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Home My WebLink AboutFOUNDATION INVESTIGATION REPORT - 96-00020 - Madison County Jail - New BuildingFOUNDATION INVESTIGATION MADISON COUNTY - COURTHOUSE EXTENSION REXBURG3 IDAHO OVERVIEW 95-131 i ne site for the Courthouse expansion is contiguous to and north of the existing Courthouse expansion that was constructed in the tate 707s. The C rthouse is located at the northeast corner of the intersection of (Dain and 2nd Easy Street in Rexburg, Idaho. Amore detailed location of the site and related topography are shown ars Figure 11, This foundation investigation was made to determine the subsurface soil conditions at the site of the proposed expansion, to develop a teasib[e foundation aifernative and make recommendations for the proposed expansion. In performing the foundation investigation, six (6) test borings were drilled at the oc shown on Figure 1. The test borings were drilled to a depth vary►�ng from 1�D to 25I ft. The test borings were used to determine the physical characteristics o� the underlying soil strata. The information obtained was used in formulatin� the prapased foundation plan far the building and rebated facilities, GENERAL SITE CONDITIONS a review of aerial photographs taken in 1973 fthat in the fast a structure had been built on the proposed site. A review of a dater aerial photograph from the late 74's indicates that a portion of the building had been removed in the late 70's to facilitate construction of the existing expansion that currently houses the City Police and the Madison County Commissioners Office. Sometime after that the remain41 ing building was utilized for storage of the County ambulances. During the date St's this building was removed and the area paved for its current use as a parking lot dor the Rexburg Police and other users of the Courthouse facilities. not The test borings indicate that portions of the building floor still d there is evidence, as shown an Figure x, that a portion of the foundations stili remain under some areas of the parking lot. 0 A copy of the aerial photographs showing the location of the old ambulances building is included in the Appendix,. The existing site is pawed with an asphalt concrete paving and is relatively flat. I Severs[ utility systems border the site on the north side of the Courthouse expansion that was constructed in the Tate 70's. These include: * power * telephone • gas ■ city sewer facilities These are shown on the site topography drawing (a separate document). SUBSURFACE GEOLOGICAL CONDITIONS In order to accurately ascertain the characteristics of the underlying strata, six (6) fore hales were drilled at the locations shown on Figure 1. These test borings were drilled with a hollow stem auger to a depth varying from 12 to 27 ft. The test borings are shown on Figure 2 through Figure 4, It should be noted that the holes were drilled prior to the building location being established and may not be directly under the proposed building. In general, the subsurface soR conditions include the asphalt concrete surface that is approximately 2-rinches thick, a 3/4 -inch grape! base of approximately 4 --inches thick, and a small amount of granular borrow. Underlying the pavement surface is a layer of native sits or silty clay which varus in thickness from Z to 7 ft. During the drilling of fest boring #4, a 4 --inch concrete slab was encountered of a depth of approximately 1 ft. Sewers( attempts were made to relocate test hole #4, but the concrete was encountered in all doles. Finally, the test hope was drilled through the concrete slab. The exact extent of the concrete slag was not determined through this investigation. In the area where the old ambulance building was located there will be obvious disturbance from the placement of the building footings and utility services. It is not known whether the building had a basement or not. The use of the building as a parking garage for the � ambulances would tend to lead one to thin assumption th he bu1111ding was an on -grade slab and possibly was a masonry structure. Underlying the native silts is an extensive depth of a s'M ilty sand to a poorly graded gravel., The sands and gravels are relatively well compacted and haves an rd blow counts varying from 16 to over 90 blows per boat. This indicates that the sands and sandy gravels are in a dense condition and will provide excellent bearing capacity for the proposed structure. Standard penetration tests were perform 8 ft, and 10 ft in all holes,, and also at 10 ft, 15 ft, ZO fit, and 25 ft in bare holes #1 and #2. The standard penetration testigves an indication of the bearing capacity of the sods that were encountered duping the drilling operation. BORE H 1 �Ef 25 4-5-9-10 ML—MH 7-12-35-32 .-20- 20-33-43-49 —20- 20-33-43-49 � 10-28-25-27 41 12-12-19-22 13 0 OM O HOLE PA_ N 15 20 LEGEND MADISON C BORE HOLE 2 ML—MH 3-6---7-8 s P 19-27-21-23 16-25-36-35 30-45-35-35 sp 20— 1 � 10-22-29-36 � 14-20-24-23 BO O OF H STANDARD P TEST ( DRILL LOGS F100"E h". FjrG 2 -.