Loading...
HomeMy WebLinkAboutCT 90-09A; AVIARA PA 5; Review Amended Map Condominium Plan; 1996-02-12i ' >1 !fi -\ 1&. ''* liSOlV£ ENGINEER AN& ENGINEERING ^ '- - ^OtOG/STOF RECORD, AND REVIEW1'' OF AMENDED &AP.AND CONDOMINIUM '<• - PLAW gORAVIARA, PLANNING AREA 5, • - « • ', „ '^ "V , T.% • t r- ' -,. » * 4^,* "'*"H ^II-A' ^* *p ,"* J" -%* ~ PETRA COSTA MESA • SAN DIEGO • PALMDALE February 12, 1996 J.N. 239-95 n n n PETRA GEOTECHNICAL INC. 3185-A Airway Avenue Costa Mesa, CA 92626 Tel: (714) 549-8921 Fax:(714)549-1438 WESTERN PACIFIC HOUSING 18001 Cowan, Suite H Irvine, CA 92714 Attention: Mr. Rich Welter Subject: Soils Engineer and Engineering Geologist of Record, and Review of Amended Map and Condominium Plan for Aviara, Planning Area 5, Lots 95, 96 and 97 of Tract No. 85-35, Carlsbad, California. References: 1) Preliminary Geotechnical Investigation, The Pacific Rim Country Club and Resort, Phase I, Carlsbad, California; report by San Diego Soils Engineering, Inc., dated January 29, 1986. 2) As-Graded Geotechnical Report, Units A through E, Aviara, Carlsbad, California; report by ICG, Inc., dated January 18, 1990. 3) Summary of Geotechnical Evaluation of Existing Alluvium, Lots 95 through 98, Carlsbad Tract 85-35, Northwest Intersection of Alga Road and Batiquitos Drive, Carlsbad, Cali- fornia; report by Petra Geotechnical, Inc., dated May 2, 1995. Dear Mr. Welter: This report is submitted to inform the City of Carlsbad that our firm, Petra Geotechnical, Inc., has been retained by Western Pacific Housing to serve as Soils Engineer and Engineering Geologist of Record with respect to all future work within our purview to be performed within Lots 95 through 97 of Tract No. 85-35, city of Carlsbad, California. The purpose of this report is to also present geotechnical recommendations for additional grading to be performed within the subject lots, and for design of building foundations and associated exterior site improvements. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 2 • •••» TT We have reviewed the referenced geotechnical reports prepared by San Diego r? Soils Engineering, Inc., and ICG, Inc. (Reference Nos. 1 and 2) with respect to as-graded soil and geologic conditions within the subject lots and we generally n, concur with their findings, conclusions and recommendations except as modified herein based on the proposed construction. We have also reviewed the -" enclosed "Tentative Map and Condominium Plan" (originally dated August 14, 1995 and revised on October 19, 1995) for the subject lots prepared by «*t ?f Hunsaker & Associates, Inc. (Plate 1). |? LOCATION AND SITE DESCRIPTION i Lots 95 and 96 are located to the northeast of the intersection of Hummingbird "i. Drive and Batiquitos Drive within the city of Carlsbad, California while Lot 97 is located to the southeast of this intersection. Lot 95 is an approximately 30- •?«• ••• to 50-foot high, descending fill-over-natural slope bordering the easterly side of Lot 96. Lots 96 and 97 are relatively level mass-graded pads.55 The descending slope (Lot 95) inclines at variable slope ratios that range up to a maximum of 2:1, horizontal to vertical. The slope is covered with a moderate to locally thick growth of native and landscape vegetation consisting of a variety of groundcover plants, shrubs and trees. No evidence of significant erosion or 7 gross instability was observed on this slope or other slopes bordering the subject lots. -m The level surface of Lot 96, located on the north side of Hummingbird Drive, has been recently bladed and is void of significant vegetation; however, the blading has resulted in a long windrow of soil containing significant organic Ml debris. This windrow is located at an approximate distance of 100 feet from the PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 3 .** top of the easterly descending slope. A temporary desilting basin also exists«*» *m within Lot 96 at the southeasterly end of the pad, adjacent to Hummingbird •«* Drive. The bottom of the basin is covered with dense vegetation and contains --** ri an accumulation of wet, soft sediments of silt. Deep erosion gullies up to 3+. feet deep were observed on Lot 96. The erosion gullies are approximately 100-.*& " to 175 feet long and extend across the pad surface in a generally north to south direction, terminating at the desilting basin. r»:••<m The level surface of Lot 97, located on the south side of Hummingbird Drive, fl is covered with a sparse to locally moderate growth of weeds. A temporary desilting basin is also located at the southeasterly end of the Lot 97. The *f, bottom of the basin is covered with dense vegetation and contains an accum- ulation of wet, soft sediments of silt. A deep erosion gully up to 2+. feet deep !„ was observed on this pad. The erosion gully is approximately 175 feet long and extends across the pad surface in a generally northwest to southeast direction 1 and terminates at the desilting basin. BACKGROUND INFORMATION Our review of the report by ICG, Inc. (Reference No. 2) indicates that mass grading was performed between April and November of 1991. Mass grading -i* within Lot 95 consisted of constructing the fill-over-natural slope. A 20- to 30- **foot-wide fill key was constructed near the mid-point of the slope to provide -ni support of the fill portion. This fill key is shown on the enclosed plan, Plate 1. *m The fill portion of the slope was constructed at a maximum gradient of 2:1, M horizontal to vertical. