It is unrealistic to construct a whole, serious poem with spondees. Consequently, spondees mainly occur as variants within, say, an anapaestic structure.
For example (from G. K. Chesterton, "Lepanto"):
This whole verse is rather unusual in structure, making it a somewhat difficult example. The following is a possible analysis, and shows the role of the spondee.
A simpler version of the first line might be:
Two short syllables are added at the beginning, and "founts" is lengthened to "fountains." These extra syllables add "filler," so that when the poem is read stress no longer naturally falls on the syllable "fount" (or, does so to a lesser degree). As a result there are unstressed syllables just before the "fall," so that naturally becomes an anapaest ("fountains fall-," duh-duh-DAH), and the "ing" slips into the following anapaest. Chesterton's version changes all this; it is less intuitive to write and has a more unusual sound. The spondee effects this.
Tennyson often made use of spondaic and pyrrhic substitutions in his work. Here are some examples:
Spondees above are "Well-loved," "This la-," "slow pru-," and "make mild."
There are two spondees in this excerpt: "blood creeps," and "nerves prick."
Another example of a poem using spondee is Gerard Manley Hopkins' Pied Beauty He marks the 6th line thusly to indicate the spondee: "And áll trádes, their gear and tackle and trim." The poem also ends with the short spondee line "Praise Him."