Basic Bearing Capacity Test to Utilize Soft Clay Soil as Efficient Material for Embankment

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 8, Issue 10, October 2018)

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Basic Bearing Capacity Test to Utilize Soft Clay Soil as

Efficient Material for Embankment

Myoung Soo Won

1

, Christine P. Langcuyan

2

, Ji Hwi Kwak

3

, Gwan Hee Choi

4

1,2,3,4

Department of Civil Engineering, Kunsan National University, Gunsan, Republic of Korea

Abstract—The abundance of clayey soil is an advantage for finding alternative material for embankments especially with the scarcity and high-cost of high-quality fill materials. Clayey soil can be suitable as embankment material when treated properly and with the addition of reinforcements. However, very limited studies can be found and only few researchers consider the potential of clayey soil as embankment material. So in the present study, utilization of soft clay soil as embankment material is proposed. Eight basic bearing capacity tests are carried out to investigate the capability of soft clay soil to support an imposed loading. Additional materials such as crushed gravels and composite geotextiles are used as reinforcements. Also, a desirable embankment material such as sand is also tested as basis for comparison. The bearing capacity test results showed that soft clay layered with crushed gravels and reinforced with composite geotextiles exhibited higher bearing capacity than the dense sand. Therefore with the favourable results obtained by the present study, it can be inferred that soft clay soil has high potential as embankment material when treated properly and when reinforcements such as crushed gravels and composite geotextiles are added.

Keywords— bearing capacity, soft clay, composite geotextiles, embankment material, mould test.

I. INTRODUCTION

Suitable materials for embankment can be any type of soil when properly treated. Granular soils like sand and gravel are highly desirable as embankment materials because they can facilitate drainage, prevent saturation, well-graded and capable of being well compacted. On the other hand, more fine materials like silt and clay are less desirable and unsuitable for use as embankment materials because of poor engineering properties (FHWA, 2008; Limsiri, 2008). However, good quality of granular materials for road embankments can be expensive and may be scarce or limited at the construction site. Thus, the need to procure and transport good quality materials to the construction site may be costly and uneconomical (FHWA, 2009).

A potential alternative material that is feasible for embankment is the clayey soil. The raw clay materials are very abundant and available near the coastal areas.

Some clayey soils are even considered as waste soil materials from the substructure excavation at the construction site. Generally, the clayey soils are not used and can be readily available near the site. Yet, very limited studies can be found concerning the feasibility of soft clay as road embankment material. Only few considers its high potential for road embankment. A comprehensive study of Limsiri (2008) paved the way for very soft organic clay for road embankment application.

Hence, clay has high potential and may be suitable as road embankments if designed and conditioned properly. High-strength clayey soil can be attained at optimum moisture content, with proper compacted and clear from unsuitable materials. However, such case is very difficult to achieve at the construction site. During construction, a clayey soil with high water content would probably be used which has poor engineering properties. High strength of soft clay is possible with the addition of reinforcements or stabilizing agents. It is expected that it should have higher bearing capacity when reinforcements are added and proper soil conditioning is applied. Several reinforcements can be considered; geosynthetics has been widely used as reinforcement of soft clay, as well as aggregates such as gravel and sand. Several studies showed that addition of reinforcement to soft clay reduces settlements and increases strength (Benmebarrek et.al., 2015; Esmaeili et.al., 2018; Le Hello and Villard, 2009).

In this present study, basic bearing capacity tests are carried out in order to investigate the capability soft clay to support imposed loading and its viability to be utilized as embankment material. Reinforcement materials like crushed gravel and composite geotextiles are used as reinforcements for soft clay. The basic bearing capacity test undertaken in this study is a preliminary experiment prior to a large model test. Moreover, there are eight cases of material samples consist of clay, sand and combinations of clay with crushed gravels and composite geotextiles that are tested.

II. TEST EQUIPMENT AND METHODOLOGY

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The mould is placed in the CBR equipment to measure the bearing capacity of the material sample at certain penetration. Penetration dial gauge is controlled using the mechanical jack with 1 mm/min penetration rate and the load is recorded from the load gauge.

The basic mould test is composed of eight (8) sample cases which are depicted in Figure 2 and Table I. Case 1 refers to full volume of soft clay having 30% normal moisture content (NMC) and specific weight, ɣ, of 17.6 kN/m3. The model case is placed on the mould in 4 layers and each layer is subjected to compaction by manually vibrating its surface for 1 minute using a 2.5 kgs metal tube with 50 mm surface diameter. Case 2 refers to full volume of clay with optimum moisture content (OMC) of 14.5% and ɣ of 20.5 kN/m3 subjected to compaction using a 4.5 kgs rammer with 55 blows on each layer. Case 3 refers to full volume of loose sand with ɣ of 13.5 kN/m3 directly placed without compaction.