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 4 Lot 96 was mass-graded as a large cut-to-fill transition pad. The fill materials were placed within the eastern portion of the pad whereas bedrock materials are exposed within the western portion. The maximum depth of fill placed is approximately 30 feet within the southeast portion of the lot. The bedrock materials exposed within the cut portions of the pad consist of alternating layers of siltstone, claystone and sandstone belonging to both the Delmar and Torrey Sandstone Formations. Lot 97 is also a cut-to-fill transition pad with fill materials underlying the easterly and southwesterly portions of the level pad. The fill materials along the eastern side of the lot are associated with an approximately 20-foot-wide top-of-slope fill key. This fill key was constructed in order to remove existing unsuitable slopewash materials located along the top of the easterly adjacent descending slope. The location of this fill key is shown on the enclosed plan, Plate 1. The maximum depth of fill placed within the lot is approximately 15 feet within the southeast corner of the lot. In a similar manner as Lot 96, the bedrock materials exposed within the cut portions of Lot 97 consist of alter- nating layers of siltstone, claystone and sandstone belonging to both the Delmar and Torrey Sandstone Formations. The geologic units underlying the site are shown on the enclosed site plan, Plate 1. REVIEW OF TENTATIVE PLAN Proposed Lot Number Changes The accompanying "Tentative Map and Condominium Plan" prepared by Hun- saker & Associates, Plate 1, indicates that 57 condominium units and a recre- ation area will be constructed within Lots 96 and 97. Lot 95 is designated as "Open Space" and will remain in its present condition. Lot numbers will be changed as follows: PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 5 *» f— ! "" 1. Lot 95: The descending slope on the easterly side of Lot 96 will remain unchanged and will be designated as "Open Space" Lots 29 through 33. '«• 2. Lot 96: Lot 96 is to be subdivided into seven lots designated as Lots 15 P through 21. Lots 15 through 20 will contain 34 condominium residential ^ units numbered 24 through 57, inclusive. Lot 21 is a recreation area. F""" 3. Lot 97: Lot 97 is to be subdivided into six lots designated as Lots 1 * through 4, 23 and 24. Lots 1 through 4 will contain 23 condominium "* residential units numbered 1 through 23, inclusive. Lots 23 and 24 will remain unchanged and are designated as "Open Space." These lots *m, represent the descending slope located on the northeasterly side of future '~ Lots 3 and 4. „ The comments and recommendations provided in the remainder of this report "* refer to the new lot numbers and residential unit numbers. Proposed Construction Building plans are not available for review; however, it is expected that the condominium units will one- and/or two-stories in height, and of woodframe construction with first floor slabs-on-grade. Other improvements proposed within the site consist of private access streets, concrete driveway areas, sidewalks, private patios, fences, and underground utilities. In addition, an approximately 3- to 5-foot-high retaining wall is proposed near the southerly property line of future Lot 2. The proposed construction within the recreation area (Lot 21) is not shown on the enclosed plan. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 6 Proposed Grading £ Maximum cuts and fills ranging up to maximum depths of approximately m and 5l/2 feet, respectively, are proposed to achieve finish grade elevations within P Lots 15 through 21. Cuts of up to approximately 3 feet deep are also proposed m at the base of the existing ascending slopes on the north side of Lots 16 and 17, U and on the west side of Lots 15 and 16. These cuts will result in maximum <•• slopes heights of approximately 9 to 12 feet. Minor 1- to 2-foot-high slopes will *4 also be constructed between some of the individual condominium building pads. at Within Lots 1 through 4, the majority of the proposed cuts and fills will range <•" from approximately 2 to 3 feet deep; however, deeper cuts and fills are **• proposed within two local areas. Within the southeasterly corner of Lot 3, fills *** of up to 8 feet deep are proposed, and a cut of up to approximately 11 feet ^ deep is proposed within the southwesterly portion of Lot 2. Approximately 2- '" to SV^-foot-high, 2:1 cut slopes are proposed along the north side of Lot 1, and on the west sides of Lots 1 and 2. Slopes ranging from approximately 1 to 6 ~ feet in height will also be constructed between some of the individual con- dominium building pads. The height of the slope that presently descends to Hummingbird Drive on the northeasterly sides of Lots 3 