Determining the Geotechnical Parameters of Stabilized Soils by Stone Column Based on SPT Results
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Department of Irrigation, University of Shiraz, Shiraz, Iran
E-mail: smazomorod@yahoo.com
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Determining the Geotechnical Parameters of Stabilized Soils by Stone Column Based on SPT Results
and Department of Irrigation, University of Shiraz, Shiraz, Iran
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ABSTRACT
Stone column is one of the soil stabilizing methods that is used to increase strength and decrease compressibility of soft and loose fine graded soils. Various methods that have been proposed to determine the bearing capacity and loading-settlement behavior of stone columns are based on the finite element analysis and experimental observations. The purpose of this paper is to present the new simple method to determine the properties of treated soil as a function of the standard penetration test number of soil and stone column.
INTRODUCTION
Since the availability of suitable construction sites decreases, and the need to utilize poor soils for foundation support and earthwork construction increases, it is necessary to strength the ground under existing structures to insure stability against adjacent excavation or tunneling, or to improve resistance to seismic or other special loadings. Therefore, soil improvement techniques have been rapidly developed in the past several years. Several methods are used to stabilize soils such as: compaction, consolidation, grouting, admixtures, thermal, reinforcement, and stone column. The ability of any of these methods to improve soil properties depends on several factors, including: soil type, degree of saturation and water table, initial relative density, initial in-situ stresses, initial soil structure, and special characteristics of the method used. In most cases the goal of treating soil is increasing shear strength and loading capacity, increasing stability and settlement control. Stone column is one of the methods used for treating fine and loose aggregates soils. Installation of stone column in loose and fine graded soils causes increasing strength and load capacity of these soils. Stone columns in saturated clay soils work as drainage system and results in decreasing consolidation time. Also stone column in loose and saturated non-cohesive fine graded soils act like a drainage system and reduce the drainage path and lessens liquefaction probability in case of earthquake occurring. Stone column systems in soft, compressible soils are somewhat like pile foundations, except that pile caps, structural connections, and deep penetration into underlying firm strata are not required, and the stone columns are, of course, more compressible. The strength of stabilized soil by stone column is consisting of soil strength and stone column strength. It is not possible to provide a sample that contains soil and stone column. In order to determine the supporting capacity and load settlement behavior of stone column foundation, finite element analyses, have been proposed. This analyses need to perform test and to determine strength parameters of stone column and soil material, modeling and performing complex analysis. In order to overcome difficulties, a new method based on standard penetration test number is introduced in this paper. Unlike finite element methods, the new method easily determine load-settlement behavior and shear strength of soils treated by stone column, with standard penetration number test.
CHARACTERISTICS OF STONE COLUMN
Most stone column installations are made using the vibro-replacement method in a manner similar to vibroflotation. A cylindrical vertical hole is made by a vibrating probe penetrating by jetting under its own weight. In some cases a dry process without water jets is used. Gravel backfill is dumped into hole in increments of 0.4 to 0.8 m and compacted by the probe which simultaneously displaces the material radillay into the soft soil. The diameter of resulting stone column can be estimated from the consumption material. It will usually be in the range of 0.6 to 1.0 m. larger columns diameter can be formed by copling two or three vibrators simultaneous. Gravel particle sizes in the range of 20 to 75 mm are typical. After installing stone columns a blanket of sand or gravel 0.3 m or more in thickness is usually placed over the top. This blanket works both as drainage layer and also to distribute uniform stresses from above structure. Stone columns may be used in clusters and rows to support walls and footings or as groups in square or triangular grid patterns with center-to-center columns spacing of 1.5 to 3.5 m. When stone columns are used in foundation construction the supporting capacity and settlement are of primary concern. When stone column used for stability purposes under embankments or in slopes, the shear strength of the columns is of primary interest. In fact stone columns transfer applied loads to extend layers of soil and cause stress distribution and reduce settlement. Design values of 20 to 30 tons per column are typical for columns in soft to medium stiff clays. The required minimum column length in soft soil can be estimated based on shearing strength along sides and end bearing capacity. The settlement of a stone column foundation depends on the cross sectional area of the column and their spacing. A load test on single columns within a group will ordinarily give a settlement in the range of 5 to 10 mm under design load. But experience and analyses indicated that the settlement of large loaded area supported by stone columns will be about 5 to 10 times greater than this.
The effectiveness of stone column installation is increasing load capacity of soil, increasing stability of slopes, decreasing foundations settlement and decreasing liquefaction potential. Some designers assume conservatively that all the applied foundation loads are carried by columns and for analysis purposes the friction angle of compacted stone columns can be taken about 35? to 40? and elastic modulus of it in the range of 40 to 70 (M pa) are appropriate. In proposed method it’s assumed that the applied foundation loads are divided between stone column and soil by relative stiffness ratio. And standard penetration test results are used to determine all necessary parameters for stone column analyses.
PROPOSED METHOD
In spite of the complexity of the load transfer mechanism in a stone column foundation and need for simplifying assumptions in analysis, the following assumptions are needed:
_ stone columns settlement is equal to the settlement of surrounding soil and applied loads to foundation are divided between soil and stone column proportional to stiffness ratio.
_under applied loads; lateral strain is formed in soil and stone column. The strength of stabilized soil by stone column is provided from interaction between soil and stone column that comes from radial strain of column and surrounding soil that is controlled by the passive resistance of the soil which can be mobilized to withstand radial bulging and by the friction angle of compacted material in the column.
