CONCEPT OF UNDER REAMED CEMENTED STONE COLUMNS FOR SOFT CLAY GROUND IMPROVEMENT

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1 IGC 2009, Guntur, INDIA CONCEPT OF UNDER REAMED CEMENTED STONE COLUMNS FOR SOFT CLAY GROUND IMPROVEMENT Y.S. Golait Professor Emeritus, Dept. of Civil Engg., RKN Engineering College, Nagpur , India. V. Satyanarayana Section Engineer (W), South East Central Railway, Nagpur , India. S.S.V. Raju Sr. Materials Manager, Consulting Engineers Group, Bangalore, India. ABSTRACT: Stone column technique is becoming increasingly popular for improvement of the ground of soft clays, silty clays, loose fine sandy silts etc. The soft clay ground improvement is, especially, the main concern in our country and it is in this case the stone column technique has great relevance. The paper highlights the new concept of semi-rigid type of cemented stone columns in soft clay for improving significantly the performance of the conventional stone columns. The laboratory model tests on unit cells of layout of such stone columns of 3 cm diameter with area replacement ratio of 9% and length ratio of 10 were performed to ascertain the degree of effectiveness of the cemented stone column systems in soft clay (black cotton soil). The results are discussed with respect to improvement in load carrying capacity and reduction in settlement. 1. INTRODUCTION Many large areas, where major construction activities are required to be undertaken, consist of deposits of soft clays, silty clays, loose fine sandy silts etc. Such deposits are effectively improved by stone columns technique (Barksdale & Bachus, 1983). This technique is becoming quite popular since last few decades for increasing the load carrying capacity and decreasing the potential settlement of structures built at such sites. (Ranjan & Rao, 1983, Datye & Nagaraju, 1977). In our country, the problems associated with soft clay soils like black cotton soils, soft coastal clays, relatively looser alluvial deposits etc. have great relevance to stone column treatment. The conventional practice of stone column construction is to install the straight shafted columns with fill material in the form of un-cemented mixture of crushed stones, gravel, coarse sand etc. These columns, in most cases, are bearing type wherein they are usually taken down to rest on relatively firm stratum. For the required improvement, it is reported that the area replacement ratio varies from about 20% to 40% (Barksdale & Bachus, 1983). The ultimate load carrying capacity of the stone column treated ground is governed, besides other factors, by the lateral passive resistance the surrounding soil can offer to lateral bulging of stone column within the upper length of about 3 to 4 diameter of column, under applied vertical load. Thus in soft soils having undrained shear strength less than 15 kpa, the stone columns may not be feasible (Rao & Nayak, 1995). The literature reports some approaches to improve the performance of conventional stone columns. Maher & Gray (1990) have shown that the strength and stiffness of a sand column can be significantly increased by incorporating randomly distributed reinforcing fibres. Madhav (1983) suggested the use of several reinforcing geofabric layers placed horizontally at different depths. Alternatively, as per his view, a rigid hollow cylinder at the top portion of column can also be used. Ranjan & Rao (1983) have proposed skirting around the stone column or group of stone columns to enhance the degree of ground improvement. Barksdale & Bachus (1983) have reported a case history of the provision of rigid stone columns of concrete. The method of construction of such columns consists of supplying cement or grout by bottom feed unit or pumping ready mix concrete to bottom of hole through feeder pipe. The experimental study of Junar & Oraccio (1991) has shown that very soft ground (with soil shear strength even less than 15 kpa) can be improved by such rigid stone columns. A novel concept of providing a semi-rigid type of stone column with enlarged bulb in its stem and formed by using an artificially prepared cemented fill material in the hole is proposed in the present study to make the method cost effective with substantial enhancement in the nature of treated soft clay ground with 356

