Experimental Study of Soft Clay Soil Improvement by Deep Mixing Method

Experimental Study of Soft Clay Soil Improvement by Deep Mixing Method

Maysa Salem Al-Qaisi Maki Jafar Mohammed Al-Waily

Al-Furat Al-Awsat Technical University, Al Musaib Technical College, Kufa 54003, Iraq

Corresponding Author Email: 
maki_jafar@atu.edu.iq
Page: 
224-232
|
DOI: 
https://doi.org/10.18280/mmep.090128
Received: 
29 August 2021
|
Accepted: 
18 November 2021
|
Published: 
28 February 2022
| Citation

OPEN ACCESS

Abstract: 

The deep method (DMM) is a soil remediation method that involves on-site ‎mixing of soil with cement and/or other materials. These compounds, which ‎are also known as "bonding materials," can be applied dry or wet. The current ‎study involves the construction of 13 laboratory models to examine the means ‎of improving soft clay soil qualities through deep mixing techniques with ‎piling foundation. In the dry condition, static loading studies on piles and ‎DMM were carried out using tow materials, cement, and lime. The model ‎experiments included a single pile as well as groups of piles and cement or ‎lime columns. There were two, three, and four piles or columns in each group. ‎The model tests revealed that deep mixing had a significant impact on ‎increasing bearing capacity by averaged times ranging from 1.23 to 2.43 times ‎for soft clay soil treated with single and groups of four cement or lime ‎columns, respectively, as well as minimizing settlement by averaged ‎percentages ranging from 33% to 89 percent. These results were comparable ‎to those obtained using pile foundations in the same manner. The outcomes of ‎the model tests were also evaluated in terms of group efficiency.

Keywords: 

clay, soft, DMM, cement, lime, pile

1. Introduction

Due to complex influencing conditions, the issue of soil ‎carrying capacity becomes crucial ‎when structures must be ‎built on soft and low strength soil deposits. As a result, ‎various ground ‎improvement methods for increasing soil ‎strength parameters have been devised [1]. Enhancing ‎a ‎soil's physical/mechanical properties without altering its ‎natural composition is referred to as ‎soil improvement. The ‎soil subbase is improved using knitted materials or other ‎methods that ‎link soil particles. In the 1970s, Japan ‎developed the DSM (Deep Soil Mixing)‎‏ ‏‎column ‎process ‎‎(Figure 1). The primary purpose of cement-soil ‎columns is to enhance the properties of local soils ‎‎(such as ‎strength and stiffness) by adding different cementing ‎materials. The most often ‎used binders are ‎cement and lime [2, 3].

DMT is one of the most common ‎and yet relatively ‎recent methods of improving the ‎strength ‎and deformation characteristics of weak soil ‎layers. ‎This method can be used in fine sand and ‎highly ‎organic soils. The procedure of DMT includes ‎mixing the ‎in-situ soil with a certain ‎proportion of cement or lime or a ‎combination of both in ‎slurry or powder form, depending ‎on ‎the initial water content. This is achieved by using ‎special ‎augers and vertical columns of ‎improved soil with ‎the desired center to center distance re ‎eventually formed. If ‎the binders are ‎used in the slurry form, the method is called ‎‎“wet DMT”; ‎on the other hand, if they are used in ‎powder ‎form, it is called “dry DMT”. The latter is only ‎applicable ‎where the water table is near ‎the ground surface. The ‎effectiveness of DMT in ‎the enhancement of soil strength was ‎the ‎motivation behind several comprehensive studies ‎on ‎different aspects of this technique from ‎basic concepts to ‎applications [4-9].

