How to Read a Bunker Site Assessment Report: Soil Borings, Water Table, and Bearing Capacity

A soil boring report is the single most important document in any underground bunker project, and most property owners receive one without knowing how to interpret what it says. Understanding the key sections of a site assessment—soil classification, water table depth, and bearing capacity—gives you the ability to evaluate whether a proposed bunker design is actually matched to your property’s conditions, or whether a contractor is working from assumptions that could cost you significantly more in the long run.

What a Site Assessment Report Actually Contains

A geotechnical site assessment for an underground bunker project typically includes several distinct sections, each addressing a different aspect of the subsurface conditions at your property. The boring logs are the core of the report—vertical records of what the drill encountered at each depth interval, from the surface down to the depth required by the project. Alongside the boring logs, the report includes laboratory test results for soil samples collected during drilling, groundwater observations recorded during and after drilling, and the geotechnical engineer’s interpretation of what all of this data means for the proposed structure.

In Southwest Missouri, site assessments for bunker projects typically require borings to a minimum depth of fifteen to twenty feet below the proposed bottom of the structure, depending on the soil conditions encountered. This depth requirement exists because the loads from an underground bunker extend into the soil well below the foundation level, and the engineer needs data from the full zone of influence to make accurate structural recommendations. A report based on borings that stopped at ten feet when the bunker floor is planned at eight feet is not providing the data the engineer actually needs.

The number of borings matters as well. A single boring tells you what the soil looks like at one point on your property. Missouri’s geology—particularly in Greene, Christian, and Webster counties where karst limestone underlies much of the region—can vary significantly across a single parcel. A thorough site assessment for a bunker project typically includes a minimum of three borings: one at the proposed structure location and two at offset locations to characterize the variability of conditions across the site. Properties with known karst features or significant topographic variation may require additional borings.

Reading the Boring Logs: Soil Classification and What It Means

The boring log is a column-by-column record of what the drill encountered as it advanced through the soil. Each row in the log represents a depth interval—typically six inches to one foot—and includes a soil description, a classification symbol, and the results of any field tests performed at that depth. The classification system used in most geotechnical reports in the United States is the Unified Soil Classification System (USCS), which assigns a two-letter code to each soil type based on its grain size distribution and plasticity characteristics.

For bunker planning in Missouri, the classifications you are most likely to encounter are CH (high-plasticity clay), CL (low-plasticity clay), ML (silt), and SM (silty sand). The CH and CL designations are particularly significant because high-plasticity clays—which are common throughout Greene, Christian, and Webster counties—exhibit the most pronounced volume change behavior with moisture variation. A boring log that shows CH material from the surface down to fifteen feet is telling you that the site has significant shrink-swell potential, which directly affects the lateral earth pressure calculations for the bunker walls and the drainage system design required to manage moisture around the structure.

The Standard Penetration Test (SPT) blow counts recorded in the boring log provide a measure of soil density and strength. The SPT N-value is the number of hammer blows required to drive a standard sampler twelve inches into the soil. Low N-values (below 10) indicate soft or loose material with limited bearing capacity. High N-values (above 30) indicate dense or stiff material with good bearing capacity. For a bunker project, you want to see N-values in the bearing zone below the foundation that are consistent with the bearing capacity assumed in the structural design. If the boring log shows N-values of 8 to 12 in the zone where the engineer assumed a bearing capacity of 2,000 pounds per square foot, the design assumptions need to be revisited. As detailed in our guide on professional site evaluation, these discrepancies between assumed and actual conditions are exactly what a thorough site assessment is designed to catch before construction begins.

Water Table Depth: The Number That Drives Everything Else

The groundwater observations in a site assessment report are among the most consequential data points for bunker design, and they are also among the most frequently misunderstood. The report will typically record two groundwater measurements: the depth at which water was first encountered during drilling (the “water on” depth) and the stabilized water level measured after the boring has been open for a period of time (the “stabilized” or “static” depth). These two numbers are often different, and both matter.

The water-on depth tells you where the drill first encountered a saturated zone, which may be a perched water table above a less permeable layer rather than the true regional water table. The stabilized depth, measured after the boring has equilibrated, is a better indicator of the actual groundwater level at the time of drilling. However, neither number tells you what the water table will be during Missouri’s wettest seasons, which is the condition that governs the design of the waterproofing system and the drainage system for the bunker.

A thorough geotechnical report for a bunker project will include a discussion of seasonal water table variation based on regional data, local well records, or piezometer monitoring if the project warrants it. For planning purposes, the design water table used in the bunker’s structural and waterproofing calculations should reflect the highest anticipated groundwater level, not the level observed on the day of drilling. In many parts of Southwest Missouri, the seasonal variation in groundwater depth can be four to eight feet or more, which is the difference between a bunker floor that sits above the water table year-round and one that experiences hydrostatic pressure against the floor slab during wet seasons.

The hydrostatic pressure implications of water table depth are direct and significant. Water exerts pressure of approximately 62.4 pounds per cubic foot. A bunker floor slab that sits four feet below the seasonal high water table is subject to approximately 250 pounds per square foot of upward hydrostatic pressure—a force that must be resisted by the weight of the slab itself, the weight of the structure above it, or a combination of both. Undersized slabs in high water table conditions are subject to uplift failure, which is one of the most common and most expensive failure modes in underground construction. Our analysis of floor slab uplift resistance covers the structural requirements in detail.

