Experimental study on Down-hole Dynamic Compaction to reinforce the deep slag miscellaneous fill foundation of highways

doi:10.21203/rs.3.rs-4612032/v1

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Experimental study on Down-hole Dynamic Compaction to reinforce the deep slag miscellaneous fill foundation of highways

https://doi.org/10.21203/rs.3.rs-4612032/v1

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Down-hole dynamic compaction is one of the important engineering technology to solve the problems of insufficient bearing capacity and uneven settlement after construction for deep slag miscellaneous fill foundation. Based on the deep slag miscellaneous fill foundation treatment project of an expressway, several groups of field tests were carried out, such as different pile forming processes and different pile spacing. Through the super heavy dynamic penetration test and foundation bearing capacity test, the reinforcement effect of down-hole dynamic compaction on the soil between piles of miscellaneous fill foundation is studied. The results show that the down-hole dynamic compaction has a significant reinforcement effect on the deep slag miscellaneous fill. After treatment, the compactness of the undisturbed slag miscellaneous fill changes from loose to medium dense or dense, and the soil compression modulus increases significantly. In addition, according to the compaction effect of soil between piles, the improvement effect of punching pile with small pile diameter on the bearing and deformation capacity of foundation is better than that of rotary digging pile with large diameter. Finally, deep dynamic compaction in the hole will have a certain disturbance on the shallow soil. In engineering applications, after the completion of the pile, the surface soil should be tamped to ensure the overall treatment effect of the foundation.

Physical sciences/Engineering

Physical sciences/Engineering/Civil engineering

Highway subgrade

Reinforcement effect

Field test

Slag miscellaneous fill

Punching pile

Rotary digging pile

In the construction of expressways, due to the restrictions of many factors, such as linear indicators, the distribution of structures along the line, and the engineering environment, it is inevitable to cross some special geological areas. Especially in some towns where mineral resources are exhausted, the waste mine slag landfill area is more common, which makes the deep buried slag miscellaneous fill meet more and more problems in the current expressway engineering construction. Due to the complex nature of miscellaneous fill, poor compactness, large partial backfill depth and short time, when highway engineering structures pass through these areas, the foundation settlement and uneven settlement are large, which seriously restricts the engineering construction and safe operation of the highway.

For the treatment of slag miscellaneous fill foundation, surcharge preloading, excavation and backfilling or replacement, dynamic compaction, deep treatment of pile composite foundation and other measures are generally adopted in the project. However, the above measures have certain application limitations. They either cannot effectively deal with deep layers of slag miscellaneous or are too costly. Based on the above problems, down-hole dynamic compaction technology has been gradually applied to the treatment of slag miscellaneous fill in recent years.

The down-hole dynamic compaction technology, based on the theory of cylindrical cavity expansion [1–5], involves the vertical compaction and lateral squeezing of fill material within boreholes by the impact of a compaction hammer during its descent. This process results in densification of the subsoil and an increase in bearing capacity [6–9].

The down-hole dynamic compaction technology was first introduced in 1994, but its depth of foundation treatment was limited to around 4m due to constraints posed by construction machinery [10]. In 2003, Professor Si [11] pioneered the down-hole dynamic compaction technology, which further increased the depth of treatment and its effectiveness. With the continuous development of construction machinery, compaction piles have been further applied and have shown promising results in various complex foundations such as soft clay foundations and deep collapsible loess foundations. This technology not only reinforces the foundation to meet upper structure bearing capacity and deformation requirements but also reduces costs and minimizes environmental pollution.

Yu R and Li E-W [12] illustrated the technical characteristics of down-hole dynamic compaction piles and compaction piles in treating collapsible loess foundation through engineering examples. The study indicated that down-hole dynamic compaction were capable of treating thicker soil layers compared to compaction piles. In terms of treatment effectiveness, the overall stiffness of the foundation after down-hole dynamic compaction was uniform, with a potential increase in bearing capacity by 2–9 times, high deformation modulus, minimal foundation settlement, and immunity to groundwater influence. Additionally, literature [13–17] has also conducted relevant research on the treatment of collapsible loess foundations and contributed to the application of down-hole dynamic compaction method for such foundations. Kong Y et al. [18] carried out an in-situ water immersion test study on a DDC composite foundation at the Malan loess site in Heping Town, Lanzhou with a collapsible soil layer thickness of 36.5m over a duration of 137 days. They recommended a reasonable spacing of 1.1m between DDC method piles with a length of 20m.

