Complete a Geotech Study to help reduce risk of collapse.
When designing the proper foundation for the tremendous loads that must be supported when building large grain storage complexes, you need to consider some of the following issues:
- Geotechnical studies and soil borings.
- Concrete strength, slump tests, and water-to-cement ratios.
- Rebar size and placement.
Understanding pile-loading capacity
Craig Thompson with WL Port-Land Systems, Inc., Pittsburgh, PA, made a number of very valid statements in his Fourth Quarter 2014 article shared in Milling Journal titled “Concrete Silo Integrity.” Craig mentioned a number of good points regarding soil-bearing and/or pile loading capacity.
He pointed out that a “500,000-bushel concrete silo is holding approximately 14,000 tons of grain. It doesn’t take a rocket scientist to begin quickly to appreciate just how important a properly-designed foundation system can be when subject to such heavy weights.
For this reason, Nationwide Insurance points out that the grain industry has been experiencing more grain bin failures in recent years. Certain people will elect to take shortcuts or try to go cheaper by not giving proper attention to the extreme soil-bearing capacities that are needed with these large bins and subsequent weights on the soil beneath them.
I want to recognize the following people who shared some excellent information on this topic:
- Ryan Michalek, risk management property engineer, Nationwide Insurance, Des Moines, IA
- Rodney Carpenter and Michael Blough, Clear Creek & Associates, Goshen, IN
- Craig Thompson, WL Port-Land Systems, Inc., Pittsburgh, PA
- Jim Voigt, JFV Solutions, Mt. Zion, IL
- Dr. Carol Jones, Oklahoma State University, Stillwater, OK
A history of geotechnical failures
Ryan Michalek of Nationwide Insurance shared a recap of one of the most infamous geotechnical failures involving a grain storage structure that happened back in 1913. The Transcona grain elevator in Transcona, MB, Canada sustained a catastrophic failure of its shallow footings, when the facility was filled for the very first time following construction.
This particular failure is a popular case study in geotechnical engineering circles, because it is literally the classic example of a structure exceeding the bearing capacity of the supporting soils,” he said. “Also adding to its infamy is the fact that the structure remained nearly 100% intact following the soil failure, even as it came to rest after being rotated a full 28 degrees from vertical, which made the elevator a popular target for photographers.
The soil investigations used at the time of its construction were minimal. Only a simple diameter plate bearing test was used to give an indication of soil-bearing capacity. Practically speaking, this rudimentary device only tested the soil material at the surface and did not account for the fact that this heavily-loaded structure would interact with soils down to a greater depth.
This failure brought to light to soil scientists of the time that soils often are stratified (not uniform with depth), and testing methods used should recognize that fact.
Michalek points out that geotechnical engineers take soil samples with a drill-like apparatus down to a depth where it is anticipated that the structure to be built will interact with the soils beneath the potential grain storage structure. From the soil samples, the geotechnical engineer identifies the soil type encountered and performs laboratory analysis to determine the suitability of the soils at the proposed site. From this analysis, recommendations are made regarding design parameters, which will facilitate foundation earthwork operations.
The geotechnical engineer’s recommendations are synthesized into a formal report and are used extensively during the structural design, earthwork, and foundation construction phases of a project.
Nationwide points out that from time to time, the insurer encounters clients who utilize a geotechnical engineer only for the preliminary site investigation, but they fail to continue to retain the engineer’s services for the actual construction operations that will take place later.
Ideally, the geotechnical engineer should be retained to provide quality control measures as construction progresses. Typical tasks may include inspection of foundation bearing surfaces, groundwater mitigation, fill placement verification, and other responsibilities.
Other quality control issues during the concrete pour include verification of the rebar size used and its proper placement, along with the proper aggregate size used in the mix.
According to Michalek, soil conditions are the single greatest unknown at a construction site, because we physically can see only the soil surface. This very cursory analysis really tells neither little about the structural properties of the soil on-site nor anything about the soil stratification at various depths below the surface.
A geotechnical engineer will examine the potential loads and calculate the best ways to support the weight of the structure filled with grain and how to transmit those loads efficiently into the foundation and grounds. Thompson points out that “the owner and engineer together should provide general oversight to ensure quality control and the structural integrity of the storage complex.&rdquo
Additional foundation issues involve soil conditions and concrete type
Brock Grain Systems covers some very valid points on foundation issues within its Owner/Operator & Maintenance Manual: “Just as important as the soil is the concrete and concentration of reinforcing steel in the foundation that will be built on that soil. Damage to a bin can result from poor soil conditions or inadequate concrete type, grade, bearing strength or construction method.”
“It is obvious that grain operators still are very willing to take the gamble on whether soils at their site are suitable to support a new grain structure, without the benefit of information that could be garnered when using a geotechnical engineer,” said Michalek.
In many cases, a licensed, independent professional engineer will need to obtain borings for soil consolidation, elastic and/or plastic deformation, shear deformations, settlement and suitability for building.
The engineer should design the foundation and floor slab accordingly to meet local building codes, determining concrete and reinforcing bar requirements. The finished design should include footing requirements, tunnel requirements and the interaction of all foundation components under load.
Differential settlement is a common problem
A number of serviceability problems can develop as a result of failures to use proper geotechnical studies. A common ailment is when differential settlement occurs, which can cause sweep augers to get hung up on ridges in the bin floor due to uneven foundations later.
Foundation repairs are very expensive, not to mention cumbersome and inconvenient for grain operators. Depending on the size of the project and the nature of the problem to be corrected, foundation repairs themselves can approach 10% to 15% of the original construction cost, and this is without factoring in any lost revenue that can be incurred with a potential loss of use of the grain storage tank.
When you consider the fee for a geotechnical engineer is typically 1.5% to 2% of the total construction cost, the idea of using a geotechnical engineer is really akin to an insurance policy. You pay a nominal fee up front to avoid the risk of paying a large sum at some point in the future.
Helpful tips for the operator
Jim Voigt, JVF Solutions, Mt. Zion, IL, shares a number of good points from an operator’s perspective:
- Be sure to take enough soil borings. The soil analysis is only as good as the individual hole you drilled. The soil characteristics can vary within three feet of each other.
- Depending on current conditions, the water table can vary greatly, particularly if you are next to a body of water that rises and falls such as a river.
- The type of foundation design required will vary with the walls and tunnels used.
- What is the total load, including bin, grain, conveyors, and other equipment on top?
- Weather conditions during construction such as extreme wet, extreme cold, or extreme heat provide additional challenges.
- To remain competitive in the bidding process, most bin manufacturers’ foundation recommendations are minimal, especially on the large bins.
- It can’t be emphasized enough to use an engineering firm that understands the dynamics of a grain bin with different discharge types and grain hanging or bridging causing uneven loading.
- Watch the bulk density of material used in calculations. Using the wrong calculation can lead to failure.
- With the amount of steel going into the ring walls, it is very important that the proper vibration is applied, particularly in larger walls that are involved in a tunnel design.
- Watch the lap and ties on the rebar.
Wayne Bauer is director of safety and security at Star of the West Milling Co., Frankenmith, MI; 989-652-9971. Photos courtesy of Wayne Bauer.