Geotechnical Engineering Guide for Contractors

The ground under your project is the one thing you cannot change after construction starts. You can swap out finishes, reroute ductwork, and adjust framing layouts on the fly. But the soil? That’s what it is. And if you don’t understand what’s down there before you break ground, you’re rolling the dice on your schedule, your budget, and your reputation.

Geotechnical engineering is the discipline that figures out what the ground is made of, how it will behave under load, and what you need to do to build safely on it. As a contractor, you don’t need to become a geotechnical engineer. But you absolutely need to understand geotechnical reports, know what the recommendations mean for your scope, and recognize when conditions in the field don’t match what’s on paper.

This guide covers the practical side of geotechnical engineering for contractors. We’ll walk through soil types, site investigation, how to read a geotech report, foundation selection, earthwork, dewatering, and how to protect yourself from the kind of below-grade surprises that wreck projects.

Why Geotechnical Engineering Matters to Contractors

There’s a saying in construction that goes something like this: the cheapest part of a building is the part you can see. Everything expensive is hidden. And nothing is more hidden, or more expensive when it goes wrong, than what’s happening below grade.

Geotechnical problems show up in a few predictable ways:

Foundation failures. If the soil can’t support the loads, you get settlement. Sometimes uniform, which is manageable. Sometimes differential, which cracks foundations, racks door frames, and can make a building structurally unsound. Differential settlement of even a half inch across a span can cause visible damage in finishes and functional problems with doors and windows.

Excavation surprises. You bid the job expecting to dig through sandy clay and hit solid rock at six feet. Now you need a rock hammer or blasting, your excavation timeline just tripled, and the cost increase is coming out of someone’s pocket. Without adequate site investigation, that someone is usually you.

Water problems. Groundwater that nobody anticipated showing up in your excavation. Perched water tables that the borings missed because they were done during a dry season. Dewatering costs that weren’t in anybody’s budget because the geotech report was either missing or nobody read it carefully enough.

Slope instability. Cut slopes that fail because the soil wasn’t strong enough to stand at the angle shown on the grading plan. Retaining walls that move because the lateral earth pressure was underestimated. Erosion that undercuts your foundations before you can get backfill in place.

Every one of these problems is preventable with proper geotechnical investigation and, just as important, with contractors who understand what the investigation is telling them.

The real cost of geotechnical surprises isn’t just the direct fix. It’s the schedule impact. A foundation redesign in the field can add weeks or months. Equipment mobilization for unexpected rock removal doesn’t happen overnight. Dewatering systems take time to design, procure, and install. And while all of that is happening, your crews are standing around, your subs are getting pushed, and your client is asking questions you don’t have good answers for.

Understanding Soil Types and Their Construction Impact

Not all dirt is created equal. The type of soil on your project site determines everything from how you excavate to what kind of foundation the building sits on to how water moves through (or doesn’t move through) the subsurface.

Here’s a practical breakdown of the major soil types you’ll encounter and what they mean for your work.

Granular Soils: Sands and Gravels

Sands and gravels are generally the most cooperative soils for construction. They drain well, compact predictably, and provide good bearing capacity when dense. They don’t swell with moisture changes and they’re relatively easy to excavate.

The catch is stability. Clean sands (sands without any clay or silt content) won’t stand in a vertical cut. They’ll slump to their natural angle of repose, which is typically between 30 and 35 degrees. That matters when you’re trying to excavate a basement or utility trench. You’ll need shoring, sloping, or benching to keep the walls from caving in.

Below the water table, saturated sands get even more interesting. They can flow into excavations (a condition called “running sand” or “flowing sand”) and in seismic zones, loose saturated sands can liquefy during an earthquake, essentially turning into a liquid and losing all bearing capacity.

Dense, well-graded gravels are some of the best foundation soils you’ll encounter. If your borings show dense gravel or cobbles at foundation level, that’s usually good news for your budget.

Cohesive Soils: Clays and Silts

Clay is the soil type that gives contractors the most headaches. It has some useful properties: it’s relatively impermeable (which is why it’s used for liner systems) and it can stand in vertical cuts better than sand. But the problems outweigh the benefits for most construction scenarios.

Expansive clay is the big one. These clays swell when they absorb water and shrink when they dry out. The volume change can be dramatic. Highly expansive clays can exert pressures of 10,000 pounds per square foot or more against foundation walls and slabs. That’s enough to crack concrete, heave floor slabs, and push foundation walls inward.

