An Evaluation of Using Void Box for Slab-On-Grade Foundation on Expansive Clay

The purpose of this study was to evaluate the performance of two types of molded paper void boxes for lightly loaded residential foundation such as slab-on-grade foundation on expansive clay. Void box is a technology to isolate the foundation from soil and reduce the effect of volume change of expansive soil on slab-on-grade foundation. As the void boxes are made of recycled waste paper, this study focused on load carrying capacity of void boxes for different moisture content, rate of water absorption of slab and beam void boxes and the deformation of void box due to the soil volume change.

In order to determine the compressive strength of void boxes as they absorb water, both types of void box were kept into humidity chamber at 100% humidity and were subjected to compression tests after absorbing certain amount of water. The rate of water absorption by the void boxes were also determined with time. A model test was also performed in the laboratory to assess the moisture induced volume change of expansive soil under the support and the void area of the void boxes.

The results showed that both type of void boxes could carry approximately 80 kPa stress at dry situation. As the moisture content increases, both types of void boxes lose their strength at a rapid rate up to a moisture content of 25%. However, the beam void boxes showed a higher load carrying capacity than the slab void boxes at a certain water content. The strain at failure of the void boxes was in the range of 9-12%. The study showed that the rate of water absorption of beam void boxes was also lower than the slab void boxes. After 28 days in the 100% humid condition, the slab void box absorbed 100% water, whereas, the beam void box absorbed 52% water. The study also showed that the moisture induced swelling of soil under the void spaces of was significantly higher compared to that under the contact areas of the void box.