The MHI/MHRA anchor pull tests were performed with soil testing taking place, using the measuring equipment available to the installer. A Consulting Geotechnical Engineer selected the appropriate anchor for the measured soil values, and they were still unable to validate industry standard of, 3,150-pound nominal working load, for helix soil anchors.

Moving on, we will address additional fallacies in the MHI/MHRA test program.

Soil Conditions

On page 1-1 of the MHRA report it states, "poor soils were chosen", yet on page C-1 of the MHRA report, it is states, "no attempt was made to locate the worst soils", which leaves it open to just how poor were the soils, that were chosen for this industry sponsored test program. The MHRA report states, "Most of the tests took place in sand or sand and silt soils, which typically do not hold anchors as well as soils with higher clay content". This statement is not entirely accurate, and would have been completely inaccurate, if the word "typically" would not have been used in this statement. We will discuss why it was necessary to include the word "typically", in their statement.

First, the statements that follow were made by Felix Yokel, from at that time, what was known as the U.S. Department of Commerce (DOC), National Building Sciences (NBS) division. This division has become known as the U.S. DOC, National Institute for Sciences and Technology (NIST). Felix Yokel is to this date, still currently employed at NIST. The study of anchors for manufactured homes, carried out by Felix Yokel et al, was by far the most extensive and comprehensive pull testing of anchors used for manufactured housing, carried out to this date. Yokel et al tested 309 various types of manufactured home anchors, at three (3) sites, a sandy, silty, and clay site, under many different load configurations, and conditions. This is almost double the number of anchors tested by MHI/MHRA. Felix Yokel et al would not have been subject to outside influences, and therefore would not have felt obligated to the Manufactured Housing Industry, or to any other industry, that might benefit from a favorable, or unfavorable report on helix soil anchors. In short, the reports issued by the U.S. Department of Commerce are some of the only reports, which we feel provide unbiased information about the performance of soil anchors.

Yokel et al. (1982)
Figure 2 compares the load-displacement characteristics of vertical anchors installed on the three test sites and pulled at a 40º angle. Note that the initial anchor stiffness on the sandy site was less than that on the silt and clay sites. However, the stiffness on the sandy site increased rapidly with increasing loads and the peak resistance was reached at a smaller displacement than that in the silt and clay site. It is noteworthy that while on the sandy and silty site the load capacity of the vertical anchors pulled at an angle tended to be higher than that of the axially-pulled vertical anchors, a similar increase in load capacity did not occur on the clay site (Figure 2). This can be explained by the fact that the compressive forces exerted in this loading mode on part of the soil mass surrounding the anchor substantially increased the shear strength of the sands and silts, which increases with increasing confining pressures, but not that of the clays, which entirely depend on cohesion and thus tends to be independent of confining pressures.

Note: References in the following statements to "full-depth", means that the anchor was installed vertically to the ground surface, to the anchors full depth, and not that the anchor that was installed 6-inches deep.

Yokel et al, 1982
On the Silt site for the vertical full-depth, 6-inch single helix anchors ranged from 3000 lb to 5750 lb and averaged 4270 lb. The resistance of the diagonally loaded vertical anchors at 4-inch horizontal displacement and 45-degree pull ranged from 1100 to 1300 lb and averaged 1260 lb.

On the Sandy site for the vertical full-depth, 6-inch single helix anchors ranged from 3900 lb to 6000 lb and averaged 4800 lb. The resistance of the diagonally loaded vertical anchors at 4-inch horizontal displacement and 40-degree load inclination ranged from 2800 to 4000 lb and averaged 3200 lb.

On the Clay site for the vertical full-depth, 6-inch single helix anchors ranged from 2300 lb to 3500 lb and averaged 3100 lb. The resistance of the diagonally loaded vertical anchors at 4-inch horizontal displacement was negligible.

Even on the SANDY SITE, where anchors were relatively stiff and the average performance of vertical anchors met the standards, many individual anchors did not meet the requirements.

Figure 3: Ultimate load capacities for 48-inch - 6-inch single helix anchors installed vertically in Sand, Silt, and Clay and loaded at a 40-degree angle to the ground. Note: these values are complete failure, and are not limited to 4-inch horizontal displacement, and 4,725 pounds is the ultimate load capacity for anchoring equipment.

