**3.1 Basics**

A Combined Pile-Raft Foundation (CPRF) is a hybrid, technically and economically optimized foundation system. It combines the bearing capacity of a foundation raft and of piles or barrettes. For the foundation of classic high-rise buildings as well as for engineering constructions like bridges and towers CPRFs can be used.

The technical regulations for classic deep foundations prevail for CPRFs as well [4]. In addition, the Combined Pile-Raft Foundation Guideline [7] has to be considered. This internationally validated guideline reflects the individual features of a CPRF and is published by the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE).

CPRFs have a very complex bearing and deformation behavior due to the interaction between the foundation elements and the subsoil. CPRFs belong to the Geotechnical Category GC 3 according to EC 7 [6].

The advantages of a CPRF, compared to a conventional spread foundation and a classic pile foundation, are the reduction of:


### **3.2 Bearing and deformation behavior**

The measurement data of high-rise buildings founded on spread foundations in Frankfurt am Main, Germany, showed, that 60–80% of the settlements arise in the upper third of the influenced soil volume. A part of the load on a CPRF is transferred py the piles from areas with a small stiffness under the foundation raft to a stiffer, deeper area of the subsoil without neglecting the bearing capacity of the foundation raft (**Figure 3**).

**Figure 3.** *Principle load transfer of a CPRF.*

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

The bearing and deformation behavior of a CPRF is characterized by the interaction between the bearing elements (foundation raft and pile resp. barrettes) and the subsoil. **Figure 4** shows all interactions of a CPRF.

A CPRF transfers the total building load Ftot,k to the piles and the subsoil. The mobilized resistance of a CPRF depends significantly on the settlement s, which is similar to a classic deep foundation. The resistance Rraft,k(s) equates to the integration of the soil contact pressure σ(x,y) under the foundation raft. The resistance Rtot,k(s) of a CPRF equates to the resistance of the foundation piles PRpile,k,i(s) added to the resistance of the foundation raft Rraft,k(s) (Eq. (2)).

**Figure 4.** *Interactions of a CPRF.*

As shown in Eq. (3), the total resistance of a single foundation pile consists of the skin resistance Rs,k,i(s) and the pile base resistance Rb,k,i(s). The skin resistance Rs,k, i(s) can be calculated by integration of the skin friction qs,k(s,z), which depends on the settlement s and the depth z.

$$\begin{split} R\_{pile,k,i}(s) &= R\_{b,k,i}(s) + R\_{s,k,i}(s) \\ &= q\_{b,k,i} \bullet \frac{\pi \bullet D^2}{4} + \int q\_{s,k,i}(s,x) \bullet \pi \bullet D \bullet dz \end{split} \tag{3}$$

The load-deformation behavior of a CPRF can be specified by the CPRF coefficient αCPRF. This coefficient declares the relation between the resistance of the piles and the total resistance and varies between 0 and 1 (Eq. (4)).

$$a\_{\rm CPRF} = \frac{\sum R\_{pile,k,i}(s)}{R\_{tot,k}(s)} \tag{4}$$

If the whole load Ftot,k is carried by the foundation raft, the CPRF coefficient is αCPRF = 0. If the whole load Ftot,k is carried by the foundation piles, the CPRF coefficient is αCPRF = 1. Related to technical and economic aspects a CPRF coefficient αCPRF between 0.5 and 0.7 can be considered as optimum. For αCPRF > 0.9 additional analyses on the piles are necessary.

The effective horizontal stresses influence the mobilized skin friction of the piles. Hence the stress level of the subsoil influences the load-deformation behavior of a CPRF. The neighboring piles, the foundation raft, and the effects during the construction of the piles influence the stress level of the subsoil around every pile of a CPRF. The soil contact pressure under the foundation raft leads to an increased stress level of the subsoil. The result is higher skin friction in the upper parts of the piles.

#### **3.3 Principle calculation method of a CPRF**

For the design and calculation of a CPRF various methods can be selected [8–14]. Up to now only numerical methods, like the Finite-Element-Method (FEM) provide calculation results that are comparable to reality.

The knowledge about the load-deformation behavior of a free, single pile is necessary for a qualified design of a CPRF [4]. Otherwise, a pile load test has to be performed. Two reasons are important for the knowledge about the bearing capacity of a free, single pile:


In situ pile load tests are required for complex construction projects and/or difficult soil conditions.

### **3.4 Monitoring of a CPRF**

Regarding the Geotechnical Category GC 3 a CPRF has to be monitored [4, 6, 7]. The monitoring program consists of geodetic and geotechnical measurements of the new building and of the vicinity and covers the construction phase and the service phase of the building. The following tasks are important:

