Study on Cohesive Soil and Fly Ash Mixture As Reinforced Earth Retaining Wall Filling Material

Vashi, Jigisha M, Research Scholar, Desai, A.K. Associate Professor, Solanki, C.H. Associate Professor, AMD, SVNIT, Surat.

The performance of filling material and its interface friction properties with the geosynthetics would directly influence the application properties of the geosynthetics reinforced earth retaining walls. Through Triaxial test experiments performance include studies of shear parameters of cohesive soil and fly ash mixture in this paper. The results indicate that the mixture of cohesive soil and fly ash have higher strength and rigidity & good interface friction. The technical performance of the mixture conform to the requirements of geosynthetics reinforced earth retaining walls, so it can be used as the filling material of geosynthetics reinforced earth retaining walls in region where specified graded sand is not available.


Because of the good engineering performance, a large number of reinforced earth retaining walls have been constructed throughout the world. Compared with the traditional gravity earth retaining walls, geosyntheric reinforced earth retaining walls have better engineering characteristics of light deadweight, beautiful shape, construction convenience etc. Especially on the soft ground, the better performance would be embodied in virtue of their light deadweight.

The national planners in India have put infrastructure development on priority. This has resulted in transport planning, widening of National Highways and new roads in the country. Thus work of Retaining Earth (RE) structures/slopes will be designed in very large numbers over different areas. Thus there is a huge possibility of RE wall being constructed for every 2 Km of 6-4 lane road of NH, State Highways where there is a need of large fill/backfill quantity of sand. But in futures sand is not likely to be a source, so there is a need for use of local waste/fill materials as backfill and hence the present study was taken up.

Filling material’s performances and interface friction properties with the geosynthetics directly influenced the application performances of the geosynthetics reinforced earth retaining walls. The standards used filling materials are cohesion less soil having percentage of fines (<0.08mm) is <15%, free-draining backfill & Soil reinforcement friction factor tan d not = 30o)12. As the filling material of geosynthetics reinforced earth retaining walls, it should have the following engineering properties: (1) good mechanical properties which include the strength and rigidity; (2) better interface friction property with the geosynthetics; (3) the material, better be lightweight.

The shear resistance is function of size, type Undisturbed/Remoulded/ critically weathered, state of drainage in shear, degree of saturation has been studied by many researchers. Following parameters for inorganic cohesive CH soils could be adopted Skempton (1948); Nagaraj (1964); Focht & Sullivan (1969); Stauffer & Wright (1984); Green & Wright (1986); Kulhawy & Mayne (1990); Kayyal & Wright (1991); etc. for preliminary analysis, for compacted soil at OMC & 95% of MDD by Proctor test and observed the range of cohesion of about 100 kPa & angle of internal friction about 20o to 23o for un-drain conditions and cohesion of about 50 kPa & angle of internal friction about 18o for drain condition. Desai, M.D, (1967) studied the expansive CH soil (low to medium) of region has average OMC 20 to 25% and MDD 1.48 to 1.52 g/cc, and this soil remoulded to 95% MDD has Cu = 80 kPa to 100 kPa, Φu = 15 to 18o. Ke Zhao, et al. (2001) studied the performances of saponated residue and added-calcium fly ash mixture in highway application. Desai, N.H (2007) performed box shear test for Fly ash and CH soil in different proportions and showing good results of cohesion in range of 2 to 11 kPa and angle of internal friction in range of 20 to 45 degree.

Through experiments, the technical performances of coshessive soil and fly ash mixture, and the shear parameters for mixture of 80% FA + 20% Cohessive soil & 75% FA + 25% Cohessive soil, were studied. The purpose was to investigate the application properties of the mixture used as filling material of geosynthetics reinforced earth retaining walls.

Experimental Work

The purpose of this experimental work is to review the previous research conducted for region in India having CH soils and combined pertinent data with results from the technical literature to develop guidelines for selection of shear strengths for reinforced cohesive-soil structures.

Soil sample of high plastic clay (CH) and fly ash was collected from GIPCL, Nani Naroli, Kim, Dist. Surat. Physical properties of soil sample were determined by standard lab tests and are represented in Table 1. (Grain size distribution as per IS 2720 Part 4, Specific Gravity as per IS 2720 Part 3 Section I, Liquid & Plastic Limit test as per IS 2720 Part 5, Compaction test as per IS 2720 Part 7, and Free Swell Index test as per IS 2720 Part 40). Physical and chemical properties of fly-ash were also tested and results are presented in Table 1.

Table: 1. Properties of Soil & Fly-ash
Test Physical Properties Chemical Properties of Fly Ash (% content)
Soil Fly-ash
Test 1 Test 2 Test 1 Test 2
% Passing
I S Sieve size in mm 4.75mm 100 100 100 100 SiO2 24.30
2.00mm 100 100 100 100 Al2O3 13.11
1.00mm 97 96 100 100 Fe2O3 17.16
0.425mm 95 95 100 100 TiO2 2.51
0.250mm 95 94 100 100 CaO 27.00
0.075mm 76 75 79 78 MgO 0.32
Specific Gravity 2.497 2.488 2.547 2.526 Na2O 1.05
Liquid Limit Immediate 58 61 44 45 K2O 0.16
LiO2 Nil
After 24 Hrs Soaking - 51 52 SO3 9.50
After 48 Hrs soaking 62 61 LOI (Loss on Ignition) 4.78
Plastic Limit 34 38 NP NP -
Plasticity Index 24 23 -
Standard Proctor Test MDD in kN/m3 16.20 16.10 1.29 1.26
OMC in % 23.5 23.5 32.0 33.0
Free Swell Index in % 50 48 -

For the investigation purpose 80% Fly Ash + 20% Soil mix and 75% Fly Ash + 25% Soil mix was decided. Two different tests namely Triaxial test, (IS 2720: Part 11: 1993/2002) and Permeability (IS 2720 Part 17:1986/2002, falling head method) were carried out for this combination. Specimens were cured for 3 days, 7 days, & 28 days and tested for the said test. Modified proctor test was conducted on 80:20 & 75:25 (F:S) mix proportion. Modified Proctor Test result graph is as shown in Fig.1. Moisture v/s Density relation for Fly Ash: Soil (80:20 & 75:25) ratio mention is given in Table 2.

