Yahia E.-A. Mohamedzein

Experimental study of the performance of plastic pipes buried in dune sand

This paper presents a laboratory model to study the performance of plastic pipes installed in dune sand. Model PVC pipes were installed in a rigid steel tank and tested. The model simulated trench construction with the pipe placed in the middle of the tank and dune sand along the boundaries and on the bottom of the tank to simulate the native soil. The bedding underneath the pipe and the backfill around the pipe were constructed using carefully placed and compacted dune sand. For comparison purposes, a well-selected coarse sand was also used as a bedding material. Different factors were considered such as type of bedding, degree of compaction of bedding, soil cover depth, magnitude of applied surface pressure, and load repetition. The experimental results showed that plastic pipes can be installed in well-placed and compacted dune sand bedding and backfill. A minimum soil cover of 2d is required to guard against adverse effects of surface loading on the pipe. The pipe behaved in elastic range during all stages of loading with small amount of unrecoverable deformations upon the removal of all loads. However, significant recovery (or rebound) occurred only after the removal of all applied surface pressure and portion of the soil cover soil up to 1d above the crown.

The use of skirts to improve the performance of a footing in sand

Abstract This paper investigates the use of skirts to improve the bearing capacity and to reduce the settlement of a circular footing resting on dune sand. The skirts are fixed to the perimeter of the footings. A series of tests were conducted on model footings in a large tank and the footings were instrumented to measure normal stresses and settlement. The test results indicate that the skirts increase the bearing capacity and reduce the settlement of the footing. The improvement in the bearing capacity is up to about 470% for a surface footing with skirt of a depth of 1.25B. The skirts also reduced the settlement of the footings to as small as 17% of the original settlement of a surface footing without skirts. A modified bearing equation was proposed for circular footings with skirts. The proposed equation agrees quite well with published experimental results.

Effects of Wetting on the Shear Strength of Plastic Silty Sands

This paper presents the results of intensive laboratory testing program investigating the effects of wetting on the drained shear strength and deformation characteristics of silty sands. The materials used were clean medium to fine grained Nile River bed sand and plastic Nile silt. The sand was mixed with 4%, 8%, 12%, 16%, 25% and 40% by dry weight of the Nile silt. The test specimens were prepared at two relative densities 30% and 79.4% and drained direct shear tests were performed on air dry and wetted “submerged” specimens. Results from the tests, i.e. shear stress versus displacement and volume change versus displacement, were analyzed and discussed. The dry specimens showed compression and then dilation during shearing. The volume change behavior of wetted silty sands depends on the silt content and the relative density. With silt content up to 25%, the wetted loose silty sands compressed during shear whereas the wetted dense mixtures showed compression and then dilation during shear. The wetted specimens with fines content of 40% showed compression only during shear. The wetted specimen with (sand + 40% fines) behaved like fine grained soil. Significant drop in shear strength caused by wetting has been measured for all tested specimens. The drop in shear strength was greater for the dense specimens compared to the corresponding loose ones. The drop showed general trend of increase with fines content.