Since we lack x-ray vision, we can’t tell from just looking at the surface what lies beneath. And what lies beneath determines the ease or difficulty of the earthwork job. The types of geological strata, as well as all information related to their physical characteristics and locations—determine how you will excavate and reutilize them, how long it will take you excavate or compact the soil, and how much it will cost you to do either.
Contractors depend on engineers or hydrogeologists to provide the most accurate information possible. This is a constant bone of contention between the two, with the contractor often left holding the bag if the engineering plans are either completely wrong or partially inadequate as the result of poor hydrogeological information. Most sites have multiple layers of geological strata with a wide range of physical characteristics. These strata are often arranged by time, chance, and natural forces into complicated formations consisting of variable thicknesses, discontinuous boundaries, and variable slopes. Each has a unique impact on the project’s soil balance and the terrain (both existing and created by the earthwork operations). These in turn have an impact on equipment and vehicle operations that affect the bottom line of any project.
Shrink, Load, and Swell Factors
The first physical attributes affecting the effort and bottom line of an earthwork project are the volume ratios relating the natural in-ground bank volumes of soil with the expansion that results from excavation and the subsequent recompaction of the soil as it is utilized during construction.
Soil volumes are measured three ways: as bank cubic yards, as loose cubic yards and as compacted cubic yards. Each is related by either the swell factor or the shrink factor of the soil. These factors are a function of the soil’s physical characteristics and have to be accurately determined prior to beginning earthwork operations in order to estimate the efforts required for hauling, stockpiling, and in-place compaction. The use of these factors assumes a homogenous material, since each soil type will have unique factors. When faced with multiple soil types needed for excavation or utilized for earthwork construction, the contractor will have to apply individual factors for each type.
A soil’s swell factor reflects the reality that the volume of soil placed by nature in the ground is not the same as the volume of the same mass of dirt excavated by the contractor and placed in the dump truck. The same mass of soil occupies more volume in the truck (loose cubic yards) than it does in the ground (bank cubic yards). The swell factor is an adjustment representing this increase in volume. However, the swell factor plays no part in the calculation of an earthworks balance. What it does is determine the subsequent hauling and stockpiling requirements.
Swell is the percentage increase in volume caused by the excavation of soil. Physically, the act of excavation breaks up the soil into particles and clods of various sizes. This creates more air pockets and results in an effective increase in the soil’s void volume. An increase in volume also results in a decrease in density. This decrease in density and increase in volume varies between soil types and is not proportional due to the initial, natural void volume of the bank soil. The swell factor is calculated as follows:
|loose cubic volume|
|————————-= 100% + swell factor|
|bank cubic volume|
For example, if the volume of loose soil is 1.25-times greater than the bank volume it occupied prior to excavation, the soil’s swell factor is 25%. The load factor is the inverse of the swell factor and can be calculated as follows:
|————————-= load factor|
|100% + swell factor|
This is equivalent to:
|————————-= load factor|
In the above example, soil with a swell factor of 25% would have a load factor of 80%. The load factor can be used to show the relationship between loose and bank cubic yards by dividing the loose volume by the load factor.
In addition to the swell factor and its associated load factor, soil also has a shrink factor. While the first two relate the volume of an equal mass of bank soil in the ground with the loose mass deposited in stockpiles or dump trucks by excavation, the shrink factor relates the initial bank soil with the volume resulting from subsequent placement and compaction of the loose soil into earthen structures. Often this ratio is not a result of natural characteristics so much as the construction specifications. For example, clay soils used to construct a high density/low permeability containment layer for landfills or lagoons are typically constructed in controlled lifts of a certain spread thickness which are then compacted to a final desired thickness. Typically, the soil is spread out over the work area in loose lifts about 8 inches thick. Multiple passes with a sheepsfoot roller are then performed to compact and knead the loose clay into a tight layer of about 6 inches thickness. This results in a post-compaction volume that is 25% smaller than that of the initial loose placement volume. The resultant shrink factor is calculated as follows:
|compacted cubic yards|
|bank cubic yards|
Suppose an earthwork operator is excavating wet clay. Assume its initial bank density to be 3,500 pounds per cubic yard and its excavated loose density to be 2,800 pounds. One ton of this soil (2,000 pounds) would occupy 0.57 bank cubic yard in the ground while its hauled or stockpiled volume would be 0.71 loose cubic yard. This results in a swell factor of 24%. Its related load factor would be 0.80. Suppose further that this clay is used to construct a lagoon liner in the manner described above with compaction reducing its volume to 0.53 cubic yard. The shrink factor, then, would be 0.93.