2 respect to increased load carrying capacity and reduced settlement. 2. UNDERREAMED CEMENTED STONE COLUMN The lateral bulging of conventional stone column under the action of axial load limits its load carrying capacity. It can thus be inferred that if a non-bulging type of fill material is used, the enhanced effects in stone column behavior are expected. The use of concrete as fill material and forming rigid vertical elements is too costly as it is similar to providing closely spaced cast in situ concrete pile foundation. As a cost effective proposition, a semi-rigid type of column formed by using a mix of conventional granular material or crushed stone aggregates treated with marginal amount of cementing admixture like cement, fly ash, lime etc. is also expected to perform well in enhancing the ground improvement effects of the unit cell of stone column layout. A provision of an enlarged bulb in the stem located either at the bottom of column or at some suitable intermediate level can have additional beneficial effects due increased bearing area. The suggested sequences in installation of such stone columns can be (i) making a bore hole of diameter 30 to 45 cm by hand operated augers or boring rig, up to a maximum depth of 10 to 15 bore diameters, (ii) forming an enlarged bulb of diameter of 2.5 times the bore diameter using an appropriate underreaming tool as is employed in construction of underreamed pile foundation and (iii) filling the hole by cemented granular mix in stages accompanied with compaction by appropriate hammer or tamping tool. The procedure is similar to construction method adopted for installing underreamed piles in black cotton soil except for the nature of fill material. The procedure is simpler and cost effective in moist clay deposits. However, for very soft clays below GWT, boring with removable casing tubes and under water mix placing technique will have to be employed. Such semi-rigid type of under reamed cemented stone column is viewed to exhibit very little possibility of bulging and to cause larger participation of soil within the unit cell in load transfer mechanism. Figure 1 shows schematically the proposed stone column unit cell. The initial studies by Gautam (2007) revealed that the granular material of gravel and coarse sand with 2% cement plus 4% fly ash formed a semi-rigid mass of properties: cohesion c0.65 kg/cm 2, Φ angle 38 and elasticity modulus Ec 600 kg/cm EXPERIMENTAL INVESTIGATIONS In order to investigate the effectiveness of cemented stone columns in soft clay soils, the laboratory model studies were carried out to evaluate the performance improvement with respect to increase in load carrying capacity and reduction in settlement of a single unit cell of stone column layout. Rigid loading base Db 2.5 ds Fig. 1: Unit Cell of Soil Stone Column System The soil used in the investigations was highly plastic clay of Nagpur region having properties: w L 65%, I P 35%, c u 0.20 kg/cm 2, Φ u 10 and elasticity modulus, Es 101 kg/cm 2. The unit cell investigated in model tests relate to field stone column layout of area replacement ratio, r a, of 9% and length ratio, r l, of 10. The area replacement ratio is defined as ratio of area of stone column to total area within unit cell, and the length ratio is the ratio of length (L) to diameter (d s ) of stone column. The model column diameter of 3 cm was selected. The rigid steel plate of 10 cm diameter was used to apply loads on unit cell. The following five types of unit cells with the aforesaid geometry were investigated: Unit cell of only soil (without treatment) VS Unit cell with conventional, straight shafted uncemented stone column SU Unit cell with straight shafted cemented stone column Unit cell with underreamed cemented stone column having bulb at the bottom. CU b Unit cell with underreamed cemented stone column having bulb at a depth 5ds below soil surface CU i All the stone columns were floating type. The experimental set up comprised of steel test tank (50 cm diameter & 50 cm height), screw jack and proving ring filled loading frame, specially designed boring and under reaming tools etc. Stone column installation procedure was simulated in laboratory model set up. The cemented granular mix used to construct the stone column in model experimentation comprised of 60% coarse sand and 40% fine sand mixed with 2% cement and 4% fly ash. 2% water content was found Soil Stone column But for CUI case Unit cell But for CUb case 357

