IN SITU SPECIFIC GRAVITY VS GRAIN SIZE: A BETTER METHOD TO ESTIMATE NEW WORK DREDGING PRODUCTION

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1 IN SITU SPECIFIC GRAVITY VS GRAIN SIZE: A BETTER METHOD TO ESTIMATE NEW WORK DREDGING PRODUCTION Nancy Case O Bourke, PE 1, Gregory L. Hartman, PE 2 and Paul Fuglevand, PE 3 ABSTRACT In-situ specific gravity is difficult to measure and often not included in the sediment geotechnical characteristic evaluation on projects. Yet in situ specific gravity combined with grain size is very important in calculating anticipated dredge production. Relying on grain size alone will tend to over-estimate hydraulic dredge production on new work projects, where sediment density tends to be greater than maintenance and/or remedial dredging projects. Blow counts provide the information to determine in situ density of the material to be dredged. This paper will discuss the relationship between in situ specific gravity, average grain size and blow counts as well as their use in calculating hydraulic dredge production. This information is applicable to dredging project design for new work and remedial dredging projects. Dredge production provides the basis for dredge selection, project budget, project schedule, residual control while influencing all aspects of the project. It is therefore important that this calculation be as accurate as possible and account for the varying material density to be expected on new work and remediation dredging projects. Keywords: Dredging, blow counts, in situ specific gravity, grain size, hydraulic dredge production, new work INTRODUCTION New work dredging projects by definition have a lack of historic dredging record as the basis for a specific production rate. For this reason, it is important to understand the elements that impact hydraulic dredge production and to be able to reasonably estimate a project production rate. This requires a bottom-up approach; using a specific dredge with existing equipment characteristics (horse power, ladder and/or booster pumps, pipeline ID diameter, cutterhead diameter, and pump efficiency) and applying it to the project (dredging depth, sediment characteristics, pumping distance, and lift to disposal site). Sediment characterization is required to confirm a dredge production rate. The calculation of a dredge production rate is generally accomplished using the D 50 grain size, specific gravity of solids, and in situ specific gravity on the waterway bed before it is dredged. While all of these project elements are important in estimating production, this discussion focuses on in situ specific gravity and grain size. It also examines how to determine these values. In-situ specific gravity is difficult to quantify by measure and often is not included in the sediment geotechnical evaluation for the project. Yet in situ specific gravity and grain size is information that must be identified to calculate a realistic dredge production. Relying on grain size alone as a value in a data base for hydraulic dredge production will tend to over-estimate dredge production on new work projects. New work sediment density tends to be greater than on maintenance and remedial dredging projects. Figure 1 shows the impact on dredge production as the in situ density is increased while all other variables are held constant. This example uses a medium size hydraulic dredge in fine to medium sand (D 50 =0.4mm) transporting the dredge slurry meters (3,000 feet). It is important that the production calculation be as accurate as possible and account for the varying material density to be expected on new work projects. Sampling for in situ density of sediments, grain size and blow counts taken from the dredge prism is the preferred source for data input. If this is not available, there is a less rigorous but acceptable approach that can be used to determine values for both equivalent grain size (D 50 ) based upon sediment descriptors and in situ specific gravity based upon sediment descriptors and blow counts. 1 Dalton, Olmsted & Fuglevand, Inc., NE 68 th Street, Suite B, Kirkland, WA 98033, , ncase@dofnw.com 2 Dalton, Olmsted & Fuglevand, Inc., Silverdale Way NW, Suite 201, Silverdale, WA 98383, , ghartman@dofnw.com 3 Dalton, Olmsted & Fuglevand, Inc., NE 68 th Street, Suite B, Kirkland, WA 98033, , pfuglevand@dofnw.com 215

