If done properly (keeping in view that small adjustments and sensitivities can create  tipping point thresholds) may help to mitigate gentrification and segregation.

Article Residential Preferences and Neighborhood Racial Segregation:...

Article From Schelling to Spatially Explicit Modeling of Urban Ethni...

Absolutely! Urban Planning can create gentrified and segregated communities, which are ethnically and socially homogeneous. This happens in most cases and in general poor planning displaces discriminated sectors of the population to the margins while the wealthiest and dominant classes occupy the most desirable sector of the city. This can be seen quite clearly in Paris, London, New York and many other major cities. 

 On the other hand, a good planning process can lead to multicultural, plural and diverse communities where different social groups and ethnicities can flourish.  

For further reading:

https://books.google.fr/books?hl=en&lr=&id=P44M4UpSzxQC&oi=fnd&pg=PR3&dq=plural+society+urban+planning+diversity+pluralism+intercultural+multicultural&ots=01XIY2vdHe&sig=PN2NZF5QdSB1ZgtOiNDXtagkh-A#v=onepage&q&f=false

http://epn.sagepub.com/content/34/6/959.short

http://onlinelibrary.wiley.com/doi/10.1111/j.1468-2427.2005.00614.x/abstract;jsessionid=DEF2C5EAF5BE2465B8DA3B27DEF18339.f03t02?systemMessage=Wiley+Online+Library+will+be+unavailable+on+Saturday+14th+May+11%3A00-14%3A00+BST+%2F+06%3A00-09%3A00+EDT+%2F+18%3A00-21%3A00+SGT+for+essential+maintenance.Apologies+for+the+inconvenience.&userIsAuthenticated=false&deniedAccessCustomisedMessage=

I just watched a formidable talk at the RSA on housing poverty and marginalization with Matthew Desmond a social scientist and ethnographer who is co-director of the Justice and Poverty Project at Harvard University. This talk discusses the housing situation in the US and the UK and explains how the poor planning decisions and lack of coherent government policies have seriously increased poverty, ethnic segregation and marginalization. I think it can be of great interest to your question! 

https://www.youtube.com/watch?v=JHb2NNfWOnI&feature=youtu.be

If we talk abstractly — that is to say without any reference to empirical realities — we can say that planning decisions may foster or hinder pluralism depending on the substance/contents of policies and programs.

In reality, however, the evidence, worldwide, shows that planning since the 1960's has not been friendly to diversity, multiplicities, or plural society (to use your words). Just think of the record of "public housing" particularly in the developing countries and in the old Soviet Union. Or think about urban renewal projects in Europe and North America. Or think about mega projects, for example dams and land reclamation. Or think about the so-called "new urbanization" policies (recently in vogue) that presume to favour sustainability and end up creating exclusive neighbourhoods affordable only to the higher income/status groups. What you find is unfortunately a dark record of anti-pluralistic outcomes.

The ardent apologists will, of course, say: these are all examples of "bad" planning. Show us, then, examples of "good" planning: ones that actively fostered diversity and co-existence.

Researches rarely, if at all, answer yes or no questions. Thinking critically about town/city planning, it is a field of study dealing with how we use and maintain our environments in settled communities. Much of the emphasis in the field of study since industrial revolution has been on balancing the physical ecology rather than social and economic factors that have to do with people who the users of the environment. That is, we have focused on how to use the environment most efficiently for most benefits rather than various dimensions of human attitudes towards the environment. Hence, I suggest that there will be few empirical studies if any that show the relationships between town/city planning as public, private, or nonprofit institutions and those social and economic factors of human users. It could be fruitful to conduct a phenomenological study of this issue from literature review or case study. I do not think cause-effect relationship study is feasible at the present theoretical base of town/city planning as field of knowledge; though it is plausible that town/city planning has to with formation of plural society. Therefore, the question could be rephrased for research as follows: How does town/city planning affect the formation of plural society?

Researches rarely, if at all, answer yes or no questions. Thinking critically about town/city planning, it is a field of study dealing with how we use and maintain our environments in settled communities. Much of the emphasis in the field of study since industrial revolution has been on balancing the physical ecology rather than social and economic factors that have to do with people who the users of the environment. That is, we have focused on how to use the environment most efficiently for most benefits rather than various dimensions of human attitudes towards the environment. Hence, I suggest that there will be few empirical studies if any that show the relationships between town/city planning as public, private, or nonprofit institutions and those social and economic factors of human users. It could be fruitful to conduct a phenomenological study of this issue from literature review or case study. I do not think cause-effect relationship study is feasible at the present theoretical base of town/city planning as field of knowledge; though it is plausible that town/city planning has to with formation of plural society. Therefore, the question could be rephrased for research as follows: How does town/city planning affect the formation of plural society?

Researches rarely, if at all, answer yes or no questions. Thinking critically about town/city planning, it is a field of study dealing with how we use and maintain our environments in settled communities. Much of the emphasis in the field of study since industrial revolution has been on balancing the physical ecology rather than social and economic factors that have to do with people who the users of the environment. That is, we have focused on how to use the environment most efficiently for most benefits rather than various dimensions of human attitudes towards the environment. Hence, I suggest that there will be few empirical studies if any that show the relationships between town/city planning as public, private, or nonprofit institutions and those social and economic factors of human users. It could be fruitful to conduct a phenomenological study of this issue from literature review or case study. I do not think cause-effect relationship study is feasible at the present theoretical base of town/city planning as field of knowledge; though it is plausible that town/city planning has to with formation of plural society. Therefore, the question could be rephrased for research as follows: How does town/city planning affect the formation of plural society?

Dear colleague,

You might be interested in discussions in France about the concept of "mixité" (social and spatial 'mixity'), that has been for decades at the core of urban policies in the country and that is now critically examined by scholars for having had effects precisely contrary to what the ideological discourse promoted.

If you don't read French, see Mustafa Dikec's work: his book Badlands of the Republic (Wiley, 2011) and, for example

http://eur.sagepub.com/content/14/4/277.short

If you do read French instead, you might like to begin with this article by Eric Charmes:

http://www.laviedesidees.fr/Pour-une-approche-critique-de-la-mixite-sociale

or with Hacène Belmessous' pamphlet: Mixité sociale : une imposture: Retour sur un mythe français (L'Atalante, 2006)

Best regards

d.