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Preliminary Geotechnical Engineering Study PDF

18 Pages·2011·0.17 MB·English
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8 July 2011 Mr. Scott Unger Executive Director Allentown Economic Development Corporation 435 West Hamilton Street, No. 331 Allentown, PA 18101 Re: Preliminary Geotechnical Engineering Study Allentown Arena Allentown, Pennsylvania Langan Project No.: 240022301 Dear Mr. Unger: This letter report presents the results of Langan Engineering and Environmental Services, (Langan’s) preliminary geotechnical engineering study for the proposed Allentown Arena in center-city Allentown, Pennsylvania. The study was performed for the Allentown Economic Development Corporation. The purpose of our geotechnical engineering study was to provide preliminary site-specific subsurface information with the potential of identifying subsurface liabilities (e.g., miscellaneous fills, soft silty/clayey layers, known karst hazards, and depth/condition of bedrock) at the due-diligence stage that may adversely affect the site’s future development. A supplemental, structure-specific geotechnical engineering study will be needed once the proposed site layout and grading is finalized. This report summarizes our findings and presents preliminary geotechnical engineering concerns and impacts associated with site redevelopment. Site Description and Existing Conditions The project site of our preliminary geotechnical subsurface investigation occupies two city blocks of Allentown. These blocks are bounded by Linden Street to the north, Hamilton Street to the south, 6th Street to the east, and 8th Street to the west. A series of single-lane alleys crisscrosses the site in a north-south and east-west alignment. The location of the site is shown on the attached Figure 1. The site is fully developed and is occupied by a combination of at-grade parking lots, 2- to 3-story residences, and multiple-story residential and commercial buildings, many of which have full basements. Of particular interest is the existing at-grade parking lot located just northwest of the intersection of Hamilton Street and 7th Street, which is the site of the former Corporate Plaza Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 2 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 office building. This former development is the site of a well-documented, very large sinkhole that occurred beneath the former office tower, resulting in substantial settlement of structural columns leading to irreparable structural distress, its condemnation and subsequent demolition. It is important to note that this sinkhole was filled with grout and also that the site was backfilled with demolition debris after the building was razed. Langan prepared a geologic and geotechnical evaluation for this sinkhole event in our report dated 21 December 1994. A synopsis of this report is presented in the ‘Previous Geotechnical Investigations’ section below. Proposed Construction The proposed development will involve the construction of an approximately 8,500-seat arena consisting of an event level, concourse level, lower suite level, upper suite level/press level, low roof, catwalk/rigging level and high roof level. An arena loading dock will be located on the northwest corner of the proposed arena, featuring a concrete cantilever retaining wall up to 16 feet high. Anticipated column loads for the arena are 1,100 kips for the columns supporting roof trusses, and 500 kips at typical interior columns. It is our understanding that the proposed arena will be located on the city block between 7th and 8th Streets between Linden and Hamilton Streets. As such, it overlaps the limits of the former Corporate Plaza development, now the large at-grade parking lot, that was the site of the massive sinkhole discussed above. Local Geology According to the Atlas of Preliminary Geologic Quadrangle Maps of Pennsylvania, 1981, Allentown East quadrangle, the site of the proposed arena development is underlain by the Allentown Formation. This bedrock formation is composed of laminated, medium grey dolomite and impure limestone, which are both carbonate rocks that are categorized as karst. Karst is a type of geologic formation characterized by carbonate bedrock such as limestone or dolomite that is susceptible to dissolution when exposed to mildly acidic groundwater. Over time, the dissolving bedrock creates the features common to karst topography, including an irregular, pinnacled bedrock surface that is often highly fractured and ground subsidence in the form of sinkholes, or dolines. Karst topography presents a unique set of challenges to site development, particularly from a geotechnical engineering perspective. Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 3 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 Previous Geotechnical Investigations A series of geotechnical subsurface investigations have been completed at the site throughout its development history. Prior to Langan’s current subsurface investigation, the field investigations have focused on identifying the subsurface conditions beneath the former Corporate Plaza development, both before and after it existed. A brief summary of the previous geotechnical investigations performed by others is provided in the subsequent sections. F&M Associates Investigation Prior to the construction of the original Corporate Plaza development, the site’s subsurface conditions were investigated by F&M Associates. F&M performed 32 borings in two separate geotechnical investigations. A series of 21 borings (TB-1 through TB-21) were drilled in August 1977, with an additional 11 borings (B-22 through B-32) drilled in August and September of 1984. The locations of the borings performed by F&M are included on the attached Figure 2 entitled “Boring Location Plan.” These borings were drilled to depths ranging from 11 to 48 feet below existing ground surface (BGS). Rock coring was performed in 19 of the borings, with core lengths ranging from 3 to 21 feet. Subsurface conditions revealed by the F&M borings consisted of up to 8 feet of uncontrolled fill material underlain by sandy silt or silty sand above a weathered limestone with variable rock surface elevations. The limestone bedrock surface was encountered from 8 to 40 feet BGS. Groundwater was encountered in two of the borings performed during the 1977 subsurface investigation, at 14 feet and 23 feet BGS in borings TB-2 and TB-3, respectively. No groundwater was encountered in the 1984 subsurface investigation. The boring logs from the F&M field investigations are provided in Appendix A. Langan Engineering & Environmental Services Study Langan conducted and investigation for the City of Allentown of the sinkhole formation that developed below the Corporate Plaza building and North 7th Street in Allentown, Pennsylvania on 23 February 1994. A copy of this report is included in Appendix B. The focus of the report was “to document the geologic conditions associated with the sinkhole, to document the sequence of events and effects of the sinkhole development, and to assess the possible causative mechanism(s) for the sinkhole”. The following are excerpts from the Executive Summary” and “Conclusions” sections of Langan’s 21 December 1994 report. “Direct observations of the sinkhole were limited to the period of 23 to 27 February 1994. The sinkhole was filled with concrete on 27 February 1994.” Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 4 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 “The sinkhole below 7th Street was an elongate, east-west-trending open depression with depths as great as 17 feet. The overall size of the depression was approximately 150 ft. by 45 ft. The sinkhole extended below the Corporate Plaza building, and resulted in the partial collapse of that building and attached parking deck. An associated sinkhole also developed below a vacant building on the east side of 7th Street. The sinkhole resulted in the collapse of the pavement of North 7th Street and severance of underground utilities. Approximately three million gallons of water were discharged into the sinkhole from the city water supply system.” “The bedrock underlying the City of Allentown consists mostly of the Allentown Formation, which is composed of limestone. Sinkholes, which are common in the Lehigh Valley, occur when overburden soil filters downward into voids in the underlying soluble limestone bedrock. The 7th Street sinkhole occurred within a soil-filled, east- west-trending trough in the irregular bedrock surface, which is a typical sinkhole situation. A review of soil boring logs from a site investigation conducted as part of the Corporate Plaza development indicates that loose soil conditions within the bedrock