Istokpoga Marsh Watershed Improvement District Minor Impoundment Project Review

Transcription

1 Section 1 Introduction 1.1 Purpose of the Design Review This document summarizes an independent design review of the 30% design plans dated May 8, 2012 for a proposed minor impoundment, as prepared by Chastain Skillman (Chastain). The approximately 250-acre impoundment is to be located in the Istokpoga Marsh Watershed Improvement District (District), approximately 0.85 miles southeast of the lake. The purpose of the impoundment is to store and treat stormwater, which will be released by the District during dry periods, as needed, for irrigation in the region. Royal Consulting Services, Inc. (RCS) was specifically requested to review the design and constructability of the above ground minor impoundment berms to determine if the design and budget were appropriate for the purpose, based on RCS experience on similar projects. Section 2 Preliminary Data Collection 2.1 Field Investigations RCS engineers performed hand borings at six locations for the proposed minor impoundment berm to visually inspect the soil layers to determine their suitability for impounding water. The boring sites selected are shown on Figure 1 and Table 1 lists the findings. Table 1 Field Investigations Summary HOLE ID GENERL LOCTION GPS COORDINTES* NORTH WEST DEPTH TO FIRST SUITBLE MTERIL (inches BLS) 1 SW Corner Center of South Side SE Corner bout 1500 Feet North of SE Corner bout 1980 Feet North of SW Corner North Side at Internal Corner *Note Hand held GPS data The borings were performed using a standard 2 ¼ diameter hand auger with a maximum investigation hole depth of 8 feet. The soil was removed from each hole in segments approximately equal to the height of the bucket and brought to the surface for evaluation. The process of boring and investigation continued until a soil layer that seemed to be suitable for construction of a berm, by visual inspection, was encountered. Page 1

2 Legend K Soil Boring rdaman Soil Boring TH-10 TH-9 6 TH-8 TH-11 TH-12 TH-7 TH-13 TH-6 5 JoySL 4 J:\0100\22\3 CD_GIS\GIS\Soil Borings.mxd 2/28/2013 TH-14 1 TH-5 TH-1 TH-2 TH-3 2 TH ,000 Feet 2,000 Copyright: 2012 Esri, DeLorme, NVTEQ, TomTom, Source: Esri, DigitalGlobe, GeoEye, i-cubed, USD, USGS, EX, Getmapping, erogrid, IGN, IGP, and the GIS User Community IMWID Berm nalysis Soils Investigation

3 2.2 Basis of Evaluation Istokpoga Marsh Watershed Improvement District The Natural Resources Conservation Services (NRCS), through their practice codes and construction specifications, provide the basis of design for most water impoundments constructed on agricultural sites. The following CS codes (construction specifications) provided a basis for this document: Embankment Pond (CS378) Earthfill (CS023) 2.3 Select Backfill The NRCS specifies that suitable earthfill material be used in a berm as Select Backfill, which is defined in CS023 as follows: Section 3 Select backfill: Select Backfill shall be material that is well graded, free of debris, roots, organic matter, and peat. Select backfill shall be material excavated for the Work (native) or may be imported. The Contractor may blend native materials to achieve a material that meets the requirements for Select Backfill. Select backfill shall be free of seeds of nuisance or exotic species. Select Backfill shall meet the following Unified Soil Classification System (STM D2487) designations: a. Water Retaining Embankments: CL, ML, SC b. Structure Backfill: SW, SP, SM Design Evaluation 3.1 Comparison of Data The Chastain Skillman 30% design plans and documentation provide two sources of soil information, SPT Borings and Muck Probes, both performed by rdaman & ssociates, Inc (rdaman). The rdaman geotechnical report provides the SPT data, which was also provided on Sheet 7 of the plan set. For comparison purposes, the RCS engineer selected the nearest SBT and proposed berm cross section sites to perform the hand borings for comparison. Table 2 Comparative Information RCS HOLE ID NEREST RDMN BORING NEREST CROSS SECTION EXISTING GROUND FT NVD88 1 TH TH TH TH TH TH Evaluation Procedure To provide an independent evaluation of the proposed berm design, the RCS analysis was conducted without coordination with the rdaman geotechnical report or the Chastain design plans. Page 3