So, for planning purposes, the earthwork contractor will have to assume that for every 100 cubic yards he excavates he will need to haul 124 cubic yards so that he will be able to place 93 cubic yards. All of these numbers affect his bottom line. The first determines the amount of the excavation effort, the second determines his hauling requirements and the third determines the overall cost of the finished project.
But suppose the geological information is wrong, and instead of encountering 100 cubic yards of wet clay, the contractor excavates 100 cubic yards of loose sand and clay having a bank density of 3,400 pounds per cubic yard and a loose density of 2,700 pounds per cubic yard. Two-thousand pounds of this material would occupy 0.59 cubic yard in the ground and 0.74 cubic yard in the truck. This results in a swell factor of 26%. So the contractor will have to haul 126 cubic yards of this material for every 100 cubic yards in the ground. (Odds are this material would not be suitable for liner construction due to failure to meet permeability requirements, so the shrink factor is not a concern.) An extra 2 cubic yards of hauling may not sound like much, but the additional cost can make or break the budgets of large-scale, highly competitive earthwork projects with tight profit margins.
Blasting, Ripping and Dewatering
The real budget buster, however, is the unforeseen encounter with hardpan material or bedrock. An insufficient hydrogeological site investigation may fail to adequately map the depths and slopes of underlying bedrock. Even normally sufficient hydrogeological investigations may miss significant changes in bedrock surfaces due to the presence of anticlines between the investigation locations. Anticlines are defined as folds with older rocks toward the center of curvature, or a fold that is convex upward or that had such an attitude at some time in its geological history. Some anticlines are so complicated they are difficult or impossible to define. When these nasty surprises are encountered, the contractor has only two choices: rip or blast.
By definition, bedrock and hardpan have essentially zero voids volume. Bank volume is as compacted as this material can get. Blasting of rock volumes, measured in bank cubic meters, is necessary so that the massive rock formations are reduced to chunks and shards small enough to be loaded into haul trucks. Like the less violent act of excavation, swell factors are applied to blasted rock to reflect the volume increase after blasting. Rock, when crushed and well-broken by explosives, expands by a factor of 35%. Therefore, for every 100 cubic yards of bank rock or hardpan, blasting will increase the volume removed from the site to 135 cubic yards. If the blasted material is to be crushed further for use as roadbed material, the final volume could be over 140 cubic yards.
In addition to the increase in volume due to its swell factor, rock blasting can also result in overbreak. Overbreak occurs when blasting operations break up rock below the planned limits of excavation. These spoils have to be removed and replaced with stable fill in order to provide a structurally sound foundation for the subsequent earthwork. The backfilling, compaction, and smooth grading of the new bottom surface are usually done at the contractor’s expense and the amounts are not typically included in the estimates of excavation quantities.
Ripping is performed with a special ripper attachment fixed to the back of a large tractor dozer. Ripping and dozing hard rock are the most difficult jobs for track-type tractors, and cause the highest repair costs. Among other problems, ripping tends to wear out undercarriage components faster than pushing or dozing applications. Geological characteristics—fractures, planes of weakness, fault lines or brittle, weathered strata—determine what types of hardpan material are suitable for ripping. Strata having massive homogenous formations with a non-crystalline structure usually have to be blasted.Neither blasting nor ripping is cheap. The most common method of rock blasting is drill and blast. Depending on the nature and extent of the rock formation, a pattern of relatively small holes will be drilled into the formation to a depth designed to ensure a complete breakup of the strata to be removed (while minimizing the potential for over breaking). Charges are set in these holes (either singularly are at multiple depths) and set with electrical fuses. The cost per bank cubic yards for blasting ranges from 40 cents to $1.50, depending on location and site conditions. Ripping can be even more expensive on a per-cubic-yard basis. This involves not just the operating costs of the ripper-equipped dozers, but maintenance and repair costs, as well. Rippers wear out very fast in extreme conditions, with ripper points often having a “lifetime” of only 10 to 12 hours of continuous use. Resultant repairs cost in excess of the equipment’s depreciation cost is common.
