2005-P09263 - land alteration - Laserfiche WebLink

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� Mr. Ray Newkirk Wayzata,Minnesota Project No. 95286 identified thai th,e original5teiner and Koppelman constcuction plan for the site inteaded for roof runoff to be directed into a drain pipe leading away from the slope and take. Site Visit 2 On September 8, Mr. Newkirk requested a second site visit due ta another heavy rainstorm event from the preceding weekend and a l�rge soil slump. Mr. S. Gale, P.E. and Mr. B. Theroux, P.E. met with Mr. Newkirk and Mr. Jim Smith of Concept Landscaping. The slope was obs�rrved to have shifted to create a slide scarp approximately 3 %2 ft lugh at the top of the slope. A level "bench" lead from the scatp to a stnaller secondary scarp approximately Y.to '/z ft in height. The concrtte stair�ase steps and pavers had buckled and slid both laterally and verticaily. A silt fence had been in�tailed a few feet behind the upper scarp at the top of the stope. Field Testing We observed the exposed subgrade silty clay soil on the face of the sc�rp. The brown siliy clay surface soils appeared to be a glacial till sail. A vane shear test was conducted withiia the soils at the base of the scarp. At a depth of approximately 6 inches, a vane shea�r stc�gth of 224 pounds per squsre foot(ps�vv�as measur�d. This value is typical of a soR cohesive soil. Approximgtely 9 inches below the base of the scarp, a vane shear strength of 130U psf was recorded. This value would be typical of a fum to stiff cohesive soil. The lateral extent of the slide vvas estimated to be approxvnately 45 to 50 ft at�lhe top of the slope. �V�I.U_ATTON Based on our field r�onnaissance, it appears thaxt the mechanism causing movement of the soil mass was related to a shear failure of the �oil. Our observadons indi�e that a shear failur�occurred causing a block of soil to shift downward snd slide towazd t�e lake. Such slides occur whe�the shear strcngth of the slope soil is reduced to a degree that it c,an no longer support its own weight. The trigger for this reduction in shear strength is aflen the introduction of watet into the soil. Unless drained properly in clay soils, aa excess of watec can tead to a shaip increase in pore pressure within the soil. The water not only s�ftens and weakens the clay, but it als� increases the weight of the soil. Because of t�e nature of clay partictes, nnce the clay is softened it takes a Don�g tune to regain strength. As�increase in�veight, such as occearred during the rainstomns,probably eaused the failure. in such situations, it becomes prudent to talce measiues to reduce the amount o�water flowing into the affected area aad direct It�way fron�n the soil. It is our opinion that water nmoff entered t�e sIide area by flowing ovecland toward the slope and through the seam space between the clay subgrade and recently placed sod. During a heavy z�ainfall�vent,a sharp rase in the influx of water cowld lead to a� increase �n�he excess pore pressure of the cIay,softenin�it and leading to a shear strength fasluure. �ie-Tec Engineering,Inc., SEPTEMBER,2005 3