and 4 will be increased by approximately 6 feet. •m CONCLUSIONS AND RECOMMENDATIONS M General•*» *" From a soils engineering and engineering geologic point of view, the subject " property is considered suited for the proposed development provided the *" following conclusions and recommendations are incorporated into the design criteria and project specifications. Furthermore, based on our review of the m PACIFIC WESTERN HOUSING February 12, 1996 r- J.N. 239-95 Page 7 M •* referenced reports by San Diego Engineering and ICG, we generally concur J^ with their findings, conclusions and recommendations with respect to as-graded ;"* subsurface soil and geologic conditions. mr Slope Stability <• f~ The upper portion of the slope within Lots 29 through 33 (existing Lot 95) is '— underlain by compacted fill materials while the lower portion consists of p bedrock materials of the Delmar Formation. Similar geologic conditions exist m on the slope that descends from Lots 3 and 4 (Existing Lot 97) to Hummingbird £| Drive. The cut slopes proposed along the north sides of Lots 1, 16 and 17, and — the west sides of Lots 1, 15 and 16 will expose bedrock materials of the Torrey jf! Sandstone Formation. The reports by San Diego Soils Engineering and ICGi „» indicate that the bedrock materials within the site dip to the west and southwest p. at inclinations of approximately 3 to 5 degrees. These bedding attitudes are — considered to be favorable with respect to the gross stability of existing and I?proposed cut slopes. .M* - During our recent site reconnaissance, no evidence of surficial slumping, erosion •• or ravelling was observed on the slopes surrounding the lots. In addition, the slopes are covered with a moderate to occasionally thick growth of native ** vegetation and landscape vegetation. Based on these conditions, the existing slopes are considered to be and surficially stable. Provided new cut and fill slopes constructed within the site are properly landscaped upon completion of grading, and all slopes are properly maintained during the lifetime of the project, all slopes are expected to remain surficially stable. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 8 .,*) Potential Effect of Proposed <* Construction on Adjoining Properties r* It is our opinion that the proposed construction and grading will not adversely £ affect the stability of adjoining properties provided that grading and construc- " tion are performed in accordance with the recommendations presented herein. r~ • """ Earthwork ft General Earthwork and Grading Specifications "*" All earthwork and grading should be performed in accordance with the follow- ffp ing recommendations prepared by this firm and with all applicable requirements *** of the municipal grading and excavation codes of the City of Carlsbad, Cal- 't** ifornia. •an !* Site Clearing •AM* Jf Prior to any grading, all significant vegetation existing within the temporary desilting basins and within the level areas of the subject lots should be stripped "; and removed from the site. The windrow of soil existing within Lot 96 contains significant organic debris and should also be removed from the site. ^ Desilting Basins All soft and wet sediments of silt should be removed to competent bearing ** materials from the bottoms of the desilting basins. The excavated materials ** may be reused for compacted fill provided they are first cleared of organic materials and air-dried to achieve near optimum moisture conditions. Required " depths of removal will have to be determined during grading; however, it is •ttf anticipated the removals may possibly be on the order of 4 to 5 feet. During PACIFIC WESTERN HOUSING February 12, 1996 ^ J.N. 239-95 Page 9 <4ttl r placement of fill within the basins, the fills should be benched into competent * materials on all sides of the basins. '• n Repair of Erosion Gullies The erosion gullies should be repaired by overexcavating the surrounding grades Q equal to the depths of erosion. The overexcavated areas should then be brought m to finish grades with properly compacted fill. .« Capped Building Pads R Cut-to-fill transitions presently exist within the large mass-graded lots, and ~ additional cut-to-fill transitions Wy^created within several individual building p"* pads as a result of the planned grading. Furthermore, onsite bedrock materials J! consist of alternating layers of nonexpansive sandstone, and moderately to •I*" highly expansive siltstone and claystone. Based on the above soil and bedrock p conditions, it is recommended that all building pads be underlain with a min- "~ imum 3-foot-thick compacted fill cap. The compacted fill caps will provide -r uniform soil bearing conditions and minimize distress to the buildings and other structural improvements related to the detrimental effects of excessive differen- tial settlement and/or expansion. Based on our review of the proposed grading, the majority of the building pads that are located outside the limits of existing ***< compacted fill will require overexcavation. The compacted fill caps should be constructed by overexcavating existing grades to a minimum depth of 3 feet at "] below planned finish grades and then replacing the excavated soil and/or bed- •«* rock materials as properly compacted fill. Horizontal limits of overexcavation should extend to a distance of at least 5 feet beyond perimeter building lines. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 10 Preparation of Existing Ground Within areas to receive fill that are located outside the limits of recommended overexcavation, the existing ground surface should first be scarified to a depth of 8 to 12 inches, watered or air-dried as necessary to achieve a uniform moisture content slightly over optimum moisture conditions, and then recom- pacted in-place to a minimum relative compaction of 90 percent. This pro- cedure should also be followed within building areas to remain at existing grade, and within building areas underlain with existing fill to be cut to grade where the depth of cut is less than 12 inches. Fill Placement All fills should be placed in lifts no greater than 8 inches in thickness, watered or air dried as necessary to achieve a uniform moisture slightly over optimum moisture conditions, and then compacted in place to a minimum relative com- paction of 90 percent. Each fill lift should be treated in a similar manner. Subsequent lifts should not be placed until the preceding Lift has been approved by a representative of this firm. In order to create fill caps that possess a relatively uniform texture and ho- mogenous expansion potential, the different soil and bedrock materials should be thoroughly blended prior to being placed as compacted fill. The laboratory maximum dry density and optimum moisture content for each change in soil type should be determined in accordance with Test Method ASTM D 1557-91. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 11 Geotechnical Observations Exposed bottom surfaces in each removal area should be observed and ap- proved by a representative of this firm prior to placing fills. A representative of this firm should also be present onsite during grading operations to verify proper placement and compaction of all fill, as well as to verify compliance with the other recommendations presented herein. Post-Grading Considerations Site Drainage Positive drainage devices such as sloping concrete flatwork, graded swales, and area drains should be provided around each residence to collect and direct all surface waters to the adjacent streets. Neither rain or excess irrigation water should be allowed to collect or pond against building walls or foundations, or within the level areas of the lots. Utility Trenches All utility trench backfill should be compacted to a minimum relative compact- tin of 90 percent. Where onsite soils and bedrock materials are utilized as backfill, mechanical compaction will be required. Density testing, along with probing, should be performed by a representative of this firm to verify adequate compaction. As an alternative for shallow trenches where pipe might be damaged by me- chanical compaction equipment, such as under building slabs, imported clean sand having a Sand Equivalent value of 30 or greater may be used for backfill PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 12 and flooded or jetted into place. No specific relative compaction will be required; however, observation, probing, and if deemed necessary, testing should be performed by a representative of this firm to verify an adequate degree of compaction. Foundation Design Recommendations Proposed Structural Setbacks The site plan indicates that a minimum horizontal setback of 10 feet will be maintained between the residential structures and the tops and toes of adjacent slopes. Provided all slopes are properly landscaped and maintained thereafter as recommended herein, this minimum setback is considered acceptable from a geotechnical point of view. Allowable Bearing Capacities An allowable bearing value of 1500 pounds per square foot may be used for design of shallow pad footings and 12-inch-wide continuous footings founded at a minimum depth of 12 inches below the lowest adjacent final grade. This value may be increased by 20 percent for each additional foot of width and depth, to a maximum value of 2500 pounds per square foot. These recom- mended allowable bearing values include both dead and live loads, and may be increased by one-third when designing for short duration loads such as wind and seismic forces. Settlement Under the above bearing values, total settlement of footings is expected to be less than 3/4 inches, and differential settlement between adjacent footings is PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 13 expected to be less than 1/4 of an inch. The majority of the anticipated settle- ment is expected to take place during construction as the loads are applied. Lateral Resistance A passive earth pressure of 250 pounds per square foot per foot of depth up to a maximum value of 2500 pounds per square foot may be used to determine lateral bearing for footings. A coefficient of friction of 0.40 times the dead load forces may also be used between concrete and the supporting