Lime is frequently ‎used to strengthen ‎cohesive soil. The shear strength of the stabilized soil ‎gradually increases ‎over time. This increase in strength is ‎influenced by soil type, lime content, lime type, the density ‎of ‎the compacted material, and curing conditions, including the ‎period between mixing and ‎compaction of the soil. Lime ‎columns can be used to support light constructions or to ‎strengthen ‎road embankments. The columns and the soil ‎contained by the columns from a more or less stiff ‎block, ‎with minor differential settlements within the block [10].‎ Fundamental metrics such as after-curing void ratio (eot) ‎and cement content (Aw) have ‎been ‎proven to be adequate ‎for characterizing the strength and compressibility ‎of ‎cement-admixed ‎clay at high water contents. The ratio ‎eot/Aw has been proved to ‎integrate the effects of clay ‎water ‎content, cement content, and curing time on the ‎strength of ‎cement-admixed clay based ‎on analysis made on the results ‎of unconfined ‎compression tests [11].

Unconfined compression experiments have traditionally ‎been performed on lime and ‎cement-treated soft clays with ‎parametric variations in the additive amount and curing ‎time. ‎The pioneering ‎work on lime-treated clays in three-dimensional systems is summarized ‎under ‎untrained, ‎drained, isotopically, and anisotropic ‎circumstances. The (p, q, e) plot as ‎typically applied for ‎the ‎understanding of triaxial data presents all data. The ‎break-up of ‎the cement bonds is observed ‎with increased ‎normal stress and the stress of the deviator. ‎The loss of ‎strength from highest to ‎critical conditions with increasing ‎shear stress is ‎also shown [12]. The 55-kilometer Bangna-‎Bangpakong Highway in Thailand has severe ‎settlement and ‎stability ‎issues. The deep mixing technique (DMM) with ‎soil-cement ‎utilizing conventional Portland ‎cement was ‎used for foundation enhancement on this ‎major arterial road. ‎The shear strength of ‎the surrounding soft clay (soil failure) ‎or the ‎shear strength of the cement pile regulates the ‎bearing ‎capacity of a single cement pile ‎‎(pile failure). Settlement ‎assessments of treated ground ‎were performed for cement ‎pile ‎lengths of 14, 16, and 18 m at 1.5, 1.7, and 2.0 m ‎spacing. The ‎‎1.5 m spacing results in ‎the least amount of ‎settlement and the most significant settlement ‎decrease ‎‎[13]. ‎Swelling of expansive soils and related foundation ‎movements causes many structures ‎‎considerable problems. ‎With the enormous clays that exist in Ankara. In order ‎to ‎increase the ‎physical properties, swelling, strength and ‎strength consolidation of that ‎clay, the performance ‎of the ‎lime column approach was examined on laboratory ‎size ‎models [14].

Figure 1. Deep mixing methods [3]

Comprehensive laboratory and field study ‎program to design a five-‎hectare area of land growth using ‎DSM are explored. Site inspection, drilling, sampling, ‎and ‎field testing using CPTU and VST were all included in the ‎scope of work. The ‎laboratory research was undertaken to ‎optimize the mix structure and consistency of the ‎cement ‎‎[15]. State-of-the-art and recent advances in Deep Soil ‎Mixing, emphasizing the ‎European perspective is discussed. ‎The debate is accompanied by a short review of ‎significant ‎historical developments that have retained their ‎importance until recently an updated general ‎classification ‎scheme for Soil Mixing that allows a comprehensive ‎understanding of the old and ‎new machinery and machines ‎designed and improved soil. The focus is on wet soil mixing ‎and ‎its application to concrete foundations such as highway ‎and railway viaducts and modern wind ‎turbines [16]. The ‎spatial correlation effects on the overall behavior of a finite ‎element analysis ‎axially loaded column, in which the ‎strength and the module of the material are regarded ‎as ‎random fields. The mean and standards difference in the ‎general strength of a full-sized column ‎might be easily ‎achieved with this method. Therefore, in determining design ‎values these results ‎are likely to be more scientific [17].‎ Two full-scale experiments to determine lime-‎cement ‎column panels' field behavior as ‎excavation support ‎are conducted. Both of these experiments ‎began with the ‎excavation of a ‎braced steel sheet pile wall backed by panels ‎of alternating lime-‎cement columns to a ‎predetermined ‎depth, followed by a stepwise increase in the load ‎added ‎behind the sheet pile ‎wall. These experiments ‎demonstrated that column-type ground ‎improvement ‎constructed as ‎panels of alternating columns within the ‎passive zone of a sheet ‎pile wall significantly ‎improves ‎stabilization and decreases structural stresses, ‎vertical and ‎horizontal displacements in the soil ‎caused by ‎excavation and stacking [18].‎‏ ‏Montmorillonite's (MMT) ‎impact on the bearing potential of soft soil reinforced by ‎the ‎columns of cement-deep mix (CDM) is designed for the ‎simulation of laboratory model. ‎The findings indicated that ‎soil concrete specimens practically linearly decrease in ‎their ‎MMT content, an unconfined compressive strength. ‎The bearing capacity is reduced ‎when the MMT ‎concentration increases[19].

