Insitu Investigation of Cinder Mound For Building Construction - A Case Study

    Cinder is a waste generated as coal residues from the blast furnace of thermal power plant. It was accumulated over a long periods near railway station, Jamshedpur, Jharkhand, India. This dumping yard looks like a mound covering an area of around 10 acres. At present, it is in the middle of inhabitants and occupying costly land. It has wide level ground on the top of mound. To utilize for construction of building over it or as a structural fill, insitu and laboratory investigations were carried out. Different tests conducted viz. Geophysical electrical resistivity test (GERT), Standard penetration test (SPT), Dynamic cone penetration test (DCPT) and plate load test (PLT). Resistivity test was carried out covering the whole area while SPT, DCPT and PLT were carried out at selected locations. It was observed that cinder is a coarse grained, non plastic and present in loose state. The paper presents and discusses the results of insitu and laboratory geotechnical investigations. Based on results, it was concluded that it may be used as a structural fill or for building construction over it.

    A.K. Sinha, V.G. Havanagi and V K Kanaujia, Central Road Research Institute, New Delhi, India

    Introduction

    Pictorial view of cinder dump yard
    Fig. 1 Pictorial view of cinder dump yard
    Cinder is a waste material generated as coal residues from the blast furnace of power plants. The material was removed from stoker fired boilers at Thermal power station, Jamshedpur, India and it mainly contains silica and oxides of metal. Slag is another waste material generated as a byproduct during the manufacturing of molten iron. It is generated due to the fusion of limestone and other fluxes with the ash from coke and siliceous and aluminous components of the ferrous ore. It contains oxides of silica, aluminum, iron and other metals. Both cinder and slag were dumped together at the dumping site, outside the plant in huge mound shape. The dump material constitutes 80% cinder and 20% slag material. Generally, it is commonly called as cinder as it mainly contains cinder. Dump is formed over a period of 70 years and looks like a mound / hillock and situated at a distance of about 2.5 km from railway station, Jamshedpur. Presently, cinder mound is surrounded by inhabitants. The slope of the cinder dump is around 450 and its height is varying from 10 m to 45 m from the existing ground level. Vegetation in the form of grass, small plants and trees has been observed at the top of the dump. Side view of cinder mound is shown in the Fig. 1. The site is located in the seismic zone II which is a zone of low seismic activity and earthquake intensity as per Indian standard code.

    As per the available literature, cinder can be used in the construction of embankments and sub grade (HRR-39, 2012). Havanagi et al. (2013) studied the feasibility of cinder in the construction of road with or without stabilization (local soil and cement). The material can also be used on track surfaces and roads to provide additional traction in winter conditions. It also employed as inorganic mulch in xeriscaping, because of excellent drainage properties and erosion resistance (Cinder Wikipedia, 2014).

    Cinders, ash, slags, etc. should be used as structural fill materials (NGSMI, 2005). Presently, it has no application and occupying costly land near the plant. Therefore, to utilise this mound for the construction of building, field investigation was carried out on the top leveled surface of the mound. Different tests were carried out viz. Electrical resistivity test, Standard Penetration Test, Dynamic Cone Penetration test and Plate load test. This paper presents the results of insitu geotechnical investigations.

    Laboratory Investigation Of Cinder

    Different geotechnical tests were carried out on the disturbed and undisturbed samples collected during the SPT i.e. (a) Natural moisture content & bulk density (b) Specific gravity test (c) Grain size analysis (d) Atterberg limit test (e) Hydraulic conductivity test and (f) Consolidation test.

    Natural Water Content And Bulk Density

    Natural water content and bulk/dry density of samples removed from (SPT) Shel by tube sampler at different depths and bore holes have been summarized in Table 1. The natural water content varies in the range of 13 – 33 % and dry density varies in the range of 10.7 – 14.2 kN/m3. The value of dry density indicates light weight of cinder material.

    Table 1: Natural moisture content and density
    BH. No. Natural Water Content (%) Bulk Density ( kN/m3) Dry Density ( kN/m3)
    1 14.2 - 23.0 12.2 – 14.3 10.7 – 12.2
    2 22.5 - 24.9 12.9 - 16.7 10.3 - 13.4
    3 14.3 - 32.6 12.2 – 14.8 9.2 – 13.0
    4 15.0 - 33.7 10.7 - 12.6 9.0 - 11.4
    5 20.2 - 30.2 11.2 – 15.5 8.4 – 12.2
    6 18.6 - 59.1 11.7 – 14.1 8.7 - 11.5
    7 19.6 - 26.8 12.0 – 22.9 10.0 – 19.0
    8 14.9 - 59.5 11.4 - 16.1 9.9 - 11.7
    9 13.7 - 29.4 13.0 – 16.1 10.6 – 14.2
    10 13.0 - 17.8 13.8 - 16.6 11.9 – 14.1

    Specific Gravity Test

    The value of specific gravity in different bore holes/depth has been summarized in Table 2 and it varied in the range of 2.19 to 2.47. The specific gravity of cinder is observed to be low. This is due to unburnt carbon present in the cinder waste.

