Dynamic pore water pressure in saturated soil due to turbine engine’s vibration

. The turbine engine that is operating will generate vibrations that are transmitted by the foundation into the soil layer. This causes the pore water pressure in the soil to increase, which causes the shear strength of the soil to decrease and can reduce the carrying capacity of the soil as a foundation resting place, which results in a reduced bearing capacity of the foundation as well. The pore water pressure in the soil consists of static pore water pressure and excess pore water pressure. Static pore water pressure is hydrostatic pressure that depend on Ground Water Level (GWL), while excess pore water pressure is the pore water pressure that occurs due to changes in undrained soil stress. Therefore, a study was conducted on the effect of gas turbine engine vibration on pore water pressure in sand and clay soils with a depth of 8,5 m (fine to coarse sand), 10,5 m (silty clay), and 12,5 m (medium to coarse sand). From the research results, the value of the pore water pressure ratio for each layer of soil was 0,32, 0,02, and 0,49, which can reduce the bearing capacity of the pile, especially in sandy soils and if the value is greater than one, it can be concluded that the soil has the potential for liquefaction.


Introduction
The operating turbine machine will generate vibrations that are transmitted by the foundation surrounding the soil layer.This causes the pore water pressure in the soil to increase, which causes the shear strength of the soil to decrease and can reduce the carrying capacity of the soil as a foundation resting place, which results in a reduced bearing capacity of the foundation as well.
Pore water pressure itself has the meaning of water pressure contained in the soil pores.The pore water pressure in the soil consists of static pore water pressure and excess pore water pressure.Static pore water pressure is the pore water pressure when the soil is stable, while excess pore water pressure is the pore water pressure that occurs due to changes in undrained soil stress.In addition, there is also an active pore water pressure, which in saturated soil has the same value as the pore water pressure (Pactive = Pwater) [1].
Generally, the pore water pressure in the soil is negative (compressive).However, due to capillary forces or undrained unloading, the pore water pressure can become positive (suction) [2].In Fig. 1, the pore water pressure is positive (suction) in the soil above the Ground Water Level (GWL) and negative in the soil under the GWL.
An increase in pore water pressure occurs in dynamic soils due to back-and-forth vibrations that cause water to not be able to pass through the soil pores quickly so that excess pore water pressure dissipation does not occur.Turbine engines as cyclic loads also have the potential to cause soil to liquefy.Symptoms occur in saturated sand *Corresponding author: veronica.325190065@stu.untar.ac.id soils with moderate to fine gradations [3].Liquefaction potential is evaluated using excess pore water pressure [4].Seed and Lee (1996) define the initial liquefaction as the point where the increase in pore pressure is equal to the initial effective confining pressure (when uexcess = σ3' or the pore water pressure ratio ru = uexcess/σ3' = 1,0) [4].
The effective confining pressure can be calculated using the following Equation 1.
Based on the laboratory test results in Fig. 2, it can be concluded that the denser soil will need more number of cyclic load to reach the maximum pore water pressure excess ratio.It can also be seen that in loose sand, initial liquefaction is reached (ru = 1,0) in only 10 cycles.Meanwhile, in dense sand, the initial liquefaction is not achieved after 17 cycles because it takes thousands of cycles with a low shear stress amplitude to reach the initial liquefaction.
The relationship between cyclic shear stress, density, and the number of liquefaction failure cycles can be depicted in laboratory cyclic strength curves as shown in Fig. 3.
Therefore, in this study, a numerical analysis based on finite element pore water pressure was carried out in saturated soils, both in clay soils and sandy soils, in dynamic soils due to the turbine engine's vibration.

Pile capacity
Decourt (1995) developed a more comprehesive correlation for calculating end bearing capacity with SPT value as follows Equation 2 [5].
where Ap = pile toe area, N b ̅̅̅̅ = average SPT value around pile toe, Kb = base factor given in Table 1.
Coyle and Castello (1981) give the Equation 3 for calculating skin capacity of foundation [6].

