LANDBLIOE HAZARD EVALUATION PORTIONS OF BHOADMOOH RESIDENTIAL RESORT PHASE I. INCLUDING W, PHASES ll AND Ill VILLAS RT HOLLOW. ESTATES LOT 9 COLORADO SPRINGS. COLORADO Prepared For: COG LAND a DEVELOPMENT COMPANY 1 0 Lake Colorado Colorado 80906 Job No. cs-nzz April 11,1997 INC. CONSULTING ENGINEERS 52w MARK manna BLVD I COLORADO SPRINGS. COLORADO name I (719)525-naua I TABIE OF CONTENTS SCOPE SITE CONDITIONS DEVELOPMENT GEOLOGIC CONDITIONS INVESTTGATION SUBSUFIFACE CONDITIONS STABILITY ANALYSIS SUMMARY CONCLUSIONS RECOMMENDATIONS LIMITATIONS FIG. 1 - LOCATION OF EXPLORATOHY BORINGS a. CROSS SECTION LOCATIONS FIG. 2 - DEPTH TO CLAYSTONE FIG. 3 . ESTIMATED CONTOURS OF EOUAL SHALE ELEVATION FIGS. 4 TI-IRU - SURFACE II. SUSSURFACE CROSS SECTION USED FOR STABIUTY ANALYSIS FIGS. 7 THFIU 11 - LOGS OF EXPLORATOHY SORINGS FIGS. 12 THRU 22 - SWELL CONSOLIDATION TEST RESULTS FIGS. 23 THRU 26 - GRADATION TEST RESULTS FIGS. 27 AND 25 - DIRECT SHEAR TEST RESULTS TABLE I - SUMMARY OF LABORATORY TEST RESULTS APPENDIX A - VILLAS AT LOG HOLLOW APPENDIX - SUMMARY OF STABILITY ANALYSIS LOT 9. BROADMOOR RESIDENTIAL COMMUNITY APPENDIX - SUMMARY OF STABILITY ANALYSIS, BARONS RIDGE APPENDIX - DIRECT SHEAR TESTING AND ACCOMPANYING BORING LOGS BROADMOOR SOUTH GOLF COURSE APPENDIX - SUMMARY OF STABILITY ANALYSIS, THIS REPORT COD LAND 1. DEVELOPMENT COMPANY CTUT CS-TJH 11 12 12 fill SCOPE This report presents the results of our analysis to evaluate the potential tor reactivation of a portion of a historic landslide and the stability of Individual slopes contained within devalopments to be located at the foot of Cheyenne Mountain between the Cheyenne Mountain Zoo and the base of the abandoned Broadmoor ski area In southwestern Colorado Springs, Colorado. The purpose of our investigation was to devaiop subsurface pro?les to perform analysis to aid the Colorado Geologic Survay (005) In their evaluation of projects within the Phase I area of the Broadmoor Fieaidntlal Community as discussed In a letter dated February 10. 1997 (File No. EP- 9743025). It has also been prepared to provide more data In accordance with the City of Colorado Springs Geologic Hazards Ordinance (96-74). This report is meant to supplement and combine previous evaluations we made within the area. Our analysis was conducted using conditions encountered In exploratory borings, the proposed development plan. laboratory testing, engineering analyses. and our experience. Changes In the scope of the development or by conditions disclosed by additional studies may make revisions In some at the conclusions and recommendations contained in this report appropriate. We should be comected to reassess our conclusions and recommendatkms It changes are made. SITE CONDITIONS The area of evaluation lays south oi the Cheyenne Mountain Zoo and north of the abandoned Broadmoor ski area. The devalopment Is also west of the Broadmoor South Coil CouraeL?enerrilu agape _a pipeline cgnnacting Fiaoltora and Reservoirs. The Investigated developments Include Lot 9 ol the Estate Lots of the Broedmoor Fleeort Community. Phases II and of the Villas at Log Hollow, and the Barons Ridge cluster home development. Figure 1 shows the various developments evaluated. aura-7:22 1 lei? Surface topography within the developments and surrounding grounds are diverse. The steeper topography exists within the eastern portion of the site and to the West of the Villas. Ground slopes In the eatern portion of Lot 9 of the estates are probably the steepest, being on the order of 2:1 (horizontal to vertical). Slopes between Lot 9 and through the eastern one-third of Lot 3 at Barons Ridge average about 3:1. Slopes in the eastern portions of Lots 4 and 5 at Barons Ridge are also about 3:1. The front (western) portions of Lot 9 and Lots 4 and 5 are comparativaiy fiat. There are some fairly steep flit slopes along the rear of lots located along the south side of Barons Ridge. These men-made fill slopes are at ratios of about 1:1 and are boulder faced. These areas are designated as "no build". The ?ll were placed under "controlled conditions?, replacing random ?ila placed during the life of the ski area. Relatively flat terracing has been graded to the front of these lots to accommodate "walk-out" baaemnta. The ground surface within much of the Villas at Log Hollow ts relatively flat to gently sloping. Slopes steepen to the west