    Table 2: Specific gravity of cinder material at different depths
    Sl. No. Borehole No. Depth (m) Specific Gravity Depth (m) Specific Gravity
    1 BH1 2.4 2.49 6.9 2.47
      5.4 2.61 9.9 2.27
    2 BH2 1.5 2.23 9 2.20
      4.5 2.19 12 2.20
    3 BH3 5.4 2.28 12.9 2.30
      11.4 2.31    
    4 BH4 3.0 2.26 10.5 2.27
      7.5 2.19    
    5 BH5 3.9 2.24 8.4 2.30
      5.4 2.31 12.9 2.98
    6 BH6 4.5 2.28 12 2.24
      6.0 2.25    
    7 BH7 2.4 2.29 6.9 2.45
    8 BH8 3.0 2.35 10.5 2.32
      6.0 2.23    
    9 BH9 3.9 2.29 9.9 3.37
      6.9 2.30    
    10 BH10 1.5 2.48 7.5 2.68
      4.5 2.60    

    Grain Size Analysis

    It was observed that cinder is coarse grained material having gravel (7 – 50 %), sand (42 - 74 %) and silt (5 - 39%) in all the bore holes with depth. The variation in the grain size distributions in the ten boreholes is given in Table 3. There exists a variation in the grain size due to isolated dumping of cinder and WRP slag at same location (yard).

    Table 3: Variations in the grain size of each boreholes
    BH. No. Gravel (%) Sand (%) Silt (%)
    1 10 – 24 56 – 75 7 - 24
    2 16 – 30 55 – 70 14 - 20
    3 12 - 26 57 – 72 10 - 24
    4 9 - 21 58 – 67 19 - 30
    5 10 - 25 60 – 74 13 - 29
    6 10 - 15 62 – 72 16 - 28
    7 13 - 50 42 – 65 8 - 26
    8 7 - 17 51 – 68 21 - 39
    9 10 - 43 48 – 72 5 – 22
    10 8 - 30 57 – 73 5 – 31

    Atterberg Limit Test

    Cinder was observed to be non-plastic in nature down to the investigated depth. According to IS classification, cinder is classified as SP i.e. poorly graded sand.

    Permeability Test

    Permeability test was carried out on compacted cinder samples as per Indian Standard method. Remolded samples were prepared at insitu density. The coefficient of permeability was determined as 6.3×10-7 m/s. The value indicates that it is a free draining material.

    Consolidation Test

    The value of coefficient of consolidation Cv was observed to be 4×10-8 m2/s under normal loading of 100 to 150 kN/m2. The value of compression index (Cc) was observed to be 0.04. The value of Cc indicates that cinder is low compressible in nature.

    Direct Shear Test

    Sample was oven dried and passed through 4.75 mm sieve. Three specimens of size 60 x 60 x 25 mm were prepared at 12.5 kN/m3 (average in-situ dry density) and 25% moisture content. The specimens were saturated and Consolidated Drained (CD) test was carried out. The specimens were sheared at the rate of 4.17×10-6 m/s (0.25 mm/min). Cinder samples were observed to be cohesionless and angle of internal friction (f) was determined as 340. The values indicate that cinder samples have high shear strength.

    Insitu Investigation of Cinder Mound

    Insitu study was carried out to investigate the feasibility for building construction over the top of cinder mound viz. Standard Penetration Test, Dynamic Cone Penetration Test, Plate load test and Geophysical Resistivity tests.

    Standard Penetration Test

    Nc values vs. depth of cinder
    Figure 2: Nc values vs. depth of cinder
    Standard Penetration Test (SPT) was carried out on the cinder dump area as per Indian Standard at ten locations (Boreholes, BH1 to BH10). Bore holes were made by hand auger upto 1.5 m depth and then advanced by shell and auger method. Casing pipes were used to stabilize the sides of the boreholes. The diameter of the boreholes was 0.1 m. After cleaning the hole, split spoon sampler was advanced by driving with a drop hammer weighing 63.5 kg falling freely through a height of 0.75 m. The number of blows (SPT N value) required for 0.30 m penetration was recorded upto 15 m depth at every interval of 1.5 m. Representative disturbed samples were collected regularly. Undisturbed cinder samples were taken from Shelby tube. Ground water table did not encounter in any of the boreholes down to the investigated depth (15m). The SPT N values were corrected (Nc) for overburden pressure as per Indian Standard. Variation of Nc with depth is shown in the Fig. 2 for BH 1 to 5.

