Ground improvement using the vibro-stone column technique

Transcription

1 Ground improvement using the vibro-stone column technique A. Kosho 1 A.L.T.E.A & Geostudio 2000, Durres, Albania ABSTRACT The vibro stone columns technique is one of the most used techniques for ground improvement processes all over the world. In recent years this technique is also used in Albania. For the first time in our country this method is used for ground improvement at Ferry Terminal of Durres, which is the biggest harbour of Albania. In this case the technique is used to reinforce silty clay and silty sand soils. There are two methods of construction in the vibro replacement technique named: the wet method and the dry method. In Durres Ferry Terminal building and yard infrastructure the dry method is used. The paper outlines the technique, the ways of application in different types of soil, and settlement and bearing capacity calculations for this case. Keywords: vibro methods, stone columns, dry method, settlement, bearing capacity 1 INTRODUCTION Albania has seen extensive growth for the past years with many infrastructure projects in the construction industry. Current technology affords many ground improvement techniques to suit a variety of soil conditions, structure type and performance criteria. These ground improvement techniques can offer alternative foundation system to the conventional pile foundation system. 2 WHAT IS A STONE COLUMN? Vibro- replacement stone columns extends the range of soils that can be improved by vibratory vibratory techniques to include cohesive soils. Densification and/ or reinforcement of the soil with compacted granular columns or stone column is accomplished by either a top-feed or a bottom-feed method. Cohesive, mixed and layered soils generally do not densify easily when subjected to vibration alone. The vibroreplacement stone column technique was developed specifically for these soils, effectively extending the range of soil types that can be improved with the deep vibratory process. With vibro-replacement stone columns, crushed stone is designed to increase bearing capacity, reduce settlement, mitigate the potential for liquefaction and improve shear resistance. 1 ALTEA & Geostudio2000, Rr. Maliq Muco L. 6, Durres, Albania. [email protected]

2 Figure 1. Bottom-Feed method of stone column construction [1]. 2.1 Vibro-Replacement technique Vibro-replacement is a technique derived by further developing the Vibro-compaction process. Soils such as pure silts/clay or mixed deposits of silts, clays and sands can not be improved by vibro-compaction because of their inability to properly respond to vibration. The vibro-replacement technique introduces a coarse grained material as load bearing elements consisting of gravel crushed stone aggregate as a backfill medium. There are two methods of construction in the vibroreplacement technique, i.e. the wet and the dry method.

3 Regarding the information that we receive, for the replacement works for Durres Ferry Terminal Building and Yard Infrastructure we adopt the dry method as described below. The essential equipment used for the vibro techniques is a depth vibrator. For this project the Beta vibrator was selected. Bottom- Feed vibro-replacement is a completely dry operation where the vibrator remains in the ground during the construction process. However, the vibrations themselves have minimal effect on cohesive soils (clays and silts), so in these and mixed soils, the penetration of the poker is followed by the construction of a stone-column. The displacement of the existing ground by the penetrating poker allows the construction of granular columns with high friction angle, so that the composite soil mass has a greater average strength and stiffness than the untreated ground. McCabe et al. [1] gave an extensive description of the method, essential parts of which is repeated next. The hole created by the poker is filled with inert crushed stone or gravel (or approved construction waste in certain circumstances) and is compacted in stages from the base of the hole upwards. There are two different approaches that may be used to construct the column, depending on the ground conditions. In the Top-Feed System, the poker is completely withdrawn after initial penetration to the design depth. Stone (40-75mm in size) is then tipped into the hole in controlled volumes from the ground surface. The column is compacted in layers (the stone is forced downwards and outwards) through continued penetration and withdrawal of the poker. The Top-Feed System is suitable if the hole formed by the poker will remain open during construction of the column. Alternatively, the stone may be fed from a rig-mounted hopper through a permanent delivery tube along the side of the poker, which bends inwards and allows the stone to leave from the poker tip. This Bottom-Feed process requires a smaller grade of stone (15 to 45mm). By remaining in the ground during column construction, the poker cases its own hole and hence is suited to ground with a high water table or running sand conditions. The process of the Bottom-Feed system is schematically illustrated in Figure 1. In addition to improving bearing capacity and reducing compressibility, stone columns installed in a uniform grid pattern will help homogenize variable soil properties, thereby reducing the potential for differential settlement. Stone columns serve a secondary function of acting as vertical drains, accelerating the dissipation of excess pore water pressures (and associated primary settlement) from the imposed loading, allowing a foundation or floor slab to be brought into service at an early stage. In addition to the savings per meter length that stone columns present over piles, the soft column heads facilitate the use of ground bearing slabs, representing further savings compared to the ground beams and suspended slabs associated with piled solutions. Plate load tests (typically 600mm diameter) are carried out on constructed columns to verify the compactness of the stone and the stiffness of the supporting ground at the top of the column. However, settlements measured may not be representative of foundation or slab behavior due to differences in the load duration and depth to which the ground is stressed. Longterm zone load tests provide a more realistic reflection of the stiffness of the ground, when a plan area of the size of a real foundation is loaded which will usually straddle several columns and the intervening untreated ground. However, due to related cost, these tests are generally reserved for marginal sites. In addition to these control measures, it should be noted that the vibrating poker itself acts as an investigating tool, which provides an additional safeguard against unforeseen ground conditions. A measure of the resistance to the penetration of the poker is fed back electronically to the rig operator, who can then match the quantity of stone supplied to the lateral resistance of the ground encountered.