Table: 2 Moisture-Density Relationships with Mix Proportion
Mix Proportion MDD in kN/m3 OMC in %
80:20 13.89 30
75:25 14.35 30

Moisture Density Relationship
Figure 1: Moisture Density Relationship for 80:20 and 75:25 Proportions

Pilot test results of triaxial test of samples, proportion in 80:20 & 75:25 (Flyash: Black soil) with 3 days, 7 days and 28 days currying period at OMC-MDD; at different cell pressure of 0.5 kg/cm2, 1.5 kg/cm2 and 2.5 kg/cm2; and at 1.5 mm/min strain rate. The values of cohesion C and angle of internal friction φ are found out from modified failure envelope of traixial test are finding out for 3 days, 7 day, and 28 days respectively. The summary of the test result are shown in Table 3.

Table: 3. Testing Result
Tests 3 Days 7 Days 28 Days
Triaxial Test - UU Cuu (kPa) Φ(º) Cuu (kPa) Φ(º) Cuu (kPa) Φ(º)
80:20 (F:C) Mix 414 47.02 365 51.13 336 56.36
75:25 (F:C) Mix 388 42 429 39 457 40
Permeability K for
80:20 (F:C) Mix
2.66E-05cm/sec (Falling Head Permeability Test was conducted on 80:20 mix proportion immediately after casting of the sample.)
Permeability K for 75:25(F:C) Mix 1.23E-05 cm/sec (Falling Head Permeability Test was conducted on 80:20 mix proportion immediately after casting of the sample.)


Through systematic experiments, the application performances of cohesive soil and fly ash mixture have been studied and presented in this paper. The main conclusions obtained are summarized below.

(1) The mixture of cohesive soil and fly ash has higher strength, rigidity, and good water stability. (2) Triaxial shear parameters of the mixture of cohesive soil and fly ash are relatively higher & meet normal design parameters of backfill. These indicate better interface friction if it’s used with geosynthetics. (3) The typical test presented, conformed that mix design of (80:20 & 75:25) can be evolved  for a site to provide low cohesion, high  Φ > 30° material for backfill in RE structures.  This requires placement at 2% less than OMC & MDD. (4) The performance of cohesive soil and fly ash mixture conform to the requirement of filling material of geosynthetics reinforced earth retaining walls. So it can be used as a good filling material for geosynthetics reinforced earth retaining walls at site where BS 8006:1995 specified material is not available.


Authors wish to express their deepest gratitude and sincere appreciation to Dr. M. D. Desai (Visiting Prof SVNIT) for his constant guidance, dedication, and encouragement. Authors would like to thank Er. H. H. Desai & Er. N. H. Desai (Owner of Unique Research Center) for providing the lab facilities for the experimental work.


  • British standard code of practice for “strengthened / reinforced soils and other fills.” BS: 8006-1997.
  • Desai, M.D. (1967), “Experience in Shear Testing for Problems of Earth Dam Foundations & Embankment Materials.” Pre-conference Symposia on Pore Pressure & Shear Resistance of Soils, INS of SMFE, New Delhi.
  • Desai, N.H, (2007), “Experimental Investigation for use of Flyash as a Major Constituent with Clay for Construction of Embankment.” M.Tech Thesis, D.D.University, Nadiad.
  • Focht, J. A., & Sullivan, R. A. (1969), “Two Slides In Over-consolidated Pleistocene Clays.” Proceedings of the Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico City, 1969, Vol. 2, pp: 571–576.
  • Green, R., & Wright, S. G. (1986), “Factors Affecting the Long Term Strength of Compacted Beaumont Clay.” Research Report 436-1, Center for Transportation Research, The University of Texas at Austin.
  • Kayyal, M. K., & Wright S.G. (1991), “Investigation of Long-Term Strength Properties of Paris and Beaumont Clays in Earth Embankments.” Research Report 1195-2F, Center for Transportation Research, The University of Texas at Austin.
  • Ke Zhao et al. (2001). “A Research on the Performances of Saponated Residue Added-Calcium Fly Ash in Highway Application”. Chinese Journal of Fly Ash Comprehensive Utilization, 14 (1):6-10.
  • Kulhawy, F. H., & Mayne, P. W. (1990), “Manual on Estimating Soil Properties for Foundation Design.” EPRI EL-6800, Research Project 1493-6, Final Report, Cornell University, Ithaca, August.
  • Nagaraj, T.S. (1964), “Soil Structure and Strength Characteristics of Compacted Clay” The International Journal of Soil Mechanics, Geotechnique, No. 2, pp: 103-114.
  • Skempton, A.W. (1964), “Long Term Stability of Clay Slopes” The International Journal of Soil Mechanics, Geotechnique, No. 2. pp: 77-100.
  • Stauffer, P. A., & Wright, S. G. (1984). “An Examination of Earth Slope Failures in Texas.” Research Report 353-3F, Center for Transportation Research, The University of Texas.
  • Vashi, Jigisha M., Desai, A.K., Solanki, C.H., & Desai, M.D. (2010), “Fly Ash as Backfill Material for Reinforced Earth Structures.” National conference on Fly ash/Futuristic Materials in Civil Engineering Construction for Sustainable Development, V.V.Nagar, Anand, India.
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