    The value of Nc is varying in the range of 1 and 22. The results indicate that the substrata are in loose state down to the depth investigated. Occasional high N-values is attributed to the presence of larger hard lumps of cinder within the substrata. A perusal of the data indicates that the substrata consist of cinder upto the investigated depth.

    Determination of Ultimate Bearing Capacity

    Ultimate Bearing Capacity was determined as per Indian Standard procedure considering the shear failure criteria (IS: 6403, 1981)] and settlement criteria (8009-Part1, 1976). The average SPT value (Ncor) was determined for a depth equal to 2 times the width of foundation from the foundation depth. In computing this value, any individual NC value more than 50 percent of the average is neglected and the average is re-calculated as per IS: 6403 (1981). Values of Nc for all loose seams is however included.

    (a) Shear failure criteria

    Cinder will fail under local shear failure as values of SPT are low. Width and depth of footing was considered as 1 to 2 m and 2 to 3 m respectively. Average corrected SPT (Ncor.) values were determined considering the SPT values from the depth 2 to 4 m from the foundation depth for each bore hole as given in Table 4. In situ average density of cinder was determined as 11.5 kN/m3 for overburden correction. The average angle of internal friction (Φ) for the depth under consideration was obtained from IS: 6403 (1981).

    The ultimate bearing capacity was calculated using Equation 1.

    qu = qN'qsqdqiq+0.5BN'γsγdγiγ W' ---(1)

    Where
    qu = Ultimate bearing capacity based on local shear failure, kN/m2
    q = Effective surcharge at the base level of foundation, kN/m2 (γd)
    γ = Bulk unit weight of foundation soil, kN/m3
    d = depth of footing, m
    W' = Correction factor for location of water table = 1 (water Table is not present)
    sc, sq, sγ = Shape factors = Inclination of the load to the vertical in degrees = 1 from IS 6403 (1981)
    dq = dγ = 1 + 0.1d/B √N Φ for > 10°, IS:64039 (1981)
    NΦ = tan2 (π /4 + Φ /2)
    Φ = Angle of shearing resistance of cinder, degrees
    Φ' = tan-1 (2/3 tan Φ), angle of shearing resistance of cinder, degrees The inclination factor shall be as under:
    iq = (1-α/90)2 = 1
    iγ =(1-α/Φ)2 = 1
    α = Inclination of the load to the vertical, degrees
    B= width of footing, m
    Nq and Nγ bearing capacity factors, obtained from IS: 6403 (1981).

    As a typical calculation (BH1) of strip footing for width = 1m, depth = 2 m, Ncor = 3 and FOS = 3, safe bearing capacity was calculated as below.

    qu = qNqsqdqiq +0.5BNγsγdγiγ W' = qNqdq+0.5BNγdγ
    q= γd = 11.5*2 = 23 kN/m2, Nq= 15, dq = dγ =1.05, B = 2m, Nγ= 17
    qu= 156 kN/m2, Safe bearing capacity, qs = 52 kN/m2
    Similarly, safe bearing capacity for other borehole location was determined and given in Table 4. Considering the results, safe bearing capacity for width of footing 1 to 2 m was determined as 52 kN/m2.

    Table 4: Safe bearing capacity (kN/m2) by shear failure criteria
    Bore hole No. Ncor
    Depth
    2- 4 m
    Φ, deg. Φ' Nq B=1m qs B=2m qs
    dq/ dγ dq/ dγ
    BH1 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH2 4 29 20 6.4 5.39 1.04 52 1.02 52
    BH3 2 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH4 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH5 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH6 10 30 21 7 6 1.04 57 1.02 57
    BH7 5 29 20 6.4 5.39 1.04 52 1.02 52
    BH8 16 32 22.7 8.4 7.8 1.05 69 1.03 69
    BH9 4 29 20 6.4 5.39 1.04 52 1.02 52
    BH10 3 28 19.6 6.4 5.39 1.04 52 1.02 52

    b) Settlement criteria

    Settlement of a footing of width B under unit intensity of pressure resting on dry cohesionless soil deposit with known standard penetration resistance value Ncor, was taken from IS 8009 (1976). Settlement per unit pressure was determined with respect to Ncor and width of footing. Safe bearing capacity was calculated considering the permissible settlement of footing (0.05 m).