Zhong P et al. [19] summarized the influencing factors of down-hole dynamic compaction method and provided an amended formula for its bearing capacity based on the principle of dynamic compaction method. Yang P [20] studied the long spiral drilling in-pile dynamic compaction crushed stone pile construction technology in garbage landfill foundation, which improved the construction quality, saved construction costs, and met environmental requirements. Sun Q-J [21] investigated the bearing characteristics of down-hole dynamic compaction piles and drilled-injection piles composite foundation using numerical analysis and theoretical research. Hu G et al. [22] achieved favorable practical results in treating soft weak foundations in a high-filling airport clay loam. Qiu C-J et al. [23] treated soft weak foundations in a clay loam airport in Wulong using down-hole dynamic compaction piles and tested the treatment effect through various means, demonstrating that the down-hole dynamic compaction method has good treatment effects for soft weak foundations. Yang X-H et al. [24] proposed reinforcing soft weak foundations using cement-lime + steel slag crushed stone rammed piles and compared compared with the mechanism of a traditional strengthening scheme of cement mixing pile. It proved its economic and overall advantages. Fan M-J [25] used down-hole dynamic compaction to treat building waste landfill foundations and studied the design parameters of deep-layer dynamic compaction.

The above research mainly focuses on the application of deep compaction in treating loess and weak soil, while there is less research on the treatment of mixed fill soil, especially deep mixed fill soil. Deep mixed fill soil is complex in composition and has varying thickness, so different construction methods (drilling or rotary digging pile), as well as corresponding construction parameters and design parameters, have a great impact on the treatment effect. Therefore, this project conducts on-site tests and selects a region of slag miscellaneous fill with a depth of over 40 meters, studying the treatment effect of different drilling methods (drilling or rotary digging pile) and different spacing and diameter of piles on deep mixed fill soil. This can provide reference for the application of deep compaction technology in deep mixed fill soil areas.

1.1 Project background

There are many slag accumulation sites along an expressway project in Henan Province, which began to be backfilled in 2014 and resumed cultivation from 2017 to 2018. The slag accumulation area along the project is wide. The slag deposit has a large void ratio, loose filling, deep thickness and uneven distribution, and the maximum filling depth is 41.2m. Due to the complex nature of miscellaneous fill, the post construction settlement is a big test for the project quality. The bridge scheme was selected in the scheme demonstration, but the negative friction of pile foundation is large and the project cost is high. Finally, the scheme of subgrade passing is adopted. The maximum filling height of subgrade slope is 24.7m.

1.2 Engineering geology

In combination with the site survey, the geological composition of the site foundation from top to bottom is as follows:

1. Slag miscellaneous fill: The surface layer is the reclamation backfill with developed vegetation, and the lower part is mainly composed of silty clay and gravelly soil alternately. Silty clay is plasticity- hard plastic state, mainly clay. It contains a large number of ferromanganese oxides and occasional calcareous nodules. The parent rock of gravelly soil is mainly composed of marl, and the main mineral components are calcite and clay minerals. Partially filled with a small amount of silty clay, accounting for about 30–50%, with large pores, loose filling and uneven thickness.

2. Strongly weathered marl: Brownish gray, strongly weathered, argillaceous texture, layered structure, undeveloped joint fissures, calcite filled fissures, karst development, mainly composed of calcite and carbonatite, rock core in column shape, column length of 6-17cm, brittle hammering sound.

3. Strongly moderately weathered marl: It is strongly weathered, grayish white to grayish brown, the rock core is broken-relatively broken, locally mixed with limestone, mostly in thin layer, limestone and dolomite are mostly in fragment shape, joint fissures are relatively developed, and the rock mass is broken.

2.1 Test scheme

At present, the construction technology of down-hole dynamic compaction mainly includes punching tamping and rotary digging tamping. As shown in Fig. 1, punching tamping are mainly carried out by tamping a small hole with a heavy hammer in advance, and then tamping and expanding the hole to the design pile diameter with filler. For rotary excavation and tamping, part of the soil is taken out by the long spiral drill to form holes, then the filler is put into tamp and expand to the design pile diameter. Punching and tamping are mainly used for small hole forming, and the depth is limited. Rotary digging and tamping are mainly used for hole forming of large diameter piles, and the pile depth can reach more than 30m. The two methods have different construction technology, and the treatment effect of the foundation is also different.