2. Experimental Works

2.1 Testing program‎ ‎

The current research included thirteen laboratory models, ‎one of which was for untreated soft ‎clay soil. Four ‎soil ‎models were treated in labs using cement columns utilizing ‎the dry method. ‎Four model tests were carried ‎out to ‎evaluate the soft clay soil after it had been treated with ‎dry ‎lime columns. The mat foundation has ‎been ‎strengthened with a pile foundation to ‎accommodate the ‎conduct of the last four model tests on the mat ‎foundation, ‎the model tests ‎were carried out using single, double, three, ‎and four cement/lime columns or ‎piles, according ‎to the ‎chosen layout. All model tests have been conducted twice to ‎verify the accuracy of the ‎test ‎results. To accomplish this, ‎ ‎three soil specimens have been prepared and two have been ‎tested ‎for all ‎investigations. The gap between results was ‎confined to less than 5%. If it exceeded 5 ‎‏%‏‎, the third ‎specimen ‎was tested and the average findings were obtained, ‎unless the data ‎were removed.

2.2 Materials

2.2.1 Soil

‎The study results revealed that 10% sand and 38% silt, ‎and 52% clay were used in carrying up ‎the model tests. The ‎soil is inorganic sandy silt clay called CL, according to the ‎Unified Soil ‎Classification System. The physical properties ‎of the soil are presented in Table 1. ‎

Table 1. Soil Characteristics‎

Property

Value

Standard

Liquid limit, L.L. (%)‎

‎42‎

ASTM D4318 [20]

Plastic limit, P.L. (%)‎

‎25‎

ASTM D4318

Plasticity index, P.I. (%)‎

‎17‎

ASTM D4318

Specific Gravity

‎2.66‎

ASTM D854 [21]

Maximum dry unit weight ‎‎(kN/m3)‎

‎19.30‎

ASTM D698 [22]

Degree of Saturation

‎100%‎

 

Optimum moisture content

‎14%‎

ASTM D2216‎ [23]

Sand content

‎10‎%

 

Silt content

‎38‎%

 

Classification (Unified Soil ‎Classification ‎System)‎

CL

ASTM D2487 [24]

2.2.2 Cement ‎

This investigation made use of sulfate-resistant Portland ‎cement. The properties of the cement ‎used according to ‎Standard Test Methods for Chemical Analysis of Hydraulic ‎Cement, ASTM,‎ C114 – 11a are mentioned in Table 2 ‎[25].

Table 2. Cement Properties‎

Property

Value

Chemical Properties

SiO2

‎22.40‎

Al2O3

‎3.75‎

Fe2O3

‎4.50‎

2.2.3 Lime ‎

‎As a stabilizer, a commercial quick lime was utilized in ‎this experiment; the physical and ‎chemical properties of the ‎lime employed are listed in Table 3‎‏ ‏according to ‎Standard ‎Specification for Quicklime and Hydrated Lime ‎for Soil Stabilization, ASTM C 977 – 03 [26]‎. ‎