If your geotech report mentions “high plasticity clay,” “CH soil,” or “high swell potential,” you need to pay very close attention to the foundation recommendations. These soils typically require deeper foundations, void space under grade beams, moisture barriers, and careful drainage design to manage the moisture fluctuations that drive the swelling cycle.

Silts fall somewhere between sand and clay in their behavior. They can be tricky because they have some cohesion (they’ll stand in a cut briefly) but they lose strength rapidly when disturbed or when water content changes. Silty soils are particularly prone to frost heave in cold climates because they have the right combination of pore size to draw moisture upward through capillary action.

Organic Soils and Peat

If your borings encounter organic soils or peat, the geotech report is going to recommend removing them. Organic soils compress significantly under load, they continue to compress over time as the organic material decomposes, and their engineering properties are unpredictable.

You cannot build on organic soils. Period. They need to be excavated and replaced with engineered fill, or you need to use deep foundations that bypass the organic layer entirely and bear on competent material below.

The excavation quantities for organic soil removal can be enormous. Organic deposits can be 5, 10, or even 30 feet thick in some locations. That’s a lot of material to remove and replace. Make sure the geotech report clearly defines the limits of organic soil so you can estimate the earthwork accurately.

Rock

Rock sounds like it should be the best foundation material, and it usually is, if your structure is designed for it. The problem with rock is getting through it when you don’t want it there.

Rippable rock (rock that can be broken up with conventional excavation equipment) is manageable but slow and expensive compared to digging through soil. Non-rippable rock requires hydraulic breakers, chemical expanding agents, or blasting. All of those add significant cost, noise, vibration, and schedule time.

Rock also introduces challenges for utilities. Trenching through rock for underground pipes and conduits is a different operation entirely from trenching through soil. The cost per linear foot can be five to ten times higher.

Your geotech report should characterize the rock, noting its type, hardness, fracture spacing, and weathering condition. That information helps you plan your approach and price it correctly.

The Geotechnical Investigation Process

Understanding how a geotech investigation works helps you interpret the results and, just as important, understand the limitations of what the report can tell you.

Desktop Study and Site Reconnaissance

Before any drilling happens, the geotechnical engineer reviews existing information. Geological maps, previous reports from nearby sites, aerial photographs, historical land use records, and topographic data. This desktop study gives them an initial picture of what to expect and helps them plan the field investigation.

A site reconnaissance visit looks at surface conditions: existing vegetation (which can indicate water tables and soil types), drainage patterns, existing structures and their condition, and any visible signs of slope movement or settlement.

Soil Borings

Soil borings are the core of any geotechnical investigation. A drill rig advances a hollow-stem auger or casing into the ground, and soil samples are collected at regular intervals (typically every 2.5 or 5 feet) and at every change in material.

The standard test performed during drilling is the Standard Penetration Test (SPT). A 140-pound hammer drops 30 inches onto a split-spoon sampler, and the number of blows required to drive the sampler 12 inches is recorded. This blow count (called the N-value) is one of the most important numbers in the geotech report. Higher N-values mean denser, stronger soil. Low N-values mean loose or soft soil.

For reference:

• N-value 0 to 4: Very loose sand or very soft clay. Expect problems.
• N-value 4 to 10: Loose sand or soft clay. May need improvement.
• N-value 10 to 30: Medium dense sand or medium stiff clay. Generally adequate for shallow foundations.
• N-value 30 to 50: Dense sand or stiff clay. Good bearing material.
• N-value over 50: Very dense or hard. Excellent bearing capacity but potentially difficult to excavate.

The number and depth of borings depends on the project. A single-family home might need two to four borings. A commercial building might need ten or more. Borings should be located where the heaviest loads will be concentrated, where cuts and fills are planned, and where variable conditions are suspected.

Laboratory Testing

Soil samples collected during drilling get sent to a lab for further testing. Common tests include:

• Moisture content: How much water is in the soil right now
• Atterberg limits: The moisture levels at which clay changes from solid to plastic to liquid behavior. These define whether a clay is low or high plasticity and predict its swell potential.
• Grain size analysis: What the soil is made of (percentages of gravel, sand, silt, and clay)
• Compaction testing (Proctor test): Determines the maximum density the soil can achieve and the optimum moisture content for compaction. This becomes your target for earthwork.
• Consolidation testing: How much clay will compress under load and how long it will take. Critical for settlement estimates.
• Shear strength testing: How strong the soil is. Used for foundation bearing capacity and slope stability calculations.
• Swell testing: For clays, how much volume change occurs when the soil absorbs water. This drives foundation design in expansive soil areas.
• Corrosion testing: pH, sulfate content, chloride content, and resistivity. Determines whether the soil is aggressive to concrete or steel and what protection measures are needed.