3 appropriate to get the homogeneous workable moist mix that could be easily filled and compacted in the hole. The results of the triaxial tests on cured cylindrical samples of the mix showed the properties: c 0.68 kg/cm 2, Φ 37 and the modulus of elasticity Ec 600 kg/cm 2. The unit cell of 10cm diameter was load tested after saturation of the soil column system. Almost identical physical state of soil in different tests was ensured with dry unit weight of 1.23 gm/cm 3 and water content of 42%. The model unit cell in the laboratory investigations corresponded to field stone column layout with triangular pattern with column shaft diameter of 30 cm, at a spacing of 1m and of length 3 m. The load-settlement curves obtained from the model tests were analyzed to work out the bearing capacity improvement factors, F b, and settlement reduction factors, F s, for all the categories of stone column systems viz. SU,, CU b and CU i. 4. ANALYSIS AND INTERPRETATION OF RESULTS 4.1 Failure Pressure Intensity The load-settlement curves for unit cells of five categories are shown in Figure 2. Fig. 2: Load-settlement Curves for Various Unit Cells The curves indicated almost direct proportionality between load and settlement initially over a considerable load range, which is then followed by a non-linear nature leading to plastic yielding characterized by continuous vertical deformation even under constant load. The failure load Q f of the system was determined by double tangent method and the values for five unit cells are: 62 kg (VS), 93 kg (SU), 113 kg (), 153 kg (CU b ) and 165 kg (CU i ). The corresponding failure pressure intensities (q f ) are: 0.79, 1.18, 1.44, 1.95 and 2.10 kg/cm 2 respectively. The prediction of failure load (Q f ) for CU i case is shown in Figure 3. Fig. 3: Determination of Failure Load for CU i Unit Cell It is seen that there is considerable increase in q f values for cemented stone columned unit cell over the conventional uncemented stone column case. For the geometry of the stone column layout considered in the present investigations, the percent increase in load carrying capacity of different soilcolumn systems with reference to VS, SU, and CU b is shown in Table 1. Table 1: Percent Increase in q f Values Reference Soil Column % Increase system system in q f values SU VS CU b CU i CU SU b 64.5 CU i 77.4 CU b 35.4 CUi 46.5 CU b CU i 7.8 It is observed that compared to 50% improvement in bearing capacity by SU, the improvements by cemented stone column systems viz., CU b and CU i are 82.3 %, 146.8% and 166.1% respectively. In other words, as compared with the bearing capacity of unit cell of conventional un-cemented stone column (SU), the bearing capacity of straight shafted cemented stone column system () is increased by 21.5% and that for CU b and CU i cases the values are 64.5% and 77.4% respectively. The underreamed bulb at bottom or at inter mediate level, improves the bearing capacity by 35.4% for CU b and 46.5 % for CU i case. This is clear indication of high degree of effectiveness of cemented stone column and by provision of underreamed bulb. The under reamed 358

4 cemented stone column with the bulb located at 5d s is found to be the most effective. 4.2 Bearing Capacity Improvement Factor The improvement in the failure pressure intensity of any soilcolumn system is proposed to be expressed by bearing capacity improvement factor (F b ), which is defined as: F b Failure pressure for soil-column system Failure pressure of untreated soft clay ground approximately two-third failure load (i.e. 2/3 Q f ) and this portion can therefore be fitted with a straight line as shown in Figure 3 for typical curve of CU i case. All such straight approximations for respective 2/3 Q f loads are given in Figure 5. The slope of the line is unit cell stiffness, k. The k values thus obtained are shown in Table-2. The values are found to increase sequentially for SU,, CU b and CU i. or F b (q f )t/(q f )u The values of F b are: 1.0 (VS), 1.50 (SU), (), (CU b ) and (CU i ). The graphical representation is shown in Figure 4. Load Q (kg) VS Settlement δ (mm) Fig. 5: Straight Line Approximation for Initial Portion of Load-settlement Curves 4.4 Settlement Reduction Factor Fig. 4: Bearing Capacity Improvement Factors F b value of for CU i means the bearing capacity of treated ground by under reamed cemented stone column with bulb at intermediate level is times the bearing capacity of untreated soft clay ground. 