Laserfiche WebLink
7A <br /> Page 37 <br /> Dr.Daniel Ponti Mary 15,2009 <br /> pag�4 C0813 <br /> of the l.andslide, which may be coincident with khe upper contact o£E where i# slopes <br /> taward the creek. However, o�tl.y a very small portion o€�iis surface appears tv have <br /> been exposed. by the trench. -- thi.s area is at the deepest portion of �he trench where <br /> water cvvered the base, thus preventing deta�led observations of ixench floor and <br /> sidewa�.ls. � <br /> Conc�usions <br /> 1. Subsurfa�e info�mation from the re�enf trench excavation, as presented <br /> by BAGG,does not preclude the pxesence of.a creekbank siump on Lot 4. <br /> �We disagxee with the conclusion presented by BAGG #ha# fhe t�ench <br /> explorati.an reveais"na evidence�a sugges#a siape faiiure". <br /> 2. BAGG's opini.on that distress to the basement walls, manifested as an <br /> open crack in the sidewall and out-of-level tilt in �he downstope wall, is <br /> due to fiIi se€tlem�nt does not preclude the presence o� a creekbank <br /> slump. In fact, the presence of fill daes not preclude the presence of a <br /> creekbank siump. Fill settlement could occur with or withQUt an � <br /> underlying l.andslide, or also could mask the effects of landslide creep <br /> movemen�. <br /> 3. Adaed to our previou.s reconnaissance analyses, the resuil:s of the trench <br /> excavation reinforces our earlier interpretation that the anomalous <br /> geologic feaiure is a pr.obable landslide and an indicafzon that the cree� <br /> banks on the project site are potentially sus�eptibXe to slope faii�re. As <br /> sta�ed in our previous lefter, erosional scarps visible along the creek <br /> banks are also indications of channel widening due to erosivn. The <br /> extensive construction of va�rious types of retaining walls, and the <br /> emplacemen# of gunite and riprap on the sIope and rhannel margins <br /> further attest to homeowners'attempts fio mitigate the effects o�erosiori. <br /> 4. Also, as stated in our previous correspondence,BAGG has nat provided <br /> missing geotechrtical data used to support their previous slope stability <br /> analyses. Without such information,, it is impossible to independently <br /> eva.l�zafe whether the fai�ure thresholds or the recommended engineered <br /> se�backs they report axe appropria�e. However we suspect, based on <br /> BAGG's vpinion that no landslide is present,that the pxesence of existing <br /> creekbank slumps is not incorporated into t�heir analyses. <br /> 5. Again, as previously noted,the probabl.e landslide is contained within th� <br /> City's 25-fovt setback, but is not cvntained wi.th�n the proposed Iimit of <br /> deveiopmenk Based.on our sife visits and a r�vie�v af the relevant BAGG <br /> repori�,it is aur opinion fhat slumps and bank erosion have occuxred on _ <br /> fhe prvject skte and may likely occur there i�t the future.From the data at <br /> hand,it is reasonabie to expect th.at bank failure wouid extend beyond 15 <br /> feet from the tap of the slape and thus may pose a significant hazard fo <br /> the creekside properties proposed for the prc�ject. <br /> .��...r._�w..�....�.w.....�.........,._.�..._�.�..,���......�._.,.�.�.M..�.....�.�....��....._....�.__._.,w.,�.....y,.�..��....__�.�.._.,.....,._,.......,...........,..�,.,�,..����������,����;. <br />