Soils and foundation structure modeling
The first thing to do is collect soil parameter data with GWL at a depth of 6.85 m.For modeling in finite element applications, soil parameters are used as shown in Table 3 with the Mohr-Coulomb material model.Specifications and machine foundation modeling data to be used for finite element modeling are shown in Tables 4-5.The illustrations of the machine foundation structure to be modeled are shown in Fig. 4-5.

Dynamic loads
There are two kinds of foundation loading, static loads and dynamic loads.The static load is in the form of a turbine engine with a size of 13 m × 5.3 m × 5.5 m and a weight of 4,876 kN.Meanwhile, for dynamic loads, dynamic parameters are used in the form of a damping ratio value of 0.22 % which is obtained as a result of the piling activity of the foundation [7] with a load in the form of vertical displacement on the pile head results of the Pile Driving Analyzer (PDA) test in Table 6 and the pile head displacement graph in Fig. 6.
In addition, there is also a dynamic load on the turbine engine itself in the form of harmonic vibration of 10.3005 kN/s and a vibration frequency of 50 Hz.

Pore water pressure analysis
Pore water pressure analysis was carried out on saturated soil with 10 cm from the outermost pile skin in the y direction at a depth of 8.5 m (fine to coarse sand), 10.5 m (silty clay), and 12.5 m (medium coarse sand) that can be seen in Fig. 7. Following that, it is determined whether the pore water pressure at that depth is suction or pressure, and its effect on the stability of the bearing capacity of the turbine engine foundation.The results of the analysis are the dynamic soil pore water pressure at the dynamic loading phase with a time interval of 5 s and the fading phase with a time interval of 0.5 s.
The results of calculating the pore water pressure from the finite element modeling for each soil layer are as follows Fig. 8-13.
Based on Fig. 8, it can be interpreted that at a depth of 8.5 m, the excess pore water pressure is positive, which means suction.Meanwhile, at a depth of 10.5 m and 12.5 m, based on Fig. 10 and Fig. 12, the excess pore water pressure is negative, which means compression.Based on Fig. 11 and Fig. 13,the increased value of active pore water pressure at a depth of 10.5 m is -40.79 kPa and at a depth of 12.5 m is -101.33 kPa, which means that the deeper the soil layer, the compressive active pore water pressure will be greater.The active pore water pressure at the point under consideration and the static pore water pressure can be plotted onto a graph of pore pressure vs. depth as in Fig. 14.From Fig. 14, it can be seen that when the soil is dynamically loaded, the active pore water pressure of the soil is relatively greater than when it is fading.In addition, it can also be seen that the change in pore water pressure (Δu) tends to be greater because it is an end-bearing pile, so that the pile load will be concentrated in the end area.The liquefaction potential can also be determined by first calculating the effective circumferential stress (σ3') per layer using Equation 1.Then, the liquefaction potential at the point under review can be identified, as shown in Tables 7-8.Based on the value of the ratio of pore water pressure (ru) is less than 1, which means that it can be concluded that in the three layers of soil observed, especially sandy soils in the first and third layers, there are increments of excess pore water pressure but liquefaction state has not occurred.This phenomenon can reduce the bearing capacity of the foundation piles because the effective stress will decrease.

Pile capacity analysis
As a result of excess pore water pressure on saturated sand soils, leads to reduced skin capacity which can be calculated in the following ways (data can be seen in Tables 9-12 The end bearing capacity is calculated at a depth of 4D (4 × 0,6 = 2,4 m ≈ 3 m) from the pile toe as follows (data can be seen in the Table 13).
4. In sandy soils that have excess pore water pressure but no liquefaction, the bearing capacity needs to be reduced by considering the value of ru.The magnitude of the multiplier for the reduction in skin capacity is about 1-ru, 51-68 %. 5.The allowable pile bearing capacity without reduction is Qa = 1,961.59kN, and with reduction is Qa =1,800.66kN.

Fig. 9 .
Fig. 9. Active pore water pressure at a depth of 8.5 m.

Fig. 10 .
Fig. 10.Excess pore water pressure at a depth of 10.5 m.

Fig. 11 .
Fig. 11.Active pore water pressure at a depth of 10.5 m.

Fig. 12 .
Fig. 12. Excess pore water pressure at a depth of 10.5 m.