Over-Excavation and Recompaction in Casper, Wyoming

A geotechnical report that says "remove and replace the upper four feet" is a quantity problem as much as a soils problem. On a 40,000 square foot warehouse building pad, four feet is nearly 6,000 cubic yards of material to dig, haul, replace and compact. Over-excavation is the work we do when the native ground is not good enough to carry the building, and it is one of the largest items on many commercial site budgets.

We remove the weak soil to the depth the engineer defines, inspect the bottom, then rebuild the section in compacted lifts with approved material. Done correctly, the result is a uniform platform. Done poorly, it is a soft spot under a floor that opens a crack three years later.

The same technique repairs failed areas after construction starts: a rutted truck court, a wet corner of a pad, or a zone where a utility trench settled and softened the surrounding soil. We take the same careful steps on a repair as on a new pad.

Why native ground sometimes has to go

Weak soils come in several forms. Organic or soft clay layers compress under load. Loose fill left from earlier work has no history and no density. Frost-susceptible silt can heave under a slab in winter, and wet pockets turn to mush when a loaded truck crosses them. Any of these can ruin a building pad if they are left in place.

We read the boring logs, walk the site, and look at the surface after rain. Proof-rolling on the stripped subgrade shows soft areas the borings missed, because borings are small samples of a large site. When the proof-roll disagrees with the report, we tell the engineer, who decides on the extent of removal.

Old industrial ground deserves extra suspicion. Earlier yards and plants were built, expanded and torn down over many years, and the fill left behind rarely matches any record. The ground at Platte River Commons, a former refinery property, is the sort of place where a boring that looks fine may sit a few feet from a trench backfilled with debris. We would rather find it with a bucket than with a slab.

Ground near the river and in the older industrial areas

Casper is located on the North Platte River, and the floodplain and terrace ground near it behaves differently from the benches and uplands. Shallow groundwater, layered alluvium and old fill are common near the river corridor and in the earlier industrial districts. Casper also lists a three-foot frost line and severe weathering, which means any pad carrying heated or cold-sensitive loads must be set on stable, free-draining material.

The city expects engineered building and site drawings, and plan review covers engineering reports. For a pad that needs over-excavation, the geotechnical report and the removal depth go into that submittal, so our field quantities have to match what the engineer designed.

Remove, inspect, replace, test

The work is excavation work at heart. It begins with exposing the area and cutting the weak layer, taking care not to overshoot the depth. We keep the bottom dry and avoid disturbing it with tracks. The engineer or tester inspects the bottom, sometimes with probing or additional borings, and approves it before we cover it.

Replacement material is placed in lifts of about eight inches loose, moisture conditioned and compacted to the specified percentage of maximum dry density. Geotextile or geogrid goes in where the plan requires it. Each lift is tested, and failing areas are reworked before the next lift goes on. The final surface is trimmed and proof-rolled, ready for the pad work described under building pad preparation and the final trim in fine grading.

Keeping the hole from growing

Over-excavation quantities have a habit of increasing. We control this by cutting in sections, inspecting as we go, and stopping at the depth the engineer approves. A wide, sloped cut that follows the failure zone costs less than a rectangular box dug to the deepest point.

We also manage water. Surface runoff and groundwater turn the hole into a pond, and a pond softens the bottom we just approved. Ditches, sumps and a graded bottom keep it workable. If rain is forecast, we do not leave a half-finished section open overnight; we close it or protect it. When imported material is part of the plan, see our notes on import, export and haul-off.

What the engineer will want in writing

Documentation matters on this scope more than on most. We record the limits of removal by station and elevation, photograph the bottom, keep the delivery tickets for the replacement material, and file the compaction test results. The 2024 International Codes are listed by the City of Casper, with amendments in Title 15 of the municipal code, and the inspector may ask for a special inspection report on fills under structures.

The record helps later too. When a floor crack shows up years from now, the owner and engineer can check what was under it. If you plan engineered fill and soil stabilization as an alternative, the same records apply.

How to budget a removal-and-replace item

The unit is cubic yards, but the cost is dominated by depth, haul and replacement material. A deeper cut needs flatter slopes and more spoil. A longer haul adds trucks. Imported structural fill is priced by the ton and delivered by truck, and the compaction work adds roller hours and testing. Fabric and grid add material cost and labor.

We recommend owners carry a contingency for this scope until the dig is open. We give a unit price for additional depth and a clear statement of what is in the base number. A project for logistics and warehouse developers with a large pad can see the quantity move by thousands of yards, and a unit price turns that into a manageable change.

Send the geotechnical report

We will price the removal depth, the replacement material and the compaction testing, and tell you how much of the quantity is certain and how much depends on what the dig shows.

Questions Owners Ask

How do you know how deep to dig?

The geotechnical report gives a design depth, and the field inspection confirms it. We dig to the depth where firm, stable soil is exposed and the engineer approves the bottom. That depth can be shallower or deeper than the report suggested.

Can the removed soil be reused?

Sometimes. If it is suitable and can be dried or conditioned, the engineer may allow reuse. Organic soil and wet clay usually cannot go back. We test before we decide, and we do not stockpile a material we cannot place.

What compaction level is typical?

Structural fill under slabs and footings is commonly compacted to a high percentage of standard or modified Proctor density, often 95 percent or more. The exact figure is in the specification, and we test to it.

How long does it take?

A few days for a small pad and several weeks for a large one. The pace depends on depth, haul distance, weather and the testing cycle. Wet or frozen weather slows it more than anything else.

Will over-excavation delay the foundation?

It can if it is discovered late. If the geotechnical report flags weak soil, we plan the removal as the first activity and keep the foundation crew off the area until the platform is tested and accepted.

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