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330 Land Company, LLC Arroyo Lago <br />9785.004.001 Design-Level Geotechnical Report <br /> <br /> <br /> Page | 20 April 16, 2024 <br />Latest Revision October 31, 2025 <br />In addition, one of the following options should be followed. <br /> <br />1. We recommend that bioretention design incorporate a waterproofing system lining the <br />bioswale excavation and a subdrain, or other storm drain system, to collect and convey water <br />to an approved outlet. The waterproofing system should cover the bioretention area <br />excavation in such a manner as to reduce the potential for moisture transmission beneath the <br />adjacent improvements. <br />2. Alternatively, and with some risk of movement of adjacent improvements, if infiltration is <br />desired, we recommend the perimeter of the bioretention areas be lined with an HDPE tree <br />root barrier that extends at least 1 foot below the bottom of the bioretention areas/infiltration <br />trenches. <br /> <br />Site improvements located adjacent to bioretention areas that are underlain by baserock, sand, <br />or other imported granular materials, should be designed with a deepened edge that extends to <br />the bottom of the imported material underlying the improvement. <br /> <br />Where adjacent site improvements include buildings greater than three stories, streets steeper <br />than 3 percent, or design elements subject to lateral loads (such as from impact or traffic patterns), <br />additional design considerations may be recommended. If the surface of the bioretention area is <br />depressed, the slope gradient should follow the slope guidelines described in Section 5.8. In <br />addition, although not recommended, if trees are to be planted within bioretention areas, HDPE <br />tree boxes that extend below the bottom of the bioretention system should be installed to reduce <br />potential impact to subdrain systems that may be part of the bioretention area design. For this <br />condition, the waterproofing system should be connected to the HPDE tree box with a waterproof <br />seal. <br />Given the nature of bioretention systems and possible proximity to improvements, we recommend <br />ENGEO be retained to review design plans and provide testing and observation services during <br />the installation of linings, compaction of the filter material, and connection of designed drains. <br /> <br />It should be noted that the contractor is responsible for conducting all excavation and shoring in <br />a manner that does not cause damage to adjacent improvements during construction and future <br />maintenance of the bioretention areas. As with any excavation adj acent to improvements, the <br />contractor should reduce the exposure time such that the improvements are not detrimentally <br />impacted. <br /> <br />5.11 LANDSCAPING CONSIDERATION <br /> <br />We recommend restricting the amount of surface water infiltration near structures, pavements, <br />flatwork, and slabs-on-grade. This may be accomplished by: <br /> <br />• selecting landscaping that requires little or no watering within 3 feet of structures, <br />slabs-on-grade, or pavements. <br />• using low precipitation sprinkler heads. <br />• regulating the amount of water distributed to lawn or planter areas by installing timers on the <br />sprinkler system. <br />• providing surface grades to drain rainfall or landscape watering to appropriate collection <br />systems and away from structures, slabs-on-grade, or pavements.