trough were present at the time of site development. The greatest depth of subsidence within the sinkhole appeared to be below column A-5 on the east side of the Corporate Plaza building. It is in this area that the first loss of soil likely occurred.” “Based on the events which led to the development of the surface depression, the nature of the collapse of the street and buildings, and the geologic conditions, Langan concludes that there was a void in the bedrock below the site of the sinkhole into which overburden soil migrated from the overlying bedrock trough. The sink through which the soil migrated was located below the east facade of the Corporate Plaza building, and the 7th Street sidewalk. The loss of soil into the bedrock resulted in an unstable condition in the overlying soil which caused the building to settle and the water main to fail which led to the collapse of the Corporate Plaza building and adjacent street, and the subsequent subsidence of the buildings on the east-side of 7th Street.” “Based on the results of our investigation and technical evaluation, we conclude that there was an initial soil loss within the soil-filled bedrock trough below the east side of Corporate Plaza and 7th Street, into a void(s) in the underlying bedrock. The resulting soil loss propagated upward from the bedrock surface, creating a conduit of loose soil, with increased hydraulic conductivity which extended upward through the soil column. The timing for this initial loss of soil is not known, and can not be estimated accurately. When the shifting soil undermined the water main, the pipe sheared under the load of the overlying soil and road pavement, and water began to drain down the conduit and a rapid loss of soil ensued. This accelerated soil loss continued for approximately three hours, after which time the street and east side of the building collapsed. After the water supply lines were shut off, soil continued to sink slowly into bedrock over the course of the next six to eight hours, which resulted in the subsidence of Column D-6 on the northwest corner of the building, and development of the sinkhole below the vacant buildings east of 7th Street.” Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 5 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 Refer to the full report and figures included in Appendix B for a complete description of the geologic conditions, sequence of events and Langan’s model of the mechanism and development of the sinkhole. Earth Engineering Incorporated Investigation A previous geotechnical subsurface investigation was performed by Earth Engineering Incorporated (EEI), documented in their Preliminary Report of Subsurface Investigation dated 5 October 2009. EEI performed nine soil borings located within the limits of the existing asphalt parking lot formerly occupied by the Corporate Plaza development. The locations of the borings performed by EEI are included on Figure 2. The nine EEI borings were advanced to the top of bedrock, as interpreted by refusal of the soil auger, ranging in depth from 23 to 51 feet BGS. A single 5-foot run of rock coring was performed in two of the EEI borings. Subsurface conditions revealed by the EEI borings consisted of a surficial layer of asphalt pavement underlain by 3.5 to 12 feet of demolition fill of variable density and composition. The demolition fill was underlain by a stratum of natural soil referred to as Stratum I, described as decomposed/weathered dolomite in the form of sandy silt to silty sand with rock fragments. The Stratum I soils extend from beneath the demolition fill to the top of the underlying limestone bedrock. Groundwater was not encountered during the EEI geotechnical subsurface investigation. The boring logs from EEI’s field investigation are provided in Appendix C. Langan Geotechnical Investigation Langan performed a subsurface investigation consisting of eight borings distributed throughout the two city blocks included in our site. The borings were completed between 4 and 5 May 2011 by Earthcore Inc., using a truck-mounted Acker Soilmax drill rig and a track-mounted Acker AD-18 ATV drill rig, both equipped with hollow-stem augers. The boring locations are shown on the attached Figure 2. All field work was completed under the direct observation of a Langan field engineer. The borings were advanced to depths ranging from 15.5 to 55 feet below existing