Groundwater Monitoring

During drilling, the geotechnical engineer notes the depth where water is first encountered and the depth where it stabilizes after a period of time. In some cases, monitoring wells or piezometers are installed to track groundwater levels over time.

A single groundwater reading during drilling has limited value because water tables fluctuate seasonally. A reading taken in August may be several feet lower than the actual high water table that occurs in March or April. The geotech report should discuss seasonal fluctuation and, ideally, base its recommendations on a worst-case (highest) water table.

This matters to you because dewatering costs are directly tied to how much water you have to deal with. If the report says the water table is at 12 feet but the seasonal high is at 8 feet and your foundation is at 10 feet, you have a dewatering problem that a dry-season boring didn’t reveal.

How to Read a Geotechnical Report

The geotech report is one of the most important documents on any project, and it’s also one of the most commonly ignored. Too many contractors flip to the foundation recommendations, note the bearing capacity number, and file the report away. That’s a mistake.

Here’s what to actually look for.

The Boring Logs

Boring logs are the graphical representation of what was found at each boring location. They show soil layers, descriptions, blow counts, sample depths, and groundwater observations. Read them. Compare what was found at different boring locations. If Boring 1 shows clay to 15 feet and Boring 2 (50 feet away) shows sand to 15 feet, you have variable conditions and need to be prepared for both.

Pay attention to the legend and soil descriptions. Terms like “loose,” “soft,” “wet,” “organic,” and “fill” are all flags that deserve your attention.

The Site Plan

The report should include a plan showing where the borings were done relative to the building footprint and site features. Check whether the borings actually covered the areas where your work will happen. If the building got redesigned after the geotech investigation and the borings don’t align with the current footprint, there may be areas of the site with no subsurface data at all.

Engineering Recommendations

This is where the geotechnical engineer translates the data into construction guidance. Key items to look for:

Allowable bearing pressure. The maximum load per square foot that the soil can safely support. This drives your foundation sizing. Common values range from 1,500 psf for soft clays to 4,000 psf or more for dense gravel.

Foundation type recommendations. Shallow footings, deep foundations (piles or drilled shafts), mat foundations, or ground improvement. If the report recommends deep foundations, that’s a significant cost driver compared to spread footings.

Earthwork recommendations. How to prepare subgrade, what compaction requirements apply, whether on-site soils can be reused as fill, and what imported fill specifications are needed if they can’t.

Lateral earth pressures. For retaining walls and basement walls, the report specifies the pressures the soil will exert. These drive the structural design of those walls.

Pavement sections. For parking lots and roadways, the report recommends pavement thickness based on subgrade strength.

Drainage recommendations. How to handle surface water and groundwater, including whether foundation drains are needed.

Seismic site class. Based on the soil conditions, the report classifies the site for seismic design purposes (Site Class A through F). This affects the structural engineer’s seismic design and can influence foundation costs.

The Limitations Section

Every geotech report has a limitations section. Don’t skip it. It tells you what the report doesn’t cover, the assumptions that were made, and the conditions under which the recommendations apply. If site conditions change (unexpected fills, different water levels, or different structural loads than assumed), the recommendations may not be valid.

This section is also your documentation if conditions in the field don’t match the report. Noting discrepancies immediately and getting the geotechnical engineer back out to evaluate is the right move. Don’t try to make field decisions about changed geotechnical conditions without engineering input.

Foundation Selection: Matching the Foundation to the Soil

The geotechnical report recommends a foundation type, and the structural engineer designs it. As a contractor, you need to understand why a particular foundation was chosen and what that means for your construction approach.

Shallow Foundations

Spread footings and strip footings work when there’s competent bearing soil within a few feet of the surface. They’re the cheapest and simplest foundation type. Your excavation is minimal, your formwork is straightforward, and the concrete volumes are predictable.

The key with shallow foundations is subgrade preparation. The bottom of the footing excavation needs to be clean, undisturbed soil at the bearing elevation specified in the geotech report. If you over-excavate, you can’t just throw the soil back in and compact it under a footing. You’ll need to fill with lean concrete or compacted granular material, and the geotechnical engineer should approve the repair.

