Increase of Bearing Capacity of Foundation Soil in Port of Koper (Slovenia)

Increase of bearing capacity of foundation soil in Port of Koper (Slovenia)

L. Battelino Water Management Institute

Maritime Engineering Department Ljubljana, Slovenia

Abstract

The Port of Koper is situated in the northern part of Adriatic Sea and is the only maritime port for Slovenia to handle its seaborne cargoes. All port terminals and structures need high bearing capacity of foundation soil in the case of shallow foundations or require deep foundations for their construction elements, which is considerably more expensive.

The entire Port of Koper was built on the area, acquired with dredging and reclaiming from the shallow sea bottom. Till 1969 the sea depth on this area was about 2m. It was reclaimed with dredged material from the sea bottom till level about + 1m msl and after that the area was filled with moulded marl and crushed rock till level about +3m msl. From the soil investigation it can be seen that the soil layers consist of weak silty and clayey soils with low bearing capacity. For the shallow foundation the bearing capacity of the foundation soil has to be improved. To increase the bearing capacity and to accelerate the consolidation, gravel piles in a polymer meshes and preloading with the embankment have been installed at the site. During the consolidation process the ground water was filtered up because of preloading embankment, using gravel piles as vertical drainage system.

The whole system was equipped with observation points for settlement observations and we were able to observe the soil consolidation degree. The results of settlement readings during preloading time on the site, were exceptional close to computational ones based on the theory of elasticity with mean modules of compressibility and to those based on the theory of finite elements.

1 Introduction

Bearing capacity of cohesive soil such as clay, clayey silt or fine sand in great extend depends on their porosity. This is the ratio between volume of voidness and volume of mineral grains per volume unit. If the porosity of such soil is very high, the foundation soils are usually very compressible and slidable. With relatively low pressures of constructions we may prevent sliding because of soil low shear strength, but we can't avoid great settlements of the constructions. Also the time development of the settlements usually has very long duration. To increase the bearing capacity and activate the great part of settlements of such foundation soils in advance, the special method of soil improvement was developed. This method was first developed specially for the soils in the area of the port of Koper and was slightly modified during the time of application. Recently this type of soil improvement was introduced for the shallow foundations of tanks for chemicals and for the embankment for stacker - reclaimer crane railway on the dry bulk terminal in the Port of Koper. Because of very long period of field observation during the consolidation period the results of soil improvement on chemical tank area is presented in this article.

2 Soil investigations

On the storage tank farm area the extensive field soil investigations were carried on. There were four borings bored out, one of them till level -40m below sea level, other till level of -20m. Borings indicated that the soil composition in the area is very heterogeneous. The original sea bottom before 1969 was about -2m msl and consisted of submerged lower part of river valleys. The upper part of soil layers (see figure

1) till depth of -30m msl consists of weak, compressible, easy kneading clayey soils. Somewhere thinly liquid consistent state is appearing and somewhere the layer of clay is interrupted by a thin silty - sandy intermediate layers. According to AC classification there are CI-OI, CH-OH, MI-ML and SU-ML soils. The weak soil layer is most usually followed by hard kneading clay and below -33m msl sandy gravel layer with clay particles appears. The upper part in depth of about 3.5m is artificial fillpartl y done with reclaiming from the sea bottom and partly filledwit h moulded marl and crushed stone.

In all the borings the vane test was executed till the level of -20m. From the result it can be seen that the undrained shear strength of the upper part of weak clayey soils does not exceed 20kPa and the residual values does not exceed 50% of the original ones. The laboratory tests were also done on some undestroyed soil samples taken from weak soil layer. With laboratory tests the following were established: natural water content, consistency limits, specific weight, natural and dry volume weight, one-axis pressure strength, deformability, permeability and shear strength. From the results the following average values are established:

- natural water content w = 40 - 45%

- liquid limit WL = 46 - 52% - plastic limit w? = 19 - 21% - plasticity index Ip = 27 - 31%

- consistency index Ic = 0.26 - 0.32 - natural volume weight y = 17.5 - 18.4kN/nf - dry volume weight yd = 12 - 13.4kN/nr*

- specific weight y, = 27.2 - 27.5kN/nf - one-axis pressure strength q» = 15 - 22 kN/nf

- permeability k = 1.3 E-9 -1.23 E-8cm/s - cohesion part of shear strength c = 15.5 - 23.3kN/nf - angle part of

shear strength 9 = 17 - 22°

All the results of soil investigation show that the soils in the area of chemical tanks are extremely compressible; hence the expected settlements of the construction foundations on such ground will be very high.