soils to determine lateral sliding resistance. An increase of one-third of the above values may also be used when designing for short duration wind and seismic forces. fi•*• Footing Observations All footing trenches should be observed by a representative of this firm to verify that they have been excavated into competent bearing soils. These observations should be performed prior to placement of forms or reinforcement. The exca- vations should be trimmed neat, level and square. All loose, sloughed or moisture-softened materials and any debris should be removed prior to placing concrete. Excavated soils derived from footing and utility trenches should not be placed in slab-on-grade areas unless they are compacted to at least 90 percent of maximum dry density. Expansive Soil Considerations The results of laboratory tests performed by ICG, Inc. (Reference No. 2) indi- cate onsite fill and bedrock materials exhibit Expansion Potentials that range PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 14 from Very Low to High as determined in accordance with UBC Standard Test No. 29-2. It is anticipated that after the dissimilar earth materials are blended and then replaced as properly compacted fill as recommended herein, the re- suiting subgrade soils will exhibit a Medium Expansion Potential. Therefore, minimum design recommendations for footings and residential floor slabs for this condition are presented below. However, these recommendations should be considered preliminary and should be confirmed by performing laboratory tests of the actual subgrade soils upon completion of grading. Based on the results of these laboratory tests, revised recommendations may be required. Furthermore, additional slab thickness, footing size and/or reinforcement may be necessary for structural considerations as determined by the project architect and/or structural engineer. 1. All exterior footings for both one-story and two-story construction should be founded at a minimum depth of 18 inches below the lowest adjacent final grade. Interior footings may be founded at a minimum depth of 12 inches below the lowest adjacent final grade. 2. All continuous footings should be reinforced with two No. 4 bars, one top and one bottom. , 3. Exterior isolated pad footings intended for support of roof overhangs such as patio covers should be a minimum of 24 inches square, and founded at a minimum depth of 18 inches below the lowest adjacent final grade. The pad footings should be reinforced with No. 4 bars spaced 18 inches on centers, both ways, near the bottoms of the footings. 4. Living area concrete floor slabs should be a full 4 inches thick, and reinforced with 6-inch by 6-inch, No. 10 by No. 10 welded wire mesh, or with No. 3 bars spaced 24 inches on centers, both ways. All slab rein- ^ PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 15 .^m forcement should be supported on concrete chairs or brick to ensure the ••Ml p desired placement near mid depth. « 5. Living area floor slabs should be underlain with a moisture vapor barrier r~*consisting of a polyvinyl chloride membrane such as 6-mil Visqueen, or .* equivalent. At least 2 inches of clean sand should be placed over the membrane to promote uniform curing of the concrete. *ir, 6. Garage floor slabs should be a full 4 inches thick and reinforced in a i" similar manner as living area slabs. Garage floor slabs should also be *•p poured separately from adjacent wall footings with a positive separation * maintained with 3/8-inch minimum felt expansion joint materials, and then »4W r? quartered with weakened plane joints. A 12-inch-wide by 18-inch-deep i grade beam should be provided across garage entrances. The grade beam •m *T should be reinforced with two No. 4 bars, one top and one bottom. 8. Prior to placing concrete, the subgrade below all living area and garage *floor slabs should be presoaked to achieve a moisture content that is 5 percent or greater above optimum moisture content. This moisture con- tent should penetrate to a minimum depth of 18 inches into the subgrade. Soluble Sulfate Analysis ***Laboratory tests performed by ICG, Inc. indicate that the majority of the onsite earth materials contain less than 0.10 percent water soluble sulfates. Therefore,-m Type II cement may tentatively be considered for concrete placed in contact **»with onsite soils. However, additional laboratory tests should be performed at X the completion of grading to verify the soluble sulfate content of the soils exposed at finish pad grade. Based on these test results, revised recommen- <$& dations may be required. - PACIFIC WESTERN HOUSING February 12, 1996 ^ J.N. 239-95 Page 16 at Retaining Wall Desism Recommendations 4* V*Footing Embedments * Footings for the retaining wall proposed near the southern property line of Lot 2 should be founded at a depth that will provide a minimum horizontal clear- ^ ance of 5 feet between the outside bottom edge of the footing and the face of the slope. •M f* "* Allowable Bearing Capacity and Lateral Resistance m f? Retaining wall footings may be designed using the appropriate allowable bear- — ing and lateral resistance values recommended for residential building footings; f| however, when calculating passive resistance, the upper 6 inches of the footings — should be ignored. !t» ••m .m Active Earth Pressures ~& Active earth pressures equivalent to a fluid having densities of 45 and 75 ,** pounds per cubic foot should be used for design of cantilevered walls retaining a level backfill and ascending 2:1 backfill, respectively. .