Dewatering is required for any excavation that experiences significant inflow of groundwater into the pit. Depending on the size of the excavation and its depths relative to the local groundwater table elevation, pumps required to drain the pit to allow for further excavation range from the tens of gallons per minute to the hundreds of gallons per minute. Most large-scale excavations will require multiple high-capacity pumps. This operation resembles the bailing of a very leaky and very large boat. Alternatives include surrounding the excavation area with horizontal interceptor drains to divert water around the site and the installation of a ring of well points designed to lower the groundwater table and allow for dry excavation. One way or another, dewatering adds significant unanticipated costs.
Now that we’ve seen some of the things that can go wrong, the problem becomes one of what can be done to minimize their potential for occurring (or if they occur, how we can minimize their adverse impact on earthwork planning).
Boring Logs and Fence Diagrams
The preferred method is a thorough hydrogeological investigation of the site. Typically, a hydrogeological investigation meets all the testing, measuring, or drilling requirements necessary to satisfy environmental regulations or scientific inquiry. However, even these standards may be insufficient to allow for adequate earthwork planning, especially in sites with complicated geology. In fact, the regulatory requirements for the number of borings per acre should be considered a minimum number for purposes of project planning.
Either way, test borings are a necessity if the planner is to have a decent idea of what lies beneath in the subsurface geology. Typically, borings should be drilled to a depth at least 10 feet below the planned bottom of the excavation. Borings along the perimeter of the site can be used as groundwater observation wells.
The locations, depths, and results of these borings should be clearly located by a surveyor who will record their northings, easting, and elevations (of both the existing ground and the top of the well casing). The log descriptions should be prepared using the Unified Soil Classification System (USCS) to describe the subsurface soil strata. The elevations of the top and bottom of each distinct soil stratum and a description of the types and contents of each stratum should be recorded on the log.
Additionally, the elevation of water in the well that is inserted after the boring is complete should be recorded to determine local groundwater conditions.
Each boring location should be shown on a topographic map. Alignments created by connecting adjacent boring locations should be drawn to create geological cross sections of the site. Such alignments should be located across the length of the site and at right-angle crossings to these first alignments. These cross sections are created by a “connect the dots” process where matching strata boundaries are connected by interpolated straight lines drawn from boring to boring. There may be instances where a stratum in one boring does not have a matching stratum in an adjacent boring (for example, a discontinuous sand lens isolated within an overall clay strata). In this case, the interpolation lines are drawn only halfway to the next boring and tapered off to show that they do not continue.
Despite their two-dimensional name, fence diagrams are actually three-dimensional representations of connected geological cross sections. The three-dimensional effect is achieved by staggering the boring locations (represented by the tops of each of the boring logs) on an isometric view of the site topography. Each log extends downward from the surface to its maximum boring depth. Matching strata surfaces are connected both along the main alignments and the secondary alignment oriented across the primary geological cross sections. The result is a pseudo-three-dimensional display of the underlying site hydrogeology on a single plan sheet.In addition to fence diagrams, the data from the borings can be used to create surfaces of the strata for plan view contours. Elevations of matching surfaces (such as groundwater elevations) at each boring are recorded at the boring locations. These elevations and the distances between adjacent boring locations can be used to create contour elevation lines delineating the strata’s surface elevations. Most computer AutoCAD systems will do this for the project planner by creating a Triangulated Irregular Network (TIN). These are triangles that represent small, continuous surface areas (like the facets of a jewel). Each corner of each triangle represents a field or hydrogeological survey point with northing, easting, and elevation coordinates. A TIN model is a series of these connected triangles and can represent the surface of a soil stratum by forming a series of contiguous, irregular triangles covering the entire surface.
TINs are used as the basis for Digital Terrain Modeling (DTM). DTM surfaces are used to generate the contour elevation lines by interpolating consistent elevations that cross the sides of each TIN triangle. Overlaying DTM surfaces can be used to define the vertical extents and the resultant depths and thicknesses of the strata bounded by these surfaces. This allows for another representation of the strata surface, the isosurface. These lines can be used to show relative depth from the ground surface to the strata’s surface or show the thickness of the strata across the site. These allow for more accurate cut-and-fill staking in the field and estimation of the in-place volume of the strata.