Bearing Capacity: What the Soil Can Actually Support

Bearing capacity is the maximum load per unit area that the soil can support without shearing or settling excessively. For an underground bunker, bearing capacity governs the design of the foundation system—specifically, whether the structure can bear directly on the native soil at the proposed depth, or whether the soil conditions require a modified foundation approach such as a thickened slab, grade beams, or soil improvement.

The geotechnical engineer calculates allowable bearing capacity from the SPT N-values and laboratory test results in the boring logs, applying safety factors that account for the uncertainty inherent in subsurface investigation. A typical allowable bearing capacity recommendation for a bunker foundation in Missouri might range from 1,500 to 3,000 pounds per square foot, depending on the soil conditions at the foundation depth. The structural engineer uses this value to size the foundation elements—a lower bearing capacity requires a larger foundation footprint to spread the load, while a higher bearing capacity allows a more compact foundation design.

What the report cannot tell you directly is whether the bearing capacity at the proposed foundation depth is uniform across the entire footprint of the structure. Soil conditions in Missouri can vary laterally as well as vertically, and a boring at one corner of the proposed bunker may encounter significantly different conditions than a boring at the opposite corner. This is why multiple borings are essential for a thorough site assessment—a single boring provides a point measurement, not a characterization of the entire site. When the boring logs show significant variation in soil conditions across the site, the structural engineer needs to account for differential settlement in the foundation design.

Comparing Site Assessment Data to Contractor Proposals

The most practical use of a site assessment report is as a benchmark for evaluating contractor proposals. A proposal that does not reference the site assessment data—or that was prepared before the site assessment was completed—is not a proposal based on your actual site conditions. It is a generic estimate that may or may not be appropriate for your property.

When reviewing contractor proposals alongside your site assessment report, look for specific references to the geotechnical data. The proposal should identify the design water table depth used for waterproofing calculations, the allowable bearing capacity used for foundation sizing, and the lateral earth pressure coefficients used for wall design. These values should be traceable to the site assessment report. If a contractor cannot explain how their design parameters relate to the geotechnical data from your property, they are not designing for your site—they are applying a standard design that may be adequate or may be significantly undersized for your actual conditions.

The comparison table below summarizes the key data points from a site assessment report and how each one should influence the bunker design:

Missouri-Specific Conditions That Affect Site Assessment Interpretation

Southwest Missouri presents several site conditions that require specific attention when interpreting a geotechnical report for bunker planning. The karst limestone geology underlying much of Greene, Christian, Stone, and Taney counties creates the potential for solution features—voids, caves, and sinkholes—that standard soil borings may not detect. A boring that encounters solid rock at twelve feet does not rule out the presence of a void at fifteen feet. Properties in areas with known karst activity may require geophysical investigation methods such as ground-penetrating radar or electrical resistivity surveys in addition to standard soil borings to characterize the subsurface adequately.

Missouri’s clay soils also present a specific challenge related to the timing of site investigation. Clay soils change their engineering properties significantly with moisture content. A boring performed during a dry summer period will show higher N-values and lower plasticity than the same boring performed after a wet spring. The geotechnical engineer should account for this seasonal variation in their recommendations, but property owners should be aware that a site assessment performed during an unusually dry period may underestimate the challenges the site presents during wet conditions. Our resource on Missouri clay soil and bunker construction covers the seasonal behavior of these soils in detail.

The depth to bedrock is another Missouri-specific factor that significantly affects bunker planning. In parts of Greene and Christian counties, bedrock can be encountered at depths as shallow as four to eight feet below the surface. A bunker designed for a ten-foot depth of cover may require blasting or specialized rock excavation equipment if bedrock is present at that depth, which can add substantially to the project cost. The site assessment report should identify the depth to rock at each boring location and provide the geotechnical engineer’s assessment of whether rock excavation will be required for the proposed bunker depth.

How the Specification Process Uses Site Assessment Data

At Bunker Up Buttercup™, the site assessment report is the foundation of every specification package we develop for clients in Southwest Missouri. The geotechnical data from the boring logs directly informs the structural calculations for wall thickness and reinforcement, the waterproofing system specification, the drainage system design, and the foundation sizing. Every parameter in the specification package is traceable to the site-specific data from the assessment report, not to generic assumptions or standard details that may or may not be appropriate for the actual site conditions.

This approach matters because it is the only way to ensure that the contractors who bid on the project are pricing work that is actually appropriate for the site. A specification package based on generic assumptions may result in bids that appear competitive but are actually priced for conditions that do not exist at your property. When the contractor encounters the actual site conditions during excavation—higher water table than assumed, softer bearing soils, unexpected rock—the result is change orders that can significantly exceed the apparent savings from the lower initial bid. A specification package grounded in actual site assessment data eliminates this source of cost uncertainty before the first contractor submits a bid.

If you have received a site assessment report for your property and want to understand what it means for your bunker project, or if you are planning a bunker and have not yet commissioned a site assessment, our team serves property owners throughout Greene, Christian, Webster, Stone, and Taney counties. We can review your existing report, identify the data points that matter most for your project, and develop a specification package that puts those conditions to work in a design that is built for your actual site—not a generic approximation of it.