4.3 Unit Cell Stiffness The settlement behavior of loading base over circular area of unit cell of soil-stone column system depends on the overall stiffness of the unit cell containing semi-rigid stone column and the surrounding soft clay soil. The unit cell stiffness, k, is defined as the pressure per unit settlement. Loading pressure intensity q Thus, k Settlement of loading base δ kg/cm For a non-linear load-settlement curve, k varies with load level. However, on close examination of observed loadsettlement curves for all five unit cells (Fig. 2), it is apparent that the curvature is insignificant up to a load level of 2 For comparing the settlement characteristics of any stone column system with that for untreated soil, the settlement reduction factor F s is introduced. This is defined as: F s Settlement per unit pressure for a unit cell of Settlement per unit pressure for a unit cell of (1/k) of soil-column system (1/k) of un-treated soil (k) of (k) of un-treated ground treated ground treated soil un-treated soil For example, if F s is 0.8 for CU b case, it means the settlement of ground treated with under reamed cemented stone columns with bulb at bottom will be 80% of the settlement of un-treated ground under same applied pressure intensity. It can also be inferred that 20% reduction in settlement will be caused by installing such columns. The values of F s and percentage reduction in settlement for all the cases are given in Table

5 Unit cell Table 2: Unit Cell Stiffness Values Stiffness (k) (kg/cm 3 ) Settlement reduction factor (F s ) Settlement reduction (%) VS SU CU b CU i It is noteworthy to find that the settlement is considerably reduced by provision of cemented stone columns. As compared to 10.8% reduction in settlement in SU case, 19.12% reduction is achieved by case. The under reamed bulb at 5ds depth further enhances the reduction in settlement to 27.8%. 5. CONCLUSIONS The provision of semi-rigid floating type of underreamed cemented stone columns is conceptualized probably for the first time for economical and effective improvement of soft clay ground. The model studies, conducted on single unit cells using a scale reduction factor of 10 for a field stone column layout of 30 cm diameter columns having area replacement ratio of 9% and length ratio of 10, revealed that both the bearing capacity characteristics and settlement characteristics of soft clay ground are significantly improved by the use of under reamed cemented stone columns. However, in order to develop this ground improvement technique, the aspects of load transferring mechanism, analysis for bearing capacity estimation, detailed behavioral features like effects of area replacement ratio, length ratio, column size etc. need to be thoroughly investigated. REFERENCES Barksdale R.D. & Bachus R.C. (1983). Design and Construction of Stone Columns, FHWA Report No. RD. 83/026, Vol. 1, Dec. 1983, Federal Highway Administration, Washington D.C. Datye K.R. & Nagaraju S.S. (1977). Installation and Testing of Rammed Stone Columns, J. Geotech Div., AE, Vol. 17. Gautam R.B. (2007). Studies on Floating Underreamed Cemented Stone Columns for Bearing Capacity Improvement of Black Cotton Soil, M. Tech Dissertation (Unpublished) submitted to Nagpur University. Juran I. & Oraccio R. (1991). Reinforcing Soft Soils with Artificially Cemented Compacted Sand Columns, J. Geotech Div., AE, Vol. 17. Madhav M.R. (1982). Recent Developments in the Use and Analysis of Granular Piles, Symposium on Recent Developments in Ground Improvement Techniques. Maher M.H. & Grey D.H (1990). Static Response of Sands Reinforced with Randomly Distributed Fibres, J. Geotech Div., AE, Vol Ranjan G. & Rao B.G. (1983). Skirted Granular Piles for Ground Improvement, Proc. VIII, European Conference on SM & FE, Helsinki. Rao N.B.S. & Nayak I.G. (1995). Model Studies on Partially Confined Sand Columns using Geogrid Tubes, Indian Geotech. Journal, Vol. 25 (3). 360