of Phases it and The grown! to the east of the Villasl between Phases It and are btween about 3:1 and 4:1 based upon topographic mapping provided. A predominant ridge and knob also exists In this area. Slopes to the west of the Barons Ridge development steepen to about 3:1. A mi-loot high out slope was made along the west side of Stone Manor Heights. north and south of Governors Point. The slope existed at ratios between sooth 0.5:1 and 1:1 and was buttressed with +a-foot diameter boulders. A predominant drainage llowed south of the Villas and the Barons Ridge developments. "Debris ?ow" In this drainage have a potential to impact the building pads within the southern portion of the Villas. The drainage flows easterly eventually bisectln?g the Rroadmoor South golf Course Into north and south One-halves. There are numerous boulder exposed and nested in this drainage. Mitigation of the hazards associated with the debris flow has been addressed lit a report prepared by Flo Engineering. cunts-rm 2 I Vegetation across the developments Include mature to pines. Scrub oak and similar shrubs are common. Cobbles and boulders are scattered across the surface. Grasses common to low mountain regions are also common. Remnants of the Old Ftotten Log Hollow picnic area once located surrounding the Intersection of Stone Manor and Governors Point have been removed. DEVELOPMENT The ovarell area to being developed for construction of relatively large. executiva-style, detached. single-family residences, smaller but luxurious cluster or pelio homes and two unit attached structures oflarge size. Respectively, the subdivisions are reterred to as Estates at Brosdmoor Fiaaort Community. Barons Ridge. and the "Miles at Log Hollow. The limits oi the various communities are shown on Fig. 1. The buildings are expented to be multi-leval ranging from ranch-style and two-story with walk-out. garden level and lull?depth basements. The ovarell concepts for grading and building siting has been to minimize disturbance ?tting streets and buildings to the topography. At the time this report was prepared. grading for the malor access road (Stone Manor Heights) had been completed, utilities Installed and roadways paved. Also developed was Barons Ridge and the Phase I area of the Villas at Log Hollow. The most signi?cant cute made occur along Stone Manor Heights. north and south of Governors Point and below Let 3 of Barons Ridge on Charles Grove. These cute Were up to 15-foot high and faced with large, dry, stacked boulders. Slopes more than about 10 feet high were laid back to ratios of about 1:1 (horizontal to vertical}. with slopes with heights less than 10 last being as step as 0.5:1. Figure 1 shows the building layouts and the grading plan. GEOLOGIC CONDITIONS Geologic conditions based upon a review of aerial photography and field reconnaissance were depicted In a report we prepared dated September 6. 1995 momammsmawm mes-7:22 3 {Job No. CS-SBOT). conditions relative to this development inciude historic landslide and debris ?ow deposits. Debris tlow deposits are contained within the area identified as ice containing the historic landslide. Storm events removed some of the landslide material and replaced them with debris flow material. Figure 1 shows the mapped content of these geologic conditions. Subsurface Investigations concluded Indicate landslide deposits consist of cleystone subject to varying degrees of weathering and disturbance. Granular soils were often found lnterlayered with the weathered claystone. The materials may have weathered to a clay-like soil. The relative stiffness or density of the materials ls erratic. both vertically and laterally. Testing Indicates the materials to be moderately to highly plastic. Investigations of the South Golf Coure we have conducted ovar the last five years indicate deep seated failures occur near the surface of the intact Pierre shale. This contact ls normally very distinctive with the shale being fissile and moist as We" as very hard. Where the upper shale is displacedI it Is claystone-tllre, having higher moisture and being softer than the underlying "intact" materials. Groundwater also commonly occurs near this interface. Localized slope failures have occurred within the oleyslone where shale ls deep and prolong wetting ofthe surficlal material occurs. Failures have occurred on slopes having ratios