Table 3. Cement Properties‎

Property

Value

CaO (%)‎

‎88.40‎

MgO (%)‎

‎1.75‎

Carbon Dioxide

‎3.45‎

Free Moisture%‎

‎0‎

Retained on (24.4-mm) ‎sieve

0

2.3 Configuration of the test

2.3.1 Soil tank ‎

The model tests were conducted in a steel test tank with ‎dimensions of 500 mm x 500 mm ‎x 500 mm and ‎constructed of steel plates 6 mm in thickness as shown in ‎Figure 2. The container ‎is sufficiently stable and ‎demonstrated no lateral deformation during soil preparation ‎and ‎testing.‎

2.3.2 The loading frame

The entire configuration, mainly consisting of the steel ‎container, loading frame, Linear variable differential transducer (LVDT), 0.001 ‎mm accurate, loading ‎cell with a capacity of 50 kN, logger data, and accessories, is ‎illustrated ‎in Figure 2.‎

Figure 2. Complete setup

2.3.3 The foundation plates and accessories ‎

The model tests were conducted on a square steel plate ‎‎150 mm wide and 10 mm thick that served as the square ‎foundation.‎

2.4 Model preparation and testing

A correlation between the water content and the soil's ‎undrained shear strength was established ‎prior to creating ‎the soil bed. This association would aid in sustaining the ‎required shear ‎intensity for each model. The shear strength ‎was determined using the unconfined compression ‎test. In ‎this research, the soil was produced at a moisture content of ‎‎28.2 percent, which ‎corresponds to undrained shear ‎strength, cu of 13 kPa (ASTM D 2166 [27]‎)‎. Following that, ‎the soil bed was prepared ‎as follows: To begin, the natural ‎soil was smashed with a hammer into small pieces and ‎then ‎allowed to air dry for 24 hours. Second, the soil sample ‎was split into 10-kilogram ‎portions and ‎sufficiently moistened. Following that, samples of moist ‎clay soil were gathered in ‎a large container and left to soak ‎for five days. During the period, the everyday temperature ‎was ‎monitored. After the required saturation period had ‎ended, the test tank was filled with saturated ‎clay soil in 10-‎cm-thick layers. To reach the necessary density of 18.25 ‎kN/m3, each layer was ‎tamped with a special tamping ‎hammer weighing 2 kg and measuring 50 mm x 50 mm. The ‎soil ‎model's final thickness was 400 mm. ‎

2.5 Construction of pile, cement, and lime columns

To begin, the soil bed's top layer was flattened. Later, a pile ‎or piles of 30 mm in diameter and ‎‎400 mm in length (i.e., ‎a length to diameter or (L/D) ratio of 13.33 was pressed into ‎the soil ‎block to the desired depth. The (L/D) ratio of the pile is an important factor in determining the pile's bearing capacity. This ratio was chosen for the pile model to fit the dimensions of the square foundation, which serves as a cap for the piles in this case, as well as to match the dimensions of the steel container. In the field applications, a ‎hollow tube is driven into the ground to the ‎required depth, ‎and the binding agent is delivered with air pressure into soil ‎holes, cracks, ‎fissures, and crevices as the tube is removed ‎‎[28]. ‎ The method has been replicated with groups ‎of ‎columns installed in the compact soil block. To construct the column, an auger was used to ‎make the hole to a depth of ‎‎400 mm. Each was ‎installed in blocks of soil using a ‎hollow ‎polyvinyl chloride (PVC) pipe, which was opened ‎on ‎both ends, of 30 mm of diameter and 550 ‎mm height. The ‎PVC length has been selected to help the ‎soil block ‎penetrate easily, creating ‎the columns with the features of ‎the compact soil blocks. ‎The advance of utilizing a PVC ‎pipe ‎was to make smooth holes in the soil block without ‎the ‎need of spiral brush cleaning the holes ‎before lime or ‎cement was filled with [14]. The columns were then filled ‎with a homogenous ‎amount of powdered cement or ‎quicklime. To form the lime piles, three successive ‎uniform ‎layers of cement or quicklime were placed and ‎lightly compacted. The center-to-center space ‎between each ‎pile and the next pile was 100 mm. The installation of a ‎single column within a ‎compacted soil block in the test tank ‎is depicted in Figures 3 to 5. ‎

