The Tella Firma foundation is a proprietary design that has been around since 2011. Similar to the pier and beam solution, Tella Firma mitigates the consequences of expansive soil by isolating the building foundation from the soil. Unlike the pier and beam solution, the Tella Firma foundation can be concrete and poured on the ground, almost as simple as a slab on grade foundation. The innovative part is the use of post tension cables and the patented Tella Firma lifting mechanisms to suspend the concrete slab a few inches above the soil, as shown in Figure 3.
Figure 3: Tella Firma Pier and Slab System
The Tella Firma foundation system follows a simple process which is explained in more detail in the schedule analysis section. The first step is to drill and pour concrete piers which reach to stable soil or bedrock. As the piers set, a Tella Firma lifting mechanism is installed at the top of each pier. The post tension slab is then poured on grade with protective sleeves to keep concrete from covering the lifting mechanisms. Once the slab is set and the cables have been stressed, a screw is inserted into each lifting mechanism and the slab can be lifted manually with a T-wrench. The lifting mechanisms can be capped for easy access should the slab need adjusting in the future.
The Tella Firma foundation is more weather resistant than the pier and beam foundation. It is a more permanent and reliable fix to expansive soils compared to the over-excavation process. In both comparisons, the Tella Firma foundation saves time, and possibly money. Because of these opportunities, it is necessary for the team to determine if the Tella Firma foundation is feasible and appropriate for use on the Sterling Ranch development.
Objectives
The following four objectives are set to guide the study into the feasibility and applicability of the Tella Firma foundation at Sterling Ranch.
- Conduct a geotechnical analysis of the Sterling Ranch soil.
- Design a Tella Firma Foundation for 3 different home layouts (Large, Average, and Small).
- Conduct a cost analysis of the Tella Firma Foundation as compared to the incumbent technology.
- Provide recommendations to improve the Tella Firma Technology.
The first objective analyzes the Sterling Ranch soil to first determine if expansive soils are present and require a solution. In the process of the geotechnical report, soil data is collected to be used in the foundation design process. Objective # 2 serves to determine if a Tella Firma foundation can be engineered within code compliance. This structural feasibility analysis is conducted on three home layouts that can be found on the Sterling Ranch development. The third objective puts a price on the Tella Firma foundations that are designed so that they can be compared to the current over-excavation process. This serves to determine financial feasibility of the Tella Firma foundation. Finally, objective # 4 draws on the in depth analysis conducted by the team to determine if there are any inefficiencies in the Tella Firma foundation that could be improved in future designs. By achieving all four of these objectives, the team can conclude whether or not the Tella Firma foundation is the right decision for Sterling Ranch to use on future developments.
Applicable Codes and Specifications
In order for the analysis to be relevant and realistic, the team conducts research within the guidelines of existing industry code. Specific codes were followed during the geotechnical analysis, concrete slab design, concrete pier design, and residential loading calculations. Additionally, best practices and educational instruction was followed for procedures that lacked code guidance.
Geotechnical Analysis: All geotechnical testing and classification was completed with the guidance of ASTM D (miscellaneous materials). One of the most referenced standards was D2487-17e1 which was used to classify the soils from Sterling Ranch under the USCS classification system.
Concrete Slab Design: The Tella Firma concrete slab was designed with guidance from ACI 318-19. In addition to guidance on the design process, ACI 318-19 provides design checks to ensure strength and serviceability, the two key criteria in a concrete structure.
Concrete Pier Design: The piers for the foundation were designed following the steps of ACI 336.3R-93. Strength and serviceability checks for the concrete pier were based on both ACI 318-19 and ACI 336.3R-93.
Residential Loading Calculations: To determine the loads acting on the Tella Firma foundation, design loads were carried through the house floor plan following the methods of ASCE 7.