as ?at as 3:1 (horizontal to vertical} where irrigation of high maintenance turf grass has occurred at the head and a toe of slope for 10+ year period. These types of failures hve occurred not only within the area containing the historic landslide but throughout the City of Colorado Springs. in comparison. slopes that have not been subjcted to long-term Wetting lrom the surface have remained stable even at ratios steeper than 2:1. Debris flow material found consisted of erratic combinations of clay, silt, sand. gravel. cobbles and boulders. The cells inciude "slope wash" materials, predominantly consisting of finer grain soils such as clay and sand or occ- urm aochOMpm mam-rm a fragments. These materials often have elevated moisture and varying stiffness or density. Alluviume predominantly Including silty and clayey sand and gravel with cobbles and boulders occur north of Governor: Point. The alluvluma normally have lower moisture contents than the debris flow and slope wash soils. The alluviums are typically more dense or still than the slope wash soils based upon penetration resistance tests. INVESTIGATION Portions of the historic landslide at elevations lower than or to the east of the devalopment hve reactivated. There have also been failures within the "claystone" that comprises much of the historic landslide debris. .lhe ?rst notable__ yell-rent was In 1933 when a portion of the ?roadmoor South Gail Coursgso?ut-h rel?the Ester bisecting drainage ot lateral ground movement._ Vie have been Invaluad In the evaluation and stabilization of the gall course since that time. Flevlewers of the Colorado Geologic Survey have been brleied on our consultation and the subsequent mitigation procedures. Since that information Is voluminous, it shall not be discussed in detail In this report. A summary conclusion Is that Intense Irrigation and poor surtsce drainage over 15: {rears-reth in. deep wet-tingoltite, subsoils. resulting In softening ot the landslide deposits and causing a significant rise or tits?groundwater table. At the base or the gait course, high. steep slopes ?over shallow shale allowed dong-term plastic eerormetlon and large strains '(horizontel movement}, resulting In development of very low of the claystone (landslide deposits] at the shale surface. knu?ng_thew?ht?zr El 1.5591-1 Daft. installed below portions of the golf course exhibiting evidence of major movement. Where 't-tralhs were installed: we saw no signs o-I surlcha-Iumovement during the spring of 1995, even though a 30-year precipitation event occurred during that spring. Some portions of the golf course where horizantal drains were not Installed movad. Including an area north of the major bisectan drainsgeway. This failure was considered to be a localized rotational failure, resulting from prolonged (153i; years) Emu?rm 5 pending of Water at the toe and crest ot a relatively steep slope. The charactristics of a deep seated movement indicative of reactivation of the landslide mass was not evident. Much of the residential development below the golf course Is contained In the same landslide complex and Is 10+ years old. We are not aware of any reported major slope failures prior to the spring of 1995. We are aware of two localized (rotational) failures that occurred well east of the golf course as wall as a failure to the south. wall outside the mapped historic landslide complex. We also Invastigsted six significant siopo failures throughout the City that occurred In the spring of 1995. In all of these cases. prolonged Wetting occurred at either the toe or crest. or both. In some Instances. the tee of the slope was steepened by cutting or erosion. . .s As part of our evaluation of the Broadmoor South Golf Course. wa drilled three boringe at the base of the Broadmoor ski area. One hole (SB-3) was located on Lot 2 of the Barons Flidge. In September of 1995. we eVeIusted surficlal geologic conditions within the Phase I ree based upon aerial photography and a field reconnaissance (Job No. 05-5807). Subsequently. four holes were drilled to ?Investigate general subsurface conditions and reported under Job No. CS-EEOM. dated actober 31. 