4 Instead, the hand borings were used to develop proposed excavation depths and berm cross sections, based on field observations only. No laboratory testing was used. The RCS engineers realize the independent investigations conducted in this manner do not meet the same level of accuracy or have the same scientific basis as the rdaman report, but believe it is sufficient for comparison and evaluation purposes. The stated goal of the RCS analysis was to establish if there is potential for a reduction in construction costs, while still meeting the NRCS criteria for Pond Embankments. For each RCS hand boring, the volume of the excavation was calculated for the adjacent berm using the following procedures: Existing ground elevation provided by closest Chastain plans cross section Bottom of excavation at depth established by hand boring to first suitable material plus 6 inches of extra excavation Excavation bottom, which is equivalent to the width between the berm toe lines 15 wide berm top at elevation NVD88 Side slopes of the berm and excavation is 3:1 Compaction was ignored since it applies equally to both berm designs 3.3 Excavation Volume Calculations Table 3 provides the amount of excavation required to remove unsuitable material under the footprint of the berm and provide a stable base for the berm at the six locations selected for the hand borings. Table 3 Excavation Volumes HOLE ID NEREST CROSS SECTION PROPOSED EXCVTION ELEVTION (FT NVD88) EXCVTION VOLUME (YD 3 /FT) DIFFERENCE (YD 3 /FT) Chastain RCS Chastain RCS Section 4 Evaluation Summary The site selected for the minor impoundment is a challenging location to construct an above ground impoundment capable of retaining water, since it requires the removal of a substantial quantity of unsuitable soil. However, the last column of Table 3 indicates that there is a potential for reducing the total earth work volume calculated by Chastain and thus the corresponding cost of the project. The percent reduction at the six sites analyzed by RCS ranged from 19% to 64% with the average being 51%. The reduction in work would primarily affect three tasks: muck removal, suitable fill Page 4

5 excavation and placement of the suitable fill. Other savings might be realized such as job overhead, office facilities, dewatering, and SWPPP plan monitoring but the savings for these items would constitute only a small portion of the overall project and will be ignored for the purposes of this evaluation. ssuming that the entire berm has the same reduction rate as calculated above and using the Chastain Preliminary Opinion of Probable Cost dated pril 28, 2012 as a basis of volumes and unit costs the following savings would then be possible for the three tasks: Table 4 Potential Cost Reduction CHSTIN 30% ESTIMTE No DESCRIPTION UNIT QUN. UNIT COST TOTL *RCS REDUCTION 12 Regular Excavation Work (Muck) 500,000 CY $6.00 $3,000,000 $1,530, Subsoil Excavation (Fill Material) 255,000 CY $2.50 $637,500 $325, Embankment 255,000 CY $2.50 $637,500 $325,125 *Note: Based on 51% average Subtotal $4,275,000 $2,180,250 10% Contingency $427,500 $218,025 Total $4,702,500 $2,398,275 The Chastain 30% design construction cost estimate for the project was $6,954,255. The RCS potential reduction of $2,398,275, would decrease the project total to $4,555,980, assuming no other scope or budget changes are made. In a similar manner of using NRCS standards, substantial cost saving could be realized for the water control structures and pump station. fitfor-purpose pump station based on agricultural criterion could provide the same functionality and safety as the proposed station while saving substantial capital and construction costs. RCS estimates that the $1,192, cost budgeted by Chastain for the pump station could be reduced by at least $500, if this design approach is taken. Finally, the redesign of the overflow weir using agricultural appropriate structures could eliminate large concrete components and thus further reduce the development cost. Page 5