In expansive soil areas, shallow foundations often include void forms under grade beams. These cardboard or foam void forms create a gap between the beam and the soil so that when the clay swells, it pushes into the void space instead of lifting the beam. The void depth is specified in the geotech report based on the expected swell. This detail is critical to get right. If the void forms aren’t installed correctly or the wrong depth is used, the grade beams will heave.

Deep Foundations

When surface soils are too weak, too compressible, or too unpredictable for shallow foundations, the design goes deep. Deep foundations transfer building loads through the weak upper soils to stronger material below. The three main types are:

Driven piles. Steel H-piles, pipe piles, or precast concrete piles driven into the ground with a pile hammer. Driven piles are fast to install and the driving resistance provides a built-in quality test. If the pile reaches the required resistance, it has the required capacity. The downsides are noise, vibration (which can damage adjacent structures), and the inability to adjust length easily.

Drilled shafts (caissons). A large-diameter hole is drilled into the ground, a reinforcing cage is placed, and concrete is poured. Drilled shafts work well in rock and in soils where driving piles would be impractical. They can carry very large loads. The construction process is more complex than piling: you need to manage the open hole (preventing cave-in with casing or drilling slurry), dispose of spoils, and place concrete properly.

Helical piles (screw piles). Steel shafts with helical plates that are screwed into the ground. They’re relatively quiet, vibration-free, and quick to install. They work well for light to moderate loads and are popular for additions, underpinning, and sites with access restrictions. Capacity is verified by monitoring torque during installation.

Each foundation type has different cost, schedule, and logistics implications. Driven piles need a crane and hammer. Drilled shafts need a drill rig that may be very large depending on shaft diameter and depth. Helical piles need specialized installation equipment but the rigs are smaller. Factor all of this into your planning, including access routes, staging areas, and the impact on adjacent work.

Ground Improvement

Sometimes it makes more financial sense to improve the existing soil rather than bypass it with deep foundations. Common ground improvement methods include:

Dynamic compaction. A heavy weight is dropped repeatedly from a crane to densify loose granular soils. Effective but creates significant vibration and noise. Not suitable near existing structures.

Vibro-compaction and vibro-replacement (stone columns). A vibrating probe is inserted into the ground to densify granular soils or create columns of compacted gravel in soft cohesive soils. Stone columns provide drainage in addition to improved bearing capacity.

Soil mixing. Cement or lime is mixed into the soil in place to improve its strength. Used for soft clays and organic soils.

Grouting. Various types of grout (cement, chemical, or compaction grout) are injected into the ground to fill voids, stabilize loose soils, or lift settled structures.

Surcharging. A load of fill is placed on the site and left for a period of time (weeks to months) to precompress soft soils before construction. The surcharge is removed after the target settlement has occurred. This is the cheapest ground improvement method but it requires time.

Ground improvement is its own specialty. If the geotech report recommends it, bring in a specialty contractor early. These aren’t operations that general excavation crews can handle, and doing them wrong can make conditions worse.

Earthwork: Getting the Ground Ready to Build

Earthwork is where geotechnical engineering meets the real world. All the borings and lab tests are just preparation for this: moving dirt, placing fill, compacting it, and creating a stable platform for construction.

Stripping and Grubbing

Before any earthwork begins, you strip the topsoil and remove any vegetation, roots, and organic material from the work area. Topsoil is not engineered fill. It contains organics that decompose and compress. It needs to be removed from any area that will support structures, pavements, or utilities.

Strip depth is typically 6 to 12 inches but can vary based on site conditions. Stockpile the topsoil separately if it will be reused for landscaping. Don’t mix it with excavated subsoil.

Cut and Fill Operations

Mass grading moves earth from high areas (cuts) to low areas (fills) to create the design grades. The geotech report tells you whether the on-site soils are suitable for reuse as structural fill and what moisture conditioning may be needed.

Most soils need to be within a certain moisture range to compact properly. Too dry and the particles won’t bond. Too wet and the soil becomes a muddy mess that won’t compact at all. The optimum moisture content from the Proctor test is your target. In dry climates, you’ll be adding water. In wet climates, you may need to aerate or dry the soil before placing it.

Fill should be placed in lifts (layers) of uniform thickness. Typical lift thickness is 8 inches loose (before compaction), which compresses to about 6 inches after compaction. Thicker lifts don’t compact adequately through their full depth, resulting in a compacted surface over loose material underneath. This is one of the most common earthwork quality failures: the surface passes the density test but six inches down the fill is still loose.