ground surface (BGS). Soil samples were collected in conjunction with SPT testing continuously in the upper 12 feet and at 5-foot intervals thereafter in general accordance with ASTM D1586. Soil samples were classified in the field and recorded on our field logs along with other observations during drilling. The top of limestone bedrock was encountered at six of the eight boring locations at depths ranging from 15 to 40 feet BGS. Borings LB-2 and LB-3 were advanced to approximately 50 feet without encountering bedrock. Once encountered, bedrock Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 6 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 coring was performed to collect samples of the limestone and to evaluate the bedrock’s condition. Between 5.5 to 25 feet of rock coring was performed in borings LB-1, LB-4B, LB-5, LB-6, LB-7, and LB-8. Cores were obtained using a NX/NQ-sized core barrel fitted with a diamond cutting bit producing 2.125-inch-diameter rock cores. Core run lengths were typically 5-feet-long. All borings were backfilled with bentonite grout in the lower 10 to 15 feet of the borehole, followed by a combination of bentonite grout and soil cuttings to within 4 feet of ground surface. The final 4 feet of borehole was then backfilled with cement grout and the pavement surface was restored with cold patch asphalt in asphalt-paved areas or with cement grout in concrete-paved areas. The boring logs from Langan’s field investigation are provided in Appendix D. Laboratory Testing Upon completion of the borings, the soil and bedrock samples were brought back to our office for further evaluation and laboratory testing. Soil classifications were verified by a senior geotechnical engineer and select samples were sent to our subcontracted geotechnical laboratory to determine index and engineering properties of the subsurface soils and bedrock. Laboratory testing was performed on six soil samples and three bedrock samples at a subcontracted laboratory and included the following: (6) Water Content [ASTM D2216]; (5) Particle Size Analyses [ASTM D422]; (4) Atterberg Limits [ASTM D4318]; (1) Percent Finer than No. 200 Sieve [ASTM D1140]; and, (3) Unconfined Compressive Strength of Rock Cores [ASTM D2938]. The laboratory test results are discussed in the following section under their respective soil strata. The complete laboratory reporting is provided in Appendix E. SUBSURFACE CONDITIONS In general, the subsurface conditions beneath surficial pavement materials consist of three soil strata overlying variable-depth bedrock. These soil strata in descending order from the ground surface include a 0 to 10 foot thick fill stratum underlain by residual soils, underlain by highly weathered limestone rock. The top of bedrock was encountered in six of the eight borings at depths ranging from 10 to 40 feet BGS. A brief description of each stratum is provided below. Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 7 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 Surficial Materials Surficial layers encountered at the boring locations consisted of asphalt or concrete pavement. Asphalt pavement was encountered at four boring locations (LB-2, LB-3, LB-4 and LB-7). The thickness of the asphalt ranged from 1.5 to 4 inches and was placed on underlying stone aggregate subbase ranging in thickness from 4 to 6 inches. The existing asphalt pavement at the at-grade parking lots is in poor condition with abundant cracking and other damage. Asphalt pavement within the public right-of-ways is in significantly better condition, with only minor cracking and other damage. Concrete pavement was encountered at three boring locations (LB-1, LB-5, and LB-6), ranging in thickness from 4 to 6 inches and was constructed on an underlying stone aggregate subbase layer ranging in thickness from 3 to 6 inches. No surficial materials were encountered at boring LB-8, where fill material was encountered beginning at ground surface. Fill Beneath the surficial layer exists a stratum of highly variable fill made up of sand, gravel, brick and concrete fragments, silt, and clay. Varying amounts of wood, asphalt fragments, glass, metallic debris, and cinders were also encountered within the fill. These fill materials were placed throughout the course of the site’s development history. The fill consisted primarily of soil constituents with some man-made material; however, there