# Geotechnical Engineering Report

## Project Information

- **Project:** Riverfront Mixed-Use Development
- **Location:** 400 Commerce Avenue, Millfield, Colorado 80001
- **Client:** Northwind Builders, LLC
- **Report Date:** November 8, 2025
- **Report Number:** GEO-2025-0042

## Executive Summary

This report presents the results of our geotechnical investigation for the proposed 8-story mixed-use building at 400 Commerce Avenue, Millfield, Colorado. The investigation included 6 borings to depths of 50 to 80 feet below existing grade and 4 cone penetration tests (CPTs).

Based on our findings, we recommend a deep foundation system consisting of drilled shafts bearing in the Denver Formation claystone. Shallow foundations are not recommended due to the presence of expansive soils in the upper 15 feet.

## Subsurface Conditions

### Soil Profile

The subsurface conditions encountered are summarized as follows:

- **0 to 3 ft:** Fill material (sand, gravel, construction debris)
- **3 to 15 ft:** Stiff to very stiff expansive clay (CH), PI = 35-52
- **15 to 30 ft:** Sandy clay to clayey sand (CL/SC), medium dense
- **30 to 50+ ft:** Denver Formation claystone, very stiff to hard

### Groundwater

Groundwater was encountered at depths of 18 to 22 feet below existing grade. Seasonal fluctuations of 5 feet should be anticipated. Dewatering will be required for construction of the below-grade parking levels.

## Foundation Recommendations

### Drilled Shafts

- Shaft diameter: minimum 30 inches
- Bearing stratum: Denver Formation claystone
- Allowable end bearing pressure: 25,000 psf
- Allowable skin friction: 2,500 psf in claystone
- Minimum embedment in claystone: 10 feet
- Design shall comply with ASCE 7-22 and IBC 2021

### Settlement

- Total estimated settlement: 0.75 inches maximum
- Differential settlement: not to exceed 1/2 inch over 50 feet
- Settlement shall be monitored during construction per ASTM D7380

## Seismic Considerations

- **Site Class:** D (Stiff Soil)
- **Seismic Design Category:** B per ASCE 7-22
- SDS = 0.169g, SD1 = 0.085g
- Liquefaction potential: Low (dense claystone at bearing depth)
- Lateral earth pressure for basement walls shall include seismic increment per ASCE 7-22 Section 11.8

## Earthwork Recommendations

### Excavation

- Temporary excavation slopes shall not exceed 1H:1V in native soils
- OSHA 1926.652 applies to all excavations deeper than 5 feet
- Shoring required if adjacent structures are within a 1:1 influence zone

### Fill Placement

- Structural fill: maximum 95% Modified Proctor (ASTM D1557)
- Moisture content: within 2% of optimum
- Lift thickness: not to exceed 8 inches (loose)
- All fill shall be tested per ASTM D6938 at minimum frequency of 1 test per 2,000 square feet per lift

## Limitations

This report has been prepared for the exclusive use of Northwind Builders, LLC for the specific project described herein. The recommendations are based on subsurface conditions encountered at the boring locations and may not represent conditions between borings.

For information only: soil conditions can vary significantly over short distances. Additional investigation may be recommended if conditions differ from those described in this report.
