By Evan Gurley
In addition to agriculture, people have historically used soil as a construction material for foundations, buildings and flood control. As early human settlements expanded, structures were erected and supported by formalized foundations. Until the 18th century, however, no theoretical basis for soil design had been developed, so the discipline relied more on past experience than science.
Leaning Tower of Pisa: something is up
Several foundation-related engineering problems, like the differential settlement of Italy’s Leaning Tower of Pisa, prompted a more scientific approach to examining the subsurface. The earliest documented advances in soil mechanics occurred in 1717 when Henri Gautier developed earth pressure theories for the construction of the retaining wall. Since then, there have been significant advancements in the science and engineering behind soil mechanics. In 1925, there was major progress with the publication of Erdbaumechanik by Karl Terzaghi (considered to be the father of modern soil mechanics and geotechnical engineering). Terzaghi developed the principle of effective stress, demonstrated that sheer strength of soil is controlled by this effective stress, and created the theories of foundation-bearing capacity and the rate of settlement due to consolidation.
Structural performance depends on firm footing
Today’s geotechnical engineers perform investigations to obtain information on the physical properties of soil and rock underlying (and adjacent to) a site and use this data to design earthworks and foundations for proposed structures. Geotechnical investigations include surface and sub-surface explorations to ensure that the proposed building site’s soils and/or bedrock are suitable for placement of precast concrete structures.
Proper installation of precast concrete products is critical for maintaining structural integrity. The modular nature and simple joint connections of precast concrete components enable fast assembly. Proper installation once on site ensures structural performance while maintaining a safe work environment.
Precast concrete products differentiate themselves from alternative products in that the load-carrying capacity of precast concrete is derived from its own structural integrity, not from the adjacent soil or consolidated backfill. A precast concrete product can be designed as a separately engineered unit or system to withstand all loading conditions, unlike other products that must rely on adjacent soils to meet specified in-service strengths.
Subgrade soil conditions will affect the overall performance of any bearing structure, therefore it is important that precasters adhere to local and industry-wide standards, codes and best practices when dealing with the actual design, site preparation and installation of precast concrete products. Producers can also assist the owner, contractor and inspectors through organized and accurate scheduling of materials, construction procedures, contract documents, plans and specifications.
Precast concrete complies with industry best practices
Precast concrete manufacturers adhere to local and industry-wide standards (ACI, ASTM, etc.) and regulations that specify precast concrete products are to be placed and installed on level, compacted gravel, sand or stone sub-bases. Compacted gravel or stone sub-bases allow water to drain away from the installed product, minimizing the possibility of settlement. Rainwater standing or running alongside an installed underground product may cause differential settlement. If soil grading is such that water runs alongside a precast concrete product during rainfalls, the water can flow under the edge of the precast structure and carry away the supporting soil.
Having a level, sound sub-base specified for placement of a precast concrete product is crucial as the weight of the structure is distributed directly to the supporting soil. Soil pressure from direct loading is greatest immediately beneath the precast concrete product, and the load distribution spreads out as the depth of the affected soil increases. Figure 1 illustrates the distribution of loads beneath a foundation/footing and points out the location of the critical compaction zone. If a product is installed with improper compaction, the critical zone would be the first place for failure due to settlement or structural shifting.
Informed installation
In order for installed precast concrete products to be as effective as possible, engineers, contractors and building officials who address underground installations should have a thorough understanding of installation best practices. Precast concrete manufacturers typically provide proper best practices and installation literature and/or assistance to the end user of the precast product, based on engineering calculations and industry guidelines. In addition to providing the appropriate literature and assistance, precast concrete manufacturers frequently check the installation sites prior to the installation or upon request.
Designing a precast concrete product with proper accuracy is just as essential as placing and installing the product on a properly prepared subgrade. Unsound design practices, whether preproduction or site preparation, can jeopardize the service life of the structure. Adherence to all local and industry-recognized codes is essential in construction and design practices; failure to follow best practices could result in costly product and site remediation.
Site preparation
Things to consider during the site preparation phase of the project:
• Review the approved site plan to confirm lot lines, precast location, length and elevations.