3 Estimation offina lsettlement s without soil improvement

The exact estimation of the shallow tank foundations settlements is a very difficult task because we have to establish the exploitation range of the tanks. In the settlement calculation the 100% utilization was taken into account. Also the settlements regarding ground fill in the surrounding were calculated. The total expected calculated settlements under tanks are 127cm. The time development of the settlements is expected to be very slow because the consolidation level according to the calculation is only 50% in 50 years. The calculation of the shallow foundations of the tanks with no improvement of the soil has also shown that the bearing capacity of the foundation soil can be jeopardized.

4 Foundation soil improvements

With respect to the relative sensitivity of designed storage tank area on settlements the relevant steps were chosen and carried out in accordance with the design demands to reduce the settlements during the exploitation. To increase the bearing capacity and to accelerate the consolidation process gravel piles in a polymer meshes and preloading fill were suggested. The system of gravel piles in triangular mesh arrangement works as vertical drainage system. In combination with suitable preloading embankment essentially speed up the consolidation process. The system works as vertical drainage and there is also radial filtration of porous water. This means, that in relatively short time the essential part of final settlements will occur.

The set period of preloading fill on the location of the storage tank area was expected to be around six months. The mound must have satisfactory height and density of the filling material in relation to the tank load and time of preloading before the construction of the tanks. It should be enable to settle the ground, so the rest of the settlements would not be critically for the installation.

5 Design of foundation soil improvements

5.1 Gravel piles

The existed ground level was +1.50m msl. From this original level the gravel piles were executed. According to the calculated rate of radial drainage the triangular mesh of 3m was chosen. Diameter of gravel piles was 40cm and they were driven onto depth of 12m. First the steel tube with lost toe was driven into the soil. After that the polymer tubular mesh was installed into the hole. Finally the hole was filled with gravel granulation of 0 - 30mm. At the end the steel pile was driven out and the execution of the second pile may begin. The micro location of the piles was only under the tanks and it can be seen on the figure 2. When all the piles were finished the area was covered with rock filter layer of 30cm, which works as horizontal drainage. In this layer the system of horizontal drainage was installed with controlled release of drainage water into shafts.

5.2 Preloading fill

Before the start of the execution of preloading mound the 19 observation points were installed on the prepared ground. Because of the foundation soil sensitivity and low bearing capacity the execution of the preloading fill has to be carried out in steps. After each step there was intermission and between the steps several observations were carried out. On the basis of the observation analysis the execution was continued. Because the designed level of the tank area was +2.5m, the firstste p of fill was till level +2.5m. It was only 1m difference, therefore there was an intermission only for the period when observations were executed. The second stage was the execution of the fill till level of +5.5m. After the observations were carried out and after the intermission of one month, the third stage started.

It was the level of mound till +7m. The load of this height of the rock preloading was equal to tank load. According to the calculation we expected the total preloading time of six months. Because of some other objective reasons not related to the design the actual time of the preloading mound was extend up to 18 months. We took this opportunity and after one year raised the fill till level +8.8m. The lay - out of preloading fill and the observation points are shown on figure 2. All the time we have measured the settlements on the observation points and the quantity of the drainage water. The water was flowing through the system of

horizontal drainages all the time and was not related to the rainy period. Nevertheless the Koper area is mostly dry.

6 Settlements

The settlements were observed through all the time of preloading period on the 19 observation points. Unfortunately the central point - 4 and also the point 9 were destroyed after three months of observation.

