**4. Examples from engineering practice**

#### **4.1 Calibration of a numerical model**

Numerical simulations using FEM have been carried out for the design of a CPRF of a new high-rise building founded in soft soil [15]. For the calibration of the numerical model a pile load test using Osterberg-Cells (O-cells) has been carried out in the project area. The test pile consisted of the upper test segment 1, the middle test segment 2 between the two O-cells, and the lower test segment 3.

In various testing phases, the O-cells were activated individually to determine the skin friction of the different layers and the pile base resistance. At test segment 3 only the lower O-cell was activated, while test segment 2 was used as an abutment to determine the skin friction and the pile base resistance. At test segment 2 the upper Ocell was activated and the lower O-cell was released to determine the skin friction. Test segment 1 was used as an abutment in this test phase. At test segment 1 the upper O-cell was activated and the lower O-cell was stiffened to determine the skin friction. Test segments 2 and 3 were used as an abutment in this test phase.

A numerical (FEM) back analysis of the pile load test was used to calibrate the numerical model of the CPRF. The FE-model of the numerical back analysis of the pile load test with the three test segments and the two O-cells is shown in **Figure 5**.

The results of the in situ pile load test and the numerical back analysis show good accordance (**Figure 6**). By this, the used soil mechanical parameters and the simplified stratigraphy, which was necessary for the numerical model, were verified.

The design of the CPRF is performed by three-dimensional, nonlinear FEsimulations. Taking into account the requirements of the load-deformation behavior the length, diameter, and the number of piles were optimized on the basis of the FEsimulations. The optimized CPRF is shown in **Figure 7**. Eighty percent of the total building load are carried by the piles and 20% of the total building load is carried by the raft. So, the CPRF coefficient is αCPRF = 0.8.

#### **4.2 High-rise building in settlement active clay**

The high-rise building Messeturm in Frankfurt am Main, Germany, is 256.5 m high and is founded on a CPRF in the settlement active Frankfurt Clay (**Figure 8**). The foundation raft has a ground view of 58.8 m 58.8 m with a maximum thickness of 6 m in the center and a thickness of 3 m at the edges. The base of the foundation raft is about 11–14 m below the surface.

**Figure 5.** *Numerical simulation of the pile load test for calibration.*

**Figure 6.** *Measurement and calculation of the pile load test (upper O-cell activated, lower O-cell stiffened).*

*Reducing Carbon Emissions by Combined Pile-Raft Foundations for High-Rise Structures*

*DOI: http://dx.doi.org/10.5772/intechopen.104559*

**Figure 7.** *FE-mesh of the optimized CPRF.*

**Figure 9** shows the CPRF with 64 bored piles with a diameter of 1.3 m. The length varies between 30.9 m in the center and 26.9 m at the edges. The total building load is about 1,855 MN including 30% of the live loads.

The subsoil in the project area consists of artificial fillings at the surface which are underlain by quaternary sand and gravel until a depth of 8–10 m below the surface. Below follows the tertiary Frankfurt Clay to a depth of about 70 m below the surface. At a depth of 4.5–5.0 m below the surface is the groundwater table. The maximum measured settlements of the foundation raft were 13 cm in the center and 7–9 cm at the edges.

The CPRF was calculated using the FEM. Thereby a section of the foundation was modeled, using the symmetry of the ground view (**Figure 10**).

The FE-calculation simulates the construction process step-by-step. These steps are the excavation of the construction pit, the construction of the CPRF, the groundwater lowering, the loading of the CPRF, and the groundwater re-increase.

For the optimization of the CPRF different pile configurations and pile length was analyzed as well as a pure raft foundation. **Figure 11** shows the comparison of the load-settlement curves of a pure raft foundation and of a CPRF.

**Figure 8.** *High-rise building Messeturm in Frankfurt am Main, Germany.*

The maximum settlements of a pure raft foundation were calculated to be 32.5 cm. The in situ measured maximum settlements of the CPRF of 13 cm correspond to the calculated maximum settlements. The calculation and the measurement data showed a CPRF coefficient of αCPRF = 0.43.