Compaction Testing and Quality Control

Compaction testing verifies that fill has been placed correctly. The standard test measures the in-place density of the compacted fill and compares it to the maximum density from the Proctor test. The result is expressed as a percentage: 95% compaction means the fill has achieved 95% of the maximum Proctor density.

Typical compaction requirements:

• Under structures: 95% to 98% of modified Proctor
• Under pavements: 95% of modified Proctor
• General site fill: 90% to 95% of modified Proctor
• Backfill in utility trenches: 90% to 95% of modified Proctor

Testing frequency varies by project specifications, but a common standard is one test per 2,500 square feet per lift, plus tests at any location that looks questionable.

Nuclear density gauges are the most common field testing method. They provide results in minutes. Some projects are moving to electronic density gauges or other non-nuclear methods due to regulatory requirements for handling radioactive materials.

When a test fails, the area needs to be reworked. That means breaking up the fill, adjusting moisture, recompacting, and retesting. This is normal and expected. What’s not acceptable is ignoring failed tests or moving on without resolution. Failed compaction under structures will eventually show up as settlement, and by then the fix is enormously more expensive.

If you’re looking for better ways to track compaction testing, inspection schedules, and field documentation across your projects, a good construction project management platform can keep everyone on the same page and make sure nothing falls through the cracks.

Dewatering: Dealing with Groundwater During Construction

Groundwater is one of the biggest wildcards in construction. If your excavation goes below the water table, you need a dewatering plan. And not just a pump in the corner of the hole.

When Dewatering Is Needed

Any time your excavation will be below the static groundwater level, you need dewatering. But “below the water table” isn’t the only trigger. Perched water (water trapped above an impermeable layer), seasonal high water, and surface water infiltration can all create dewatering needs even when the boring logs suggest you’re above the water table.

Signs you have a water problem developing:

• Seeping or flowing water on excavation faces
• Soft, pumping subgrade that won’t support equipment
• Boiling or heaving at the bottom of the excavation (water pressure pushing soil upward)
• Slopes sloughing or caving
• Water accumulating faster than your sump pump can handle

Dewatering Methods

Sump pumping. The simplest method. Dig a sump at the low point of the excavation and pump it out. Works for minor seepage and shallow dewatering. Doesn’t work well when you need to lower the water table significantly because the flow of water through the soil can carry fine particles, causing settlement outside the excavation.

Wellpoints. A series of small-diameter wells installed around the perimeter of the excavation, connected to a header pipe and vacuum pump. Wellpoints can lower the water table by about 15 to 18 feet from a single stage. For deeper excavations, multiple stages can be installed at different elevations. Wellpoints are the workhorse of construction dewatering.

Deep wells. Large-diameter wells with submersible pumps. Used for deeper dewatering or in soils where wellpoints can’t produce enough flow. More expensive to install but can handle higher flow rates and deeper drawdowns.

Cutoff walls. Rather than pumping water, cutoff walls block it from reaching the excavation. Sheet pile walls, slurry walls, or jet grout walls can create a relatively impermeable barrier around the excavation. Water is then pumped only from within the enclosed area. This approach is common in urban areas where lowering the water table outside the site could cause settlement damage to neighboring buildings.

Dewatering Permits and Environmental Considerations

Pumped groundwater has to go somewhere. Most jurisdictions require a dewatering permit that specifies where the water can be discharged, what quality standards it must meet, and what volumes are allowed.

If the site has any history of contamination, the groundwater may contain chemicals that prevent it from being discharged to storm sewers or surface water. Treatment may be required before discharge, which adds significant cost and complexity.

Dewatering can also affect neighboring properties. Lowering the water table on your site lowers it on adjacent sites too. If those sites have structures on shallow foundations in compressible soil, the lowered water table can trigger settlement. This is a real liability issue. Pre-construction surveys of adjacent structures are smart practice any time significant dewatering is planned.

Retaining Walls and Lateral Earth Pressure

Any time you hold back soil, whether it’s a basement wall, a retaining wall along a property line, or shoring for a temporary excavation, you’re dealing with lateral earth pressure. The geotechnical report provides the design pressures, but understanding the concepts helps you coordinate with the structural engineer and identify potential problems in the field.

Types of Lateral Earth Pressure

At-rest pressure. The pressure soil exerts against a wall that doesn’t move. Basement walls that are braced by floor slabs typically see at-rest pressures.

Active pressure. The lower pressure that develops when a wall moves slightly away from the soil. This is the design case for most retaining walls because they’re allowed to deflect slightly.