2 Figure 1. Dredge production example GRAIN SIZE Grain size is one of the most fundamental sediment characteristics. Sediment grain sizes for a new work project should be determined for multiple depths that reflect the thickness of differing sediment deposits. In an effort to best represent the variable sediment density and commensurate grain sizes for a new work project, the samples should be obtained at depths where the rate of bore advancement is noted to change. The standard penetration test (SPT), and subsequent recovery of sediment for grain size and other tests should be obtained at the approximate same depth and material type. The equivalent grain size or D 50 value is used to estimate dredge production. D 50 is the grain size where 50% of the sample is coarser and 50% of the sample is finer. Differing from maintenance dredging, new work sediment typically does not have a uniform grain size for total depth of dredging, and will vary significantly between areas or elevations of the dredge prism. The design engineer must consider the bank height and average cut thickness when calculating a D 50 grain size. As a result the design engineer can opt to base the estimate on the most difficult dredge thickness conditions, or on an average of the total number of samples that represent the layering of varied sediment types for the depth of new work dredging. As an example the design of a deep draft berth area that has never been dredged before must now be dredged between depths of -4.9 meters to 15.5 meters (-16 feet to -51 feet). The grain size sampling for a representative grain size must first consider the depth of cut for each pass of the dredge until the total 1.07 meters (35 feet) of dredging is completed. The dredge cutterhead for a large pipeline dredge may be 3.0 meters (10 feet). The logic, in this case, is to sample every 3.0 meters (10 feet), and to obtain both an SPT test and sediment characteristics that includes grain size. Table 1. Material description vs grain size (USACE, 1963) Sediment Descriptor Grain Size (mm) Coarse gravel Fine gravel Coarse sand Medium sand Fine sand Silt Clay <0.005 The equivalent sediment size and in situ density is based upon a visual examination of sediment samples, driller remarks on rate of advance, the rattling of the pipe in oversize rock and gravels, the description of the bore sediment captured, and the sampling for grain size measurement from several elevations. Table 1 summarizes the US Army Corps of Engineers (USACE) definition of grain size ranges for various material types. Grain size must be measured for varying sediment conditions. 216

3 IN SITU SPECIFIC GRAVITY Specific gravity is a dimensionless unit defined as the ratio of the density of a material and the density of water at a specified temperature. In situ specific gravity of sediment is determined from an undisturbed sediment sample, which is often difficult to obtain. The in situ specific gravity of sediment in new work projects is typically denser (heavier) than frequently maintained areas with new deposit materials even though both new work and maintenance sediment can have similar D 50 values. Denser materials are more difficult to excavate, requiring greater break-out force, and also requiring more horse power on the pump to move the sediment and water slurry through the discharge pipeline to the disposal area. Higher in-situ specific gravity translates to more weight (solids) per cubic yard of solids and water slurry. The Standard Penetration Test (SPT) gives a numerical measure, called the n value, as the total blows required from a 63.5 kg (140 pound) hammer dropped a distance of 76.2 cm (30 inches) on to the drill rods. The n value is the number of blows of the hammer that drive the rod a distance of one foot, and is known as the penetration resistance. The Standard Penetration Test drives a 5.1 cm (2 inch) outside diameter split spoon or split barrel sampler into the material to be sampled a distance of eighteen inches. The blows for the first six inches are usually not counted in establishing n, with n based on the blows required to drive the sampler from 15.2 to 45.7 cm (6 to 18 inches). When in situ specific gravity measured by the SPT is not available, sediment descriptors can be used to estimate an n value and determine an approximate sediment density to calculate dredge production. Tables 2, 3 and 4 from USACE publications provide guidance in selecting an appropriate in situ specific gravity value for dredged sediment. Table 2. Material description vs average in-place density (USACE 1985) Material Description Average In-place Density* Mud (clay) and silt 1200 g/l Mud (clay) and silt 1300 g/l Mud (clay) and silt 1400 g/l Loose sand 1700 g/l Loose sand 1900 g/l Compacted (dense) sand 2000 g/l Stiff clay 2000 g/l Compacted shell 2300 g/l Soft rock 2400 g/l Blasted rock 2000 g/l *Sediment density (g/l) divided by 1000 is equivalent to specific gravity. Table 3a. Compactness descriptor vs SPT n-value for sand and silt sediments (USACE, 1995) Compactness Descriptor SPT N-Value (blows/30 cm (12 in)) Very Loose 0-4 Loose 4-10 Medium (firm) Dense Very dense Over

