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ainly drawn to the studies on lexical chunks in argumentative writings. Thestudies on the use of lexical chunks in expository writings are comparatively in a small volume.Therefore, it is worthwhile to explore the role of lexical chunks played in expository writings.
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Chapter Two Literature Review

This chapter reviews the literature associated with the main areas of interest in this study.These areas are firstly, lexical chunks and SLA; secondly, corpus and corpus linguistics; andthen, research studies concerned with corpus-based studies on lexical chunks at home andabroad; finally, there is a brief summary to the whole chapter.

2.1 Lexical chunks and SLA

The study of lexical chunks in SLA has been a prominent research area in the secondlanguage field. A number of studies (Diao, 2004; Duan, 2008; Biber et al., 2007) have clearlyshown that there is a substantial amount of formulaic language, namely lexical chunks, inactual language use. As what Ellis (2000) once pointed out that L2 learners acquire a largenumber of lexical chunks, which they use to perform communicative functions, are importantto learners; also the lexical chunks contribute to the fluency of the learners’ unplanned speech.Hyland (2008:4) states that “lexical chunks are both central to the creation of academicdiscourse and a key factor in successful language learning.” Thus, an conclusion could bedrawn that lexical chunk plays a significant role no matter in language acquisition,comprehension, or in the language production.

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2.2 Corpus and corpus linguistics
Corpus is a collection of texts stored in an electronic database, which provides a largeempirical database of discourse so that analyses are based on naturally occurring structures andpatterns of language use rather than intuition or perception. A corpus study could be treated asthe study on language and language variety that the corpus itself represents for, and theconclusions that drawn from the research could be expanded to all the language and languagevariety.Various kinds of corpus and quite a number of corpora are now available for differentresearch purposes. Here, a brief introduction of corpus types will be given. General corpus,often regarded as reference corpus, helps to reveal linguistic features of certain corpora, likeBritish National Corpus (BNC), American National Corpus (ANC) and so on. Specializedcorpus, also known as special purpose corpus, is established by authentic corpora data of acertain domain. Learner corpus is made up of language data of non-native language learners. Itcan be mainly divided into spoken corpus and written corpus. There are influential learnercorpora in China like Chinese Learners’ English Corpus (CLEC), constructed by Gui & Yang,2003; Spoken and Written English Corpus of Chinese Learners, SWECCL 1.0 & 2.0,established by Wen in 2005, 2008, and so forth. There are other corpus types like synchroniccorpus, diachronic corpus, native speakers’ corpus, bilingual corpus and multilingual corpusetc..
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Chapter Three Research Methodology......14
3.1 Research questions.... 14
3.2 Research design.........14
3.2.1 Corpus used..... 15
3.2.2 Tools used........17
3.2.3 Research procedures....... 18
3.3 Summary.... 29
Chapter Four Data Analysis and Discussion....31
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