    As a typical calculation, for B = 2 m, N = 5, Settlement/unit pressure (N/m2) = 1/9.7 m = 0.103 m (Sf) was taken from IS: 8009 (1976).

    For the computation of settlement of foundation resting at certain depth, a correction should be applied to the calculated Sf in the form of a depth factor from IS: 8009 (1976). This correction factor was taken as 0.89 (L/B = 100, B = 1 m), and 0.94 (L/B = 100, B = 2 m)

    The corrected settlement was determined as = 0.94*0.103 =0.097 m

    For 0.05 m permissible settlement of footing, safe bearing capacity = 0.05/.097= 52 kN/m2. The summary of results from different bore holes is given in Table 5.

    Table 5: Safe bearing capacity (kN/m2) by settlement criteria
    Bore hole Ncor Safe bearing capacity
    Width of footing
    (1m)
    Width of footing
    (2m)
    BH1 3 NA NA
    BH2 4 NA NA
    BH3 2 NA NA
    BH4 3 NA NA
    BH5 3 NA NA
    BH6 10 478 415
    BH7 5 112 52
    BH8 16 545 495
    BH9 4 NA NA
    BH10 3 NA NA
    NDCPT values vs. depth of cinder
    Figure 3: NDCPT values vs. depth of cinder

    The safe bearing capacity was calculated only for bore holes BH6, BH7 and BH8, as Ncor for all other boreholes were less than 5. The safe bearing capacity by settlement criteria for 1 m width of footing was taken as 112 kN/m2 while for 2m width of footing it was taken as 52 kN/m2. Lower value for safe bearing capacity was taken 52 kN/m2 for both 1 m and 2 m width of footing. It was concluded from SPT that the safe bearing capacity at the depth of foundation is 52 kN/m2 from both shear and settlement criteria.

    Dynamic Cone Penetration Test (DCPT)

    Dynamic cone penetration test was carried out as per Indian Standard (IS 4968 Part1, 1976). DCP Test locations were selected as nearer as possible to SPT test locations for better comparison of results. In this test, a steel cone having a diameter of 0.05 m and an apex angle of 600 attached to a rod of lesser diameter was penetrated into the dump by giving blows from a 63.5 kg hammer falling freely through a height of 0.75 m (similar to SPT). The number of blows (NDCPT) required for 0.30 m penetration was recorded. In this test, continuous NDCPT values were recorded down to the termination depth of 15 m.

    Typical variation of no. of blows (DCPT N values, NDCPT) with depth of penetration for DC1 to DC5 is shown in the Fig. 3. Average NDCPT were observed to around 6 for all the locations except for DC6 and DC10. The average NDCPT value for the depth of 2 to 4 m from the foundation depth (2 to 3 m) were determined and given in Table 6.

    Table 6: NDCPT values from the foundation depth of 2 m at different locations
    Depth (m) DC1 DC2 DC3 DC4 DC5 DC6 DC7 DC8 DC9 DC10
    2.1 9 9 7 3 1 24 4 6 1 11
    2.4 4 31 6 3 1 14 3 5 1 5
    2.7 4 25 6 1 1 13 1 5 3 47
    3 3 12 4 3 1 20 2 7 3 4
    3.3 3 5 2 3 2 17 3 7 2 8
    3.6 3 4 3 2 2 17 1 5 6 9
    3.9 4 2 5 2 1 15 2 6 5 7
    4.2 4 3 5 3 2 20 3 7 5 6
    4.5 6 3 3 3 1 10 2 6 5 7
    4.8 6 4 5 3 1 24 2 8 6 8
    5.1 9 2 5 2 3 40 3 8 3 8
    5.4 12 5 3 3 2 41 2 31 4 6
    5.7 10 7 5 3 4 46 8 70 7 11
    6 7 11 5 3 3 47 12 52 16 26

    Conversion of NDCPT value to NSPT value.

    The NDCPT values were converted into equivalent NC (SPT) values from the given equations.

    NC (SPT) = NDCPT/1.5 for depth upto 3m

    NC (SPT) = NDCPT/1.75 for depth 3m to 6 m

    Ultimate Bearing Capacity

    Average NDCPT is calculated for each DCPT (DC1 to DC10). In computing the value, any individual value more than 50% of the average calculated was neglected and the average re-calculated. Conversion of average NDCPT value to average Ncor (SPT) values were carried out as discussed in section 3.2.1. Safe bearing capacity of strip footing was determined similarly as discussed in section 3.1.1. Results from shear and settlement criteria were given in Tables 7 and 8 respectively.