Passive pressure. The higher pressure that develops when a wall pushes into the soil. This is the resistance that keeps a retaining wall from sliding forward.

Surcharge Loads

Don’t forget about loads on top of the retained soil. Construction equipment, stockpiled materials, adjacent building foundations, and traffic loads all add surcharge pressure to retaining walls and shoring systems. The geotech report should specify surcharge values, but if your actual loads exceed what was assumed, the wall may not be adequate.

This is a common field coordination issue. The geotech report assumes a certain surcharge, the structural engineer designs the wall for that surcharge, and then somebody parks a loaded concrete truck right at the top of the retained cut. That kind of load can exceed the design surcharge by a factor of two or three.

Keep heavy equipment and material storage away from the tops of retaining walls and excavation shoring. If you need to work near the edge, check with the engineer on allowable loads and setback distances.

Drainage Behind Retaining Walls

Water pressure behind a retaining wall can double the load on the wall compared to the soil pressure alone. That’s why drainage is critical. Nearly every retaining wall needs a drainage system consisting of:

• A granular drainage layer or geocomposite drain board behind the wall
• A perforated drain pipe at the base of the wall
• An outlet for the collected water

If the drainage system gets clogged, blocked, or was never installed correctly, water builds up behind the wall and the extra pressure can cause failure. This is one of the top causes of retaining wall collapses.

Make sure your crews understand the drainage details and don’t skip or shortcut them. Document the drainage installation with photos before backfilling. Once the wall is backfilled, you can’t see whether the drainage was done right. Keeping organized photo documentation on every project makes it easy to prove the work was done correctly if questions come up later.

Common Geotechnical Problems and How to Handle Them

Even with a thorough investigation, the ground will surprise you sometimes. Here’s how to handle the most common issues.

Unexpected Soil Conditions

You’re excavating and the soil doesn’t match the boring log. Maybe you hit rock where the report said clay, or soft material where it showed dense sand. This happens because borings are point samples. They tell you what’s at that exact spot, not what’s between borings.

What to do:

1. Stop work in the affected area
2. Document the conditions with photos and notes
3. Notify the owner, engineer, and geotechnical engineer immediately
4. Request a site visit from the geotech engineer
5. Get revised recommendations in writing before proceeding

This is a changed condition, and it may entitle you to additional compensation and time depending on your contract terms. The key is documentation. Keeping detailed daily logs and photos of unexpected conditions protects your position if the issue becomes a dispute.

Unsuitable Subgrade

You reach foundation level and the soil is softer than expected. The geotechnical engineer comes out, probes the area, and recommends over-excavation and replacement with compacted gravel. Now you need to figure out the extent of the unsuitable material, remove it, import and place structural fill, compact and test it, and get the engineer to approve it before you can form footings.

This work is almost always outside your original scope and should be handled as extra work. Track the quantities carefully: hours, equipment, imported material, and testing costs. Keep the geotechnical engineer involved for the duration of the repair.

Water in Excavations

Your hole is filling with water and you need to pour concrete tomorrow. Rushing concrete placement into standing water is never the right answer. Water dilutes cement paste, weakens concrete, and creates voids. Dewater the excavation first, even if it means delaying the pour.

For footings, pump the water down and keep it below the bottom of the footing until the concrete has set. For drilled shafts, the engineer should specify whether to place concrete dry (after dewatering) or using tremie methods (placing concrete under water). The method affects the concrete mix design and placement procedure.

Settlement After Construction

If settlement occurs after construction, the cause is usually one of three things: inadequate compaction of fill, consolidation of soft soils that wasn’t fully accounted for, or changes in groundwater conditions. Post-construction settlement investigation requires the geotechnical engineer to evaluate the situation, often with additional borings and testing.

If you documented your earthwork compaction testing and it all passed, you have a strong defense against claims that the settlement was caused by your work. This is another reason why keeping thorough records matters. A well-organized project management system helps you store and retrieve test results, inspection reports, and field documentation when you need them.

Slope Stability and Grading Considerations

Building on or near slopes adds a whole layer of complexity. Literally. The forces acting on a slope are constantly trying to pull the soil downhill, and construction activities can tip the balance toward failure if you’re not careful.