Figure 3. Drilling the hole

Figure 4. Insertion the PVC

Figure 5. Fill the hole with cement or lime powder

2.6 Model testing procedure

At the end of the 24 hours, the footing assembly was placed ‎in position so that the center of the ‎footing coincides with ‎the center of the soil bed for all model tests (single or ‎groups of pile ‎and cement or lime columns). Meanwhile, ‎the incremental load of 50 N was then applied ‎centrally on ‎the footing. Each incremental load was left for (4 minutes) ‎or until the settlement ‎was no longer consistent, whichever ‎came first according to (ASTM. 2013a.). In the following step, a ‎linear variable ‎optical transformer with a resolution of 0.001 mm was ‎installed on the ‎foundation to document the settlement at the ‎ending of the period for each incremental load.

Figure 6. Three columns group after testing

Furthermore, the incremental load was added until the settlement reached approximately equal to 22 percent of the width of the footing, or about 33 mm, exceeding the permissible settlement limit for the purpose of studying the stress-strain relationship. Figures 6 to 9 show the column groups after testing.

Figure 7. Three columns group after extraction from the soil

Figure 8. Four columns group after testing

Figure 9. Three columns group after extraction from the soil

3. Presentation and Discussion

3.1 Definition of failure load

A single model test was conducted on an untreated ‎saturated soft clay soil. ‎ The footing was ‎placed on the soil ‎surface and gradually loaded until failure. The effects of ‎applied pressure in ‎‎(kN/m2) vs. settlement ratio (S/B) ‎‎(where S represents footing settlement and B ‎represents ‎footing width) are shown in Figure 10. As shown in ‎the graph, the settlement ratio increases ‎approximately ‎linearly with increasing stress. ‎This action was predicted, as ‎an increase in load ‎would enhance the rate of consolidation ‎and produce the settlement, as many studies ‎hypothesized. ‎Local shear failure appears to be the mechanism of failure ‎‎(Figure. 10).‎ There have been numerous methodologies provided for ‎defining the ultimate bearing capacity ‎and failure of shallow ‎foundations. The following are the two most important: ‎

Figure 10. Applied pressure versus settlement ratio (S/B) for a footing resting on untreated soil‎

Terzaghi's 1947 suggestion, in which failure was ‎defined at a load equal to 10% of the model ‎footing width ‎‎(or pile diameter). Numerous researchers have endorsed this ‎proposal.

Tangent proposal, in which failure is defined as the ‎point at which the two tangents of the ‎load-settlement curve ‎connect. The first is tangent to the curve's beginning portion, ‎while the ‎second tangents to the curve's bottom flatter ‎component. ‎

The schematic design utilized to determine the failure ‎load in both ways is shown in Figure 10, with ‎the Terzaghi ‎method yielding a value of 106 kN/m2 and the Tangent ‎method yielding a value of ‎‎122 kN/m2.

 When applying the Terzaghi equation to determine the ‎ultimate bearing capacity of a shallow ‎foundation on ‎saturated clay soil, the result is:

$Q_{u l t .}=c N_{c}$

Qult. $=17 \times 5.7=96.9 \mathrm{kN} / \mathrm{m}$    (1)

‎While the ultimate bearing capacity of the soil as ‎determined by the Hansen technique is Qult. =17 x 5.14=87.38 kN/m.‎

Because the ultimate bearing capacity determined from a ‎criterion of 10% B settlement failure is ‎comparable to ‎Terzaghi's method for predicting failure load, this method ‎will be used to calculate the ‎maximum load that the model ‎footing can withstand.