    Table 7: Safe bearing capacity (kN/m2) by shear failure criteria
    Bore hole No. NDCPT
    (avg.)
    NSPT
    (corr.)
    Φ
    deg.
    Φ' Nq B=1m qs B=2m qs
    dq
    or dγ
    dq
    or dγ
    BH1 6 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH2 6 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH3 5 3 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH4 3 2 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH5 2 1 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH6 25 15 32 22.4 8.4 7.8 1.05 69 1.03 69
    BH7 3 2 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH8 6 4 29 19.6 6.4 5.39 1.04 52 1.02 52
    BH9 4 2 28 19.6 6.4 5.39 1.04 52 1.02 52
    BH10 8 5 29 19.6 6.4 5.39 1.04 52 1.02 52

    The safe bearing capacity value was determined as 52 kN/m2 for the entire Bore holes except BH6 (69 kN/m2) (1 & 2 m footing width). The safe bearing capacity was calculated only for bore holes BH6, BH10 as NC (SPT) for all other boreholes were less than 5. The safe bearing capacity by settlement criteria for width of footing 1m is taken as 112 kN/m2 while for 2m width it is taken as 52 kN/m2.

    Table 8: Safe bearing capacity (kN/m2) by settlement criteria
    Bore hole NDCPT NSPT Corr. Safe bearing capacity
    Aver. Width of footing (1m) Width of footing (2m)
    BH1 6 3 NA NA
    BH2 6 3 NA NA
    BH3 5 3 NA NA
    BH4 3 2 NA NA
    BH5 2 1 NA NA
    BH6 25 15 534 489
    BH7 3 2 NA NA
    BH8 6 4 NA NA
    BH9 4 2 NA NA
    BH10 8 5 112 52

    It was concluded from the DCPT that safe bearing capacity at depth of foundation considering the shear and settlement criteria of cinder dump is 52 kN/m2.

    Plate Load Test

    Plate Load Test (PLT) was carried out as per Indian Standard (IS 1888,1982) at six locations with plate size 0.60 x 0.60 m2. The size of pit excavated for testing was 3 m x 3 m i.e. five times the width of the test plate. The depth of foundation was kept 3 m. Loading platform was erected using wooden planks with sand bags placed on these platforms to provide reactions of adequate magnitude. Water was sprinkled for saturation of cinder before keeping the plate for testing. View of Digging of test pit, loading arrangement and testing process are shown in Figs. 4 - 6 respectively.

    Digging of test pit is in progress

    For measuring the settlement of plate, four dial gauges were positioned at the corners of the plate and fixed on datum bars which rested on the sides of the pits at sufficiently large distances from the edge of the plate as shown in the above Fig. 6.

    Table 9: Loading intensity with final settlement
    PLT No. Loading Intensity
    (kN/m²)
    Final Settlement (mm)
    1 166 Continuing
    2 208 32.16
    3 611 14.71
    4 388 35.44
    5 416 30.49
    6 222 56.65

    A seating load of 7 kN/m2 was applied and maintained for no further settlement. The first increment of the static load of 100 kN/m2 was applied. This load was maintained constant for a period till no further settlement occurred or the rate of settlement became less than 3.33×10-7 m/s (0.02 mm/min). The final/stable readings of the dial gauges were recorded. The next higher stage loading was applied and the above procedure was continued till the maximum specified settlement of 0.025 m or ultimate loading intensity of 600 kN/m2 achieved. Variation of loading intensity with settlement is shown in the Fig.7 (PLT1 to PLT4). The values of loading intensity applied in different tests and corresponding final settlement observed is given in Table 9. It was observed that curves are linear in the earlier stages but flattens out after some time, and there is no clear point of failure. This shows that material is in loose to medium dense. In this condition, settlement was plotted as abscissa against corresponding load intensity as ordinate along logarithmic scales. This gives two straight lines. The intersection was considered as ultimate bearing capacity of cinder. Accordingly, load-settlement curves on a log-log plot are shown in Fig. 8.

    Load-settlement curve

    Determination of Ultimate Bearing Capacity

    Ultimate Bearing capacity is determined by considering the shear stress and settlement criteria.

    (a) Shear stress criteria.

    From the Figs. 7 & 8, it was not possible to find out the ultimate bearing capacity even after log-log plotting.

