Reinforcement effects of ground treatment with dynamic compaction replacement in cold and saline soil regions

Yu Zhang , JianKun Liu , JianHong Fang , AnHua Xu

1. Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China

2. Beijing Jiaotong University, Beijing 100044, China

3. Qinghai Research Institute of Transportation, Xining, Qinghai 810001, China

1 Introduction

Saline soils of highway foundations have an average soluble salt content of over 0.3% at 1 m depth. The effect of saline soils to this foundation depends on the composition and properties of salt, the relation of salt to water, and the influence of natural conditions to the foundations stability.

With increased development and construction in most areas of northwestern China, a higher demand for saline soils foundation engineering is needed. Saline soils present increased complexity compared to frozen soils, expansive soils,and damp subsiding loess. Saline soil features include melt shrinkage, salt bulge and corrosiveness, all harmful to highway foundations and producing huge financial losses. Thus,choosing the right reinforcement method depends on factors such as salt content, groundwater levels, and meteorology.

In recent years, the Dynamic Compaction Replacement(DCR) method is widely used to consolidate soft soil foundations for short periods and simple construction. Numerous studies have obtained useful results from the DCR treatment of soft soils (Huanget al., 2006; Liuet al., 2006; Phear and Harris, 2008; Donget al., 2009). However, reinforcement mechanism of the DCR is not well researched in saline soils of the salt lake regions of China, where theoretical study is far behind engineering practices. Thus, it is difficult to design the correct theory and method. Moreover, the construction of the DCR lacks successful experience in this region.Based on the Qarhan–Golmud Highway project in saline soils, combined with working mechanism of pile (pier)-soil theory, the mechanism of DCR reinforcing highway saline soils roadbed was studied. The results prove that this foundation treatment method can not only improve soil strength,but also help the upward migration of salt water, thus providing a reference to designers.

2 Project summary

The Qarhan–Golmud Highway is located in the saline soil region of the Qarhan Salt Lake, Qinghai Province,China. This region has intense solar radiation, extended daylight hours, day-night temperature changes dramatically,and groundwater level is high. Under an arid climate, both dry and low wet saline soils have shrinkage features, such as ground lowering and deformation, enhanced under soil pressure weight or supplemental surface pressure (Liuet al., 2006). The highway stratum soil has a low liquid clay limit and a soft plastic state. The main ingredients are clay with minor amounts of clay powder, and a thin chloride salt crystal layer over strongly saline soil. Soil thickness is 2–10 m and bearing capacity of basic allowable values (σ) is 130–200 kPa. Salt crystals in this soil dissolve easily, are highly corrosive and produce shrinkage. These conditions produce a lowering and unfavorable natural foundation. Therefore, a reinforcement method of the DCR was applied in order to determine DCR capacity easily and effectively.

The main design parameters of DCR reinforcement methods are replacement pier diameter of 2.3 m and depth of 2 m (obtaining an area replacement ratio of about 19.2%),pier spacing of 5 m, tamping energy for 3,000 kN, and single pier design capacity greater than 300 kPa. DCR composite foundation design value is greater than 150 kPa.

3 Field test

A plate loading test was conducted on single gravel piles,single pile composite foundations and the soil between the piles. Parallel test groups are presented in Table 1. Loading and unloading test standards and procedures, loading tests curves and determined load results are "geotechnical engineering survey standards" (GB50021-2001, 2002).

In the plate loading test, the adopted load bearing plate is a rigid circular plate. Single gravel replacement pier and the soil between the piers bearing plate area is 0.785 m²(ECFTM, 2000; Gong, 2008).

The bearing capacity is determined by loading the failure loads. The former failure load level is the ultimate load and the allowable bearing capacity is equal to the ultimate load divided by safety factor; also the P-S curve can be drawn,andP0is set when a clear inflection point on the curve is the foundation bearing capacity. However, most of the experimental curve inflection point is not obvious; thus, a relative sedimentation method is adopted. The bearing capacity of the soil between the piers was determined by taking the relative deformation (S/D = 0.015), and single gravel replacement pier was determined by taking the relative deformation(S/D = 0.04). Calculated data is presented in Table 1.

Table 1 Data of loading tests

4 Composite foundation test data analysis

4.1 DCR bearing capacity analysis

Figure 1 shows the loading tests curves for single replacement pier and the soil between the piers. The curves are gradual smooth and regular which reflect the characteristics of granular materials.

For the DCR composite foundation load test, the bearing capacity of a single gravel replacement pier is 539.21 kPa; the soil between the piers bearing capacity is 262.63 kPa.

In accordance with the single pile and the soil between the piles bear the principles of the upper load, DCR composite foundation bearing capacity can be calculated as follows(ECFTM, 2000):

where:Rcf,RpfandRsfrespectively represent the composite foundation, single piles (piers) and the soil between piles (piers)standard value of bearing capacity;R1andR2respectively represents the piles reflection (piers) and the soil between piles(piers), the actual bearing capacity of soil correction coefficient relates to the foundation soil conditions and other factors. This also can be seen as the single pile and the soil between the piles that bears the upper load (Zheng, 2003; Li, 2005; Yanet al.,2005). Previous studies show that the single gravel pile bearing capacity is 402.89 kPa; the soil between the piles bearing capacity is 120.05 kPa; single pile composite foundation bearing capacity is 233.35 kPa.R1λ1andR2λ2equal to 1.5 (Zhanget al.,2013).