4 Table 3b. Compactness descriptor vs SPT n-value for high plasticity clay (USACE, 1995) Compactness Descriptor SPT N-Value (blows/30 cm (12 in)) Fluid 0 Very soft 0-2 Soft 2-4 Medium 4-8 Stiff 8-15 Very stiff Hard Over 30 Table 3c. Compactness descriptor vs SPT n-value for low to medium plasticity clay and clayey silts (USACE, 1995) Compactness Descriptor SPT N-Value (blows/30 cm (12 in)) Fluid 0 Very soft 0-4 Soft 4-8 Medium 8-14 Stiff Very stiff Hard Over 50 CASE STUDIES Case Study No 1, New Work vs Maintenance Dredging with Uniform Material Case Study No. 1 provides an example of a project with 1.5 meters (5 ft) of new work material overlaid by 1.5 to 3.0 meters (5 to 10 feet) of maintenance dredging. Grain size analysis for the surface maintenance material provided an average D 50 = 0.03 mm and core borings provided maintenance material descriptors of fine grained sandy silt, very fine grained sand, clayey silt with USCS classifications of ML and CL. The new work subsurface deepening sediments recovered by the borings provided similar material descriptors of fine grained sand and sandy silt with USCS classifications of SM, SP, and SC. Material in the new work depths was also described as loose. Blow counts were provided but non-standard methodology had been used. In situ specific gravity measurements were not provided in the geotechnical information. The dredge had a five foot cutterhead diameter. The dredge cuts were set at 1.5 meters (5 feet) cut thickness. The actual maintenance dredging production rate was m 3 /hr (295 cy/hr) and the new work production rate was m 3 /hr (253 cy/hr). For this project, the original maintenance production rate calculations used an in situ specific gravity of 1.5 and the average D 50 = 0.03 mm. This over-estimated the production. Looking at Table 4, below, for Silty Sand, well graded and Uniform Inorganic silt provides a saturated specific gravity of 1.8. Recalculating the maintenance dredging production using the Table 4 value for in situ specific gravity provided an average theoretical rate of m 3 /hr (305 cy/hr), which is within 5% of the actual rate, m 3 /hr (295 cy/hr). The original new work production rate calculations used D 50 = 0.2 mm and in situ specific gravity of 1.9, based upon the core boring descriptors. Even though it was new work the core borings described the material as loose. Table 2, above provides in situ specific gravity for loose sand as 1.7 to 1.9 and compacted sand as 2.0. Using an in situ specific gravity of 1.9 provided a theoretical average production rate of m 3 /hr (295 cy/hr) but changing the in situ specific gravity to 1.95, provides a theoretical average production rate of m 3 /hr (255 cy/hr) versus the actual rate of m 3 /hr (253 cy/hr). 218

5 Table 4. Typical weight-volume properties of soils (USACE) Soil Description State Void Water Unit Weight Porosity Ratio Content Dry Saturated (n%) (e) (w%) PCF Kg/m 3 PCF Kg/m 3 Ref* Well-graded silty, sandy Loose gravel Dense SOW Glacial till, mixedgrained. Firm PHT Sand, mixed-grained Loose Dense PHT Well-graded sand, Loose subangular Dense SOW Well-graded sand, fine Loose to coarse, clean Dense Uniform sand Loose Dense PHT Uniform sand, fine to Loose medium, clean Dense Silty sand, well graded Loose Dense Sand & silt, micaceous Loose Dense SOW Windblown silt (loess) Firm PHT Uniform inorganic silt Loose Dense Loose Organic silt Dense Sandy or silty clay Soft Stiff Glacial Clay Soft Stiff PHT Clay (30 50% clay Soft sizes) Stiff Slightly organic clay Soft PHT Very organic clay Soft PHT Organic clay (30 50% Soft clay sizes) Stiff Montmorillonitic clay Soft PHT * => Hough (1957), PHT => Peck, Hanson, and Thornburn (1974), SOW => Sowers (1979) Case Study No. 2, New Work with Variable Material by Area and Depth Case Study No. 2 provides an example of a new work dredging project that had significant variance in types of material to be dredged. The material varied by area and by depth. The project had grain size distribution and core boring data. Core boring locations are shown in Figure 2. The upland disposal area was a significant distance from the disposal area and during most of the project 1 to 2 boosters were used. 219