What Makes Slopes Fail

Slopes fail when the driving forces (gravity, water pressure, surcharge loads) exceed the resisting forces (soil shear strength). The most common triggers for slope failure during construction are:

• Cutting into the toe of a slope without adequate analysis or retaining structures
• Adding load to the top of a slope by stockpiling materials or placing fill
• Changing drainage patterns so water infiltrates into the slope
• Removing vegetation that was providing root reinforcement and intercepting rainfall
• Vibration from heavy equipment or pile driving

If your project involves cutting into a hillside or building near the top or bottom of a slope, the geotech report should include a slope stability analysis. This analysis calculates a factor of safety for the slope under various conditions (static, seismic, with and without water). A factor of safety of 1.5 is the typical minimum for permanent slopes. Temporary construction slopes may be allowed at lower factors of safety, but never below 1.3.

Temporary Cut Slopes

OSHA requires that excavation slopes meet specific angles based on soil type:

• Stable rock: Vertical (90 degrees)
• Type A soil (cohesive, unconfined compressive strength 1.5 tsf or greater): 3/4:1 (53 degrees)
• Type B soil (cohesive, UCS 0.5 to 1.5 tsf, or granular): 1:1 (45 degrees)
• Type C soil (cohesive, UCS less than 0.5 tsf, or granular with water): 1.5:1 (34 degrees)

These are maximum angles. The actual stable angle may be less depending on site-specific conditions. Surcharge loads, water, and layered soil conditions can all reduce the stable angle below the OSHA maximums.

Knowing your soil type for OSHA classification purposes is another reason to actually read the geotech report. If you’re working in Type C soil and your crews are cutting slopes at 1:1, you have a safety violation and, more importantly, a genuine risk of cave-in and injury.

Erosion and Sediment Control

Freshly graded slopes erode. Fast. And erosion isn’t just an environmental compliance issue (though the fines for sediment discharge can be substantial). Erosion can undermine foundations, clog drainage systems, and destabilize slopes.

Install erosion control measures as quickly as possible after grading. Silt fence at the toe, erosion control blankets on slopes steeper than 3:1, and seeding or hydroseeding as soon as the final grade is established. Temporary diversion berms and swales should direct runoff away from freshly graded areas.

The geotech report may include recommendations for permanent slope protection, such as riprap, gabions, or reinforced soil slopes. These should be incorporated into the grading plan from the beginning, not treated as an afterthought.

If you’re juggling earthwork across multiple jobs, tracking what’s been graded, what needs erosion control, and what’s waiting for inspection gets complicated fast. A scheduling tool that gives you visibility across all your active projects makes it much harder for things to slip through the cracks.

Geotechnical Considerations for Specific Project Types

Different project types have different geotechnical priorities. Here’s a quick rundown of what to watch for on common project types.

Residential Construction

Residential projects often get minimal geotechnical investigation, and that’s where problems start. A $3,000 geotech investigation for a $500,000 house is cheap insurance. The most common residential geotechnical issues are:

• Expansive soil causing foundation and slab-on-grade damage
• Inadequate compaction of fill in subdivisions built on regraded sites
• Slope creep or shallow landslides on hillside lots
• Septic system failures due to inadequate percolation

For residential work, make sure the geotech report addresses foundation type, slab design, surface drainage, and any special considerations for the specific lot. Cookie-cutter foundation designs applied across an entire subdivision without lot-specific evaluation are a recipe for callbacks.

Commercial Buildings

Commercial projects typically have more thorough geotechnical investigations and more complex foundation systems. Key issues include:

• Higher structural loads requiring deeper foundations
• Larger footprints meaning more variable subsurface conditions
• Below-grade spaces (parking garages, basements) requiring dewatering and waterproofing
• Pavement design for parking lots and loading areas
• Environmental considerations if the site has previous industrial use

The coordination between the geotechnical engineer, structural engineer, and contractor is critical on commercial projects. Changes in one discipline (say, adding a mezzanine that increases column loads) ripple through to the others. Keep communication lines open and make sure everyone is working from the same version of the geotech report.

Infrastructure and Site Development

Roads, utilities, and site grading are all geotechnically sensitive. Trench backfill for utilities is one of the most common sources of settlement problems in new developments. Every pothole over a utility trench is evidence of inadequate backfill compaction.

For road construction, the geotech report provides the subgrade strength (usually expressed as a California Bearing Ratio or CBR) and the pavement section design. Weak subgrades need thicker base courses or geogrid reinforcement.

Mass grading for subdivisions can involve enormous volumes of cut and fill. Settlement of deep fill sections is a real concern. If fills exceed 10 to 15 feet, the geotech engineer should evaluate whether the fill itself will settle under its own weight and whether the native soil below the fill can handle the added load.