The Formula(1)also shows that gravel replacement pier and the soil between the piers can better play the bearing and the ability to resist deformation when saline soils were treated by DCR reinforcement. The field test was not conducted on the loading plate tests of DCR composite foundation because it is quite difficult to test. According to the area replacement rate,bearing plate round area of single pier composite foundation is equivalent to 21.63 m2, and the load weight of more than one thousand tons, which is difficult to test. Fortunately the bearing capacity of DCR composite foundation can be obtained through bearing capacity of single gravel replacement pier and the soil between the piers, and the aforementioned single pile composite foundation bearing capacity correction coefficient.The bearing capacity of a single gravel replacement pier and the soil between the piers, the area replacement rate and the coefficient correction capacityR1λ1andR2λ2were substituted into Formula(1); the bearing capacity of DCR composite foundation was 476.56 kPa.

Figure 1 Curves of loading tests to single replacement pier and the soil between the piers

4.2 Pier-soil stress ratio η and load share ratio δ

Stress ratio refers to the piers and the soil between the piers.This ratio provides an important indicator of composite foundation for both the pier and the soil between the piers bear load(Zhanget al., 2005). The stress ratio is affected by factors such as:pier and the soil between the piers stress-strain relationship, construction quality, soil properties, pier diameter, and graded grave.It can be calculated as follows:

whereEpandEsare piers modulus and the soil between the piers deformation modulus. According to data in Table 1, DCR composite foundation pier-soil stress ratio is 2.05.

The pier-soil load share ratio refers to the pier and the composite foundation of the assumed ratio of the load in the composite foundation. It can be calculated as follows:

Formula(2)substituted into Formula(3)can obtain:

Formula(4)shows that the pier-soil load share ratioδis concerned only with pier soil stress ratioηand replacement ratem. In the case of constantm,δvalue will increase with an increase ofηvalue; In the case of constantη,δvalue will increase with an increase ofmvalue. Thus, the calculated piers-soil load share ratioδis 32.8% in the DCR composite foundation. Thus,most of the load was borne by the soil between the piers in the DCR of the piers. This also shows that compaction effect of the soil between the piers is the main reason for the improvement of the bearing capacity of composite foundation. The piers on the stress concentration are a secondary cause for bearing capacity of composite foundation improvement.

4.3 Analysis and comparison of composite foundation deformation modulus

Deformation calculation of composite foundation is to calculate the value of the deformation modulusE. In addition, deformation modulus of composite foundation is determined by deformation modulus of pile (piers) and the soil between the piles (piers) as follows:

TheR1λ1andR2λ2values are the same as in Formula(1),then substituted into Formula(5)which provided the following:

whereEcs,EpandEsare the deformation modulus of composite foundation, deformation modulus of piers and the soil between the piers, respectively. TheR1λ1andR2λ2values are the same as in Formula(5), the DCR reinforcement treatment replacement ratiomand pier-soil stress ratio were substituted into Formula(6)and calculated. The deformation modulus of DCR composite foundation is 20.53 MPa.

Due to the location of the salt lake regions, and distribution of subsurface groundwater, DCR was beneficial to have formed the gravel piles for drainage.

4.4 DCR settlement reduction coefficient

Settlement reduction coefficient refers to settlement after DCR treatment in the natural clay soil and the ratio of the natural foundation original settlement. This response to the effect of settlement reduction after reinforcement in the natural clay soil can be calculated as follows:

After calculation, DCR composite foundation settlement reduction coefficient is 0.83. Settlement reduction coefficient is only concerned with the replacement rate and piles-soil stress ratio, coordination of the two will determine the size of settlement reduction coefficient. Construction quality is guaranteed as long as the settlement has been reduced after the foundation is reinforced (Liu and Zhang, 2003; Niu, 2005).

5 Conclusions

DCR composite foundation was applied in saline soils and a field experiment was conducted in this area. The strength and working mechanism of pier-soil and deformation modulus of the composite foundation are analyzed after reinforcement, and are summarized as follows:

(1) DCR composite foundation can form a good drainage channel in containing groundwater in saline soil regions. This,can reduce soil groundwater level and help the upward migration of salt water, at the same time, improve soil strength, reduce the impact of salt expansion and provide good reinforcement effect.

(2) Gravel piers are flexible, and both piers and the soil between the piers bear the upper load. DCR composite foundation pier-soil stress ratio is 2.05.

(3) Through the coefficient, the bearing capacity and deformation modulus of DCR composite foundation can be determined. DCR can effectively reduce the settlement of the natural saline soil foundation. This data and analysis method can be used by practicing engineers.

The authors wish to acknowledge the support and motivation provided by National 973 Project of China (No.2012CB026104), and National Natural Science Foundation of China (No. 41171064) and (No. 41271072).

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