Surfaces and Strata, Reality and AutoCAD
This information is invaluable to both the planner and the operator. The volume is used to price the project in the planning stage, while the staking is used to guide the project in the field. There are quite a few AutoCAD and similar programs that provide this information to the earthwork project planner.
The Scientific Software Group provides both boring log and fence diagram software. The company’s gINT software is a borehole logging and geotechnical database management system that utilizes relational database software built especially for geotechnical and geo-environmental applications. It creates fully customizable database structures and user-defined reports, boring logs, 2D and 3D fence diagrams, histograms, graphs, and tables. Scientific Software’s Hydrogeo Analyst allows for groundwater and borehole data management along with visualization technology. QuickCross/Fence module allows the user to create 2D cross sections and 3D fence diagrams, and its drawing tools can edit the drawings right on the preview screen.
LAgEQ (LANGAN gINT EQuIS) is an application developed by Langan Engineering and Environmental Services Inc., integrating Microsoft Excel, gINT, EQuIS Geology, Microsoft Access, and AutoCAD to jump-start database projects using gINT and EQuIS Geology applications. This software package can expedite the population of complex hydrogeological data bases without requiring more from the operator than a basic understanding of Microsoft Excel. The interface is an Excel form that mimics a typical boring log. Once he data has been entered, LAgEQ allows output of the data with gINT to generate finished boring logs and fence diagrams.
Integrated Geologic Modelling Ltd. of the United Kingdom develops software for visualization of geological, geochemical, geophysical, and borehole data. Its Geoexpress software integrates hydrogeological and borehole data in 3D space, allowing for full-color data presentation in a graphical format. GeoExpress also has the ability to perform interactive analysis, making this product a powerful tool for the interpretation of earth science data. Being PC-based, GeoExpress can be used in the field to assist the integration and interpretation of data sets typically used in exploration. Its inherent versatility allows data to be imported from ASCII and ODBC sources in addition to a range of formats from other major exploration software packages.
Porpoise Media’s Well Logger software provides an easy means of drafting boring logs and fence diagrams. Designed for ease of use, it has a simple user interface with an easy-to-use spreadsheet interface with drop-down boxes for simplifying data entry of each borehole. Data entry allows the input of information concerning borehole lithology, samples collected, well construction, or borehole backfilling details, as well as general information about the project and boring. User-defined layouts or predefined layouts allow for quick graphical production.
Boring-Log.Com (a subsidiary of the Scientific Software Group) provides software for borehole logging, fence-diagram plotting, and site characterization. Its gINT is a powerful relational database software package tailored for the needs of geotechnical data management, enabling users to create fully customizable database structures, user-defined report formats, or boring logs. Its QuickGIS organizes lithology data for a set of borings into a table of X-Y-Z coordinates and geological layers. These data can then be exported to ArcView, other GIS systems or QuickCross/Fence.
Geosoft produces a geology display software package, Target, which allows for quick and effective visualization of subsurface drilling and borehole data. Both earthwork project planners and exploratory drillers can use this program to evaluate site hydrogeology. It can quickly generate sections in any orientation while processing data from thousands of holes. Its data mapping capabilities make quick analysis and processing possible. It is also compatible with other geotechnical software packages such as acquire.
Minimizing the Unknowns
The cost of digging a hole or constructing an earthen dike depends on several factors: swell increasing hauling costs, shrink increasing compaction costs, the need for blasting or ripping increasing excavation costs or the cost of dewatering impacting the overall budget. Naturally, the more that is known about a site, the less potential for budget-busting surprises. This usually means an increase in the number and depth of borings and test pits performed at the site to establish a model of the local hydrogeology.
However, nobody’s site investigation budget is unlimited. And even if borings were done every 100 feet in a tight grid across the site, the contractor still won’t know everything he needs to know; surprises are still possible. Unknowns can never be completely eliminated, only minimized.All these software systems and analytical methods described above can greatly reduce the time needed to analyze a site and provide the information that an earthwork contractor needs to evaluate and estimate the cost of a job. In addition to accuracy, they provide speed of analysis. Time is always of the essence in an ever accelerating business world. Which brings us to a saying that is opposite to the one that began this article but is equally true: “He who hesitates is lost.”