1995. Conditions encountered In those borings are presented in Appende A. .. In the summer of 1996. We Installed and piezometers on Lot 9 of the Estates and Let 4 at Barons Ridge to gather information for evaluation of the occurrence of lateral ground movaments. Subsurface conditions encountered In those borings are presented In Appendices and C. Concurrently. holes were )drilled to evaluate subsurface conditions to develop design and construction recommendations for foundations and ?oors within the Phase I Villas at Log Hollow development. We have also drilled boringe for most of the lots located within the Estates. Boring logs for holes drilled within the Villas are shown In Appendix A. Two test borings drilled to Investigate subgrade conditions for the design of Stone Manor autos-1m {i . Heights wars extended to depths to encounter shale. The logs of those borings are also presented In Appendix A. Using the above subsurface information gathered and strength data developed during our work for the Broadmoor South Golf Course, stability analysis was conducted for Lot 9 of the estates, Lot 4 at Barons Barons and the south two thirds of the Villas. The analysis considered groundwater conditions that existed at the time of our Investigations. The results of those analyses are presented in Appendices and C. After the work was reviewed by the Colorado Geologic Survey and after several discussions between the C05 and CTUl'hompson, inc., the CBS submitted a letter to the City of Colorado Springs outlining the Survey's criteria for reevaluation of the areas located within the historic landslide. The investigation conducted for this report was designed to fulfill that criteria. After drilling additional borings requested and after conducting analysis, the initial data was revieWed with the CBS and additional work conducted. Graphical logs of holes drilled for this Investigation are presented on Figs. 7 through 11. All of the boringa shown throughout this report were drilled using a 4-inch diameter. continuous flight, truck-mounted, power auger. At frequent intervals, penetration resistance tests were made using a California sampler to evaluate the reistlva stiffness or density of the materials encountered, to identify layers, and to obtain samples. Drilling operations were under the supervision of our representative. in most instances, the logging individual was involved with our evaluation of the Broadmoor South Golf Course. Where exploratory borings were advanced to depths that encountered "intact" hale, slotted or perforated PVC pipe was installed to allow long-term measurements of groundwater conditions. The Colorado Geologic Survey had recommended continuDUs sampler or coring methods to detect the slide plans. These methods had been tried during our investigation of the south golf course and again for this investigation. Based upon acumaoevumemmurm 7 our experience with 100+ borings located within this deposit, the contact between the displaced and Intact materials is very distinctive. More "sophisticated' sampling and drilling methods tend to yield data that is, In our opinion. lndeclsivs or inconclusive, often not confirmed by deep deformation determinations such as incitnometers. SUBSUHFACE CONDITIONS Throughout the area investigated, subsurface condition consisted of a variable thickness at alluvial or debris tlowleiope wash materials overlying claystone. in many Instances the claystone appears to have been displaced as part of the historic landslide event. The cisystone is underlain by hard. Pierre shale. Throughout a portion of the Phase ii and Phase Iil areas of the Villas, and much of Barons Ridge, the natural surliclal materials consist oi claystone. The thickness of the alluvlum or debris liowieiope materials become thicker, approaching the southerly drainage channel as well as within the more gently sloping ground north of Governors Point. Figure 2 shows our estimated extent of surlictel cleystone and the thickness of the overburden materials at our test hole locations. Using the topography presented and the depth to the hard shale. we developed an estimate of the contours of equal shale eleVation as shown on Fig. 3. The contours tor the shale appear to follow the ground eurfce contours. Borings for this investigation were drilled in February and March, 1997, which is historically a relatively dry time of the year. Groundwater measurements we have made both in this vicinity and at the golf course suggests that groundwater levels found probably represent seasonal lows. No groundwater was measured in about half ofthe