    (b) Settlement criteria

    Considering the permissible settlement of strip footing (Sf = 50 mm), the corresponding settlement of plate (Sp) is calculated as below.

    Sp = Sf [ Bp (Bf + 0.3) / Bf (Bp + 0.3) ]2

    Where, Sp = Settlement of test plate in, m
    Sf = Settlement of footing = 0.05 m (permissible)
    Bp = Size of the test plate = 0.6 m
    Bf = Size of footing in, m

    Ultimate bearing capacity of footing was calculated considering the width of footing (1 & 2 m). For typical strip footing of width 1 m and depth of 2 m, settlement of plate was calculated 38 mm. Corresponding to the settlement of 38 mm, from Fig.8 (PLT1), the ultimate bearing capacity was noted 139 kN/m2 and safe ultimate bearing capacity (FOS = 3) was estimated as 46 kN/m2. Similarly, settlement of 29 mm was calculated for strip footing of width of 2 m and corresponding ultimate loading was observed to be129 kN/m2. Accordingly, safe bearing capacity was determined 43 kN/m2 Results are summarized in Table 10 for PLT.

    Table 10: Safe bearing capacity (kN/m2) by settlement criteria
    Tests Width of footing, m
    1 2
    PLT 1 46 43
    PLT 2 180 67
    PLT 3 NA 117
    PLT 4 NA 126
    PLT 5 NA 133
    PLT 6 63 53

    The safe bearing capacities for width of the footing 1m and 2 m were taken as 54.5 kN/m2 (Average of 46 and 63 kN/m2) and 54 kN/m2 (Average of 43,67, 53 kN/m2) respectively.

    Geophysical Electrical Resistivity Test (GERT)

    To know the denseness of sub soil, electrical resistivity test was also carried out as per Indian Standard. Resistivity measurements were made by driving four electrodes about 0.10 to 0.15 m into the ground at pre-selected electrode spacing. In this method, a current 'I' is passed through cinder dump and it is distributed within a large hemispherical mass. The portion of current that flow within the cinder mass produces a voltage drop. The resistance of sub-soil is calculated as ratio of the voltage drop to the current which can be measured using a Digital Earth Resistance Tester. The Electrical resistivity sounding at a field location provides the value of apparent resistivity at different depths of sub surface strata. The apparent resistivity up to any particular depth is a function of true resistivity and thickness of various strata through which the current passes. True electrical resistivity of any material is the resistance between the opposite faces of cube of unit dimension of that material. The Electrical resistivity is expressed by the equation.

    p=EA/IL = RA/L

    Where, p= electrical resistivity (ohm-m), E = potential drop (Volts), A = area of cross section of sample, I = Current (Ampere), L = length (meter) and R = resistance.

    Inverse slope resistivity test
    Figure 9: Inverse slope resistivity test
    Vertical Electrical sounding tests were conducted using Schlumberger's circuit array method at different locations. The interpretation of vertical electrical sounding data plot was done by using a computer software using inverse slope method and analysis were carried out with reference to approximate resistivity value of geo-materials for different locations. The plot shows the thickness of different layer according to resistivity value lying in different zone (Fig.9). The plot was analyzed and a typical GERT results is given in Table 11. The lower resistivity value was observed of sub cinder strata at different depth and locations. It was concluded that material is in very loose state. The intermittent increase of higher resistivity value was also observed which was due to slag waste mixed in the cinder.

    Table 11: A typical GERT results
    Depth (m) r (ohm-m) Zone Description
    0 to 17 9999* HD  Higher resistivity value due to Coarse cinder mixed with Steel slag waste.
    17 to 19 9 WZ  Lower resistivity value due to fine cinder at very loose / moist state.
    19 to 23 9999* HD Erroneous readings due to presence of magnetic substances i.e. steel slag
    23 to 27 7 – 21 WZ  Lower resistivity value due to fine cinder at very loose / moist state.
    27 to 29 9999* HD  Erroneous readings due to presence of magnetic substances i.e. steel slag
    29 to 31 18 WZ  Lower resistivity value due to fine cinder at very loose / moist state.
    31 to 33 9999* HD  Erroneous readings due to presence of magnetic substances i.e. steel slag
    33 to 35 12 WZ  Lower resistivity value due to fine cinder at very loose / moist state.
    35 to 42 9999* HD  Erroneous readings due to presence of magnetic substances i.e. steel slag
    42 to 43 444 SFZ Resistivity value due to Coarse & fine cinder mixed at dense state.
    43 to 46 9999* HD  Erroneous readings due to presence of magnetic substances i.e. steel slag