Case 4 refers to full volume of dense sand with ɣ of 15.7 kN/m3 placed in 4 layers wherein each layer is subjected to compaction by manually hitting the side of the mould using a rubber hammer for 1 minute. Case 5 refers to the combination of clay and crushed gravels with ɣ of 17.3 kN/m3. It consists of 4 alternating layers of NMC clay and crushed gravels. The compaction of NMC clay is done by manually vibrating its surface for 1 minute using a 2.5 kgs metal tube with 50mm surface diameter. The crushed gravel layers are not subjected to any compaction method to avoid mixing with the NMC clay layers. Case 6 has the same combination with Case 5 but with ɣ of 16.1 kN/m3 and the addition of 3 pieces of composite geotextiles placed in between layers of NMC clay and crushed gravels. Case 7 consists of 4 layers of NMC clay with composite geotextiles placed in between layers whose ɣ is 17.1 kN/m3. Lastly, Case 8 is done similar to Case 6 but with an additional composite geotextiles inserted at the last layer of crushed gravels whose ɣ is 15.9 kN/m3.

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Case 1 Case 2 Case 3

Case 4 Case 5 Case 6

Case 7 Case 8

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TABLEI

SPECIFICATIONS OF MOULD TEST SAMPLE CASES

Parameters Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8

Clay OMC

1

-  - - - -

NMC2  - - -    

Sand Loose - -  - - - - -

Dense - - -  - - - -

Crushed Gravel - - - -   - 

No. of Composite

Geotextile Layers - - - 3 3 4

Compaction Method manual surface vibration 4.5kgs rammer (55blows) none rubber hammer (side) manual surface vibration manual surface vibration manual surface vibration manual surface vibration Unit Weight, ɣ

(kN/m3) 17.6 20.5 13.5 15.7 17.3 16.1 17.1 15.9

1_{OMC – stands for Optimum Moisture Content which is equivalent to 14.5%}

2

NMC – stands for Natural Moisture Content which is equivalent to 30.0%

III. TEST MATERIAL SPECIFICATIONS

All the materials used in this study consist of sand, crushed gravel, clay and composite geotextile. The main material for this study is the clayey soil with 30% NMC. The clay soil was classified using U.S.C.S. as inorganic clay (CL) with Gs=2.637, LL=45.0%, PL=23.6%, PI=21.4%, and OMC=14.5% (see Table II). The particle size distribution of the clay is shown in Figure 3 wherein 97% passes through #200 sieve. The clay soil used in the study was taken near the Gunsan Train Station. A desirable embankment material such as sand is also tested as basis for comparison.

The sand used in this study have a specific gravity, Gs, equal to 2.715 and particle sizes are finer than No.20 sieve but generally retained in No.40 sieve (see Figure 3). The sand was classified using U.S.C.S. as poorly-graded sand (SP) whose sieve analysis results are tabulated in Table III. For the reinforcements, the crushed gravels are used in the study. The crushed gravel was classified using U.S.C.S. as poorly-graded gravel (GP) whose sieve analysis results are tabulated in Table III. The particle size distribution of the crushed gravels are shown in Figure 3 with 99.7% passing through 13 mm diameter sieve and 97% retained in No.4 sieve. Lastly, the composite geotextile used in this composed of non-woven and woven geotextiles whose total thickness is 2 mm and tensile strength is 150 kN/m.

TABLEII PROPERTIES OF CLAY SOIL

Specific Gravity, Gs Percentage Passing #200 Sieve Liquid Limit, LL Plastic Limit, PL Plasticity Index, PI Optimum Moisture Content, OMC Normal Moisture Content, NMC U.S.C.S. Classification

2.637 97% 45.0% 23.6% 21.4% 14.5% 30% CL

TABLEIII

PARTICLE SIZE DISTRIBUTION OF SAND AND CRUSHED GRAVEL

Materials U.S.C.S. Classification D60 (mm) D30 (mm) D10 (mm) Cu Cc

Standard Sand SP 0.63 0.51 0.44 1.4 0.94

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FIGURE3.PARTICLE SIZE DISTRIBUTION OF CLAY,SAND AND GRAVEL

IV. RESULTS AND DISCUSSION

The results from the basic bearing capacity tests are shown in Figure 4 until 12.5 mm penetration for all the 8 mould test sample cases. The graph showed reasonable bearing capacities in Log2 scale. For better comparison of the each cases, analysis of the bearing capacities at 2.5 mm, 5 mm, 7.5 mm and 12.5 mm penetration are depicted in Figure 5. The results of Cases 5 to 8 shall be compared with the bearing capacities exhibited by the sand models, Case 3 and Case 4, and full volume of clay model, Case 1. It can be observed from Figure 5 that Case 5 obtained 83%, 100%, and 217% higher bearing capacities than Case 3 for 5 mm, 7.5 mm and 12.5 mm penetration, respectively. Also, Case 5 exhibited 50%, 175%, 200% and 375% higher bearing capacities than Case 1 for 2.5 mm, 5 mm, 7.5 mm and 12.5 mm penetration, respectively. This can be inferred that adding crushed gravel to the NMC clay enhanced soft clay’s strength due to the friction from the crushed gravel and its compressive strength. It is also remarkable that the combination of crushed gravels and NMC clay (Case 5) is stronger than dense sand (Case 4) by 36% at the end of 12.5 mm penetration. Moreover, adding 3 pieces composite geotextiles in between layers of NMC clay (refer to Case 7) increased the bearing capacities by 25% to 75% compared to full NMC clay (refer to Case 1).