3.2 Discussion of results (SRR and BIR)

The remolded soil sample had a shear strength of 13 ‎kN/m2 and was placed in a cubic soil test tank ‎with a side ‎length of 50 cm and a square footing with 15 cm side. Based ‎on the testing program, the ‎model tests were divided into ‎four categories to determine the influence of deep ‎foundations and ‎deep maxing method (DMM) as one of the ‎technical approaches for enhancing soft soil qualities ‎on ‎bearing capacity and settling, as follows:‎

For an untreated soft clay soil sample, one model test was ‎performed.‎ Four soil model tests were conducted by using a deep ‎foundation model, initially using a single pile ‎and then ‎groups of columns (two, three, and four). ‎Four model tests were carried out for soil treated by a ‎single cement column using the dry ‎method, followed by a ‎group of columns (two, three, and four) ‎. A single lime column was used to treat soil using the dry ‎method, followed by (two, three, and ‎four) groups of lime ‎columns.‎ The fitting of a polynomial of degree four is used to draw the curve connecting the real points in Figure 11; additionally, the style is used in all subsequent figures. Figure 11 depicts the distinct difference in behavior ‎between untreated soil and soil treated with a single ‎pile, ‎cement column, or lime. This is visible when the settlement ‎is analyzed using the settlement ‎reduction ratio (SRR), equal ‎to the settlement of untreated soil divided by the settlement ‎of soil ‎reinforced with a pile or column of cement or lime. ‎Looking at Table 4, it was evident that while ‎employing a ‎single pile, the averaged settlement reached half at the ‎failure load level, whereas the SRR ‎of soil treated with a ‎cement or lime column fell at an average of 32%. ‎

To determine the influence of a single pile, cement ‎column, or lime on bearing capacity, a ‎factor known as the ‎bearing improvement ratio (BIR) was utilized, which equals ‎the treatment ‎soil bearing capacity to the bearing capacity of the natural ‎soil without any treatment. Terzaghi's assumption ‎was used to ‎calculate the bearing capacity at failure corresponding to a ‎drop equal to 10% of the ‎foundation's width. The BIR can be calculated using the previous definition as shown in the following equation.

Table 4. Settlement reduction ratio (SRR) for single and groups of the pile, cement, and lime‎ columns

No. of ‎‎column

Pile

Cement Column

Lime Column

Single ‎

‎52.0%‎

‎30.8%‎

‎34.8%‎

Two

‎83.3%‎

‎67.0%‎

‎68.8%‎

Three ‎

‎87.4%‎

‎87.0%‎

‎87.0%‎

Four ‎

‎93.4%‎

‎89.5%‎

‎88.4%‎

$\mathrm{BIR}=\frac{\mathrm{Q}_{\mathrm{utr}}}{\mathrm{Q}_{\mathrm{u}}}$    (2)

where, Qutr denoted the ultimate bearing capacity of soil founded on a pile or treated with cement or lime columns, and Qu denoted the ultimate bearing capacity of soil founded on a square footing without any treatment.

The table ‎clearly shows a convergence in the percentage of increase ‎in ‎bearing capacity when employing a single pile, cement ‎column, or lime column, and it is ‎roughly equivalent to ‎‎20%. ‎‎To determine the influence of a single pile, cement ‎column, or lime on bearing capacity, a ‎factor known as the ‎bearing improvement ratio (BIR) was utilized, which equals ‎the treatment ‎soil settlement to the settlement of the natural ‎soil without any treatment.

Terzaghi's assumption ‎was used to ‎calculate the bearing capacity at failure corresponding to a ‎drop equal to 10% of the ‎foundation's width. Table 5 ‎clearly shows a convergence in the percentage of increase ‎in ‎bearing capacity when employing a single pile, cement ‎column, or lime column, and it is ‎roughly equivalent to ‎‎20%.‎

Table 5. Bearing improvement ratio (BIR) ‎for single and groups of ‎ the pile, cement, and lime‎ columns