borings. At most locations where water was found,I it occurred 20 or more feet below the ground surface. Based upon our experience, We expect groundwater occurs at the surface of the herd shale and seasonally near the base at the alluvium and debris flow materials. Groundwater exists within the debris flow/slope wash materials as in?uenced by ?ows in the adjoining drainage. Groundwater also flows through fissures and fractures within the displaced claystone. one LANDI STABILITY ANALVSIS Slope stability through this protect has been evaluated by analyzing surface and subsurface cross-sections developed along not less than seven various profile lines as shovvn on Fig. 1. The analyis for four of these cross-sections were included In previous reports and are replicated in Appendices and C. After reviewing the comments contained In Colorado Geologic Survey's letter of February 10th, three additional cross-sections were analyzed as shown on Fig. 1. The surface and subsurface profiles used in the analysis are presented on Fig. 4. We also conducted analysis on two profiles previously evaluated. Figures 5 and 6 show cross-sections used for a portion of the slope east of Lot 4 st Barons Ridge and across the eastern portion of Lot 3 of Barons Hldge. As requested by the Colorado Geologic Survey. We conducted our stability analysis considering a relatively high groundwater table. We expect that If the groundwatertable used was to develop, it would likely result In surface seepage nd water in the basement conditions. We havs made recommendations for provisions of aubclrains beneath sanitary sewers and around below-grade residential areas to help collect and discharge water to aid In avoiding development of a high groundwater table. We have also made recommendations to limit vegetation that requires supplemental watering to survive after the period. Surface waters are also to be conveyed to lined channels to provide for the rapid removal of runoff and elimination of pending. For the purposes at comparison, two of the profiles also considered lowsr groundwater tables. Most of our previous analysis considered lower groundwater levels. The Colorado Geologic Survey requestd that we conduct our analysis utilizing the lower bound for the landslide materials. eliminating the strongerl overlying alluvlum and debris flow/slope wash materials. Whilethe cross- seotions presented on Fig. 4 show the occurrence of these higher strength materialsI the strength parameters used for the materials Were the same as those Used for the displaced ciayslone. warm 9 During our recent ?eld Investigation, two samples ofthe "displaced" claystone ware subjected to laboratory direct shear testing. The results of that testing are presented on Figs. 27 and 28. We believe obtained from one sample (TH- 15 at 34 feet, Fig. 27) Is representative at the more clayey debris flow/sicpa wash material. The results from the other sampl compares well with the results of some of our testing of claystone from the golf course. Appende present the results at direct shear tests conducted on samples of displaced ciaystone obtained from the golf course. For the purposes of our evaluation, we utilized angles of internal friction of 11 and 14 degrees with cohesion Intercepts of and 700 pet. The physical computational analysis was conducted using a computer aided program (Slope/W. Geo-Slope International). When performing our analysis. we selected both rotational or circular failure surfaces and sliding block and wedge analysis. Circular or arc failure modes utilized a random search mode, allowing for computation of the lower bound factor of safety for the glven criteria and an Iaopach plot of the factor of safety for the given analysis range. In the plate which utilized the sliding block and Wedge. both random search and specified surfaces were utilized. Appendix contains the results of the Individual analyses that yield the lower factors of safety for the various areas analyzed. SUMMARY Table in Appendix summarizes calculated factors of safety for the various strength parameters and sections analyzed. For cross-section 1. factors of safety varied between 1.42 and 2.79 for the 700 condition. If wetting conditions similar to the golf course occur for a long period of time at the head and/or toe of steep slopes, large horizontal strains could result In the factor of safety falling to