THE KAOLINIZATION QUESTION

Alteration of a normally hard consolidated argillite rock formation to a soft, decomposed white shale is a geologic feature known as kaolinization. Cliff Kaye of the USGS put engineering geologists on the alert for this condition in the Boston Basin in 1967 where it has been found in over 20 locations sometimes as deep as 300 feet below ground level. The impact of an undiscovered kaolinized zone in a foundation can change the bearing capacity of a spread footing in rock or a drilled shaft in rock from 40 tons per square foot to as little as 8 tons per square foot according to most building codes in the northeast. Normal protocol during controlled inspections of footings is to remove decomposed materials by additional rock excavation, or to drill a few extra feet to obtain sound rock. At late stages of the foundation construction, it is often impractical to change the size of the footing, and the shoring or dewatering system may not be capable of being extended to greater depths. Prediction of this condition is an important role for our profession. Geologically, there is a scientific conundrum as to whether the kaolinization is a result of past hydrothermal alterations of deep origin, or if the alteration is a result of intense surface weathering during a long interglacial period. Some light is shed on this problem by recent subsurface investigations and construction monitoring of drilled foundations near Orient Heights, East Boston. While kaolinized zones were not found, evidence of deep rounded pebble zones, greensands, cemented zones, and deeply weathered regolith indicative of pre-Wisconsin glaciations were discovered preserved in the rugged harbor topography. This paper discusses the geomorphic setting of the drumlins and coastlines contributing to these unusual deep drilling conditions and reviews the impact on foundation construction.