**» Drainage •mi A pipe and gravel subdrain should be installed behind all retaining walls to m prevent entrapment of water in the backfill. Perforated pipe should consist of 4-inch minimum diameter PVC Schedule 40, or ABS SDR-35, with the perfora- ^ tions laid down. The pipe should be encased in a 1-foot-wide column of 3/4-inch to !1/2-inch open-graded gravel extending above the wall footings to a minimum „ height equal to one-third the wall height, or to a minimum height of 1.5 feet above the footings, whichever is greater. The gravel should be completely PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 17 wrapped in filter fabric consisting of Mirafi 140N, or equivalent. Solid outlet pipes should be connected to the subdrains and then routed to a suitable discharge area. Backfill The recommended active earth pressures are based on the physical and me- chanical properties of the onsite soils which may be used for wall backfill. All retaining wall backfill should be placed in 6- to 8-inch-thick horizontal lifts, watered or air dried as necessary to achieve near optimum moisture conditions, and compacted in-place to a minimum relative compaction of 90 percent. Flooding or jetting of backfill materials should be avoided. Observations, probing and testing should be performed by a representative of this firm to verify adequate compaction of all wall backfill. Temporary Slopes To facilitate construction of retaining walls, temporary slopes less than approximately 4 to 5 feet high may be cut vertical. However, where excavations exceed these heights, the upper portions should be maintained at a maximum gradient of 1:1, horizontal to vertical. Temporary slopes constructed at the above configurations are expected to remain stable during construction of the retaining walls; however, all temporary slopes should be observed by a repre- sentative of this firm for any evidence of potential instability. Depending on the results of these observations, revised temporary slope configurations may be necessary. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 18 Exterior Concrete Flatwork Thickness and Joint Spacing To reduce the potential of unsightly cracking related to the effects of mod- erately expansive soils, concrete sidewalks and patio-type slabs should be at least 4 inches thick and provided with construction joints or expansion joints every 6 feet or less. Concrete paving for all major access driveways within the development should be at least 5 inches thick and provided with construction joints or expansion joints every 10 feet or less. Concrete driveway slabs at individual garage entrances and in parking stalls should be at least 4 inches thick and also provided with construction joints or expansion joints every 10 feet or less. Reinforcement Consideration should be given to reinforcing all concrete patio-type slabs, driveways and sidewalks greater than 5 feet in width with 6-inch by 6-inch, No. 10 by No. 10 welded wire fabric, or with No. 3 bars spaced 24 inches on centers, both ways. The reinforcement should be positioned near the middle of the slabs by means of concrete chairs or brick. Edge Beams (Optional) Where the outer edges of concrete flatwork are to be bordered by landscaping, consideration should be given to the use of edge beams (thickened edges) to prevent excessive infiltration and accumulation of water under the slabs. Edge beams, if used, should be 6 to 8 inches wide, extend 8 inches below the tops of the finish slab surfaces, and be reinforced with a minimum of two No. 4 bars, PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 19 m one top and one bottom. Edge beams are not mandatory; however, their inclu- * sion in flatwork construction adjacent to landscaped areas will significantly reduce the potential for vertical and horizontal movements and subsequent * cracking of the flatwork related to the effects of high uplift forces that can "" develop in expansive soils. Perimeter Subdrains Along Major Driveways p Where the perimeter edges of major driveways are bordered by landscaping, ' m perforated pipe and gravel subdrains should be considered behind the curbs to f? collect water that may accumulate below the slabs. The subdrains should be — connected to local area drains or storm drain inlets, or outletted to the curb f| face to dispose of the water. Perforated pipe should consist of 4-inch diameter « ABS SDR-35, or PVC Schedule 40 with the perforations laid down. The pipe «i, should be embedded in one cubic foot per running foot of 3/4 to !1/2-inch open- « graded gravel wrapped in filter fabric. Filter fabric should consist of Mirafi '„ 140N, or equivalent. The bottoms of the subdrain trenches should extend