Geotechnical and Structural Material Analysis for a Four-Storey Residential Development in Annex, Eldoret

Abstract

This study evaluates the geotechnical properties of reddish gravel material intended for a foundation base and the compressive strength of concrete used for ground floor slabs at the Annex Site. A comprehensive series of laboratory tests were conducted, including Particle Size Distribution (Dry Sieve), Atterberg Limits (BS 1377 Part 2), Moisture-Density Relations (AASHTO T99), California Bearing Ratio (AASHTO T193), and Concrete Compressive Strength (BS 1881). The subgrade soils exhibited Liquid Limits ranging from 43% to 57% and Plasticity Indices between 8% and 13%, indicating low to medium plasticity. Compaction tests revealed Maximum Dry Densities (MDD) ranging from 1493 kg/m³ to 1654 kg/m³, with Optimum Moisture Contents (OMC) between 17.6% and 24.2%. Soaked CBR values at 95% MDD varied across the site from 16% to 28%, demonstrating adequate to good bearing capacity, though noticeable spatial variability exists. Furthermore, Class 20/20 concrete specimens yielded an average 28-day compressive strength of 21.2 N/mm², successfully meeting structural design requirements. Overall, the materials are suitable for their intended construction applications, provided site-specific moisture variations are properly managed.

Introduction

Soil samples were collected from four locations, and standard laboratory tests (particle size distribution, Atterberg limits, compaction tests, and California Bearing Ratio tests) were performed to assess plasticity, density, moisture behavior, and load-bearing capacity. The results showed that the soil has low to medium plasticity and CBR values ranging from 16% to 28%, indicating it is generally suitable as a foundation subgrade, with the “Lower Left Water Logged” area performing best.

Concrete samples (Class 20/20 mix) were tested for compressive strength at 7, 14, and 28 days. Strength increased steadily over time, reaching an average of 21.2 N/mm² at 28 days, which exceeds the required design strength of 20 N/mm², confirming structural adequacy.

Conclusion

This laboratory investigation confirmed that the reddish gravel at the Annex Site is a suitable material for use as a foundation base. The soils exhibit manageable plasticity and deliver acceptable soaked CBR values (16% to 28%), supporting the initial hypothesis. Additionally, the concrete testing verified that the mix design is structurally sound, exceeding the required Class 20/20 benchmark with a 28-day compressive strength of 21.2 N/mm². To ensure long-term stability, it is recommended that strict moisture control and proper compaction protocols are enforced on-site, particularly in regions exhibiting higher optimum moisture contents.

References

[1] American Association of State Highway and Transportation Officials [AASHTO]. (2022). Standard Method of Test for Moisture-Density Relations of Soils Using a 2.5-kg (5.5-lb) Rammer and a 305-mm (12-in.) Drop (AASHTO T99). Washington, DC: AASHTO. [2] American Association of State Highway and Transportation Officials [AASHTO]. (2022). Standard Method of Test for The California Bearing Ratio (AASHTO T193). Washington, DC: AASHTO. [3] British Standards Institution [BSI]. (1990). Methods of test for soils for civil engineering purposes. Classification tests (BS 1377-2:1990). London, UK: BSI. [4] British Standards Institution [BSI]. (1983). Testing concrete. Method for determination of compressive strength of concrete cubes (BS 1881-116:1983). London, UK: BSI.

Copyright

Copyright © 2026 Mary Ngusye Muendo, Benedict Shivachi Shikhuma , Cate Njeri Gikunga, Dr. Eng. Clement Kiptum , Mr. Mbugua . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.