Until the construction of the Messeturm the ultimate skin friction qs of bored piles in Frankfurt Clay was estimated to 60–80 kN/m<sup>2</sup> for 20 m long piles, based on pile load tests. At the piles of the Messeturm, an average skin friction qs of 90–105 kN/m<sup>2</sup> was measured. At the pile toe, a maximum skin friction qs of 200 kN/m2 was measured.

*Reducing Carbon Emissions by Combined Pile-Raft Foundations for High-Rise Structures DOI: http://dx.doi.org/10.5772/intechopen.104559*

**Figure 9.** *Ground view (left) and cross-section (right) of the CPRF.*

A pure pile foundation would have required 316 piles with 30 m in length and a diameter of 1.3 m. In comparison to the realized CPRF with 64 piles and an average length of 30 m, a pure pile foundation would have required much more material, time, and money. Regarding the CO2 emission, the CPRF saved about 10,000 tons of concrete. With the estimation, that the average cement ratio is about 300 kg/t of concrete, the CPRF saved about 6000 t of CO2.

#### **4.3 High-rise building on a steep slope**

The high-rise building Mirax Plaza in Kiev, Ukraine, consists of two high-rise buildings, each of them with a height of 192 m (**Figure 12**). The subsoil consists of artificial fillings to a depth of 2–3 m, which are underlain by quaternary silty sand and sandy silt with a thickness of 5–10 m. Below follow tertiary silt and sand with a thickness of 0–24 m. Then follows tertiary clayey silt and clay marl of the Kiev and Butschak formation with a thickness of about 20 m, which is underlain by tertiary fine sands of the Butschak formation. The groundwater level is about 2 m under the service. The soil conditions and a cross-section of the construction project are shown in **Figure 13**.

Two pile load tests have been carried out on the construction site to verify the skin and the base resistance of the deep foundation elements and for the calibration of the numerical simulations. The piles had a length of 10 m and 44 m and a diameter of 0.82 m. The soil properties that resulted from the back analysis were partly three times higher than indicated in the geotechnical report. The results of the numerical back analysis and the load tests show good accordance (**Figure 14**).

Tower A has a foundation raft of about 2000 m<sup>2</sup> and an overall load of about 2200 MN. **Figure 15** shows the calculated settlements of the three-dimensional FEM simulation.

The raft is located at a depth of 10 m below the surface in Kiev clay marl. The barrettes go through the Kiev clay marl and reach the tertiary fine sands.

**Figure 10.** *FE-mesh of numerical simulation.*

The outer barrettes have calculated loads between 41.2 MN and 44.5 MN. The inner barrettes have calculated load between 22.1 MN and 30.7 MN. This is typical behavior of a CPRF. The barrettes at the edge of the foundation raft have a higher

*Reducing Carbon Emissions by Combined Pile-Raft Foundations for High-Rise Structures DOI: http://dx.doi.org/10.5772/intechopen.104559*

**Figure 11.** *Measured and calculated settlements.*

**Figure 12.** *Mirax Plaza in Kiev, Ukraine.*

stiffness due to the bigger volume of the activated soil. They get more of the total load. The calculated CPRF coefficient is αCPRF = 0.88. The settlement-relevant load of 85% of the total load will lead to maximum settlements of about 12 cm. The estimated pressure under the raft is about 200 kN/m<sup>2</sup> (center) and 400 kN/m2 (edges).

The calculated base pressure under the barrettes is about 4130 MN/m2 (center) and 5100 MN/m<sup>2</sup> (edges). The estimated skin friction increases with the depth reaching 150 MN/m<sup>2</sup> (center) to 180 kN/m<sup>2</sup> (edges).

The foundation of Mirax Plaza is the first authorized CPRF in Ukraine. The CPRF reduced the number of barrettes from 120 with 40 m length to 64 with 33 m length. Regarding the CO2 emission, the CPRF saved about 15,000 tons of concrete. With the

**Figure 13.** *Soil conditions and cross-section of Mirax Plaza.*

**Figure 14.** *Measurement and calculation of the in situ load test.*

estimation, that the average cement ratio is about 300 kg/t of concrete, the CPRF saved about 9000 t of CO2.
