Reliability research of soil moisture content measurement by microwave oven drying method for earthen monuments

The deterioration of ancient earthen monuments are highly related to the change in soil moisture content, so there is a practical need to find a way to measure accurately and easily the moisture content at heritage site. This paper reports the test results of soil water content measured by microwave drying method (MDM) compared with the standard method, the oven drying method (ODM), using soil samples including earthen monuments, saline soil and loess soil. The origin of test error by MDM is analyzed based on liquid limit test, plastic limit test, salt content test results. It has been found that the error of soil moisture content measured by MDM is increased with the increase in soil plasiticity compared with that measured by ODM; but the maximium error is restricted within 2%, satisfying the requrement of national code. Soil salt content is responsible for increasing the drying rate of the soil in MDM, but has no effect on the final masurement of soil moisture content. As a final conclusion, this research indicates that the MDM is a reliable method for soil moisture measurement in heritage conservation for its advantages of accuracy, speediness and convinence.