    Suitability of Cinder for Construction
    Building Construction

    Type of Foundation


    The type of foundation depends upon the configuration of loading points, loading intensity at the foundation level and subsoil condition. The results of the investigation indicate that the subsoil consisting of cinder is in a loose state down to the depth investigated. Notably, dry densities of the cinder is in the wide range of 8.4 to 19 kN/m3 despite the specific gravity is in the limited range of 2.19 to 2.61. This is indicative of high variability in the void ratio of the cinder with maximum void ratios being over 1.5. Such high void ratios are indicative of a flocculated type of particulate structure which is typically formed due to the presence of ionic bonds or possible chemical bonds between the particles. The fact that the undisturbed samples disintegrated on extrusion adds to the uncertainty about the nature of bonds between the cinder particles, if any. However, in view of the fact that the cinder consists predominantly of sand and gravel fractions, the load settlement behavior can probably be considered to be similar to that of a sandy soil. Accordingly, the load settlement behavior can be accessed from results of Standard Penetration Tests as is typically carried out for foundations resting on sandy soil strata. Assessment of load settlement behavior from plate load test was satisfactory as the settlement observed in the various plate load tests show a large range with bearing capacity failure (i.e. continuing settlement under a loading intensity of only 166 kN/m2) observed in one test while in another test, even at a high loading intensity of over 600 kN/m2, the settlement had been only about 0.015 m. This is indicative of large variability in the behavior of the cinder under applied load. In view of the above discussion, the recommendations for shallow foundation will have to be provided by giving careful and due consideration to all aspects of shear and settlement behavior of the cinder substrata. To avoid the probable detrimental effects of differential settlement, isolated footing is not suitable.

    Therefore, the option of constructing the proposed structures in brick masonry (i.e. load bearing wall construction) laid on Reinforced Cement Concrete Strip Footings is advisable with single storied. Additional reinforcements can be provided in the brick masonry to provide ductility to the structure so that the structure can withstand significant differential settlements. As an alternate, the proposed structures are made to rest on Raft foundations.

    The foundation surface should be watered for at least 24 hours. The top slush should then be removed and the surface compacted using plate vibrators preferably of low frequency and high mass. If any loose pockets are observed, the same shall be filled with brickbats/ gravel and compacted well. Foundations can subsequently be placed over such a prepared surface.

    Depth of Foundation

    The minimum depth of foundation depends upon the following factors.

    Top loose zone – loose sub soil should be scrapped. As the cinder substrata is in a loose state down to the depths investigated, foundation depth for the proposed structure is rest at a minimum depth of 2 m below the existing ground level. Excavation should be carried out by ordinary method, down to the foundation depth with necessary side slopes.

    Safe bearing capacity depends upon the allowable settlement. Permissible settlement of footing 0.05 m is considered to evaluate the safe bearing pressure for strip footings.

    Safe bearing capacity was evaluated considering the strip footing width of 1 to 2 m and depth of 2 m by shear failure and settlement criteria for all the three methods i.e. SPT, DCPT and PLT, brief summary of values are given in Table 12, details of which is given in section 2.

    Table 12: Summary of safe bearing capacity (kN/m2) from different methods
    SPT DCPT PLT
    Shear Criteria Settlement criteria Shear Criteria Settlement criteria Shear Criteria Settlement criteria
    52 48 52 48 55 43

    Considering the above results, the average safe bearing capacity of footing is considered as 50 kN/m2.

    Limitations

    The substrata consisting of dumped cinder was observed to be smoldering beyond a depth of about 2m below the existing ground level which was evident in the pits excavated for plate load tests and also the bailer and drilling rods which became hot and could not be touched by bare hands, while progressing the boreholes. Smoldering is due to unburnt carbon particles which start burning when exposed to atmosphere. This may cause slow settlement of cinder dump area.

    As a Structural Fill

    Considering the geotechnical properties of cinder (gradation, Atterberg limit, density, strength parameters - cohesionless & ϕ = 340), it is suitable as a structural fill material.

    Reinforced retaining wall: It may be used as a back fill material for the construction of reinforced earth retaining wall as frictional angle is high. Density of the material is low, which has additional advantage as it will produce less horizontal earth pressure on the wall. Gradation and PI properties are also suitable as a backfill material of retaining wall as it contains mainly sand size particles and PI < 6.

