composed of amorphous silica.) The second test was performed on intact (?) limestone of the Burlington Formation. (Geologists used the word "intact" to describe rock that is free from defects such as joints, bedding planes, etc.) Unfortunately, no other soil or rock information was given. For these two ground conditions, the anchor performed well with respect to the NFPA 501A-1975 requirements [69]. For the anchor described above, a hole is drilled by whatever means is feasible, keeping the hole to a minimum diameter so that when the expandable type anchor is inserted, it has a minimum of clearance. With this particular anchor, a hydraulically applied compression load deforms the anchor plate and increases the normal force between the anchor and the surrounding hole; this provides a large frictional resistance against pull-out. In principle, this type of anchor is very similar to other types of expanding rock anchors, but it appears to work well in "stiff" or "hard" soils. Besides the load test data presented in figure 7.12, the only other test data obtained on the pull-out capacity of expanding rock anchors is for one case in a blue shale of unknown strength. Using Using (probably improperly) an expanding anchor of the type shown in figure 4.5, the maximum pull-out capacity was 3,000 pounds (13 kN) while using the multiple internal split tube type anchor, capacities of 16,000 and 19,000 pounds (7 to 85 kN) were obtained with less than one inch of deflection. Further proprietary information could not be obtained. 7.5 DISCUSSION OF TEST DATA Several things should be apparent to the reader regarding the determination of pull-out capacity from field tests. Although many tests have been performed, the actual prediction of pull-out capacity of anchors is far from precise. Just because anchors meet the NFPA [69] requirements for pull-out load at a minimum deflection in a specific soil deposit in some states, does not necessarily mean that similar pull-out capacities will be achieved in another part of the state for the same visual soil classification. Samples of the best pull-out load test data have been presented. However, very few pieces of literature available from anchor manufacturers contain enough information on soil shear strength and depth of embedment to corroborate present pull-out capacity hypotheses by back calculation. Although pull-out load tests for possibly up to seven or eight soil classes may meet the requirements of a state agency, it is felt that they would not pass the scrutiny of a soils engineer if he were given the responsibility to design the anchor for a given pull-out capacity and deflection. In other words, the pull-out test data that have been shown cannot be used to further the state of the art since the test data do not provide sufficient information on the strength and characteristics of the soil. Clearly, what is needed is a standard for field pull-out tests which would require the reporting of the load-displacement characteristics, the pull out capacity and the pertinent soil parameter. Facing page: Soils need to be identified to design against advers environmental factors such as expansive and freezing soils. In addition to being required to resist wind and flood induced loads, anchors are subjected to other environmental effects which not only exert loads but reduce load resistance as well. Some of these effects are discussed below. Flood effects such as scour, soil saturation, or wave impact are beyond the scope of this report. In order to design against adverse environmental factors such as expansive soils or freezing soils, the soils need to be identified. The following paragraphs discuss the appropriate site investigations required to identify potential soil problems and make recommendations for adequate design. 8.2 ANCHORS IN EXPANSIVE SOILS The geotechnical engineering literature contains much information (for example, Reference 83) on the design of Building foundations in expansive soils that is also applicable to mobile homes. An expansive soil is one that changes volume seasonally due to the addition or removal of water. During the "wet season", the soil absorbs water and swells, raising any lightweight foundation that happens to be above it. Likewise, during the dry season, the soil shrinks due to evaporation and, as a result, settlement occurs. If this movement, both upward and downward, occurs at different rates and different amounts on the same structure, considerable distortion may occur which will cause architectural distress and possible damage to the structure as well. The main objectives when building on an expansive soil are to either maintain a constant soil water content or to support the structure on a stable soil stratum. If the water content remains stable, neither shrinkage nor swelling will take place. Seasonal changes in the weather and the watering of lawns and gardens both contribute to changes in the soil water content. Sources of information on expansive soil are given by Hilf, [49] and the Canadian Manual on Foundation Engineering [80]. Further problems of heaving are discussed in the next section. Though it is conceivable that mobile home foundations in expansive soil could be affected by the relative movement between the anchors and the mobile home, no evidence of significant damage to mobile homes could be found. It is, however, reasonable to assume that seasonal adjustment of strap tension is advisable in some areas. |