No. of ‎‎column

Pile

Cement Column

Lime Column

Single ‎

‎1.23‎

‎1.20‎

‎1.20‎

Two

‎2.00‎

‎1.50‎

‎1.60‎

Three ‎

‎2.27‎

‎2.33‎

‎2.27‎

Four ‎

‎2.83‎

‎2.43‎

‎2.43‎

Figure 11. S/B versus applied pressure for single pile and cement column

A load is imparted to two piles or two cement columns or ‎lime, as shown in Figure 12. According to Table 3, the ‎percentage of settlement reduction was doubled when ‎compared to employing a single pile or column. As a ‎result, ‎the settlement ‎ reduction ratios for ‎piles, cement ‎columns, and lime columns, correspondingly, ‎were 83.3%, ‎‎67.0%, and 68.8%respectively.‎ The bearing capacity doubled ‎due to utilizing two piles, ‎and this BIR ratio ‎was increased to 1.50 for cement ‎columns ‎and 1.60 for lime columns ‎(Table 5). ‎With ‎the ‎increase in the surface area of the material with ‎increased hardness،‎ the effect ‎becomes ‎more ‎pronounced. In terms of settlement and ‎bearing coefficients, cement, and lime columns have ‎made ‎great ‎strides forward.

Figure 12. S/B versus applied pressure for two piles and cement columns

When three piles or three columns of cement or lime are used. As shown in Tables 4 and 5 and ‎Figure 13, the ‎growth in the ratios of decreasing settlement and increasing carrying capacity ‎followed the same pattern as in ‎Figures 11 and 12. Additionally, it was noted that the ‎improvement values in the prior two ratios were very similar ‎for all three approaches. The ‎average percentage of settlement was 87%. The relative rigidity of the columns ‎in the ‎untreated soil between the columns governs the block's settlement (soil + cement or lime ‎column). In this ‎configuration, the carrying capacity rose by 130%.‎

Figure 13. S/B versus applied pressure for three piles and cement columns

When the number of piles was increased to four (Figure ‎‎14), the settlement reached the lowest value in all ‎preceding ‎groups, with an average (SRR) of 93% for all load levels ‎when compared to ‎untreated soil, while the (SRR) was 89% ‎percent for soil treated with cement columns and ‎nearly ‎identical when using lime columns. This related to the ‎stiffness of the reinforced soil ‎block encompassed by the ‎cement or lime columns determine the settlement of ‎constructions ‎supported by cement or lime columns and the ‎load from the structure will be transferred to ‎the ‎surrounding soil around the block's perimeter prior to ‎the consolidation of the soil beneath or ‎around the block [10].‎

Figure 14. S/B versus applied pressure for three piles and cement columns

While the bearing capacity of these groups increased by 1.43 times for cement or lime columns ‎and by 1.83 for ‎piles. In addition, here is evidence of the efficacy of cement ‎and lime column ‎techniques in significantly increasing the ‎bearing capacity of soft clay soil in proportions ‎comparable ‎to those achieved with deep foundations represented by ‎piles. When economics are ‎considered, cement and lime are ‎a viable economic alternative, particularly in ‎developing ‎countries experiencing a large reconstruction ‎movement, due to their low cost and ease of ‎implementation ‎in comparison to deep foundations, which are more ‎expensive and require ‎complex implementation methods, ‎particularly in loose and soft soils. Not to mention the ‎fact ‎that raw materials are abundant naturally and need little ‎work to extract, particularly the ‎component known as lime, ‎which is abundantly available in Iraq and throughout the ‎world. The widespread usage of lime in ancient ages, most ‎notably in the Abbasid era's palaces, ‎demonstrates this ‎clearly. Additionally, the techniques of cement and lime ‎columns ‎contribute to the continual development of the ‎qualities of the soft soils found throughout ‎central and ‎southern Iraq. ‎ All values of the settlement reduction ratio ‎and bearing improvement ‎ratio for all groups of piles, ‎cement columns, and lime columns are presented in Figures ‎‎15 and 16. ‎