to a depth of at least 6 inches below the bottoms of the slabs or 6 inches below any underlying base materials, whichever is greater. The upper 8 to 12 inches of the trenches should be backfilled with compacted native soils to provide a relatively impermeable cap above the gravel. Ml Tree Wells K4 Tree wells are not recommended in concrete flatwork areas since they intro- duce excessive water into the subgrade soils. m PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 20b — *" Subgrade Preparation m ?»» As a further measure to minimize cracking and/or shifting of concrete flatwork, '*» the subgrade soils below concrete flatwork areas should be compacted to a min- m f~» imum relative compaction of 90 percent and then thoroughly moistened prior **to placing concrete. The moisture content of the soils should be 5 percent or "~ greater above optimum moisture content and penetrate to a depth of approx- -*» imately 12 inches into the subgrade. Flooding or ponding of the subgrade is not f? considered feasible to achieve the above moisture conditions since this methodsi» — would likely require construction of numerous earth berms to contain the water. f| Therefore, moisture conditioning should be achieved with sprinklers or a light — spray applied to the subgrade over a period of several days just prior to pouring f| concrete. A representative of this firm should observe and verify the density — and moisture content of the soils, and the depth of moisture penetration prior i'4 to pouring concrete. m * Planters .* The surface of the ground in all planter areas located within 5 feet of building foundations, should be maintained at a minimum gradient of 2 percent to miti- gate excessive infiltration of water into the foundation soils. "fli .«•Preliminary Structural Pavement Sections ** ^ Based on the physical characteristics of onsite soil and bedrock materials, an R- Value of 40 was estimated in order to provide preliminary structural pavement .„ sections for streets within the site. In addition, a Traffic Index of 5 was used for the proposed interior residential streets within the property. Based on this * Traffic Index value and an estimated R-Value of 40, the structural pavement section presented below in Table A was computed in accordance with State of PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 21 California design procedures. The pavement section consists of hot mix asphalt (HMA) underlain by a suitable aggregate base (AB). Since grading will be per- formed within the site, this pavement section should be considered tentative and verified by testing of actual subgrade soils existing at grade at the completion of grading. In addition, it should be noted that the pavement section thickness will need to be approved by the governing agency. TABLE A Road Description Interior Streets Design T.I. 5.0 R-Value 40 Thickness (in.) HMA 3.0 AB 4.0 Prior to placing aggregate base, the subgrade soils should be compacted to at least 90 percent relative compaction. Final subgrade compaction should be performed just prior to placing aggregate base, and after all utility trench backfills have been compacted and then tested by the project geotechnical consultant to verify adequate compaction. Aggregate base materials should be either Crushed Aggregate Base, Crushed Miscellaneous Base, or Processed Miscellaneous Base conforming to Section 200-2 of the Standard Specifications for Public Works Construction (Green- book). The materials should be brought to a uniform moisture near optimum moisture, and then compacted to at least 95 percent of the maximum density determined in accordance with California Test Method No. 216. Asphaltic concrete materials and construction should conform to Section 203 of the Greenbook. PACIFIC WESTERN HOUSING February 12, 1996 J.N. 239-95 Page 22 Future Improvements and/or Grading If additional site improvements are considered in the future other than those discussed herein, our firm should be notified so that we may provide design recommendations to minimize movement, settlement and/or tilting of the struc- tures. Design considerations are particularly critical where future structures may be planned on or near the tops of descending slopes. Potential problems can develop when drainage on the pads is altered in any way such as by placement of fill and construction of new retaining walls, walk- ways, patios, landscape walls and planters. Therefore, it is recommended that this firm be engaged to review the final design drawings, specifications and grading plan prior to any new construction. If we are not provided the oppor- tunity to review these documents with respect to the geotechnical aspects of new construction and grading, we can take no responsibility for misinterpre- tation of our recommendations presented herein. This opportunity to be of service is sincerely appreciated. Please call if you have any questions pertaining to this report. Respectfully submitted, PETRA GEOTECHNICAL, INC. '©'avid Hansen Senior Staff Engineer Robert W. Ruff Principal Geologist CEG 1165 DH/RWR/SJ/nls cc 1996\200V239-9SA.LTH Siamak Jafroudi Principal GE 2024