were instances in which the fill consisted entirely of brick and concrete debris. The predominantly brick-concrete debris fill areas were only encountered within the limits of the former Corporate Plaza development that once occupied a portion of the site. In addition to these demolition debris materials, possible below-grade elements of the former construction, such as basement floor slabs and exterior foundation walls, may have been encountered at boring LB-4, where multiple offsets of the drilling equipment were necessary to avoid obstructions. Auger refusal was encountered at approximately 6 feet BGS at LB-4 and LB-4A. The obstruction encountered at borings LB-4 and LB-4A was not encountered at LB-4B. This obstruction may also be the grout used to fill the massive sinkhole since it was mapped at approximately this location. LB-8 also encountered a near-surface obstruction and is also roughly within the mapped limits of the sinkhole. The fill stratum was encountered in all borings except for LB-5, ranging in thickness from 3 to 10 feet. The SPT N-values varied from 3 blows per foot (bpf) to refusal of the sampling equipment, at 50 blows over 0 inches of sampler penetration, indicating extreme variability in the in-place density of the fill materials. The average N-value of 21 bpf indicates a medium- dense state of in-situ density, but is not truly representative of the in-place density of this layer. Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 8 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 Soils laboratory testing was performed on two split-spoon samples of the soil component of this stratum collected during Langan’s preliminary geotechnical investigation. The natural moisture content ranged from 7.2% to 12.0% and the fines content (silt and clay-sized particles) ranged from 2.6% to 62.3% in the tested specimens. Laboratory testing results and field observations show that the fill stratum consists of poorly-graded gravel with sand [GP] and sandy lean clay [CL]. A summary of the laboratory testing results for samples within this layer is provided in Table 1. TABLE 1 - LABORATORY TEST RESULTS FOR FILL Boring/ Depth Water % Passing Atterberg Limits Laboratory Description Sample (ft) Content (%) #200 LL PL PI Poorly-graded GRAVEL with LB-3/S-4 6-7.7 12.0 2.6 -- -- -- sand [GP] LB-6/S-2 3-5 7.2 62.3 36 22 14 Sandy lean CLAY [CL] Residual Soil A stratum of fine-grained soil was encountered beneath the surficial material at boring LB-5 and beneath the fill material encountered at the remaining borings except for boring LB-8, where the fill is underlain by bedrock. This soil layer is residual material that mantles the weathered bedrock and is derived from the chemical weathering of the underlying limestone rock. The residual soil generally consisted of an upper zone of brown to brown-orange clayey silt with trace amounts of fine gravel and sand, and a lower zone of brown to yellow silt with varying amounts of friable weathered rock fragments. This stratum ranged in thickness from 2 to 36 feet where penetrated, and continued beyond the investigated depths of 48.3 and 51 feet at borings LB-2 and LB-3, respectively. SPT N-values collected within this stratum ranged from 2 bpf to 78 bpf and averaged 21 bpf, indicating a very stiff state of consistency. Higher N-values were typically observed in samples containing higher percentages of gravel-sized particles. Soils laboratory testing was performed on three split spoon samples of the residual soil stratum. The results of this laboratory testing are included below in Table 2. The natural moisture content ranged from 19.8% to 32.8% and the fines content (silt and clay sized particles) ranged from 49.0% to 75.8% in the tested specimens. The index test results indicate that the residual soil ranges from lean clay with sand [CL], silty sand [SM], and sandy silt [ML]. Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 9 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 TABLE 2 - LABORATORY TEST RESULTS FOR RESIDUAL SOIL Boring Depth Water % Passing Atterberg Limits Laboratory Description /Sample (ft) Content #200 LL PL PI (%) LB-1/S-3 4-6 21.5 75.8 31 22 9 Lean CLAY with sand [CL] LB-3/S-11 34-36 19.8 49.0 NP NP NP Silty SAND [SM] LB-4B/S-3 10-12 32.8 66.3 NP NP NP Sandy SILT [ML] Weathered Rock Weathered limestone bedrock was encountered below the residual soil in five boring locations (LB-1, LB-4B, LB-5, LB-6, and LB-7) at depths ranging from 9 to 36 feet BGS. The weathered rock stratum was from 2 to 6 feet thick and consisted of tan-grey gravel and sand with silt. The weathered rock was characterized by frequent refusal of the sampling equipment (i.e. 50 blows over less than 6 inches of sampler penetration). SPT N-values collected within the weathered rock ranged from 30 bpf to 50 blows over 0 inches of sampler, indicating a very dense state of relative density. Laboratory testing was performed on one sample of the weathered rock stratum. The results of this laboratory testing are included below in Table 3. The natural moisture content was 5.8% and the fines content (silt and clay sized particles) was 18.6% in the tested specimen. The index test results indicate that the weathered rock is silty gravel with sand [GM]. TABLE 3 - LABORATORY TEST RESULTS FOR WEATHERED ROCK Boring Depth Water % Passing Atterberg Limits Laboratory Description /Sample (ft) Content #200 LL PL PI (%) Silty GRAVEL with sand LB-6/S-5 9-9.9 5.8 18.6 -- -- -- [GM] Limestone Bedrock Limestone bedrock was encountered in six of the eight borings performed during Langan’s geotechnical investigation, at depths ranging from 10 to 40 feet BGS. Once encountered, rock cores were obtained of the bedrock. Coring lengths ranged from 5.5 feet to 25 feet. Rock core recovery ranged from 15% to 100%, and rock quality designation (RQD) ranged from 0% to 75.0%. A summary of the rock coring performed is provided below in Table 4. The observations collected during the rock coring runs reflect a highly variable weathering profile of the limestone bedrock, as is expected in soluble carbonate rock formations that result in karst geology. At borings LB-4B and LB-8A, the observed weathering was minimal. No loss Preliminary Geotechnical Engineering Study 8 July 2011 Allentown Arena Page 10 of 18 Allentown, Pennsylvania Langan Project No.: 240022301 of wash water was observed in these two borings, and recovery and RQD values were relatively high. The rock that was cored in the remaining borings exhibited more extensive weathering, as evidenced by the consistent loss of wash water, frequent soil seams or highly weathered seams within the cores, and the highly fractured nature of the recovered rock cores. Open voids were encountered in two boring locations: a 2-foot-high void was encountered at boring LB-6 from 23 to 25 feet BGS, and a 3-foot-high void was encountered at boring LB-5 from 41 to 44 feet BGS. TABLE 4 – ROCK CORING SUMMARY Depth Length REC RQD Boring Run Comments (ft) (ft) (%) (%) 1 21-22.5 1.5 100 38.9 No return of wash water for LB-1 2 22.5-26 3.5 47.0 36.9 Soil seam 22.5 to 23 feet, and 25 to 26 feet 3 26-31 5 62.0 8.3 Soil seam 26 to 26.3 feet LB-1 Highly fractured 31.5 to 34 feet, highly 4 31-35.5 4.5 100 0.0 weathered 35 to 35.5 feet 5 35.5-41 5.5 69.7 13.6 Highly weathered 37.5 to 38 feet 1 29-34 5 94.2 35.0 Water return throughout coring LB-4B LB-4B 2 34-39 5 95.0 75.0 Weathered zone 36.5 to 37 feet No return of wash water for LB-5. VOID 41 to 1 40-45 5 30.0 9.2 44 feet LB-5 2 45-50 5 70.0 39.2 3 50-55 5 83.8 55.0 Highly fractured, weathered zone 53.5 to 55 No return of wash water for LB-6. Soil seams at 1 15-20 5 48.3 0.0 16.9 to 17.1 feet, 18.3 to 18.4 feet, 18.8 to 18.9 feet, and 19.2 to 19.9 feet. Highly fractured from 21 to 23 feet. VOID 23 to 2 20-25 5 30.0 8.3 25 feet. LB-6 3 25-30 5 15.0 0.0 Highly fractured zone 25-30 feet Soil seam 30 to 31 feet. Highly fractured, highly 4 30-33.5 3.5 60.5 0.0 weathered from 31 to 31.5 feet. 5 33.5-35 1.5 83.3 0.0 Highly fractured 33.5 to 35 feet 6 35-40 5 98.3 45.8 No return of wash water for LB-7. Highly 1 23.5-26 2.5 53.3 0.0 fractured 23.5 to 26 feet. 2 26-29.5 3.5 47.6 0.0 Highly fractured 26 to 29.5 feet. LB-7 3 29.5-31 1.5 100 0.0 4 31-34.5 3.5 100 11.9 Highly fractured 33.8 to 34.5 feet. 5 34.5-36 1.5 100 30.6 6 36-41 5 81.7 10.0 1 10-10.5 0.5 100 0.0 Water return throughout coring LB-8A LB-8A 2 10.5-15.5 5 85.0 56.7

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331. Allentown, PA 18101. Re: Preliminary Geotechnical Engineering Study The project site of our preliminary geotechnical subsurface investigation .. rock-socketed drilled shafts (caissons) and micropiles, meet this requirement.
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Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.