Controlling Settlement Adjacent to Pile Supported Structures
It is becoming common to support residences and other relatively light buildings on piles or other deep foundations. This usually occurs on lake front properties that have poor soils, such as muck or marl. Conventional shallow foundations are not suitable for support of buildings, including lightly loaded buildings, on these soils because they will settle excessively.
If the deposits of poor soils are not excessively deep, then several deep foundation alternatives may be economically viable. These include helical piers, auger cast piles, or driven timber piles.
Because pile supported structures usually experience little or no settlement, any natural consolidation of soils adjacent to a pile-supported building can damage or cause problems with slabs or other site features placed on the native soils without the support of deep foundation. These problems can be exacerbated by placing fill soils on the site or next to the pile supported structure.
Because fill soil is relatively heavy, over time, it will compress underlying soft native soils and cause settlement. This is most apparent when a sidewalk settles excessively next to a building, causing cracking and a trip hazard for pedestrians. It can also be seen as parking lot pavements built on fill soil settle around manholes because the manhole is relatively light compared to the fill soil.
The photograph to the left shows settlement of a concrete approach to a garage. The settlement, cracing and displacement of the concrete approach resulted from consolitation of the loose soils adjacent to the supported garage floor slab. Several approaches can be taken to minimize the problem.
First, if fill is required adjacent to the building, it should be placed early in the project. During construction, the weight of the fill will begin to consolidate the soft soils thereby reducing the potential settlement when concrete slabs are placed later in the project. This approach is a type of soil pre-loading and can be accelerated by placing additional fill to increase the load on the soft soils. One of the drawbacks of this method is the time required for the settlement to stabilize.
Another approach is to avoid placing rigid surfaces adjacent to the structure. In some instances, aggregate surfaces or brick pavers can be used instead of concrete sidewalks. This will allow placement of additional aggregate when the surface settles or re-installing brick pavers after settling and placing additional fill under the pavers.
The photograph on the right is an example of displacement and cracking of a concrete slab. The damage resulted from consolidation of soft or uncompacted fill adjacent to pile cap for a deep foundation. Like the other examples, the pile supported foundation is stable but the adjoining sidewalk has settled causing it to rotate and tip upwards. If a doorway had been constructed in this location, it would have obstructed the door movement and caused a tripping hazard.
As previously described, minimizing this problem can be done by pre-loading the soil next to the foundation or using an alternative to a rigid surface adjacent to the foundation.
Another alternative is shown in the sketch below. This approach uses a reinforced concrete slab to connect to the foundation. The slab will have the strength to support loads if the soils beneath the slab settle. If compressible soils extend a considerable distance from the building, there may me some cracking and settlement where the reinforced concrete meets non-reinforced concrete. However, this cracking and settlement will take place away from the building instead of adjacent to it. It will also create a gentle transition across the settled soil instead of an abrupt transition.
These control methods should be discussed with your geotechnical and structural engineer during the planning process to develop the best approach for your particular situation.