## 5 Conclusions  
This paper presents the results of an investigation into the potential utilization of TBM muck generated from Doha's Metro Gold Line in different construction applications such as concrete coarse aggregates, fill material under buildings and road fill/subgrade. Physical, chemical and mechanical tests were conducted on the raw TBM material, and the results were compared to the acceptable specifications limits of QCS 2014. This research could launch the start of development of draft clauses to formalize and permit the use of TBM muck as a constituent material in construction. Significant cost savings would be realized if such muck could be used as a local aggregate alternative in lieu of importing aggregates from other countries. The following conclusions are warranted at this time:  
* The gradation of the raw TBM muck does not comply with the QCS 2014 standard gradation requirements for concrete aggregates. Hence, screening and sieving are needed before any attempt to use the muck as aggregate replacement in concrete. However, the TBM muck gradation complies with the QCS 2014 requirements for fill materials under buildings and road fills and subgrades. * The SEM images of the raw TBM muck showed that the particles with clear irregular contours do not exceed a maximum of 50 \(\upmu\)m; however, the gradation results showed that almost all of the particles are larger than 63 \(\upmu\)m. Hence, digital images at larger scale were taken in order to observe the overall morphological properties of the muck. Such images showed mixtures of rounded to angular and elongated shaped particles. * It could be also noted from the SEM images that the muck particles have a relatively large number of voids, and hence, high water absorption values were determined. It could be also observed that the muck surface texture is rough, which may lead to a better bond with cement paste in concrete mixtures. Further testing of concrete mixtures containing TBM muck as a partial or full aggregate replacement is essential to investigate such behavior. * According to the EDX analysis, the silicon (Si) is a dominant component of the TBM muck and its central differs from the silicon content in other coarse and fine aggregates (sand) as reported in the literature. This suggests that the concrete mixtures with TBM muck may exhibit different thermal properties such as the Coefficient of Thermal Expansion (CTE). Hence, further testing on actual concrete mixtures containing TBM muck should be directed toward investigating the thermal properties, specifically. * The testing results showed that the raw TBM muck meets the requirements of concrete coarse aggregates, except for the water absorption, loss by magnesium sulphate soundness, loss by Los Angeles abrasion and the acid-soluble sulphate. The effect of such deviations on the overall behavior of concrete mixture should be investigated by replacing different percentages of concrete virgin aggregates with TBM muck in order to study the effect of such replacement on the fresh and hardened properties of concrete. Addition of HRWRs to the concrete mixtures containing TBM muck is suggested to tackle the high-water absorption/demand. * The liquid limit and plasticity index of the raw TBM muck were higher than the permissible limits of QCS 2014 for both fill materials under buildings and road fills/subgrades, which may limit its use in such construction applications. However, improvement of TBM truck is possible through stabilization or via blending with other high-quality soils or aggregates. Further work should be directed toward this end. * Utilizing TBM truck in road sub-bases and bases was not investigated in this research work. However, there is a good potential that TBM truck could be utilized in this application, and further research is needed. The authors contributed to this research article as follows: literature review, A.A.T., R.T. and M.O.M. ; muck sampling and collection, H.A.A.-M., M.A.-A. and A.A.T. ; experimental work and testing, G.M.A.-Z., M.A.-A. and A.A.T.