After every step of preloading mound (+2.5m, +5.5m, +7m, +8.8m) the intermission was introduced and on the diagrams on figures 3 and 4 it can be seen how the execution of the next stage accelerates the consolidation and enlarges the settlements. Only the settlement profiles through the centre line are shown, on the figure 3 observed points 4, 9, 12, 15 and 18 and on the figure 4 points 1, 2, 3, 4, 5 and?. All the observed points can be divided into two groups. First points located on the lower part of the preloading (+2.5m) and outside the gravel piles area (1,

7, 12, 18) and second points on the higher part of preloading fill (at least +5.5m and higher) and inside the gravel piles area (2, 3, 5, 9, 15). The analysis below takes into consideration only the points inside gravel piles.

6.1 First and second stage of preloading fill (+2.5m, +5.5m)

After first and second stage the intermission of one month was done. During this period the average settlements of 40cm were activated. When the observed curve (see fig. 3, 4) started to turn the next stage of the preloading took its place.

6.2 Third stage of preloading fill (+7m)

The third stage was preloading fill till level +7m. As it was already mentioned, the period of preloading was extremely long. So the third stage lasted around ten months. During this time the average settlements of 100cm were activated. From the curve it can be seen that after four months when the average settlements were about 78cm, the slope of the curve became less steep. It means the activation of the settlements is slower and if we want to increase the speed of consolidation, we have to increase the preloading.

6.3 Forth stage of preloading fill (+8.8m)

The last step of preloading fill was put onto the area because the start of the construction was postponed and so we had time to increase the bearing capacity of the soils beyond the calculated one. The highest level of preloading fill was observed during the period of eight months and the average settlements that were activated through the whole period of eighteen months were around 130cm.

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1 Conclusions

After eighteen months the preloading mound was removed from the site. The existing ground level of the area was left to be at +2.50m. As it was already explained before, the average settlements are 130cm. This value of settlements is very close to the computational one, which would be activated under tank load without soil improvement during exploitation of the storage tanks. So we may say, with chosen soil improvement - installation of gravel piles and preloading mound with preloading load more than actual exploitation load, we had succeeded to activate nearly 100% of calculated settlements. As it was mentioned before the time of preloading mound was very long, three times longer than the predicted one. From the analysis, presented in diagrams (fig. 3 and 4) it appears that after predicted preloading of six months, the average settlements were around 95cm. This activation of settlements is around 75% of the final, although the preloading load was equal only to the final tank load. Comparison of the results after the total time of preloading with the results after predicted time (ratio 3:1), with the preloading load taken into account (ratio 130% : 100% of tank load), indicates, that the predicted time was well chosen. With soil improvement and 100% of exploitation load the 75% of settlements were activated in reasonable time. Today the installation of tanks for chemicals was already built and has been in use since September 1998. The control observation points on the tank foundations show that the settlements during exploitation are negligible.

References

[1] Sovinc, I, Vogrincic, G., Modern Methods and Systems in Design,

Construction, Maintenance and Revitalisation of Building, Shallow Foundation on Soils with Low Bearing Capacity. University of Ljubljana, Slovenia, Institute for Mathematics, Physics and Mechanics. Research publication, 1987. (in Slovene)

+ 2.0- + 1.50m

±0 0 FILL FILL FILL FILL j 9 A L/-\iHi CI-CH bH— IvH:— CH Ml -4 0- CI-OI CH-OH Ml — ' )| CI-ML fi O CI-ML-SU SU

-8.0- CH-OH ML Ml

1 -100

-12.0- '. CH-OH

-14.0- MH-OH ML-OL CI-OI -16.0- : ML

CH-OH -18.0- . -20 0 CH-MH

-22.0-

-24.0-

-26.0-

-28.0- 1 MI-ML -30.0- CH -32 0- _ 1 CH-MH

-34 0- MI-CI

360- SC-GC

-38.0- SM -40.0-

Figure 1: Soil profile

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Figure 2: Gravel piles and embankment lay-out

Maritime Engineering and Ports II 339

5.2.96 15.5.96 23.8.96 11296 11.3.97 19.6.97 27.9.97 5.1.98

12 15 .18

Figure 3: Observed settlements on points 4, 9, 12, 15, and 18

42 S E

-1600

5.2.96 15.5.96 23.8.96 1.12.96 11.3.97 19.6.97 27.9.97 5.1.98 date

Figure 4: Observed settlements on points 1,2,3, 4, 5, and 7