Question: what was the london olympic stadium built on
[{'answer': 'soft clay', 'provenance': [{'wikipedia_id': '2179597', 'title': 'London Stadium', 'start_paragraph_id': 11, 'start_character': 60, 'end_paragraph_id': 11, 'end_character': 69, 'bleu_score': 1.0, 'section': 'Section::::Design and construction.:Olympic design.:Original structure details.\n'}]}, {'provenance': [{'wikipedia_id': '2179597', 'title': 'London Stadium', 'start_paragraph_id': 11, 'start_character': 0, 'end_paragraph_id': 11, 'end_character': 558, 'bleu_score': 0.9260989517395081, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4653483663165870978}, 'section': 'Section::::Design and construction.:Olympic design.:Original structure details.\n'}]}]