Many homes in Central Texas are constructed on gently to moderately sloping topography. Unless a split-level design is being used, a fill pad will be required to reach planned slab subgrade elevation. Two types of fill pads are commonly referred to by Capital Geotechnical Services and be characterized as:
"Forming Fill": A fill soil installed to permit the installation of formwork to cast the concrete beams and slab for the concrete pour. There is no formal documented compaction (i.e. specific compaction equipment, thin lift thickness, notable number of compaction roller passes, quality control testing) and commonly there is no formal control of material selection (type of soil), although some control of material type and some level of compaction and moisture conditioning should be performed as noted on this page.
"Properly Compacted Fill": A fill pad installed with proper compaction and material control. "Proper" compaction implies constructing in thin lifts with a notable number of roller passes, and performing quality control (QC) testing of material selection, and QC testing of material placement = verify adequate density and moisture content.
Many residential slabs on sloping sites have interior grade beams cast on forming fill. Careful attention to this condition, however, is generally not provided by Builders, Structural Engineers, and Geotechnical Engineers, all responsible for the performance of the foundation system. If forming fill is placed in a loose and dry condition, notable settlement can occur after an initial wetting event due to the highly compressible condition of the fill soil. Some simple experimental data is provided below.
Beware of Forming Fill
One of the premier quarry pits in the Austin area producing good quality soils used for construction is the Texas Crushed Stone operation in Round Rock. One product they produce is the 7-mesh dry screenings" which is a silty fine to coarse sand produced from crushed limetsone screenings.
When placed in a loose and dry condition, however, this type of soil exhibited 14% to 15% settlement upon wetting (i.e. 3.5 inches for a 2-ft thick fill pad).
When placed with some nominal level of compaction, however, the compressibility of the forming fill (still installed relatively dry) upon initial wetting was reduced to 5.6% to 6.5%. If some moisture conditioning is performing during installation (i.e. adding water), the potential settlement is further decreased.
A more granular product from TCS is the "1.25 inch screenings" material that is a silty gravel with sand. This is an excellent product for use as "select fill" in construction and as a fill pad under a house foundation slab.
Unfortunately, a granular product such as a silty gravel with sand installed in a loose and dry condition also has relatively high compressibility when subjected to initial wetting and has exhibited 15% to 16% settlement in our experiment.
If installed with some nominal compaction and very minor wetting, however, the TCS 1.25-inch screenings product exhibited 3.5% to 9.6% settlement.
Notable settlement of a forming fill installed in a loose and dry condition is generally only a one-time occurrence and will only be experienced if the fill is exposed to a significant amount of wetting. One positive characteristic of a crushed limestone product such as the TCS 1.25-inch or 1.75-inch screenings is that after initial wetting, and after subsequent drying, the material experiences some cementation that transforms the material into a hard and stiff product and exhibits relatively high stiffness under light load. The cementation is generally caused by pozzolanic reactions from the calcium carbonate content and silt and clay fraction of the limestone base product after water was added. Fill soils that are not limestone-based will not experience this cementation and will maintain relatively low stiffness after initial wetting and drying. Soils that are not limestone-based include alluvial soils from local river area quarries, or crushed metamorphic rock from Llano area sources.
What can cause notable wetting of a forming fill pad? Poor drainage from such conditions as (1) a recessed planter allowing water to infiltrate into subgrade below the perimeter beam, (2) roof gutters discharging into a planter without any proper discharge design out of the planter, (3) ground surface sloping down towards the upslope sides of the fill pad, (4) a leaking water supply line or leaking sanitary sewer line. A poorly installed shower or bathtub drain can cause significant problems if slight settlement of teh fill allows the vertical drain element to drop and cause separation at the shower or bathtub drain at slab penetration, allowing significant quantities of water to discharge into the fill soil.
If a significant wetting event has not (yet) occurred within the history of a fill pad installed loose and dry and the structure, it is likely no notable settlement or deflection has occurred and the house has not exhibited any notable distress and is performing well. This is why you might hear of many homes being installed with just forming fill and not having had problems. But based on forensic studies we know settlement does occasionally occur where interior settlement (slab deflection) is evident and there is the site does not include expansive clay geology.
Our geotechnical engineering reports provide recommendations and commentary for designing and constructing a foundation slab on forming fill and on compacted fill (various options). Most of our competitors do not and thereby expose the developer or owner to risk and frustration. If you are building a spec home, custom home, duplex, or apartment building, please contact us and we will be happy to serve as your Geotechnical Engineer to help you with not just the successful completion of the project, but long term durability of your product as well.
This is a view of a plumber's tunnel that was excavated under a spec home slab that had exhibited only minor interior deflection, to access a plumbing leak. The house had developed minor cosmetic cracking and some minor door function issues (doors sticking or swinging open) which was enough to instigate a claim by the homeowner against the Builder. The excavation revealed that the forming fill had settled (the void under the nearby uniform thickness slab areas was 2 to 3 inches) and it was determined that seepage from a recessed planter without any proper drainage caused significant wetting of the forming fill. The settlement caused a shower drain pipe to drop and separate, and subsequent bath water subjected the fill under the bathroom area to significant wetting. The pipes you see in the photo are grout pipes (the trench was backfilled with grout after the plumbing was repaired).
Here is an example of a house pad being "properly compacted" in Austin, Texas. The fill is compacted behind the bag-fill formwork for the tall downslope perimeter beam-wall that is formed as you proceed up.
Here is an example of a gravelly forming fill for a custom home site being compacted with a vibratory plate compactor. This can be characterized as nominally compacted fill if done sporadically without any specific controls. It can be characterized as properly compacted if done in thin lifts and documented control. This house was on a sloping site (tall downslope perimeter beam and formwork) (photo by Alon Toker for the JLC).
This photograph of the same project shows a small ride-on vibratory steel drum roller being used to compact the fill pad in between the perimeter formwork. This equipment can produce a properly compacted fill if controls are documented.
This custom home site (Capital Geotechnical project in Belvedere development near Bee Cave) included the use of properly compacted select fill within perimeter formwork and bag-fill (not loose forming fill).
Here is another photograph of the proper compaction in action at the same site.
Here is a photograph of the proper compaction near the perimeter beam formwork and bag-fill. Compaction is performed before excavation for interior grade beam trenches.
For difficult to reach areas (i.e. after plumbing is installed or after interior grade beam trenches are excavated), proper compaction can still be achieved for repairing disturbed areas by using walk-behind mechanical rammers (tampers) such as this one.