• Schedule preconstruction meetings.
• Verify the on-site soil conditions.
• Call the local utility companies to confirm the location of underground utilities.
• Obtain all necessary building permits.
• Confirm drainage to avoid erosion or buildup of water behind the structure.
• The contractor/installer is responsible for the positive drainage away from the precast structure during construction.
Excavation
Excavation is one of the first factors that should be addressed in the design of a subgrade.1 During excavation, the contractor should be required to remove any materials that do not comply with the IBC2 code (rubbish debris or organic material) or other specified codes and standards. Removing materials and soils listed in the IBC codes helps create a level, firm subgrade and eliminates settlement or shifting (causing cracking) that can occur on sites with poor substrate.
It also must be realized that while excavating the soil, excavation forces loosen up the adjacent undisturbed soil by mixing air into the substrate, which decreases soil density. A decrease in soil density results in a loss in the load-bearing capacity, or the loss of the ability of the soil to adequately support loads. Proper compaction of the disturbed native soil or imported fill material is needed for excavation-affected soils to regain their intended design-density properties. All excavation and consolidation factors, geotechnical considerations and related variables must be taken into account in the design stage.
Foundation preparation
Foundation soils should be excavated as required to the dimensions shown on the plans. Soil should be inspected by the geotechnical engineer to confirm that the bearing soils are similar to the design criteria. Foundation soils are to be proof-rolled and compacted to a minimum of 95% of the maximum dry density (ASTM D698, Standard Proctor) and inspected by the owner’s engineer3 prior to placement of leveling pad materials. The contractor is required to replace any unsuitable soils discovered during excavation at the direction of the geotechnical engineer.
Backfill
The engineer specifies the compaction of all fill materials in proper lifts, whether native soil or a granular fill material, to provide a dense, flat subgrade. If the fill material is not compacted uniformly, uneven settling of the precast concrete product can occur, resulting in cracking. If the engineer calls for backfill lifts that are too deep to facilitate proper compaction, the product resting on fill soil could be subject to leaning, buckling and possible collapse.
Compacted fill material
Compaction is the process by which the strength and stiffness of soil may be increased and permeability may be decreased by a specific degree of consolidation. There are six reasons why proper fill materials and compaction methods are essential for designing a sound subgrade for an installed precast concrete product. Properly compacted subgrade soil:
1. Provides stability;
2. Reduces water seepage and swelling;
3. Reduces settling of soil;
4. Prevents soil settlement and frost damage;
5. Increases the load-bearing capacity; and
6. Provides good drainage properties that can prevent weakening of the foundation.
If improper fill materials are used, adverse effects such as settlement, differential settlement, leaning or tipping of the installed precast concrete product may occur. Differential settlement will damage the installed precast concrete product by producing (usually vertical, possibly diagonal or stair-stepped) cracks and other symptoms of structural movement. Improper use of compaction machinery may inflict damage to the installed precast concrete product in the form of breaks, buckling or leaning.
Conclusion
Some precasters do not deal directly with the site preparation and installation of underground products. Rather, they rely on seasoned geotechnical engineers who understand how a precast concrete product is to be installed according to local and industry-wide codes/regulations dealing with the existing soil conditions and best practices.
Precasters who do it all, including the site preparation and installation of a product, realize the importance of proper installation and best practices and ensure that local codes and regulations are met. For example, precast concrete retaining wall producers have developed an excellent field installation best practices manual4 and continue to educate the industry with the development of literature and their continued involvement with standards development.
1 See “Prevent Excavation Cave-in Fatalities,” by Evan Gurley, March-April 2012 Precast Inc. magazine.
2 International Building Code
3 “Engineer” refers to the owner’s designated organization or trained and experienced individual with authoritative charge over engineering functions and responsibilities.
4 NPCA’s Precast Modular Block (PMB) for Retaining Wall Systems, Field Installation Best Practices Manual, is an example of available technical support literature. Visit https://precast.org/wp-content/uploads/2012/10/PMBInstallBPM.pdf (case-sensitive).
Evan Gurley is a technical services engineer with NPCA.
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