In order to study the influence of hole forming method, pile diameter and pile spacing on the foundation reinforcement effect, a total of 8 groups of test piles were carried out, as shown in Table 1. Among them, 4 groups of punching piles which the soil were punched a hole of 0.5m firstly and filled material and compacted the hole to 0.8m were formed, with pile spacing of 2.2m, 2.4m,2.7m and 3.1m respectively; 4 groups of rotary digging piles which the soil were digged a hole of 1.2m firstly and filled material and compacted the hole to 1.8m were formed, with pile spacing of 3.7m,4.0m,4.5m and 4.6m respectively;

12 piles are arranged and 3 test piles were located at the center of each group, as shown in Fig. 2.

Table 1

Experimental program
Test pile group number	Hole Forming Method and Diameter (m)	Pile diameter (m)	Pile spacing (m)
Group 1	Percussion drill 0.5	0.8	2.2
Group 2	Percussion drill 0.5	0.8	2.4
Group 3	Percussion drill 0.5	0.8	2.7
Group 4	Percussion drill 0.5	0.8	3.1
Group 5	Rotary 1.2	1.8	3.7
Group 6	Rotary 1.2	1.8	4.0
Group 7	Rotary 1.2	1.8	4.5
Group 8	Rotary 1.2	1.8	5.2

The original slag miscellaneous fill at the site consists primarily of silty clay and marl clasts with no adverse engineering properties. Therefore, this down-hole dynamic compaction backfill is directly backfilled with slag from the vicinity of the site and from rotary excavation after preliminary screening to ensure that the maximum particle size of the backfill is less than 1/3 of the pore diameter. Meanwhile, in the process of pile formation, the thickness of each filling material is 2m, and the termination criterion is that the penetration of the last blow of repeated tamping is less than 15cm.

2.2 Test contents

(1) Foundation compactness test

From previous engineering experience, dynamic penetration is an important means to detect the effect of foundation compaction and reinforcement. Influenced by the particle composition of the slag backfill at the test site, the super-heavy dynamic penetration test (shown in Fig. 3) was used on site, and the degree of foundation soil compaction was evaluated according to the test index value of N₁₂₀ (see Table 2). In order to test the effect of down-hole dynamic compaction on the encryption of the soil between piles, super-heavy standard penetration tests were conducted on the soil between piles at the center distance from the center location before and after pile formation. Each group of 3 holes, the hole position is located in the center of the test pile, as shown in Fig. 1. During the test, the number of strikes was stopped when the number of strikes exceeded 50 strikes for three consecutive times.

Table 2

Classification scheme for soil compactness for super-heavy dynamic penetration
Super-heavy dynamic penetration index value N₁₂₀	Compactness	Super-heavy dynamic penetration index value N₁₂₀	Compactness
N₁₂₀ ≤ 3	loose	11 < N₁₂₀ ≤ 14	dense
3 < N₁₂₀ ≤ 6	slightly dense	N₁₂₀ > 14	Extremely dense.
6 < N₁₂₀ ≤ 11	medium dense

(2) Load bearing capacity test

Before the construction of test piles, the original foundation was subjected to flat plate load test to derive the bearing capacity of the natural foundation. After 14d after the end of test pile construction, the bearing capacity test of single pile composite foundation was conducted by pile load method (as shown in Fig. 4). Each group is not less than 3 piles and the test piles are located in the center of each group as shown in Fig. 2.

3.1 Effect of soil compacting between piles

In order to evaluate the foundation treatment effect of down-hole dynamic compaction, touchdown strike number N₁₂₀ was used to evaluate the degree of compactness of the in-situ soil before reinforcement and the reinforced inter-pile soil. Meanwhile, in order to analyze the reinforcing effect of different pile forming processes, the additional volume of compacted soil per square meter was used for evaluation. The compaction factor λ is defined as being the volume of additional rammed-in soil per unit length of hole/area treated by a single pile. The calculated λ for test pile groups 1 to 8 are 12.0%、10.1%、8.0%、6.0%、11.9%、10.2%、8.1% and 6.0% respectively.

(1) Effect of different hole formation methods on soil compacting between piles

In order to evaluate the effectiveness of the two methods of hole formation in compacting the soil between the piles, the weighted average number of penetration blows of the soil between the piles was used for the study. The coefficient β was defined as the ratio of the weighted average number of penetration hits of the soil between piles after reinforcement to the weighted average number of penetration hits of the soil between piles before reinforcement. The trend of β for the two pile forming methods is shown in Fig. 5.

