Yes, there is an online platform called EduRev that provides full-length mock tests based on previous years' papers for the Civil Engineering optional subject.

EduRev:
- EduRev is an educational platform that offers a wide range of study materials, including mock tests, for various competitive exams, including UPSC.
- It is a trusted platform used by millions of students and educators.
- EduRev provides access to a vast collection of previous years' question papers for the Civil Engineering optional subject.
- The platform allows you to attempt full-length mock tests that are designed to simulate the actual exam environment.

Benefits of using EduRev for full-length mock tests:
- Familiarity with Exam Pattern: By attempting mock tests based on previous years' papers, you can become familiar with the exam pattern, types of questions, and the overall difficulty level.
- Time Management: Mock tests help you practice time management skills by setting a time limit for each test. This allows you to improve your speed and accuracy in solving questions within the given time frame.
- Self-Assessment: After attempting each mock test, EduRev provides detailed solutions and explanations for all the questions. This allows you to assess your performance, identify your strengths and weaknesses, and work on improving them.
- Performance Analysis: EduRev also provides a performance analysis feature that gives you insights into your overall performance, subject-wise performance, and topic-wise performance. This helps you identify the areas where you need more practice and focus your preparation accordingly.
- Real Exam Experience: EduRev's mock tests provide an experience similar to the actual exam, which helps you build confidence and reduce exam-related anxiety.

Conclusion:
Using EduRev's online platform, you can access full-length mock tests based on previous years' papers for the Civil Engineering optional subject. These mock tests help you prepare effectively by familiarizing you with the exam pattern, improving time management skills, and allowing self-assessment and performance analysis.