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January 31,1930 <br />Dear Mr. Carlson, <br /># <br />'4-S tl <br />I has e examineu tiie sKecches of the retaining wail (seawall) <br />you provided, and have made the following observations and <br />conclusions. <br />This wall can be described as a she'^twall with anchor. The <br />wall is subject to eartii pressures but not to any significant <br />surcharge such as vehicles or buildings on the backfill side. <br />The wall is subject to possible ice and wave action depending <br />on the season and water level. <br />The primary modes of failure for this type of construction <br />are overturning, underturning, and material failure. <br />The usual analysis of this type of construction involves a <br />calculation of the active and passive forces (earth <br />pressures) on the wall. These calculations will vary somewhat <br />depending on the values used for the density and type of <br />soil, water levels etc. The dimensions and design of this <br />wall (4 ft total height with a 2 ft imbediment) however, is <br />such that most design guidelines and references would not <br />recommend the need for the use of anchors. <br />I did however, make some elementary calculations of the earth <br />pressure forces on the wall. Attachment A illustrates that <br />the cable anchors are more than capable of resisting the <br />earth pressure overturning fore; , e\'en when we assume no <br />structural assistance from the wall itself, i;hen they are <br />spaced no more than 8 ft apart. The 5 additional cabled <br />anchors you installed after the initial construction provides <br />a consistent anchor factor of safety of nearly 2 along the <br />entire seawall length. <br />The cabled anchors add assurance that the wall will not <br />overturn, but they present an increased vulnerability to <br />underturning if the soil on the shoreline side is washed away <br />thereby reducing the wall imbediment. <br />It is my opinion that the most likely _ause for premature <br />failure (failure before the expected lifetime of the treated <br />wood materials), would be vinderturning after wave or current <br />action has removed soil on the lake side of the wall. To <br />prevent this from happening, a minimum imbediment of 2 ft <br />should be maintained for a minimum of 4 ft e.xtending out <br />toward the lakeshore from the wall. <br />It is also very important to have adequate drainage provided <br />for from b**hind the wail to prevent freeze damage due to <br />water trapped behind t)i*^ wall. This is par t icu Lir ly important