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Home My WebLink AboutB13-0508 REV3 Geotech letter CORRECTIONS REQUIRED i � , �� %��'�j,� �� I m�„ i � iin ., C I„ri,l,� ,I�,il �.� �tri/��V V�� [h �n�:9l0 9+� 793ti HEPWORTH-PAWLAK GEOTECHNICAL F,��„��i y�� :mai I�. h�,�<<en{�i�:;��.��ir�:h.o,in Apri128, 2014 Transwest Construction Attn: Mike Scrivens P. O. Box 8812 Avon, Colorado 81620 �uihcC�cu;�u��s��s�ccn;truciion.e��u� -- --- - Job No. 114 106A Subject: Observation of Excavation and Drain Design, Garage Addition and New Entryway, Cross Residence, Lot 5, Block 7, Lionshead Second Filing, 126 Forest Road, Vail, Colorado Dear Mike: As requested, the undersigned, a representative of Hepworth-Pawlak Geotechnical observed the excavations at the subject site on April 3 and 10, 2014 to evaluate the materials exposed for foundation support The findings of our observations and recommendations for the foundation and underdrain designs are presented in this report. The services were performed in general accordance with our agreement for professional engineering services to Transwest Construction, dated April 2, 2014. Proposed Construction: The existing garage footing bearing level and floor slab are being lowered about 4 feet in elevation. A new entxyway to the xight (south) of the garage is being constructed and also lowered about 4 feet in elevation. Ground floor areas will be slab-on-grade. The new foundations have been designed to be supported on spread footings assuming an allowable bearing pressure of 2,500 psf. The new garage foundation walls are to be constructed in phases to reduce the potential for undermining of and distress to the existing structure. As part of the new entryway construction, an area of underpinning with a temporary column is also planned. Site Observations: On April 3, excavation for the new garage area and entryway was underway. Tl�e garage excavation was along the east wall and had been cut in one level about 4 feet below the existing floor slab elevation. The new entryway area had only the existing entryway removed and the excavation was at about the same elevation as the garage floor slab. The materials exposed in the bottom of the excavations consisted primarily of hard shale and limestone bedrock. We understand in some of the upper areas of the entryway there is up to about 4 feet of free draining (minus 3/a inch size) gravel below the slabs and footings. The bedrock materials were to hard to obtain l�<ir�.er 303-541-%11�1 � l;ulora�luSE,rin�s �71�)-63i-�56Z • Sil��erthorne 970-468-I)8�) Transwest Construction Apri128 , 2014 Page 2 undisturbed samples for swell consolidation testing. Free water was encountered in the garage addition excavation only and the exposed materials were moist to wet. Recommendations: Considering the conditions exposed in the excavations and the nature of the proposed construction, spread footings placed on the undisturbed bedrock designed for an allowable bearing. pressure of 2,500 psf should be adequate for support of the proposed additions . There could be some differential settlement with respect to the existing structure which should be considered in the design. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. All fill and loose disturbed materials in footing areas should be removed and the bearing level extended down to the undisturbed hard bedrock. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for on-site soil and well broken bedrock, or suitable nnported gravel, as bacl�ill. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind below grade walls and prevent wetting of the lower levels . A detail of the drain system we recommended behind the below grade basement walls is attached as Figure 1 . The drain system behind the foundation walls and below the footings should be collected by an interior 4 inch diameter perforated PVC pipe surrounded by draining gravel and sloped to a suitable gravity outlet. Structural fill placed within floor slab areas should consist of imported free draining gravel compacted in lifts . Exterior backfill placed around the structure should be compacted and the surface graded to prevent ponding within at least 10 feet of the building. Additional Site Observation and Recommendations: When we visited the site on April 10, excavation in the garage area and for the new entryway had continued. The garage addition had two new foundation wall areas along the east side constructed with the behind and below foundation drain "system constructed as generally per our recommended detail. The interior drain had yet been installed and the groundwater, and water from an existing drain in the southeast corner, was being collected and discharged by a sump and pump system. The entryway area had been excavated down about 3 feet from the previous observed grade with hard shale bedrock continuing to be exposed in the bottom and no free water observed. The excavation had steepened the 12 to 15 feet high cut slope to the south to near vertical. There was considerable run-off flow down the excavation cut face from snowmelt on the slope above and we recommended the slope be shored. We also Job No. 114 106A C�UPt@Ch Transwest Construction Apri128, 2014 Page 3 recommended the undermined portion of the foundation to the east of the cut slope be underpinned as soon as possible by designed per the structural engineer. Limitations: The recommendations submitted in this repart are based on our observation of the materials exposed within the foundation excavations and do not include subsurface exploration to evaluate the subsurface conditions within the loaded depth of foundation influence. This study is based on the assumption that materials beneath the footings have equal or better support than those exposed. The risk of fou�idafion movement may be greater than indicated in this report because of possible variations in the subsurface conditions. In order to reveal the nature and extent of variations in the subsurface conditions below the excavations, drilling would be required. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH - PAWLAK GEq�'��VICAL, INC. `��, "'c�. 0 R�9�0s°es / • s °'� y � °io �. y ' 8 �,' � � Y � David A. 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