Working with the Geotechnical Engineer

The geotechnical engineer is one of your most valuable resources on a project, but only if you actually use them. Here’s how to get the most out of that relationship.

Before Construction

Review the geotech report thoroughly before you finalize your bid or GMP. If anything is unclear, call the geotechnical engineer and ask. They’d rather answer questions before construction than deal with problems during construction.

Flag any areas where the borings seem sparse or where conditions are described as “variable.” These are your risk areas. Consider requesting additional borings if the investigation doesn’t cover critical areas of the site.

Include allowances in your estimate for geotechnical unknowns: unsuitable soil removal, dewatering, and additional compaction testing. The geotech report gives you a reasonable basis for these allowances, but they should never be zero.

During Construction

Have the geotechnical engineer on call for site visits during earthwork and foundation construction. Many projects include a geotechnical observation scope where the engineer’s field representative observes critical operations like:

• Foundation subgrade preparation and approval
• Proof rolling of subgrade
• Fill placement and compaction testing
• Pile driving or drilled shaft construction
• Shoring installation
• Dewatering operations

The geotechnical engineer’s field approval of subgrade conditions before concrete placement is one of the most important quality checkpoints on any project. Don’t pour without it.

If field conditions differ from the report, get the geotechnical engineer out immediately. Don’t try to interpret changed conditions yourself, and definitely don’t proceed hoping it will be fine. The cost of a site visit is trivial compared to the cost of a foundation problem.

After Construction

Keep a copy of the geotechnical report in your project files permanently. If settlement or other geotechnical issues develop years later, that report is essential documentation.

Also keep your compaction test results, inspection reports, and any field observation memos from the geotechnical engineer. This documentation package is your evidence that the work was done in accordance with the geotechnical recommendations. Having all your project documentation organized in one place through a solid document management system means you can pull up test reports and field notes years after the project closes.

Budgeting for Geotechnical Risk

One of the biggest mistakes contractors make is treating geotechnical costs as fixed numbers. They’re not. Subsurface conditions are inherently variable, and your budget needs to account for that variability.

What to Include in Your Estimate

Definite costs:

• Mobilization and setup for earthwork equipment
• Mass excavation and grading based on plan quantities
• Import or export of material based on cut/fill balance
• Compaction testing at specified frequencies
• Foundation construction per the structural drawings

Contingency items (based on geotech report risk assessment):

• Unsuitable soil removal and replacement (estimate a range based on boring data)
• Rock excavation if borings indicate rock near excavation depth
• Dewatering if water table is near or above excavation level
• Additional compaction testing for problem areas
• Geotechnical engineer site visits beyond the base scope

A common approach is to carry a geotechnical contingency of 5% to 15% of the earthwork and foundation budget. The exact percentage depends on how much subsurface data you have, how variable the conditions are, and what the consequences of a surprise would be.

Contract Protections

Your contract should address changed subsurface conditions. The standard AIA and ConsensusDocs contracts include differing site conditions clauses that entitle the contractor to additional compensation when actual conditions differ materially from what was indicated in the contract documents (including the geotech report) or from what a reasonable contractor would have expected.

If your contract doesn’t have a differing site conditions clause, you’re carrying all the risk for whatever is underground. That’s a significant financial exposure that should be reflected in your price.

Document everything. The contractor who can show photos, daily reports, and contemporaneous records of changed conditions is in a much better position than the one who just submits a change order six months later based on memory. Keeping all your field documentation organized and accessible through your project management software isn’t just good practice, it directly protects your bottom line.

Putting It All Together

Geotechnical engineering touches every phase of a construction project, from initial site selection through final grading and landscaping. As a contractor, you don’t need to be a soils expert. But you do need to:

1. Read the geotech report. All of it. Before you price the job.
2. Understand the soil conditions and what they mean for your excavation, foundations, and earthwork.
3. Budget for uncertainty. The ground always has surprises. Build contingency into your numbers.
4. Communicate with the geotech engineer. Use them during construction, not just as a report writer.
5. Document everything. Changed conditions, field observations, test results, and approvals.
6. Know when to stop. If conditions don’t match the report, stop and get engineering input.

The contractors who get this right don’t just avoid problems. They build reputations as the crews who know what they’re doing, the ones owners and engineers trust with the difficult sites. That reputation is worth more than any single project.

The ground under your building doesn’t care about your schedule or your budget. But if you respect what it’s telling you, do your homework, and work with competent geotechnical professionals, you can build on almost anything. Safely, on budget, and without the kind of surprises that make you question your career choices.