Table 3

Compression modulus of soil after treatment
Pile forming method	Distance between piles	Pile spacing/pile diameter	Compression modulus (MPa)	Improvement over in situ soil (%)	Note
percussion drill	2.2	2.8	72.28	226.6	Compression modulus of in-situ soil 22.13MPa
	2.4	3.0	52.22	136.0
	2.7	3.4	36.88	66.7
	3.1	3.9	32.16	45.3
rotary	3.7	2.1	47.50	114.6
	4.0	2.2	39.83	80.0
	4.5	2.5	29.80	34.7
	5.2	2.9	23.90	8.0

From Fig. 5, it can be seen that the compacting effect of pile by percussion drill is larger than that of rotating into holes under the same compacting factor λ. From the interpolation method, the β of pile by percussion drill is 4.4 when λ is 6.0%, which is 46.7% higher than that of rotating into holes of 3.0; the β of pile by percussion drill is 11.2 when λ is 12.0%, which is 60.0% higher than that of rotating into holes of 7.0, and the improvement effect is remarkable. And with the increase of λ, the β of percussion drill increases nonlinearly, and the compaction effect becomes more and more significant. However, the β of rotating into holes increases almost linearly with λ, once again proving that the compaction effect of percussion drill is better than that of rotary digging pile. On the one hand, this is related to the way of soil extraction for rotating into holes to form holes. The soil around the hole is unsupported during hole formation, and the soil stress is released. And when the filler is rammed and expanded into the hole, the soil is squeezed, resulting in the soil between piles being subjected to the destructive effect of unloading + loading for a short period of time, which is not conducive to the transfer of the squeezing load to the surrounding. On the other hand, the diameter of rotating into holes pile is larger than that of percussion drill. According to the relationship between pile spacing and pile diameter in the project, rotating into holes pile spacing is larger, which in turn leads to smaller soil encryption effect between pile centers at the midpoint. Therefore, in terms of the uniformity of reinforcement and reinforcement effect, the reinforcement effect of the process of rotating into holes by percussion drill is better than that of rotating into holes for miscellaneous soil in terms of the reinforcement uniformity and reinforcement effect.

The slag miscellaneous fill has non-homogeneity and there is some variation in the number of penetration blows for each group of foundation soils. Therefore, the 24 test holes of the site in-situ soil were averaged by depth-weighted average. The calculated penetration hit number of the in-situ soil was 2.7, and the weighted compression modulus of the foundation was calculated to be 22.13 MPa according to the literature ^[14]. The weighted compressive modulus of each group of composite foundations after down-hole dynamic compaction treatment is shown in Table 3. As can be seen from Table 3 after the down-hole dynamic compaction treatment, the compression modulus of the soils in each group was improved. In the eight groups of tests, the lowest increase was nearly 8.0%. As for the pile by percussion drill method, the compression modulus was increased by 226.6% when the pile spacing was 2.2m, and the improvement effect was significant. It can be seen that down-hole dynamic compaction has a good effect on improving the deformation properties of slag miscellaneous fill. In addition, for punching piles, the increase in compressive modulus is 45.3%~226.6% when the change in pile spacing/diameter is 2.8 ~ 3.9, while for rotary digging piles, even when the pile spacing/diameter is smaller than that of punching piles, the increase is only 8.0%~114.6%. Therefore, the ratio of pile by percussion drill to pile diameter can be increased appropriately under the condition of meeting the design requirements of post-work settlement in engineering applications.

(2) Effect of compacting soil between piles of different depths

The penetration numbers of foundation soil before and after treatment by different pile forming methods at different depths were obtained by averaging the penetration numbers of three groups of test piles, as shown in Figs. 6 and 7. As can be seen from the figure, the density of the original miscellaneous fill does not vary much with depth, and the penetration number is relatively small, generally between 1 and 3. The location of some penetration number mutation should be caused by the larger blocks of slag miscellaneous fill. As can be seen from Table 2, the original miscellaneous fill is basically in a loose state. And after the down-hole dynamic compaction treatment, the penetration number of soil between piles showed a substantial increase. And the deeper the depth, the greater the increase due to the limiting effect of the upper soil body. In 2 ~ 3m depth, it can reach more than 6 hits, and in 8 ~ 10m, it can reach about 50 hits. After the treatment of down-hole dynamic compaction, the foundation soil, except for the upper 2 ~ 3m range, basically reaches the state of medium dense to dense, the reinforcement effect is remarkable.

In addition, the penetration number of pile by percussion drill increases rapidly with depth at basically the same pile spacing/diameter. The penetration number was greater than 50 at a depth of 7m. The penetration number of pile by percussion drill increases gradually with depth, but the increase is smaller than that of percussion drill. And it reaches 50 blows at about 10.5m, which is about 3.5m deeper than the pile by percussion drill. Therefore, the encryption effect of pile by percussion drill is significantly better than that of rotary digging pile at depth.

