SPT Testing in Milwaukee: Subsurface Data for Foundation Design

Milwaukee's subsurface is dominated by glacial till, lacustrine clay, and the underlying Niagara Dolomite, with bedrock depth varying from less than 10 feet near the lakefront to over 100 feet inland. The Standard Penetration Test (SPT) remains the most practical method for logging these complex soil profiles. We run the SPT per ASTM D1586 using a safety hammer with calibrated energy ratio, recording blow counts every 2.5 feet through the overburden. N-values in the Estuarine clay units often fall below 8 near the Menomonee Valley, requiring careful interpretation for settlement analysis. For deep fill zones around the Port of Milwaukee, combining SPT data with a CPT test refines the stratigraphic boundary between anthropogenic material and natural deposits. Our field crews have extensive experience navigating tight urban sites in Walker's Point and the Third Ward.

Corrected SPT N60 values in Milwaukee's glacial till typically range from 12 to 35 above the water table, dropping significantly in saturated lacustrine clay.

Our approach and scope

The freeze-thaw cycles typical of a Great Lakes climate influence near-surface soil structure, making seasonal timing relevant for SPT interpretation in Milwaukee. We sample with a standard 2-inch OD split spoon and log the Unified Soil Classification per ASTM D2487 as drilling progresses. In areas underlain by compressible organic silt, such as portions of Bay View and the Harbor District, N-values must be corrected for overburden pressure before any bearing capacity calculation. This corrected data feeds directly into shallow foundation design, and for sites where spread footings prove marginal, the mat foundations option can be evaluated using the same SPT dataset. The test also provides disturbed samples suitable for grain size analysis, which helps confirm the drainage characteristics of granular fill layers commonly encountered in redeveloped industrial parcels.

Local geotechnical context

A four-story mixed-use building planned near Brady Street encountered N-values of 4 to 6 in a soft silty clay layer at 18 feet depth. The initial geotechnical report flagged excessive total settlement under the proposed column loads. Without the SPT data, the structural engineer would have assumed a much stiffer bearing stratum. The design team pivoted to a deep foundation solution, using the SPT blow counts to estimate pile driving resistance and select the appropriate pile length. Ignoring low N-values in Milwaukee's buried valley deposits invites differential settlement that manifests as cracked partition walls and misaligned elevator rails within the first five years of occupancy. The cost of a supplemental boring program after structural framing is complete far exceeds the cost of a thorough initial SPT investigation across the building footprint.

Technical parameters

Parameter	Typical value
Standard	ASTM D1586 / AASHTO T 206
Hammer type	Safety hammer with ERi calibration
Sampler	Standard split spoon, 2" OD, 1.375" ID
Depth interval	Every 2.5 ft (0.76 m) or at stratum change
N-value reporting	N60 corrected for energy and overburden
Soil classification	ASTM D2487 (USCS) per sampled interval
Typical rig access	Track-mounted CME-55 or equivalent

Other technical services

Standard Penetration Test Drilling

Mobilize a track or truck-mounted drill rig for SPT borings to depths of 20 to 100 feet. Includes split-spoon sampling at 2.5-foot intervals, N-value recording, and soil description by an on-site geologist.

N60 Correction & Foundation Report

Correct raw N-values for hammer energy, overburden pressure, and rod length per ASTM D6066. The report computes net allowable bearing capacity and estimated settlement for shallow footings.

Liquefaction Screening Package

Evaluate SPT data against the Seed-Idriss simplified procedure for the design earthquake. The screening covers clean sand, silty sand, and low-plasticity silt lenses identified in the boring logs.

Quick answers

What is the typical cost of an SPT boring in Milwaukee?

For a single SPT boring to 30 feet depth in the Milwaukee area, the cost ranges from US$480 to US$720, which includes mobilization within the city, the drilling crew, split-spoon sampling, and a factual data report with N-values and soil descriptions. Deeper borings, difficult access sites, or multiple borings per day affect the unit rate.

How deep do SPT borings need to go for a typical Milwaukee building?

The IBC requires borings to extend through all compressible strata and at least 10 feet into competent bearing material. In Milwaukee, where bedrock is shallow near the lake, 30 to 50 feet is common; inland sites with thick glacial deposits may require 60 to 80 feet to pass through the soft lacustrine clay.

How do you handle groundwater during SPT drilling?

The driller records the water level encountered during boring and again after 24 hours for a stabilized reading. In Milwaukee, the groundwater table often sits within 5 to 10 feet of the surface near the rivers and lake. We use hollow-stem augers to keep the hole open and prevent caving in saturated sand layers.

Can SPT data be used to classify the seismic site class?

Yes, the IBC and ASCE 7 use the average N-value in the upper 100 feet to assign Site Class A through F. In Milwaukee, corrected SPT N60 values from multiple borings are averaged over the top 100 feet to determine if the site falls into Site Class C (dense soil/soft rock) or Site Class D (stiff soil), which directly influences the seismic design spectrum.