Quality of stone columns depends on many factors including, but not limited to, (a) quality of the gravel used, (b) amount of stones (gravel) used / average diameter of the column, and, most importantly, (c) the post – improvement density of the soil. During the installation process, it is very hard to predict the post – improvement density. Generally in sandy soils, the peak amperage reading obtained at each lift is considered as an indirect measure of the soil density. However, in low permeable silty soils, the current reading, most of the time, does not reach the high values seen at sandy sites. Therefore, in such soils, diameter of the column is the prime concern during the installation process. As a result, stone columns built in silty soils are of nearly uniform diameters, while, those in sandy soils are of non – uniform diameters.
In both sands, and silts, minimum of one test section is done prior to production.
Using post – improvement SPT / CPT test results, the effectiveness of the stone column system is evaluated and, if necessary, modifications are made.
Simulation of near – field conditions are very difficult, and the process is very complicated. However, in order to qualitatively illustrate the changes in current readings observed at the site, some simplified correlations between soil bearing capacity and friction were assumed.
Yu and Mitchell (1998) has analysed many methods available in the literature to predict cone penetration resistance of cohesive and cohesionless soils. Among those methods they found the model, based on ultimate bearing capacity of the soil, introduced
by Durgunoglu and Mitchell (1975) predicts the cone resistance well close to measured ones at shallow depths, which is more relevant to liquefaction mitigation problem. The most simplified form of this relationship is Nq=0.194 EXP[7.629 tan φ’], where, Nq=qc’/σv0’, and qc’ is effective cone tip resistance (Durgunoglu and Mitchell 1975).
Thevanayagam et al. (2003d) reported friction angles of an array of sand – silt mixes tested at 100 kPa initial confining pressure. Fig.6.17 shows these values against equivalent relative density values.
0.0 10.0 20.0 30.0 40.0 50.0
0.0 20.0 40.0 60.0 80.0 100.0
(Dr,c)eq (%)
φ' (Deg.)
OS00 OS07 OS15 OS25 Trend
b=0.4 σc'=100 kPa
Fig.6.17 Effective Friction Angle vs. Equivalent Relative Density
Using Eq.6.4, power consumption by the probe for imparting energy into the soil can be calculated. If the relationship suggested by Durgunoglu and Mitchell (1998) is used to qualitatively evaluate the cone penetration resistance in a silty sand with 40% initial equivalent relative density (φ’ = 30 deg., Fig.6.17), it would be about 700 kPa for depth corresponding to 100 kPa initial confining pressure. Field observations reveal that the probe and the follower tubes system used to install a 10 – 15 m deep column weighs about 2.4 tons, or about 21 kN. This weight could apply only about 170 kPa static
pressure on the ground. Assuming the cone penetration resistance can be extrapolated for the vibratory probe, which is of around 0.4 m diameter, it could be conveniently concluded that the probe cannot penetrate into this soil by it’s self-weight alone. It needs some additional work to counter soil resistance, however no pushing is involved in the stone column installation process.
When the probe is vibrating with an amplitude of up to about 14 mm horizontally (FHWA 2001), the tip of the probe pushes the soil radially outwards. Since the tip is tapered, it easily penetrates further, expanding the cavity large enough so that the portion of the probe above the tip could follow with minimal resistance. If this phenomenon is considered as the work done against the vertical soil resistance, and if it is assumed the work done by the self-weight is almost consumed by the side friction, about 2.6 kW of additional energy would be required for a penetration rate of about 3 cm/s. Field observations indicate that the supply voltage is around 400 V, although expected to be near 480 V. Hence, the current consumption would be around 6-7 A.
As the soil gets denser it’s friction angle also increases, increasing the resistance to penetration. This effect is included in the numerical simulation model considering an effective zone for the soil resistance of about two diameters as of the cavity, and calculating the weighted average of the friction angle within the effective zone. In addition to the energy consumption described in this section, it was observed at several stone column installation sites that the current reading never falls below the ‘free hanging amperage’, i.e. current reading when the probe is vibrating in the air. This reading ranges usually between 110 and 120 A. It is unclear, though, whether the energy corresponding to this current reading is wasted even when the probe is inside the ground, or a portion of
this energy is imparted into the soil. However, in the current simulations, it is assumed that no energy corresponding to the free hanging amperage is imparted into the soil.
Fig.6.18 shows a comparison of current reading for the simulations shown in the Table 6.4. The current reading at 400 s shows a very high value, which corresponds to the conditions just before any excess pore water pressure is generated. However, in reality, by the time the probe reaches the considered depth of 12 m, there would be some excess pressure already generated, and therefore, this current reading cannot be obtained at the field. Beyond this time, the current readings show the general trend observed at the corresponding soil sites.
At a silty soil site, due to it’s low hydraulic conductivity, there will always be high excess pore pressures during the installation process. Since the density of such soils would not increase much during this time (Ref. Table 6.4), the increment in current reading is only due to the resistance by the gravel, during reinsertions. However, in a sandy soil, the soil resistance would increase considerably, and therefore the amperage also would increase. Generally, peak reading of around 200 A is observed at silty sites, and that of around 250 A is observed at sandy sites.
150 200 250 300 350
400 450 500 550 600
t (s)
C u rr e n t (A ) Sand: k=1E-5 m/s Silt: k=1E-6 m/s
Fig.6.18 Comparison of Current Readings: Sand and Silt