A piled raft foundation combines two foundation-building techniques into a hybrid design suitable for specialized circumstances. The raft, or spread, foundation spreads the load of the building across the ground. Think of it as a raft floating on the ground supporting a structure. Piles are columns extending below the ground surface that connect at the top of the building. A piled raft foundation uses both methods to support the building.
When a Raft Foundation Is Used
Architects use raft foundations when no solid layer of soil exists at or near the ground surface. This means the surface soils are not stable enough and the bedrock is too deep to reach for a conventional foundation. The raft foundation spreads the load across the ground
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How Piled Foundation Is Used
Piles are driven into the ground to a stable platform. This is commonly bedrock but can be other stable material. Engineers determine the number and size of the pile based on the weight and size of the building. The piles anchor the building to the bedrock insulating it from any movement of the upper level soils.
When a Piled Raft Foundation Is Used
Builders use piled raft foundations when the stability of the material that anchors the piles is questionable or when stable material does not extend under the entire building. The piled raft foundation offers the floating foundation partially anchored for support.
Design and Construction
The design of the piled raft foundation usually is complex and is performed by architects or engineers after soil testing. Placement of the piles is not always uniform or regular. Some designs call for specific placement of the piles to take advantage of stable subsoils or bedrock. Calculating the load-bearing capacity of the piled raft foundation requires knowledge of the position and strength of the piles, the stiffness and thickness of the raft and soil properties.