The settlement of unstabilized soft soils can be computed as
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(1) |
Where S is the settlement, q is the average applied stress to soil and Mc is the confined (constrained modulus) of the soil. Mc depends on elastic constant and Poisson’s ratio and expressed by
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(2) |
Where Ec is the Young’s modulus and nc is Poisson’s ratio assigned to the soil.
Assuming the average Poisson’s ratio of clay is 0.3. Therefore, Eq. 2 would be
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(3) |
The elasticity modulus of fine graded soils using SPT test, can be computed as suggested by Shultz and Horn (1967)
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(4) |
By substituting Eq. (4) in Eq. (3) it’s possible to express elastic constant of surrounded soil’s as a function of the standard penetration test number
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(5) |
Confined elastic constant of stone column material would be
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(6) |
where Es is elastic constant of stone column material and ns is its Poisson’s ratio.
Elastic constant of coarse aggregate soils based on standard penetration number, can be computed by
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(7) |
(D’Appolonia et al. 1969) where Ns is number of blows obtained from SPT test on stone column.
Poisson’s ratio for coarse aggregate soils defined as a function of angle of internal friction (Kulhawy, 1984), that angle of internal friction of coarse aggregate soils can also be obtained from standard penetration test (Robertson and Campanella, 1983).
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(8) |
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(9) |
substituting Eqs.(7), (8) and (9) in Eq. (6) confined elastic constant of stone column material also can be obtained as a function of Ns.
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(10) |
If ratio of Nc is assumed, the quantity n can be obtained from Eqs. (5) and (10). Also, from Figure (1) the quantity n which can be obtained with different values of Nc and Ns.
Figure 1. Stress distribution ratio (n) for different value of Nc and Ns.
Assuming that stone column settlement is equals to surrounding soft soil, stress distribution in soil and stone column is proporational to their relative stiffness ratio.
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(11) |
Where A is total cross section, As is stone column’s area, Ac is soil’s area and Q is total applied load to foundation, and ratio of stone column’s area to soil’s area is defined as 
.
Therefore, it concluded that the applied stress to soil after installation of stone column will be equal to
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(12) |
And the settlement of stabilized soil can be computed by equation (1).
Since the settlement is assumed to be directly proportional to the applied stress, decreasing settlement due to stone column installation is equal to stress decreasing in soft soil. If one considers the settlement ratio of stabilizezed soil to that of primary loose soil, defined as k
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(13) |
settlement of untreated soil (S) is computed from equation (1). Knowing the allowable foundation settlement S/, the required area for stone column can be obtained as
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(14) |
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(15) |
Maximum allowable stress for stone column is defined as
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(16) |
where Cu is undrained shear strength of untreated soil and is computed as [Sowers (1968)].
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(17) |
In stone column computations a safety factor of 3 is considered. So allowable stress in stone columns in term of number of blows of SPT test can be computed by equation (18).
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(18) |
The shear strength of treated soil can be expressed as a combination of shear strength of stone column and soil as
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(19) |
where ts is shear strength of column and tc is shear strength of untreated soil. ts and tc are computed from Mohr-coulomb’s failure criteria. Since the cohesion of stone column’s coarse aggregate material is zero, the shear strength of stone column material is equal to
where fS is angle of internal friction of stone column’s material and sns is effective vertical stress in stone column. fS is computed from equation (9) considering number of blows in standard penetration test, in this case ts equals:
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(20) |
The shear strength of untreated soil is obtained by
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(21) |
can be obtained from equation (17) considering number of . And also can be computed as [Olsen and Farr (1986)].
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(22) |
Considering Eqs. (17), (21) and (22) shear strength of untreated soil based on is defined as
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(23) |
substituting Eqs. (20) and (23) in Eq. (19) the shear strength of stabilized soil with stone column based on number of blows of SPT test is obtained (Nc) in soil and (Ns) in stone column.
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(24) |
where sns and snc are vertical effective stress in stone column and soil at failure surface, which results from transferred stress to desired depth and it’s magnitude can be computed from different stress distribution theories.
COMPARING RESULTS OF PROPOSED METHOD WITH FINITE ELEMENTS METHOD
Assuming Nc = 5–15 for soil and Ns = 25–50 for stone column and regarding to Eqs. 5 and 10 and Figure 1, stress distribution ratio between stone column and soil is obtained about 2–9.
Also maximum allowable load for a stone column with 0.8m diameter due to Eq. 17 computed and equal to 15-45 tons. The results from finite element analysis maximum allowable load is between 20-30 tons [Mitchell and Huber(1985)].
Comparing result shows a satisfactory agreement between proposed method and finite element method.
CONCLUSION
The stress distribution ratio between stone column and soil calculated by the proposed method about 2-9 which depends on soil type and stone column material, whereas finite element analysis results shows this ratio is about 2-6.
Maximum allowable force for a stone column which is computed by the proposed method is about 15-45 tons, and its accurate analysis results shows that this value between 20-30 tons that results from both methods are satisfactory with each other.
In loose noncohesive fine graded soils in case of earthquake, there is a probability of liquefaction occurring. Stone column installation in these soils cause shortening in drainage path and pore water pressure is rapidly disspated. If loose soil liquefies, it will lose its shear strength and will work as a fluid with high density and water pressure will be hydrostatic. In case of stone column existence in this condition, hydrostatic pressure difference which results from liquefied soil with pressure from applied over load on ground surface, work as lateral limiting load for stone column, and will result in forming passive strength in stone column. In this case, vertical stress which column can carry will be 
times more.
REFERENCES
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