CE357 Lab 7 Direct Shear

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University of Texas *

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357

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Mechanical Engineering

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Dec 6, 2023

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Laboratory 7 Direct Shear To: Ahmet Mert Kavala From: Nicholas Garcia, neg752 Partners: Jaci Vance, Mason Sirianni, Nhat Anh Jang Date: 11/3/2023 Objective The primary purpose of this lab exercise was to be able to perform a direct shear test on a specimen of dense sand and loose sand. A direct shear test is used to measure the shear stress at a failure for a corresponding normal stress as well as defining the Mohr-Coulomb failure envelope along with the cohesion and angle of internal friction parameters. These hear characteristics of the soil are important properties to keep in mind when doing construction work as these properties give an indication of the soil ’ s strength. Procedure Differences between Manual and Lab For the most part, there weren’t really a lot of differences between our experimental procedure and the procedure outlined in the manual. The main difference that I observed was that in the lab manual it called for running two tests, one for the dense sand and one for the loose sand, whereas in the lab, we did only one test in total. Additionally, the test only ran for 10 minutes total, whereas in the lab manual, it ran longer. We also didn’t do as many of the calculations and plotting outlined in the lab manual as we were going to do them post-lab using the provided data. Results and Discussion Data: Loose Sand

Dense Sand Plots: Loose Sand

Dense Sand Sample Calculations 𝐴??𝑎 𝐶??????𝑖?? 𝐹𝑎???? = = 2 𝜋 (??? −1 ( 0 63 ) − ( 0 63 ) √ 1 − ( 0 63 ) 2 ) = 1 𝐶???????? 𝐴??𝑎 = 𝐶??????𝑖?? 𝐹𝑎???? ∗ 𝐴??𝑎 = 1 ∗ 3,117.24531 = 3,117.24531 ?? 2 𝐶???????? 𝑁???𝑎? 𝑆????? = 𝑁???𝑎? 𝑆????? ∗ 𝐴??𝑎 𝐶???????? 𝐴??𝑎 = 109.1 ∗ 3,117.24531 3,117.24531 = 109.1 ?𝑃𝑎 𝐶???????? 𝑆ℎ?𝑎? 𝑆????? = 𝑆ℎ?𝑎? 𝐹???? 𝐶???????? 𝐴??𝑎 ∗ 1000 = 0 3,117.24531 ∗ 1000 = 0 ?𝑃𝑎 𝑆???? = tan(𝜑) −→ 𝜑 = ?𝑎? −1 (?????) = ?𝑎? −1 (0.4836) = 25.8 𝑜

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I would say that these results are reasonable or typical for the most part. The comparison of each curve justifies the idea that more weight being used to hold the sample soil down will need more shear stress to cause shear failure within the soil sample. Therefore, the shear stress at failure increased as the effective stress increased. It is also expected that the curves of the loose sand are not as defined as those of the dense sand. This is because a stress failure is much more apparent in readings for a dense soil due to the high compactness as compared to a loose soil sample. Possible errors from this experiment include the soil sample used in the direct shear test was too thick, not including the weight of the hanger in the normal force or the level arm being improperly balanced, a student performing the test failed to notice the motors ability to shut off when deformations become too large, and the creation of the loose sand soil sample. Lab Questions 1) All necessary plots and sample calculations for this question will be located in the results and discussion section of this lab report. 2) All necessary plots for this question will be located in the results and discussion section of this lab report. After analyzing the curves, with increasing normal stress, the magnitude of pure shear stress increased. We can also see that horizontal displacement slightly decreased as the normal stress increased. 3) While I have gone through the majority of errors in the results and discussion section of this lab report, I would say the most significant error would be the fact that it is very easy to assume that the sample failed when in fact it has not failed which could lead to inaccuracies during the lab. Therefore, it is critical that students double check and make sure that the sample has actually failed before they continue the analysis of the specimen. 4)

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