4 3 BEHAVIOUR AND DESIGN OF STONE COLUMNS Most types of ground improvement are intended to work with the existing ground whereas rigid inclusions (piles) are intended to bypass the ground to some extent. While stone columns will transmit some load to the soil by shear stresses (along the column-soil interface) and end bearing (at the column base), the predominant load-transfer mechanism (unless the column is very short) is lateral bulging into the surrounding soil. Cylindrical Cavity Expansion Theory (CCET) is applied to many geotechnical problems, most notably to the interpretation of the pressuremeter test which measures horizontal stresses in the ground, see Wroth [4]. CCET has also been used to model the bulging behaviour of granular columns leading to predictions of bearing capacity and settlement performance Bearing Capacity Hughes and Withers [5] performed pioneering laboratory studies of sand columns in a cylindrical chamber containing clay, and used radiography to track the deformations in and outside the column. They found that CCET represent the column behaviour very well, and proposed that the ultimate vertical stress (q) in a stone column can be predicted by: 1+ sinφ' 1 sinφ' q= ( σ ' + 4 ) ro c u (1) Figure 2. Priebe s basic improvement factor Priebe s settlement improvement factor, n, defined as 2 a function of the friction angle of the stone φ', the soil s Poisson s ratio and an Area Replacement Ratio dictated by the column spacing (Figure 2), [3]. The area replacement ratio is defined as A c /A, where A c is the cross-sectional area of one column and A is the total cross-sectional area of the unit cell attributed to each column (Figure 3). A c /A is geometrically related to the column radius (r) and column spacing (s) according to: 2 A r c = k A s (2) where k is π or 2π/ 3 for square or triangular column grids, respectively. where φ' is the friction angle of the stone infill, σ' ro is the free-field lateral effective stress and c u is the undrained shear strength Settlement Absolute and differential settlement restrictions usually govern the length and spacing of columns, and the preferred method of estimating post-treatment settlement in European practice, was developed by Priebe [2], based upon CCET. Although this method is strictly applicable to an infinite array of columns and empiricicalin its development, it is found to work well for most applications. Figure 3. Typical column arrangements, triangular grid (left) and square grid (right) 2 n = settlement without treatment / settlement with treatment

5 Priebe s improvement factor n may be derived from the chart shown in Figure 3 (note the reciprocal Area Replacement Ratio A c /A is used). However, corrections should be applied to allow for the compressibility of the column aggregate and influence of the pressure gradient along the soil-column interface. 4 PLATE BEARING TEST The plate bearing test is a loading test carried out by using a plate on treated ground, essentially used as a control of workmanship. The maximum load, which should be applied to a 600 mm diameter plate is 11 ton. The load should be applied in five equal increments. Following each application of load the settlement should be measured at intervals of one minute until no change is detected and then at intervals of 5 minutes. The load should be held for 10 minutes or until two successive readings at 5 minutes intervals are the same, whichever is the greater. The maximum load is held for 15 minutes or until three successive readings at 5 minutes intervals are the same, whichever is the greater. Table 1. Loading Levels, example of one test made in Durres Ferry Terminal Level Load kn Settlement mm Deformation mm Midukus N/mm unload CONCLUSIONS The Albanian construction industry has been later than many of its European counterparts in recognising the technical and economic advantages that Vibro Stone Columns can provide. Albania has an abundance of soft estuarine and alluvial soils and these may be improved sufficiently to allow standard foundations to be constructed at shallow depth, without the need to resort to deep piling. Where ground conditions are suitable, stone-column solutions have been shown to be more cost effective than trench fill in excess of 2m depth. In addition, stone columns can offer considerable contract program savings over other ground improvement methods, such as preloading and vertical drains. As with all geotechnical projects, a thorough site investigation with adequate information on soil strength and compressibility is essential [6]. ACKNOWLEDGEMENT I would like to thank Skender Allkja, A.L.T.E.A. & Geostudio 2000, Laboratory Testing for Construction Materials and Geotechnical Study, for the valuable information, and to Luljeta Bozo for reviewing the paper. REFERENCES [1] McCabe, B.A., McNeill, J.A., and Black, J.A. (2007) Ground improvement using the vibor-stone column technique. Presented at the Joint meeting of Engineers Ireland West Region and the Geotechnical Society of Ireland, NUI Galway. [2] Priebe, H.J. (1995) The design of Vibro Replacement, Ground Engineering (Dec), pp [3] Priebe, H.J. (1976) Evaluation of the settlement reduction of a foundation improved by Vibro- Replacement, Bautechnik, No. 5, pp [4] Wroth, C.P. (1984) The interpretation of in situ soil tests, Geotechnique, Vol. 34, No. 4, pp [5] Hughes, J.M.O. and Withers, N.J. (1974) Reinforcing of soft cohesive soils with stone columns, Ground Engineering, Vol. 7, No. 3, pp [6] UNI EN (2006) Industry Standards and Regulations: Execution of special geotechnical works- Ground treatment by deep vibration. Internet

6