6 Figure 2. Case study no. 2 core boring locations As a result of the variance in the materials, the material description and blow counts was analyzed for each boring. Based upon the sediment descriptions in the core borings a D 50 value was determined for each material layer and the blow counts were used with the descriptors to determine an in situ specific gravity. Figure 3 illustrates a compilation of the core boring data that was then used for this analysis. A weighted average D 50 and in situ specific gravity was calculated for each boring location within the dredging area. In order to estimate the dredge production, an isopach was created from the weighted average D 50 values, as shown in Figure 4. This overlaid the dredge plan and production rates were calculated for individual and separable dredging areas. Using this approach, the calculated theoretical dredging rate for the contracted dredging equipment was within 7% of the actual dredging rate for the project. Case Study No. 3, Dredge Production versus New Work Cut Thickness Case Study No. 3 provides an example of new work project conditions that go beyond the sediment D 50 and in situ density of the dredge material. This project had fine-grained, loose (low in situ specific gravity) materials identified to be dredged. It was a new work project, first time dredging. The actual production rate was significantly less than the theoretical production rates, based solely on grain size and in situ specific gravity of the dredged sediment. On this project, the more important project feature influencing dredge production was the shallow thickness of the dredged material and a subsurface hard rock material immediately below the specified dredge depth, as shown in Figure

The dredge will not achieve its theoretical production based upon sediment characteristics because this project is controlled by the inability of the dredge to physically move through the project.

7 The dredge will not achieve its theoretical production based upon sediment characteristics because this project is controlled by the inability of the dredge to physically move through the project. In this new work project the dredge cannot advance fast enough to feed the material to the pump and realize full production. There are other sediment characteristics that could impact dredge production, and reduce the expected dredge rate based on in situ density and grains size. One example is the new work presence of highly plastic fine grained clay materials. This sediment can form clay balls while moving through the discharge line, and subsequently the sediment grain size is not a D 50 value obtained from the borings. In this case, while fine-grained materials may indicate high dredge production rates, but clay balls form and the material behaves like a heavier coarse grained material. Figure 3. Case study no. 2 core boring analysis 221

8 Figure 4. Case study no. 2 dredging area D 50 isopach Figure 5. Case study no. 3 dredging area D 50 isopach 222

9 CONCLUSIONS New Work dredging projects require knowledge on the in situ density and the grain size to determine expected sediment production rates. A maintenance dredging mentality often overlooks the importance of the sediment density information. The in situ density can be estimated based on physical descriptors of the sediment, but the preferred approach is to provide borings within the New Work dredge prism, and meters (2-5 feet) below the dredge cut elevations as a minimum, with in situ measurement of sediment density. Blow counts, in situ density, grain size, visual observations, and sediment description are required. In situ sediment density measurements are critical New Work sediment information and often over looked. REFERENCES Hough, B.K., Basic Soil Engineering, The Ronald Press Company, New York, Peck, R. B., Hanson, W. E., & Thornburn, T. H., Foundation Engineering, New York, Ny, John Wiley & Sons, Sowers, G. F., Introductory Soil Mechanics and Foundations, Geotechnical Engineering, USACE Dredging Fundamentals, Proponent Sponsored Engineer Corps Training (PROSPECT) Training course materials. USACE (1963), ENG form 2087, 1 May USACE (1985),Table B-2, ER , Apr 85. USACE (1995),Table 1, DRP-2-13, January

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B-1 BORE LOCATION PLAN. EXHIBIT Drawn By: 115G BROOKS VETERINARY CLINIC CITY BASE LANDING AND GOLIAD ROAD SAN ANTONIO, TEXAS.

B-1 BORE LOCATION PLAN. EXHIBIT Drawn By: 115G BROOKS VETERINARY CLINIC CITY BASE LANDING AND GOLIAD ROAD SAN ANTONIO, TEXAS. N B-1 SYMBOLS: Exploratory Boring Location Project Mngr: BORE LOCATION PLAN Project No. GK EXHIBIT Drawn By: 115G1063.02 GK Scale: Checked By: 1045 Central Parkway North, Suite 103 San Antonio, Texas 78232