between 0.56 and 1.49. Most calculated factors of safety for the "golf course" condition were below 1.2. Calculated factors of safety for cross-sections analyzed within profile 2 exceed 2.0 for the 700 condition. Values dropped generally to less than 1.2 for momsosmmcouam aura-7m 10 the 0 condition. At crossusection a, calculated factor of safety for the high groundwater condition varied behveen1.?? and 2.54 for the 700 pef. Again. If conditions develop like at the golf courseI the factor at safety drops to less than 1.2 In most Instances. For the cross-section that runs across Lot 3 at Barons Ridge from Lot 9. the calculated factors of safety varied between 1.03 and 1.21 for the 700 condition. These calculated factors of safety were very similar to those calculated In our previous analysis. Our new analysis, east of Lot 4 extended further to the east than that utilized In our previous analysis. New calculated factors of safety ware between 1.45 and 1.74 where they generally exceeded 2.0 In the previous analysis. Both enaiyses Indicate factor of safety of less than 1 for the 0 condition. CONCLUSIONS The wetting conditions at the Broadmoor South Golf Course that lead to rectlvatlen of the historic landslide do not exist within the development analyzed. However. some of the topographic conditions, particularly In the eastern portion of Lot 9 of the Estates and across portions of Lot 3 of Barons Ridge, are similar to those where movement or failures have occurred after 1 13+ years ol Inte?e Irrigation or pending of water. Based uponthe analysis. It Is our opinion that a maiorlly om: ground contained with Phases II and of the Villas at Log Hollow and Barons Flidge should remain stable even with abnormally high seasonal groundwater fluctuations, provided severely intense Irrigation does not occur over large areas and that pending of water over large areas are not allowed to occur. The analysis does also indicate that there Is a higher risk of slope movement In the area of Lot 9 of the Estates and across Lot 3 of Barons Ridge should high groundwater levefs develop. mmamoeumoouvm emu-rm 11 At this time, we recommend avoiding construction on Lot 9 of the Estates. Let a of Barons Ridge and the ground In Barons Ridge to the east of the Fischer- Penrose pipeline (Lots 4 and 5). We believe more Information should be gathered regarding seasonal groundwater fluctuations along with continued monitoring of the installed lnolinometars. We believe the monitoring period should last at least one normal precipitation season. After that time. the analysis and data should be reviewed to evalute stability. We have stated In our numerous reports that it is imperative that extra ordinary measures be taken for development within the historic landslide complex to avoid significant surface water being allowed to infiltrate the ground, resulting in a rise in the groundwater table and softening of the subsoils like that which has occurred at the golfoourse. We hav resommended installation of active underdraln systems below sanitary sewers. We have also recommended installation of foundation drains around individual units that discharge into the system. We also recommend that roof downspouts and drains convey runoff rapidly to lined channels such as streets to promote rapid removal of runoff in controlled manners. Vegetation around structures should be native to the area and require limited supplemental watering after the establishment period. LIMITATIONS The conclusions and recommendations contained in this report were developed based upon conditions disclosed by exploratory borings. results of laboratory testing. engineering analyses and our experience. We are available to discuss surface and subsurface drainage. We beiieva this report was prepared using methods and procedures consistent with other professionals practicing in this area at this time. No other warranty, express or implied' la mode. mumsoevaomamcousm crurc-sm 12 If we can be of further service In discussing the contents of this report the from a please call. i . CTUTHOMPSON, [Magi Air: 4) . I William c. Ha?ni?grer. FEE-K Principal - WCHzec (5 capies) LOT 4 BARONS RIDGE 6520 r- Cho?es Grove cub?de?soc 6580 6540 6500 Elevation (ft) 5450 40 80 120 150 200 240 280 320 360 400 440 480 520 550 Horizontal Distance (ft) SURFACE AND SUBSURFACE CROSS SECTION USED FOR ANALYSIS Job No CS- 7322