There is a notably lower ultimate, and nominal load capacity for clay soils than for both sand and silt soils. So why do clay soils perform so poorly? We will let William Kovacs of Purdue University, and Felix Yokel of NBS, Department of Commerce answer this question. In a study, called "Soil and Rock Anchors for Mobile Homes - State-of-the-Art Report", it is stated by Kovacs et al:

Kovacs et al, 1979
Shear strength of cohesive soils can be shown to be inversely proportional to the water content. The higher the water content, the lower will be the shear strength of the soil.

We are sure everyone will agree that most storms, in which dangerous high winds will be present, in general have moisture in the form of rain, associated with them, which reintroduces water to the clay soil. It is unlikely that clay soils 30-inches or deeper will be affected for days by surface water introduced by rain, on any given day. It is however highly likely that the first 12 inches of clay soil, which can be affected by organic matter, root systems, and by other disturbances, will be adversely affected by moisture during the storm event. This is at a level in the soil, where stabilizer plates, that are intended to resist horizontal displacement, engage the soil.

While a person would naturally tend to believe that clay soils would provide greater resistance to both inclined and vertical loads, this assumption is not based on the available data. Based on the data available from previous anchor studies, it appears that the best soils for obtaining higher load values, under inclined loads, were chosen for the MHRA study, not the worst. It is notable that in the anchor studies, which we have been able to locate, that have taken place since 1982, when Felix Yokel tested anchors in clay soils, that the testing of anchors in clay soils, which provided for very low load values in previous studies, has been excluded from testing studies. This seems at odds, since the testing by Yokel et al indicates, that further study in clay soil needed to take place, or perhaps it was decided that when clay soils are present, manufactured housing is not.

The MHRA study is void of any information with which to determine if the use of a 12-inch or 17-inch stabilizer plate was responsible for the increase in performance. As was just shown, sand, silt, or sand and silt soils can increase the performance of helix soil anchors, under an inclined load. There was no comparison testing performed by the MHI/MHRA of anchors without stabilizer plates, for these test sites. This is needed to determine what portion of the increase in the performance of the anchor, was the result of the stabilizer plate, and what portion was the result of other factors. It has been noted in previous studies that the stabilizer plates only increased the load capacity of the anchors, from between 3 and 6 percent, while the MHI/MHRA study makes it appear, that the stabilizer plates are solely responsible for the increases in the performance of the anchors.

It was shown from the testing performed by Pearson et al, 1991, that there is an increase in the performance of anchors with 3/4-inch shafts, versus anchors with a 5/8-inch shaft. This is logical since the 3/4-inch shaft would be stiffer and therefore offer more resistance to horizontal displacement than would the anchors with a 5/8-inch shaft. We will limit Figure 4, comprised of data taken from the report by Pearson et al, 1991, for making this point. Pier stability beyond a 4-inch horizontal displacement cannot be assured, and is the reason for this limiting criteria.

(Pearson et al, 1991)
Pearson et al carried out their testing of 48-inch single helix and double helix anchors w/ 12-inch stabilizer plates in sandy soil described as loose-to-medium dense sand. They obtained far lower load values at a 4-inch displacement for 48-inch Single Helix anchors with 5/8-inch diameter shafts, than for the same configuration, under the same load configuration, of those with 3/4-inch diameter shafts.

Do not let the high load value obtained for 3/4-inch shafts mislead you into believing, that this is the solution to the problem. First off, this has known for years. The most likely reason that the industry and/or government agencies have not simply prohibited the use of 5/8-inch shafted anchors, is likely the same reason they have not prohibited the use of painted anchors, which have been shown to have little chance, of providing corrosion control for ground anchors. It would cost the installer several dollars more per anchor, to do a job with 3/4-inch shafted and galvanized anchors. This would either reduce the profit margin, or contribute to narrowing the gap between the cost of this product, and site built housing, provided the cost were passed to the consumer. Second, the load values obtained were in sand, and are not significantly different the values obtained by Yokel, some 9-years earlier, in sand. While there is an apparent improvement in sand, it is not likely, that this same improvement would be realized in clay. As was discussed earlier, it is likely that in sand, at least in part, the increase in performance was related to the increase in the surface area of the shaft, since this would act to increase the confining pressures of granular soils.

The MHI/MHRA anchor spacing chart does not distinguish between the use of 3/4-inch and 5/8-inch diameter shafts for anchors, and both are available to installers for use. Given what is being proposed by the MHI/MHRA, it would have been appropriate to have included within the tables of load values, in section D, of their report, information about the anchor used for each given load value. This is the only way to determine what role stabilizer plates played in the anchor's performance, at the selected test sites, and make an informed decision.