path and the middle two wires connected to the voltage path. The test current is selected as low-frequency alternating current.

The electrochemical effect and kinetic phenomenon of direct current (DC) will alter the structure of the soil and the chemical composition of the pore water, the water content of the soil and other factors, thereby rendering the resistivity test results inaccurate [37]. Accordingly, an alternating current (AC) was selected for the test, with a low frequency chosen to circumvent electrode polarization.

The resistivity is calculated using the following method:

$$\rho =\mathrm{R}{\displaystyle \frac{S}{L}}$$

where is the resistivity of the specimen (Ω∙ m); R is the resistance of the specimen (Ω); S is the cross-sectional area of the current through the specimen (m²); and L is the distance between the electrode sheets (m).

Once the resistivity test is complete, the unconfined compressive strength test is initiated. After conducting the compressive strength test, the resulting crushed samples are utilized for moisture content testing. The water content is determined by calculating the mean of the values obtained from the three groups of samples taken during the test.

6.2. The Changing Law of Resistivity

Cemented soil is a complex multi-phase structure. The conductor within the cemented soil is constituted mainly of cement particles, soil particles and pore water. The interaction between these three components will largely affect the resistivity index of cemented soil. The objective of this test is to ascertain the correlation between cemented soil resistivity, unconfined compressive strength and water content.

Figure 8 illustrates the correlation between the unconfined compressive strength and resistivity of the cemented soil core samples obtained from the field test. It can be observed that the proportion of cement in cemented soil samples increases in line with the rise in unconfined compressive strength. This process results in a more complete hydration reaction of the cement, which in turn gives rise to an increase in resistivity [38].

The high cement content of some samples has resulted in a strength exceeding 10 MPa, accompanied by a markedly elevated resistivity. A linear correlation exists between strength and resistivity, as illustrated in Equation (4), where the vertical axis represents the unconfined compressive strength (MPa), the horizontal axis depicts the resistivity (Ω·m) and the R² value is 0.87.

$$\mathrm{y}=8.4+19\mathrm{x},\hspace{1em}{R}^2=0.87$$

The aforementioned outcomes can be attributed to the elevated cement content within the cemented soil. This has the effect of enhancing the cementing process, whereby the soil particles are cemented together, resulting in larger cemented soil aggregates and a reduction in pore space. Following the curing process, a portion of the cement powder fills the pores, thereby reducing the ratio of pore space within the soil.

The addition of cement powder will result in a chemical and physicochemical reaction with the pore water, thereby reducing the water content of the cemented soil. The electrical conductivity of pore water is significantly higher than that of soil particles and cement powder. Therefore, it can be concluded that the electrical conductivity of cemented soil is affected by the combination of water content and pore connectivity. As the cement content rises, the pore ratio reduces, and connectivity is compromised, accompanied by a reduction in water content. This results in a decline in cemented soil conductivity and an increase in resistivity [39].

The strength and resistivity of the cemented soil obtained from this test exhibited a linear functional relationship, and the measured data points also demonstrated a certain degree of dispersion on the correlation graph.

Furthermore, the strength and resistivity values were found to be significantly correlated with one another, as evidenced by the linear functional relationship observed between the two variables. The samples used in this test were obtained from an engineering site, where a range of factors, including variations in soil quality, cement mixing and mixing uniformity, may have influenced the strength and resistivity index of the cemented soil. These factors reflect the inherent and uncertain aspects of engineering construction.

Some scholars have reported that there is an obvious linear relationship between the strength and resistivity of cemented soil or a nonlinear relationship such as the power exponential relationship. This discrepancy can be attributed to a number of factors, including the variation in geological conditions between the dependent projects and the discrepancy between indoor and field tests [40].

7. Conclusions

This study investigated the formation of field-tested DMJ piles by evaluating the unconfined compressive strength of core samples. It evaluates the statistical distribution of strength and the variations in strength with depth. Additionally, it employs resistivity testing to assist in the assessment of cemented soil strength and reaches the following conclusions.

• The dosage of cement has a considerable impact on the quality of formation and the uniformity of strength of DMJ piles. A reduction in cement dosage will result in a significant reduction in the strength of the pile formation at the center of the pile in comparison to the strength at the side of the pile;
• The distribution pattern of pile strength for DMJ piles is observed to adhere to a normal distribution, with a 5% level of significance, as determined by the K-S test. The results of the SOF calculation indicate that the SOF of DMJ pile side points is 5.5 and 6, respectively. Additionally, there is no discernible correlation between depth and the strength distribution of the pile side points within the depth range of the pile body. Conversely, the strength distribution of the pile side is more discrete, while the strength distribution of the pile core is more uniform, exhibiting a stronger correlation with depth;
• The strength of DMJ pile core samples demonstrates a linear correlation with resistivity. The assessment of the strength of DMJ cemented soil piles can be conducted using the resistivity index, which is a simple and readily measurable parameter.