Identification and quantification of the degradation of historic earthen ‘cob’ structures in Hampshire, United Kingdom

: using a novel spray test method incorporating 3-D laser scanning

Abstract

This research was aimed at identifying and quantifying erosion in earthen walls through laboratory testing of blocks made from materials used for constructing historic earthen ‘cob’ structures in Hampshire, United Kingdom. A review of literature highlighted issues with conventional methods for testing and reporting erosion of earthen blocks/walls; therefore, a novel method for testing durability (erodibility), which combines the use of a ‘modified’ spray test and 3-D laser scanning was developed for this research. In addition, the eroded materials were analysed for PSD. The overall test was structured to assess the performance of three materials (Barton Clay, London Clay and Chalk) using spray cycle and duration as the control measures, i.e., samples were tested for five different duration (15, 30, 60, 120 and 180-seconds) and repeated for five cycles (C1, C2, C3, C4 and C5).
From the results, a new parameter, Eroded material* (Em) for quantifying the rate and degree of erosion in earthen materials was developed and proposed. This was done to enhance correlation of the surface topography results, giving context to factors such as the wall thickness during comparison and overall data reporting. Using Em a range of values were generated from the surface topography results, which in turn allowed for impact rating of the blocks at different stages of erosion
The Chalk was found to be most susceptible to erosion amongst the three materials; having the highest rate of Em per second. Total Em of Chalk at the end of the test was also far bigger than those of the Barton Clay and London Clay; Chalk 30% (severely eroded), London Clay 3% (increasingly eroded) and Barton Clay 2.5% (increasingly eroded). However, the impact rating was found to be same for all three materials in 15- and 30-seconds test; meaning long spray duration had far more damaging impact on the Chalk than the Clays.
The PSD analyses produced eroded particle patterns, which can be grouped into short (15-seconds), intermediate (30 to 120-seconds) and long (180-seconds) spray duration. These patterns were further broken-down to the individual particle-sizes, which acted as ‘markers’ in the overall PSD behaviour.

Date of Award	Jul 2021
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Gareth Michael Swift (Supervisor), Andy Gibson (Supervisor) & Robert John Inkpen (Supervisor)