ranging from 17 to 46 inches (0.43 1.17 m). All of these tests were performed in one type of soil having the Unified Soil Classification symbol SM-SC (see Section 5.1 for further discussion on soil classifications). Only 3 of the 22 pull-out tests performed met the combined criteria of pull-out load and maximum deflection according to the mobile home standard [69]. These 3 tests were on helix anchors that were installed by first digging a hole, installing the anchor and then backfilling and compacting (not a typical procedure for Helix Anchors). Thus, "variations in the backfilling and compacting procedures, the only variable in the installation method used for the anchors . .. may have been responsible for the 3 anchors which met performance specification . . ." [105].

Vann and McDonald [102a] present a thorough discussion and analysis of damage experience to mobile homes by wind storms, as well as recommendations for the design, construction and anchoring of mobile homes.

The references mentioned thus far may be considered the primary works on the anchoring of mobile homes in soil and rock. However, a close review of these references will demonstrate that very little information from a geotechnical engineering viewpoint has been published.

Facing page: Measuring horizontal displacement in anchor test.

This study primarily concerns itself with the geotechnical engineering aspects of the performance of soil anchors. The anchors are typically made of metallic materials and usually will have a strength which is equal to or higher than the capacity of the soil to resist deformation. Assessment of the structural load capacity of various anchors discussed herein is not within the scope of this report.

3.2 ASSESSMENT OF AVAILABLE SOIL AND ROCK ANCHOR HARDWARE

Product data of existing anchoring hardware have been collected from military research laboratories, commercial anchor companies, and telephone, utility and power companies.

3.3 REVIEW OF THE THEORETICAL ASPECTS OF ANCHOR HOLDING CAPACITY

Available references that discuss theoretical aspects of anchor pull-out capacity and design procedures for the various types of anchors that are currently used in industry are reviewed. Parameters considered include the various soil types and environmental conditions that one would find under field situations. The environmental conditions include frost heave, swelling soils, and chemical or electrical attacks on the anchor material.

3.4 REVIEW OF AVAILABLE FIELD AND LABORATORY TEST DATA ON ANCHOR CAPACITY

Detailed test data, which include load-displacement curves for various types of anchors in various soil types were obtained from the geotechnical engineering literature as well as from various commercial anchor companies that were contacted. Where applicable, pull-out capacity is correlated with soil properties as determined by field or laboratory tests and with proposed analytical approaches in order to determine whether there is a basis for a realistic, rational design approach.

3.5 GEOTECHNICAL ASSESSMENT OF FIELD AND LABORATORY TEST DATA

The above mentioned laboratory and field data are critically reviewed to provide a preliminary assessment of the applicability of present design methods. A program of research with the objective of relating pull-out resistance to basic soil properties, installation torque or resistance in the Standard Penetration Test [12] is proposed.

Facing page: Many of the anchors on the market are specifically designed for mobile homes.

There are a multitude of anchor types on the market today. Many of these anchors have been specifically designed for mobile homes while others, designed for other purposes, could also be used to anchor mobile homes. Much of the technology and many of the products were developed primarily for military, power, and communications applications. The communications applications include the guying (tying down) of telephone

and transmission lines as well as the securing of cables on the ocean floor. Some of the mass-produced anchors to be discussed have been developed specifically for mobile home applications, while many other designs resulted from backyard or garage operations by single owners to meet the needs of the mobile home industry. State agencies that enforce the mobile home anchoring laws usually require anchor manufacturers to submit plans and specifications of their anchors as well as field pull-out test data to show that the anchors will meet the minimum requirements. Typically, manufacturers will submit their anchors to a testing laboratory where pull-out tests are conducted in various types of soil.

4.2 ANCHORS DEVELOPED FOR MILITARY APPLICATIONS

4.2.1 General

The military has various needs for soil anchors. Applications include large tents, inflatable structures, various pieces of equipment that require restraint (such as weapons), and specialized membranes used to cover poor soil for use as landing pads. In addition, the Air Force uses anchors to hold down aircraft and the Navy uses anchors to moor vessels. These latter anchors require substantial load capacities and are outside the scope of this report. Many of the anchors and associated hardware for military use have the additional requirements of light weight, portability, and ease of installation and possibly retrieval under sometimes unfavorable conditions.

Berus [21] presents test results of anchor holding capacities for 14 commercial anchors, 8 experimental tie down anchors, 7 dead man anchors, and 12 ship and helix anchors. Tests on ten of the ship anchors were performed on scale models while the rest were performed on prototype anchors. Unfortunately, the soils were only very generally classified such as "hard," "dry" or "wet sand" or just plain "sand." The tests, nonetheless, covered a wide range of anchor types, sizes (areas of elements resisting pull-out), depths and soil types. Of all the ground