Laserfiche WebLink
73-157 <br />Park Construction -2- May 28, 1987 <br />You indicated that the bottom of the existing landfill material <br />is at about elevation 968+. The existing surface elevation of <br />the undisturbed wetland area is at about elevation 972+ feet. <br />This 4-foot difference in elevation probably represents what we <br />would expect to be the amount of settlement that has resulted <br />from the weight of the existing landfill material compressing the <br />organic subsoils below. You indicated that the existing landfill <br />material is about 14 feet thick. <br />Judging from the length of time that the existing landfill <br />material has been in place, and your indication that there have <br />been no previous stability problems, it is our opinion that the <br />Zproposed addition cf 5 feet of mineral soil above the top of the <br />existing landfill grades presents a very slight risk of shear <br />failure. Our experience with projects where additional fill is <br />placed above existing landfill materials above swamp deposits has <br />resulted in additional settlement, but no shear failures. The <br />composition of the existing landfill material is such that it <br />probably possesses a rather high internal angle of frictivi, and a <br />rather high shear strength. The settlement that ha,_• occurred <br />over the past 30 years has also increased the shear strength of <br />the upper zone of organic soils which lessens the risk of shear <br />or punching failure as a result of placing an additional <br />surcharge load. Thus, based on this information, it is our opi- <br />nion that the addition of 5 feet of fill over the area presents a <br />very slight risk of embankment failure. <br />Tt should be noted that soil borings with power equipment have <br />not been taken in the existing :andfill area to evaluate the com- <br />position and strength of the existing landfill material and the <br />strengths and depths of the underlying organic soils. If the <br />assumptions we have made are inaccurate, our evaluation of the <br />potential risk may need to be re-evaluated. Additional borings <br />and tests at this time do not appear warranted for the grading <br />planned. However, at a minimum, we recommend that prior to <br />placing any fill, several settlement monitoring plates be <br />installed in various areas, preferably around the edges of the <br />existing landfill limits, and that the additional fill then be <br />placed in two stages of 2 1/2 feet per stage. Between lifts, the <br />monitoring plates should be referenced to aid in evaluating the <br />rate of settlement caused by the addition of new fill. Possibly <br />a period of one to four weeks should be set aside between staged <br />fill applications to allow more time for monitoring the plates. <br />If the additional mineral fill extends over areas that are pre- <br />sently virgin wetland, the risk of embankment failure is signifi- <br />cantly higher and a much more controlled filling technique will <br />be appropriate. <br />1 1 � <br />