Figure 15. Settlement Reduction Ratio, (SRR) for Piles, cement and lime columns

Figure 16. Bearing improvement ratio (BIR) for piles, cement and lime columns

3.3 Efficiency(η)of pile, cement and lime column groups

Efficiency(η)of Pile, Cement and Lime Column ‎Groups Similarly to piles, the efficiency of a group (E) of ‎cement or lime columns is defined as the ‎‎ratio of the group's ‎capacity to each individual cement or lime column ‎multiplied by the ‎number of columns in the group. On how ‎much load a single pile of cement or lime ‎column ‎could ‎carry, and how much load could be carried by a group of ‎cement or lime columns ‎with a ‎settlement of 10% B, was ‎determined using the formula‎: ‎

$\eta=\frac{Q_{n}}{n Q_{s}}$    (3)

where, η = efficiency of the pile, cement or lime group; Qg = ‎axial capacity of pile group; Qs= ‎axial capacity of single ‎pile; n = number of piles, cement or lime in group.‎

‎Table 5 illustrates the ultimate load of single and ‎multiple piles, cement, and lime in response to a 10% B ‎settling (B: width of footing)‎. Table 6 shows the ‎calculations of group efficiency for ‎all model tests. It can be ‎seen from these tables that the group ‎efficiency is decreased ‎with increasing the numbers of pile and cement or lime ‎column. For ‎example, the group efficiency of two, three, and ‎four cement columns are (0.63, ‎‎0.0.65, 0.51). These ‎tables ‎demonstrate that the efficiency of lime column ‎groups is approximately similar to cement columns. Table 7 shows that the group efficiency values of pile ‎groups are larger than the soil improvement techniques, ‎cement, and lime columns groups.‎

Table 6. The ultimate load of single and groups of the pile, cement, and ‎lime columns

No. of ‎‎column

Pile, N

Cement Column, N

Lime Column, N

Single ‎

‏3700‏

‏3600‏

‏3600‏

Two

‏6000‏

‏4500‏

‏4800‏

Three ‎

‏6800‏

‏7000‏

‏6800‏

Four ‎

‏8500‏

‏7300‏

‏7300

Table 7. The efficiency of groups of the pile, cement, and ‎lime columns

No. of ‎‎column

Pile

Cement Column

Lime Column

Two

0.81

0.63

0.67

Three ‎

0.61

0.65

0.63

Four ‎

0.57

0.51

0.51

4. Conclusions

(1) The implementation of the technique of deep mixing ‎‎(DMM) utilizing cement columns ‎or ‎lime by the dry method ‎to improve the soft clay soil resulted in a ‎considerable ‎reduction in ‎sinking as well as a significant ‎increase in bearing capacity. ‎

(2) The settlement reduction ratio (SRR) while using ‎pile foundations in soft clay soil was ‎‎equal to 52% at the ‎failure load level, however settlement in clay soil samples ‎treated ‎with ‎cement or lime column was reduced by an ‎average of 32 percent. When the ‎preceding ‎approaches ‎ were ‎utilized, the bearing capacity improved by about 20%.

(3) When groups of two cement or lime columns are ‎used instead of a single column, the ‎SRR ‎value is doubled. ‎The bearing capacity was doubled due to the usage of two ‎piles. ‎Also, for soil ‎treated with cement or lime columns, it ‎increased by 1.50 and 1.6 times, ‎respectively.

(4) In the context of all groups, the settlement value for ‎the four-pile group (an averaged ‎SRR ‎of 93% for all load ‎levels compared to untreated soil) was lower than for ‎previous ‎groups, while ‎the settlement value for the soil ‎treated with cement columns and lime ‎columns (an SRR ‎of ‎‎89%) was nearly identical. In the same manner, the ‎bearing ‎capacity of these groups improved ‎by 143% for ‎cement and lime columns and 183% for ‎piles.‎

(5) According to the results of the examination models for each of the cement and light columns, there is no discernible difference between them in terms of improving bearing capacity and reducing settlement of the soil treated with each of them.

The efficiency of the group decreases as the number ‎of columns grows. Two, three, and ‎‎four cement columns ‎have group efficiency of (0.63, 0.0.65, 0.51). Furthermore, ‎pile ‎groups ‎have higher group efficiency values than soil ‎improvement approaches, cement, ‎and lime ‎column.

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