    Embankment construction/filling low laying area: From the plate load test, it was observed that negligible settlements occurred for a loading of 24 kN/m2 (traffic and pavement loadings). It is a sand size cohesionless material having very good drainage property. This material may be used for the embankment construction as a per the IRC SP 58 (2001). As the material is light weight, it will produce less pressure on the subsoil which leads to less settlement. This material is suitable for the construction of embankment as well as filling of low lying area.

    Cinder should be used in the core whereas the local soil is proposed to be used as side cover. The provision of side cover of thickness 2 m is found necessary as the exposure of non plastic mix to open environment would lead to surface erosion. Stability analysis of cinder embankment was carried out under different saturation conditions i.e. partially saturated, HFL and steady seepage. Different parameters and cross sectional details are considered for the analysis viz. height = 5 m, cover thickness - 2m, loading = 24 kN/m2, horizontal (ah= 0.05) and vertical (av = 0.025) acceleration seismic factors as per IRC 6 (2000). The factor of safety values was observed to be more than one.

    Conclusions

    A geotechnical study of cinder dump area, Jamshedpur was carried out to investigate its utility for building construction and as a structural fill. Different methods of sub soil investigations viz. Standard Penetration tests, Dynamic Cone Penetration tests, Plate Load Tests and Electrical Resistivity Tests were carried out. Results were analyzed and brief conclusions are summarized below.
    • Cinder was observed to be coarse grained and non-plastic material. Cinder is classified as SP i.e. poorly graded sand. It contents gravel (7– 50%), sand (42 - 74%) and silt (5 - 39%) in all the bore holes with depth.
    • The natural water content varies in the range of 13 – 33% and dry density varies in the range of 10.7 – 14.2 kN/m3.
    • The value of specific gravity was observed to be in the range 2.19 to 2.47. The specific gravity of cinder is observed to be low. This is due to unburnt carbon present in the cinder waste.
    • The coefficient of permeability was determined as 6.3×10-7 m/s. The value indicates that it is a free draining material.
    • The value of coefficient of consolidation Cv was observed to be 4×10-8 m2/s under normal loading of 100 to 150 kN/m2. The value of compression index (Cc) was observed to be 0.04. The value of Cc indicates that cinder is low compressible in nature.
    • The average safe bearing capacity of footing was observed to be 50 kN/m2 by SPT or DCPT or PLT.
    • The lower resistivity value was observed at different depth. It was concluded that material is in very loose state. The intermittent increase of higher resistivity value was also observed which is due to slag waste mixed in the cinder.
    • It is used a structural fill in the construction of reinforced retaining wall, embankment and filling low laying area.
    • Reinforced cement concrete strip footings or raft foundations are recommended as a suitable foundation for a single storey building construction. Depth of foundation is recommended as 2 m below the existing ground level.
    Acknowledgement

    The authors are thankful to the Director, CSIR-Central Road Research Institute, New Delhi for giving permission to publish this paper. The support provided by GTE staff members of CSIR-CRRI is also acknowledged.

    References
    • Cinder Wikipedia (2014). http://en.wikipedia.org/wiki/Cinder
    • Havanagi, V.G., Sinha, A.K., Kanaujia, V. K., Ranjan, A. and Mathur, S. (2013), “Cinder waste material for the construction of road”, Journal of Indian Highways, Vol. 41(4), pp 69-72.
    • Highway Research Record (HRR)- 39 (2012), General report on road research work. Published by Indian road congress, New Delhi, India.
    • IS: 1888 (1979), Method of load test on soil. Published by Bureau of Indian standard, New Delhi, India.
    • IS: 4968-Part 1 (1976), Method of subsurface sounding for soils: Dynamic method using 50mm cone without bentonite slurry. Published by Bureau of Indian standard, New Delhi, India.
    • IS: 6403 - 1981, Code of practice for determination of bearing capacity of shallow foundations. Published by Bureau of Indian standard, New Delhi, India.
    • IS: 8009 - Part1 (1976), Code of practice for calculation of settlements of foundations: Shallow foundations subject to symmetrical static vertical load. Published by Bureau of Indian standard, New Delhi, India.
    • IRC 6 (2000). Standard Specifications and code of practice for road bridges, section II – loads and stresses. Published by Indian Road Congress, New Delhi, India.
    • IRC SP-58 (2001), Guide lines for fly ash embankment construction. Published by Indian Road Congress, New Delhi, India.
    • National Guide to Sustainable Municipal Infrastructure (NGSMI) (2005). Reuse and Recycling of Road Construction and Maintenance Materials. Published by Federation of Canadian Municipalities and National Research Council.

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