(3) Effect of different pile spacing on compacting soil between piles

In order to evaluate the effect of pile spacing on the soil between piles reinforcement, the ratio of penetration number before and after reinforcement was used for analysis. Figure 8 and Fig. 9 show the soil between piles encryption effect with different pile spacing under two different pile forming methods. As can be seen from the figures, the penetration number ratio in the upper 4m depth range is not very sensitive to the pile spacing, and the reinforcement effect becomes more and more significant with the reduction of pile spacing below 4m. In addition, as can be seen from Fig. 5, for the 3rd group of test piles, the reinforcement effect of the lower part is smaller than that of the upper part, which is analyzed that the location is mainly due to the fact that the soil is more dense in the depth range of 5 ~ 9m and basically reaches a slightly dense state, while the group of test piles with a larger pile spacing has a slightly poorer effect on the extrusion of soil between piles.

3.2 Analysis of single pile composite foundation bearing capacity

The static load test was used to test the on-site foundation bearing capacity and single pile composite foundation bearing capacity. The characteristic values of bearing capacity of each group of three single pile composite foundations satisfy that the extreme difference does not exceed 30% of the average value, so the average value of each group of test piles is taken for analysis. The test results are shown in Fig. 10.

As can be seen from Fig. 10, the effect of down-hole dynamic compaction on foundation bearing capacity is obvious. The foundation bearing capacity increases gradually with the increase of compact coefficient and it showed an upward convex nonlinear increase. However, the growth rate of punching pile is significantly greater than that of rotary digging pile. Under the pile by percussion drill method, the foundation bearing capacity was increased by 35.4% with a compactness factor of 12%. For rotary digging pile, the foundation bearing capacity was increased by 19.7%. The foundation bearing capacity was substantially increased under both pile forming methods. As far as the improvement of foundation bearing capacity is concerned, compared with rotating into holes, the pile by percussion drill has some advantages. However, in terms of compression modulus, the improvement effect of the two methods is not significant. It is analyzed that the use of the above two pile forming techniques both produce certain disturbance to the shallow soil, while the load plate of the load test has limited influence depth, which cannot bring out the advantage of percussion drilling to compact the soil in the lower part. Therefore, in engineering applications, the surface soil should be compacted after the completion of pile forming.

(1) The down-hole dynamic compaction has good technical and environmental advantages, and is suitable for dealing with deep miscellaneous fills. Materials such as slag crushed stone materials can be used in situ, solid waste utilization, economic and environmental protection. This construction technology has the advantages of low vibration, low noise, no pollution, no nuisance, etc., and is suitable for the existence of vibration-sensitive areas around the project, so the scope of application is wider.

(2) The effect of down-hole dynamic compaction on the encryption of slag miscellaneous fill is remarkable. From the study of this paper, the compactness of the original foundation is basically loose state, and after treatment, its compactness is greatly improved. The soil is encrypted from loose state to medium dense and dense state, and the compression modulus of the soil is improved significantly, which will have a substantial reduction in the post-work settlement of the project.

(3) According to the improvement of compression modulus and bearing capacity of soil between piles, pile by percussion drill is more significant due to soil squeezing effect. The improvement of bearing capacity and deformation capacity of foundation by percussion drill with small pile diameter is better than rotating into holes with large diameter.

(4) The down-hole dynamic compaction produces certain disturbance to the shallow soil. In engineering application, after the completion of pile forming, the top soil layer should be compacted to give full play to the effect of down-hole dynamic compaction on the middle and lower soil layers. When only considering the pile spacing, the foundation bearing capacity improvement factor for pile by percussion drill can be taken as the high value of the range, and the low value for rotating into holes is appropriate.

Authors’ contributions

All authors contributed to the study of this project. "Tao Chen"wrote the main manuscript text and "Weizhong Wang "prepared figures 1-5. All authors reviewed the manuscript.

Funding

This research was supported by Natural Science Foundation of Henan Province (242300421253); Key Scientific Research Program of Higher Education Institutions in Henan Province (24A580002); Key Research and Development Special Project of Henan Province (231111322100)；The Civil Engineering brand specialty construction project of private universities in Henan Province (ZLG201702).

Data availability

Corresponding author could be contacted if someone wants to request the data from this study.

Competing interests

The authors declare that they have no competing interests.

Received: 20 June 2024

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Experimental study on Down-hole Dynamic Compaction to reinforce the deep slag miscellaneous fill foundation of highways

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Abstract

Figures

0 Introduction

1 Project overview

2 Field test

3 Discussion of test results

4 Conclusion

Declarations

References

Additional Declarations

Status:

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