A recent infill project near the Rillito River encountered loose, unconsolidated sands at just four feet below grade—material that conventional borings had missed entirely. The general contractor halted excavation after a trench wall sloughed, and the geotechnical engineer called for an exploratory test pit program to map the lateral extent of the problematic zone. Exposing a vertical face twelve feet deep let the team photograph strata, collect undisturbed block samples, and run pocket penetrometer readings every eighteen inches. In Tucson’s basin-and-range setting, where Holocene alluvium drapes cemented Pleistocene pediment, exploratory test pit work reveals textural transitions, caliche lenses, and buried channel deposits that standard SPT borings can smear or bypass. Our laboratory supports these investigations with ASTM D2487 soil classification, in-situ density checks, and the plate load test when bearing capacity confirmation is needed directly on the excavated floor.
An exploratory test pit converts subsurface uncertainty into a cross-section you can photograph, sample, and measure with a tape.
How we work
Field crews working the Tucson metro quickly learn that shadows matter. Summer excavation between 10 a.m. and 3 p.m. requires continuous sidewall misting to prevent desiccation cracking of fat clays, while winter morning work in the Tanque Verde fan demands extra shoring because overnight moisture loss tightens the crust but leaves deeper silt seams cohesionless. We log every exploratory test pit using the Unified Soil Classification System, photograph the full face with a scale bar and north arrow, and collect representative samples from each distinct horizon. Depth ranges typically reach fifteen feet with a standard backhoe, though we extend beyond twenty feet using long-reach excavators when paleochannel sands or the truncated edge of a Pleistocene terrace need documentation. Every log includes Munsell color, moisture condition, consistency or relative density, caliche hardness grade, and GPS coordinates tied to the project datum.
Local ground factors
Tucson’s post-war expansion pushed subdivisions onto distal bajada slopes where soil profiles shift from coarse proximal gravels to fine, shrink-swell clays within a few hundred feet. Older commercial strips along Speedway and Oracle Road sit on compacted fill placed before modern compaction standards existed. Omitting an exploratory test pit on these parcels leaves the engineer blind to buried demolition debris, undocumented utility trenches, and perched groundwater lenses that form above shallow caliche hardpan after monsoon storms. OSHA Subpart P mandates benching, sloping, or shoring for any excavation deeper than five feet, and a competent person must inspect the pit before personnel entry. A collapsed excavation wall in unsaturated sandy silt can trap a worker in seconds; this is not a theoretical hazard in Tucson’s soil profile, where cohesion drops sharply once moisture content crosses the plastic limit.
Quick answers
How deep can an exploratory test pit go in Tucson’s basin-fill soils?
With a standard backhoe, we routinely reach fourteen to fifteen feet below grade in the uncemented alluvium common across the Tucson basin. Long-reach excavators extend that to twenty-two feet, though OSHA Subpart P requirements for benching and shoring become more demanding beyond fifteen feet. Depth is ultimately limited by the water table—which in central Tucson can appear at thirty to fifty feet—and by the presence of thick caliche horizons that may require ripping before excavation can continue.
What is the typical cost for an exploratory test pit investigation in Tucson?
A single exploratory test pit with full logging, photographic documentation, and representative sampling generally falls between US$490 and US$770, depending on depth, access conditions, and the number of disturbed versus undisturbed samples collected. Projects requiring multiple pits, extended reach depths, or nuclear density testing on the floor and backfill will shift the total toward the upper end of this range.
Can an exploratory test pit replace a standard boring program?
For shallow foundations, utility corridors, and pavement subgrade investigations, exploratory test pits often provide more actionable information than borings alone because the continuous vertical exposure reveals thin seams, variable cementation, and lateral transitions that split-spoon samples frequently miss. For deep foundations or liquefaction assessment below twenty feet, test pits complement but do not replace SPT borings or CPT soundings.
What safety measures apply to test pit excavation in Tucson?
All pits deeper than five feet are benched, sloped, or shielded in accordance with OSHA Subpart P. A competent person inspects the excavation daily and after any rain event—particularly important during Tucson’s monsoon season when brief intense storms can saturate sidewalls. Atmospheric monitoring is conducted before entry when pits exceed four feet, and access ladders are placed within twenty-five feet of any worker.