It is also remarkable that Case 8 obtained 31%, 138% and 321% higher bearing capacities than Case 4 for 5 mm, 7.5 mm and 12.5 mm penetration, respectively.

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Additionally, the present study conducted the bearing capacity test with sand for comparison with the proposed clay materials and soft clay with reinforcement materials. We used sand since granular soils are highly desirable as embankment materials because they can facilitate drainage, prevent saturation, well-graded and capable of being well compacted. As expected, Case 4 showed higher bearing capacity than Case 3 (see Figure 7) which implies that manually compacted or dense sand is stronger than loose or not compacted sand. This can be inferred that compaction apparently induced higher bearing capacity that is why compaction is very important on the construction site. It is also remarkable that Case 3 showed constant bearing capacity of 75 kPa while Case 4 exhibited a peak value of 212 kPa and gradually decreased.

Furthermore, the Cases 5 to 8 are combination of NMC clay and reinforcement materials such as crushed gravels and composite geotextiles. Figure 8 results showed that adding composite geotextiles in between layers of crushed gravel and soft clay increased the bearing capacity.

Here, Case 6 has increased up to 53% of bearing capacity after composite geotextiles are added compared to Case 5. Also, adding one more layer of composite geotextile, Case 8, exhibits 111% higher bearing capacity than Case 6 at the end of 12.5 mm penetration. So far, Case 8 exhibited favourable bearing capacity which could be desirable for field application. It can be inferred than NMC clay reinforced with crushed gravels has increased bearing capacity higher than loose sand, and adding composite geotextiles has increased bearing capacity higher than dense sand. It can be inferred that the parametric strength of crushed gravels and composite geotextiles may have attributed to the increase of the soft clay’s bearing capacity. The lone soft clay is weak and has poor engineering properties, thus, may not be suitable as embankment material. However, the soft clay layered with crushed gravels and reinforced with composite geotextiles exhibited high bearing capacity which may imply that it can be utilized as efficient embankment material.

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FIGURE5.BEARING CAPACITY ANALYSIS AT 2.5MM,5MM,7.5MM AND 12.5MM PENETRATION

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FIGURE7.COMPARISON OF BEARING CAPACITY RESULTS FOR CASE 3 AND CASE 4

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V. CONCLUSION

The basic bearing capacity test undertaken in this study is a preliminary experiment prior to a large model test. There are 8 cases of material samples consist of clay, sand and combinations of soft clay with crushed gravels and composite geotextiles. The results showed that:

1. The bearing capacity of the clayey soil was influenced by the water content. At optimum moisture content, the bearing capacity of clay is extremely high. Yet, when clay is saturated, the bearing capacity is relatively low. 2. The bearing capacity of standard sand was affected by

the compaction and density. Dense sand has higher bearing capacity than loose sand. Well-compacted clay (OMC) exhibited extremely high bearing capacity. However, proper compaction on saturated clay is difficult which greatly affects its bearing capacity. 3. The bearing capacity of the soft clay soil was improved

by the addition of reinforcements. The soft clay soil layered with crushed gravels and reinforced with composite geotextiles exhibited relatively high bearing capacity despite having high water content. Therefore, it can be inferred that soft clay soil has high potential as embankment material when treated properly and when reinforcements such as crushed gravels and composite geotextiles are added.

Acknowledgement

This study was carried out as a part of the technology innovation development project of the SME (C0565624, Development of Technology to Utilize High-Strength Clay Soils as Efficient Soil Materials).

REFERENCES

[1] Benmebarek, S., Berrabah, F. and Benmebarek, N. 2015. Effect of geosynthetic reinforced embankment on locally weak zones by numerical approach. Computers and Geotechnics. 65, 115-125. [2] Esmaeili, M., Naderi, B., Neyestanaki, H.K. and Khodaverdian, A.

2018. Investigating the effect of geogrid on stabilization of high railway embankments. Soils and Foundations. 58(2), 319-332. [3] FHWA. 2008. User Guidelines for Waste and Byproduct Materials

in Pavement Construction. Federal Highway Administration Research and Technology. FHWA-RD-97-148. https://www.fhwa.dot.gov/publications/research/infrastructure/struct ures/97148/app4.cfm. Accessed 23 July 2018.

[4] FHWA. 2009. Design of mechanically stabilized earth walls and reinforced soil slopes – volume I. FHWA-NHI-10-024 FHWA GEC 011-Vol I. National Highway Institute, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.

[5] Le Hello, B. and Villard, P. 2009. Embankments reinforced by piles and geosynthetics – numerical and experimental studies dealing with the transfer of load on the soil embankment. Engineering Geology. 106(1-2), 78-91.