� �� ■ 95131 I I It was generally found that once the layer of sift had been penetrated that blow counts were adequate to develop a goad fearing eapacE�y dor the �#ructure khat is anticipated. The sibs that overlie the sands and sandy gravels have blow counts as low as +6 blows per ft. The drilling operation did not encounter groundwater and it is not anticipated that groundwater will be a problem at normal basement levels if one is to be constructed. FOUNDATION CONSIDERATIONS It is understood that the proposed structure is to be a moderately loaded building of masonry construction with masonry bearing walls and either singe or two-sto�-y design. �n either case, the type of building will induce moderate loads to the foundation, Dui to the suitable bearing rapacity �f the subsurface materials, the use �� spread footings drove to be the Inst satisfactory foundation type for the proposed stru��ure. Ta#a� �e#��ement is nafi expected to exceed Y2 -inch and differe��ia! settlement should not exceed �/4-inch. Na problem is � anticipated with seismic loading of the structure. In the area where the flId ambulance garage use to be, all existing foundations, floor slabs, and other debris needs to be removed prior o �nitiating construction an this building. The area where footingsare removed should be backfilled with an imported granular borrow and compacted to the required Specifications. � Because of the varying depth of silt that overlays the natural sandy g�-avefs #wo (2) alternatives dor foundations were deve[ODed, The fi0 rst alternative would provide for the .,� perimeter footings extending down a a depth of at leash 42 -inches below finished. grade in order to Provide for adequate frost protection. The footings would rest either on the silts or the sandy graved materials which could be encountered in the 42 --inch depth. The lag of borings indicates the relative thickness of the silt materia. The area directlyunder the footings should be loosened, brought to optimum moisture and recot-npacted to at least 96% of a Standard Proctor ASTM D-698 or 77%Q of Relative Density dor free draining materials. interior footings could be placed on the existing sift materials ihey are brought to optimum moisture, thoroughly loose 3 and then compacted to 96% of a Standard Proctor ASTM b- 698. It was determined from the foundation investigation that the silt materials under the asphalt surfacing will have a moisture content in excess of optimum and ma need to be aerated to seduce the moisture contend for proper compaction. If this method is utilized a bearing capacity of 17U0 lbs/ft2 can bSS d for all square or continuous footings. A second alternative that was eualuated was for the removal of the existing silt material fr under load bearing arias to the interface between the silk and the sandgravel om This will require excavations varying from 2 to 7Y2 ft in depth. The excavations will materto ial. be at feast as wide as they are djnnm o provide adequate spreading of the toads that are induced on the imported sons. The excavated area should then be brought to optimum moisture content and compacted to at feast g6% of a Standard Procor 77% 0# Relative density, The area under all footings should then be backfilled in lifts not exceeding with an imported granular., free draining granular borrow and compacted to 9��a of g�rnches a Standard Proctor or o� Relative Density. If this method is utilized, a bearin �a lbs�#t' can be utilized for design of square of contiguous footings. � pacify of 4,000 If during excavation and removal of the footings and foundations of parking building a basement, sumps, septic tank, or ether objects the old ambulance should be remolded to their full depth. After which the, o area should be mencountered, they with a granular barrow as required under spread footinvz,. compacted and refilled CONSTRUCTION RECOMMENDATIONS Prior to any construction of the proposed building, the area under the s re shout stripped of all asphalt or any other decomposable material that may be encountered.� be the footings of the ambulance building need to be removed and the �r Also ,..._ compacted. area ref,lled and All materials under any major or minor structures sho+d bi'i campac#ed to at leastSiandat�d Proctar ASTiV� D--698 0r at lest 77�Q of Relative Densityas determined 96% �� a D-4253, whrch ever apples. by ASTM The Contractor's compaction Operation should be monitored in the field bplace �n— testing taken under the direction of a qualifii+�d svils engineer. y density Utility trenches that are excavated under tine structure, pay.0 ing or sidewalk ar com+pacted �o the same requirements. gas should be If angrade slabs are used in the structure, it is recommended that the silt materi dabs be removed to a depth Df at leash 6—inches below the bll�'1111orn �f the stab a� under the brought to optimum moisture and compacted to at feast 96% of a Star�da�d Pro �d the area area brought to grade with a free draining granular material compacted to at leastathe F�elaiive density.77% of Due to the high moisconeni of the si1ts, it is also recommended that5 mil Uapor barrier be paced beneath the free draining mater�a�s �,�. as a m�n��n�m a propagation. it is not anticipated that ar�y dm"mcu�� foundationproblems eliminate moisture at the site and that conditions should proceed readily with standard construction be encountered structron techniques. Parking lots, roadways, and other paved areas should have the gilt material rema depth of at least 15 -inches below the finished asphalt grade. The area should��o � brought to optimum moisture and compacted to at least 96&-/- D-698. of a Standard p then be The paving structure should consists a# at leash a 9—inch layer �t 2—Inch rctor AS�`M gradatifln granular firee draining maferial, a� 4 -inch layer of 3/4 -inch crushed maximum course, and a 2 -inch plant mix surface course. The"granular barrow should be base is at least 77%a of relative den-sity- The �/4 base should be campac�ed to at feast 95% Modified Proctor ASTM D—].557. The asphalt should be Compacted to at feast 96% of a # � 50 flow Marshall. it is also recommended that the asphalt mix design be 1r�-encgradation.h maximum uompaction of native silts under minor structures., such as side to a minimum of 96% of a Standard Proctor ASTM D-698., walks, curbs, e#c., h vUld tae